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Deployment Guide

Red Hat Enterprise Linux 5

Deployment, configuration and administration of Red Hat Enterprise Linux 5

Edition 11

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Abstract

The Deployment Guide documents relevant information regarding the deployment, configuration, and administration of Red Hat Enterprise Linux 5.

Introduction

Welcome to the Red Hat Enterprise Linux Deployment Guide.
The Red Hat Enterprise Linux Deployment Guide contains information on how to customize your Red Hat Enterprise Linux system to fit your needs. If you are looking for a comprehensive, task-oriented guide for configuring and customizing your system, this is the manual for you.
This manual discusses many intermediate topics such as the following:
  • Setting up a network interface card (NIC)
  • Configuring a Virtual Private Network (VPN)
  • Configuring Samba shares
  • Managing your software with RPM
  • Determining information about your system
  • Upgrading your kernel
This manual is divided into the following main categories:
  • File systems
  • Package management
  • Network-related configuration
  • System configuration
  • System monitoring
  • Kernel and Driver Configuration
  • Security and Authentication
  • Red Hat Training and Certification
This guide assumes you have a basic understanding of your Red Hat Enterprise Linux system. If you need help installing Red Hat Enterprise Linux, refer to the Red Hat Enterprise Linux Installation Guide.

1. Document Conventions
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In this manual, certain words are represented in different fonts, typefaces, sizes, and weights. This highlighting is systematic; different words are represented in the same style to indicate their inclusion in a specific category. The types of words that are represented this way include the following:
command
Linux commands (and other operating system commands, when used) are represented this way. This style should indicate to you that you can type the word or phrase on the command line and press Enter to invoke a command. Sometimes a command contains words that would be displayed in a different style on their own (such as file names). In these cases, they are considered to be part of the command, so the entire phrase is displayed as a command. For example:
Use the cat testfile command to view the contents of a file, named testfile, in the current working directory.
file name
File names, directory names, paths, and RPM package names are represented this way. This style indicates that a particular file or directory exists with that name on your system. Examples:
The .bashrc file in your home directory contains bash shell definitions and aliases for your own use.
The /etc/fstab file contains information about different system devices and file systems.
Install the webalizer RPM if you want to use a Web server log file analysis program.
application
This style indicates that the program is an end-user application (as opposed to system software). For example:
Use Mozilla to browse the Web.
key
A key on the keyboard is shown in this style. For example:
To use Tab completion to list particular files in a directory, type ls, then a character, and finally the Tab key. Your terminal displays the list of files in the working directory that begin with that character.
key+combination
A combination of keystrokes is represented in this way. For example:
The Ctrl+Alt+Backspace key combination exits your graphical session and returns you to the graphical login screen or the console.
text found on a GUI interface
A title, word, or phrase found on a GUI interface screen or window is shown in this style. Text shown in this style indicates a particular GUI screen or an element on a GUI screen (such as text associated with a checkbox or field). Example:
Select the Require Password checkbox if you would like your screensaver to require a password before stopping.
top level of a menu on a GUI screen or window
A word in this style indicates that the word is the top level of a pulldown menu. If you click on the word on the GUI screen, the rest of the menu should appear. For example:
Under File on a GNOME terminal, the New Tab option allows you to open multiple shell prompts in the same window.
Instructions to type in a sequence of commands from a GUI menu look like the following example:
Go to Applications (the main menu on the panel) > Programming > Emacs Text Editor to start the Emacs text editor.
button on a GUI screen or window
This style indicates that the text can be found on a clickable button on a GUI screen. For example:
Click on the Back button to return to the webpage you last viewed.
computer output
Text in this style indicates text displayed to a shell prompt such as error messages and responses to commands. For example:
The ls command displays the contents of a directory. For example: 
Desktop    about.html    logs     paulwesterberg.png
Mail    backupfiles    mail     reports
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The output returned in response to the command (in this case, the contents of the directory) is shown in this style.
prompt
A prompt, which is a computer's way of signifying that it is ready for you to input something, is shown in this style. Examples:
$
#
[stephen@maturin stephen]$
leopard login:
user input
Text that the user types, either on the command line or into a text box on a GUI screen, is displayed in this style. In the following example, text is displayed in this style:
To boot your system into the text based installation program, you must type in the text command at the boot: prompt.
<replaceable>
Text used in examples that is meant to be replaced with data provided by the user is displayed in this style. In the following example, <version-number> is displayed in this style:
The directory for the kernel source is /usr/src/kernels/<version-number>/, where <version-number> is the version and type of kernel installed on this system.
Additionally, we use several different strategies to draw your attention to certain pieces of information. In order of urgency, these items are marked as a note, tip, important, caution, or warning. For example:

Note

Remember that Linux is case sensitive. In other words, a rose is not a ROSE is not a rOsE.

Note

The directory /usr/share/doc/ contains additional documentation for packages installed on your system.

Important

If you modify the DHCP configuration file, the changes do not take effect until you restart the DHCP daemon.

Warning

Do not perform routine tasks as root — use a regular user account unless you need to use the root account for system administration tasks.

Warning

Be careful to remove only the necessary partitions. Removing other partitions could result in data loss or a corrupted system environment.

2. Send in Your Feedback
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If you find an error in the Red Hat Enterprise Linux Deployment Guide, or if you have thought of a way to make this manual better, we would like to hear from you! Submit a report in Bugzilla (http://bugzilla.redhat.com/bugzilla/) against the component Deployment_Guide.
If you have a suggestion for improving the documentation, try to be as specific as possible. If you have found an error, include the section number and some of the surrounding text so we can find it easily.

Part I. File Systems
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File system refers to the files and directories stored on a computer. A file system can have different formats called file system types. These formats determine how the information is stored as files and directories. Some file system types store redundant copies of the data, while some file system types make hard drive access faster. This part discusses the ext3, swap, RAID, and LVM file system types. It also discusses the parted utility to manage partitions and access control lists (ACLs) to customize file permissions.

Chapter 1. File System Structure
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1.1. Why Share a Common Structure?
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The file system structure is the most basic level of organization in an operating system. Almost all of the ways an operating system interacts with its users, applications, and security model are dependent upon the way it organizes files on storage devices. Providing a common file system structure ensures users and programs are able to access and write files.
File systems break files down into two logical categories:
  • Shareable vs. unshareable files
  • Variable vs. static files
Shareable files are those that can be accessed locally and by remote hosts; unshareable files are only available locally. Variable files, such as documents, can be changed at any time; static files, such as binaries, do not change without an action from the system administrator.
The reason for looking at files in this manner is to help correlate the function of the file with the permissions assigned to the directories which hold them. The way in which the operating system and its users interact with a given file determines the directory in which it is placed, whether that directory is mounted with read-only or read/write permissions, and the level of access each user has to that file. The top level of this organization is crucial. Access to the underlying directories can be restricted or security problems could manifest themselves if, from the top level down, it does not adhere to a rigid structure.
Red Hat Enterprise Linux uses the Filesystem Hierarchy Standard (FHS) file system structure, which defines the names, locations, and permissions for many file types and directories.
The FHS document is the authoritative reference to any FHS-compliant file system, but the standard leaves many areas undefined or extensible. This section is an overview of the standard and a description of the parts of the file system not covered by the standard.
Compliance with the standard means many things, but the two most important are compatibility with other compliant systems and the ability to mount a /usr/ partition as read-only. This second point is important because the directory contains common executables and should not be changed by users. Also, since the /usr/ directory is mounted as read-only, it can be mounted from the CD-ROM or from another machine via a read-only NFS mount.

1.2.1. FHS Organization
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The directories and files noted here are a small subset of those specified by the FHS document. Refer to the latest FHS document for the most complete information.
The complete standard is available online at http://www.pathname.com/fhs/.
1.2.1.1. The /boot/ Directory
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The /boot/ directory contains static files required to boot the system, such as the Linux kernel. These files are essential for the system to boot properly.

Warning

Do not remove the /boot/ directory. Doing so renders the system unbootable.
1.2.1.2. The /dev/ Directory
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The /dev/ directory contains device nodes that either represent devices that are attached to the system or virtual devices that are provided by the kernel. These device nodes are essential for the system to function properly. The udev daemon takes care of creating and removing all these device nodes in /dev/.
Devices in the /dev directory and subdirectories are either character (providing only a serial stream of input/output) or block (accessible randomly). Character devices include mouse, keyboard, modem while block devices include hard disk, floppy drive etc. If you have GNOME or KDE installed in your system, devices such as external drives or cds are automatically detected when connected (e.g via usb) or inserted (e.g via CD or DVD drive) and a popup window displaying the contents is automatically displayed. Files in the /dev directory are essential for the system to function properly.
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Table 1.1. Examples of common files in the /dev
File Description
/dev/hda The master device on primary IDE channel.
/dev/hdb The slave device on primary IDE channel.
/dev/tty0 The first virtual console.
/dev/tty1 The second virtual console.
/dev/sda The first device on primary SCSI or SATA channel.
/dev/lp0 The first parallel port.
1.2.1.3. The /etc/ Directory
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The /etc/ directory is reserved for configuration files that are local to the machine. No binaries are to be placed in /etc/. Any binaries that were once located in /etc/ should be placed into /sbin/ or /bin/.
Examples of directories in /etc are the X11/ and skel/: 
/etc
   |- X11/
   |- skel/
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The /etc/X11/ directory is for X Window System configuration files, such as xorg.conf. The /etc/skel/ directory is for "skeleton" user files, which are used to populate a home directory when a user is first created. Applications also store their configuration files in this directory and may reference them when they are executed.
1.2.1.4. The /lib/ Directory
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The /lib/ directory should contain only those libraries needed to execute the binaries in /bin/ and /sbin/. These shared library images are particularly important for booting the system and executing commands within the root file system.
1.2.1.5. The /media/ Directory
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The /media/ directory contains subdirectories used as mount points for removable media such as usb storage media, DVDs, CD-ROMs, and Zip disks.
1.2.1.6. The /mnt/ Directory
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The /mnt/ directory is reserved for temporarily mounted file systems, such as NFS file system mounts. For all removable media, please use the /media/ directory. Automatically detected removable media will be mounted in the /media directory.

Note

The /mnt directory must not be used by installation programs.
1.2.1.7. The /opt/ Directory
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The /opt/ directory provides storage for most application software packages.
A package placing files in the /opt/ directory creates a directory bearing the same name as the package. This directory, in turn, holds files that otherwise would be scattered throughout the file system, giving the system administrator an easy way to determine the role of each file within a particular package.
For example, if sample is the name of a particular software package located within the /opt/ directory, then all of its files are placed in directories inside the /opt/sample/ directory, such as /opt/sample/bin/ for binaries and /opt/sample/man/ for manual pages.
Packages that encompass many different sub-packages, data files, extra fonts, clipart etc are also located in the /opt/ directory, giving that large package a way to organize itself. In this way, our sample package may have different tools that each go in their own sub-directories, such as /opt/sample/tool1/ and /opt/sample/tool2/, each of which can have their own bin/, man/, and other similar directories.
1.2.1.8. The /proc/ Directory
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The /proc/ directory contains special files that either extract information from or send information to the kernel. Examples include system memory, cpu information, hardware configuration etc.
Due to the great variety of data available within /proc/ and the many ways this directory can be used to communicate with the kernel, an entire chapter has been devoted to the subject. For more information, refer to Chapter 5, The proc File System.
1.2.1.9. The /sbin/ Directory
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The /sbin/ directory stores executables used by the root user. The executables in /sbin/ are used at boot time, for system administration and to perform system recovery operations. Of this directory, the FHS says:
/sbin contains binaries essential for booting, restoring, recovering, and/or repairing the system in addition to the binaries in /bin. Programs executed after /usr/ is known to be mounted (when there are no problems) are generally placed into /usr/sbin. Locally-installed system administration programs should be placed into /usr/local/sbin.
At a minimum, the following programs should be in /sbin/: 
arp, clock,
halt, init,
fsck.*, grub,
ifconfig, mingetty,
mkfs.*, mkswap,
reboot, route,
shutdown, swapoff,
swapon
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1.2.1.10. The /srv/ Directory
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The /srv/ directory contains site-specific data served by your system running Red Hat Enterprise Linux. This directory gives users the location of data files for a particular service, such as FTP, WWW, or CVS. Data that only pertains to a specific user should go in the /home/ directory.
1.2.1.11. The /sys/ Directory
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The /sys/ directory utilizes the new sysfs virtual file system specific to the 2.6 kernel. With the increased support for hot plug hardware devices in the 2.6 kernel, the /sys/ directory contains information similarly held in /proc/, but displays a hierarchical view of specific device information in regards to hot plug devices.
1.2.1.12. The /usr/ Directory
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The /usr/ directory is for files that can be shared across multiple machines. The /usr/ directory is often on its own partition and is mounted read-only. At a minimum, the following directories should be subdirectories of /usr/: 
/usr
   |- bin/
   |- etc/
   |- games/
   |- include/
   |- kerberos/
   |- lib/
   |- libexec/
   |- local/
   |- sbin/
   |- share/
   |- src/
   |- tmp -> ../var/tmp/
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Under the /usr/ directory, the bin/ subdirectory contains executables, etc/ contains system-wide configuration files, games is for games, include/ contains C header files, kerberos/ contains binaries and other Kerberos-related files, and lib/ contains object files and libraries that are not designed to be directly utilized by users or shell scripts. The libexec/ directory contains small helper programs called by other programs, sbin/ is for system administration binaries (those that do not belong in the /sbin/ directory), share/ contains files that are not architecture-specific, src/ is for source code.
1.2.1.13. The /usr/local/ Directory
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The FHS says:
The /usr/local hierarchy is for use by the system administrator when installing software locally. It needs to be safe from being overwritten when the system software is updated. It may be used for programs and data that are shareable among a group of hosts, but not found in /usr.
The /usr/local/ directory is similar in structure to the /usr/ directory. It has the following subdirectories, which are similar in purpose to those in the /usr/ directory: 
/usr/local
	|- bin/
	|- etc/
	|- games/
	|- include/
	|- lib/
	|- libexec/
	|- sbin/
	|- share/
	|- src/
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In Red Hat Enterprise Linux, the intended use for the /usr/local/ directory is slightly different from that specified by the FHS. The FHS says that /usr/local/ should be where software that is to remain safe from system software upgrades is stored. Since software upgrades can be performed safely with RPM Package Manager (RPM), it is not necessary to protect files by putting them in /usr/local/. Instead, the /usr/local/ directory is used for software that is local to the machine.
For instance, if the /usr/ directory is mounted as a read-only NFS share from a remote host, it is still possible to install a package or program under the /usr/local/ directory.
1.2.1.14. The /var/ Directory
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Since the FHS requires Linux to mount /usr/ as read-only, any programs that write log files or need spool/ or lock/ directories should write them to the /var/ directory. The FHS states /var/ is for:
...variable data files. This includes spool directories and files, administrative and logging data, and transient and temporary files.
Below are some of the directories found within the /var/ directory: 
/var
   |- account/
   |- arpwatch/
   |- cache/
   |- crash/
   |- db/
   |- empty/
   |- ftp/
   |- gdm/
   |- kerberos/
   |- lib/
   |- local/
   |- lock/
   |- log/
   |- mail -> spool/mail/
   |- mailman/
   |- named/
   |- nis/
   |- opt/
   |- preserve/
   |- run/
   +- spool/
       |- at/
       |- clientmqueue/
       |- cron/
       |- cups/
       |- exim/
       |- lpd/
       |- mail/
       |- mailman/
       |- mqueue/
       |- news/
       |- postfix/
       |- repackage/
       |- rwho/
       |- samba/
       |- squid/
       |- squirrelmail/
       |- up2date/
       |- uucp
       |- uucppublic/
       |- vbox/
|- tmp/
|- tux/
|- www/
|- yp/
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System log files, such as messages and lastlog, go in the /var/log/ directory. The /var/lib/rpm/ directory contains RPM system databases. Lock files go in the /var/lock/ directory, usually in directories for the program using the file. The /var/spool/ directory has subdirectories for programs in which data files are stored.
Red Hat Enterprise Linux extends the FHS structure slightly to accommodate special files.
Most files pertaining to RPM are kept in the /var/lib/rpm/ directory. For more information on RPM, refer to the chapter Chapter 12, Package Management with RPM.
The /var/cache/yum/ directory contains files used by the Package Updater, including RPM header information for the system. This location may also be used to temporarily store RPMs downloaded while updating the system. For more information about Red Hat Network, refer to Chapter 15, Registering a System and Managing Subscriptions.
Another location specific to Red Hat Enterprise Linux is the /etc/sysconfig/ directory. This directory stores a variety of configuration information. Many scripts that run at boot time use the files in this directory. Refer to Chapter 32, The sysconfig Directory for more information about what is within this directory and the role these files play in the boot process.

Chapter 2. Using the mount Command
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On Linux, UNIX, and similar operating systems, file systems on different partitions and removable devices like CDs, DVDs, or USB flash drives can be attached to a certain point (that is, the mount point) in the directory tree, and detached again. To attach or detach a file system, you can use the mount or umount command respectively. This chapter describes the basic usage of these commands, and covers some advanced topics such as moving a mount point or creating shared subtrees.

2.1. Listing Currently Mounted File Systems
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To display all currently attached file systems, run the mount command with no additional arguments: 
mount
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This command displays the list of known mount points. Each line provides important information about the device name, the file system type, the directory in which it is mounted, and relevant mount options in the following form:
device on directory type type (options)
By default, the output includes various virtual file systems such as sysfs, tmpfs, and others. To display only the devices with a certain file system type, supply the -t option on the command line: 
mount -t type
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For a list of common file system types, refer to Table 2.1, “Common File System Types”. For an example on how to use the mount command to list the mounted file systems, see Example 2.1, “Listing Currently Mounted ext3 File Systems”.

Example 2.1. Listing Currently Mounted ext3 File Systems

Usually, both / and /boot partitions are formatted to use ext3. To display only the mount points that use this file system, type the following at a shell prompt: 
~]$ mount -t ext3
/dev/mapper/VolGroup00-LogVol00 on / type ext3 (rw)
/dev/vda1 on /boot type ext3 (rw)
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2.2. Mounting a File System
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To attach a certain file system, use the mount command in the following form: 
mount [option…] device directory
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When the mount command is run, it reads the content of the /etc/fstab configuration file to see if the given file system is listed. This file contains a list of device names and the directory in which the selected file systems should be mounted, as well as the file system type and mount options. Because of this, when you are mounting a file system that is specified in this file, you can use one of the following variants of the command: 
mount [option…] directory
mount [option…] device
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Note that unless you are logged in as root, you must have permissions to mount the file system (see Section 2.2.2, “Specifying the Mount Options”).

2.2.1. Specifying the File System Type
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In most cases, mount detects the file system automatically. However, there are certain file systems, such as NFS (Network File System) or CIFS (Common Internet File System), that are not recognized, and need to be specified manually. To specify the file system type, use the mount command in the following form: 
mount -t type device directory
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Table 2.1, “Common File System Types” provides a list of common file system types that can be used with the mount command. For a complete list of all available file system types, consult the relevant manual page as referred to in Section 2.4.1, “Installed Documentation”.
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Table 2.1. Common File System Types
Type Description
ext2 The ext2 file system.
ext3 The ext3 file system.
ext4 The ext4 file system.
iso9660 The ISO 9660 file system. It is commonly used by optical media, typically CDs.
jfs The JFS file system created by IBM.
nfs The NFS file system. It is commonly used to access files over the network.
nfs4 The NFSv4 file system. It is commonly used to access files over the network.
ntfs The NTFS file system. It is commonly used on machines that are running the Windows operating system.
udf The UDF file system. It is commonly used by optical media, typically DVDs.
vfat The FAT file system. It is commonly used on machines that are running the Windows operating system, and on certain digital media such as USB flash drives or floppy disks.
See Example 2.2, “Mounting a USB Flash Drive” for an example usage.

Example 2.2. Mounting a USB Flash Drive

Older USB flash drives often use the FAT file system. Assuming that such drive uses the /dev/sdc1 device and that the /media/flashdisk/ directory exists, you can mount it to this directory by typing the following at a shell prompt as root: 
~]# mount -t vfat /dev/sdc1 /media/flashdisk
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2.2.2. Specifying the Mount Options
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To specify additional mount options, use the command in the following form: 
mount -o options
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When supplying multiple options, do not insert a space after a comma, or mount will incorrectly interpret the values following spaces as additional parameters.
Table 2.2, “Common Mount Options” provides a list of common mount options. For a complete list of all available options, consult the relevant manual page as referred to in Section 2.4.1, “Installed Documentation”.
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Table 2.2. Common Mount Options
Option Description
async Allows the asynchronous input/output operations on the file system.
auto Allows the file system to be mounted automatically using the mount -a command.
defaults Provides an alias for async,auto,dev,exec,nouser,rw,suid.
exec Allows the execution of binary files on the particular file system.
loop Mounts an image as a loop device.
noauto Disallows the automatic mount of the file system using the mount -a command.
noexec Disallows the execution of binary files on the particular file system.
nouser Disallows an ordinary user (that is, other than root) to mount and unmount the file system.
remount Remounts the file system in case it is already mounted.
ro Mounts the file system for reading only.
rw Mounts the file system for both reading and writing.
user Allows an ordinary user (that is, other than root) to mount and unmount the file system.
See Example 2.3, “Mounting an ISO Image” for an example usage.

Example 2.3. Mounting an ISO Image

An ISO image (or a disk image in general) can be mounted by using the loop device. Assuming that the ISO image of the Fedora 14 installation disc is present in the current working directory and that the /media/cdrom/ directory exists, you can mount the image to this directory by running the following command as root: 
~]# mount -o ro,loop Fedora-14-x86_64-Live-Desktop.iso /media/cdrom
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Note that ISO 9660 is by design a read-only file system.

2.2.3. Sharing Mounts
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Occasionally, certain system administration tasks require access to the same file system from more than one place in the directory tree (for example, when preparing a chroot environment). To address such requirements, the mount command implements the --bind option that provides a means for duplicating certain mounts. Its usage is as follows: 
mount --bind old_directory new_directory
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Although the above command allows a user to access the file system from both places, it does not apply on the file systems that are mounted within the original directory. To include these mounts as well, type: 
mount --rbind old_directory new_directory
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Additionally, to provide as much flexibility as possible, Red Hat Enterprise Linux 5.10 implements the functionality known as shared subtrees. This feature allows you to use the following four mount types:
Shared Mount
A shared mount allows you to create an exact replica of a given mount point. When a shared mount is created, any mount within the original mount point is reflected in it, and vice versa. To create a shared mount, type the following at a shell prompt: 
mount --make-shared mount_point
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Alternatively, you can change the mount type for the selected mount point and all mount points under it: 
mount --make-rshared mount_point
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See Example 2.4, “Creating a Shared Mount Point” for an example usage.

Example 2.4. Creating a Shared Mount Point

There are two places where other file systems are commonly mounted: the /media directory for removable media, and the /mnt directory for temporarily mounted file systems. By using a shared mount, you can make these two directories share the same content. To do so, as root, mark the /media directory as “shared”: 
~]# mount --bind /media /media
~]# mount --make-shared /media
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Then create its duplicate in /mnt by using the following command: 
~]# mount --bind /media /mnt
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You can now verify that a mount within /media also appears in /mnt. For example, if you have non-empty media in your CD-ROM drive and the /media/cdrom/ directory exists, run the following commands: 
~]# mount /dev/cdrom /media/cdrom
~]# ls /media/cdrom
EFI  GPL  isolinux  LiveOS
~]# ls /mnt/cdrom
EFI  GPL  isolinux  LiveOS
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Similarly, you can verify that any file system mounted in the /mnt directory is reflected in /media. For instance, if you have a non-empty USB flash drive that uses the /dev/sdc1 device plugged in and the /mnt/flashdisk/ directory is present, type: 
~]# mount /dev/sdc1 /mnt/flashdisk
~]# ls /media/flashdisk
en-US  publican.cfg
~]# ls /mnt/flashdisk
en-US  publican.cfg
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Slave Mount
A slave mount allows you to create a limited duplicate of a given mount point. When a slave mount is created, any mount within the original mount point is reflected in it, but no mount within a slave mount is reflected in its original. To create a slave mount, type the following at a shell prompt: 
mount --make-slave mount_point
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Alternatively, you can change the mount type for the selected mount point and all mount points under it: 
mount --make-rslave mount_point
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See Example 2.5, “Creating a Slave Mount Point” for an example usage.

Example 2.5. Creating a Slave Mount Point

Imagine you want the content of the /media directory to appear in /mnt as well, but you do not want any mounts in the /mnt directory to be reflected in /media. To do so, as root, first mark the /media directory as “shared”: 
~]# mount --bind /media /media
~]# mount --make-shared /media
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Then create its duplicate in /mnt, but mark it as “slave”: 
~]# mount --bind /media /mnt
~]# mount --make-slave /mnt
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You can now verify that a mount within /media also appears in /mnt. For example, if you have non-empty media in your CD-ROM drive and the /media/cdrom/ directory exists, run the following commands: 
~]# mount /dev/cdrom /media/cdrom
~]# ls /media/cdrom
EFI  GPL  isolinux  LiveOS
~]# ls /mnt/cdrom
EFI  GPL  isolinux  LiveOS
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You can also verify that file systems mounted in the /mnt directory are not reflected in /media. For instance, if you have a non-empty USB flash drive that uses the /dev/sdc1 device plugged in and the /mnt/flashdisk/ directory is present, type: : 
~]# mount /dev/sdc1 /mnt/flashdisk
~]# ls /media/flashdisk
~]# ls /mnt/flashdisk
en-US  publican.cfg
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Private Mount
A private mount allows you to create an ordinary mount. When a private mount is created, no subsequent mounts within the original mount point are reflected in it, and no mount within a private mount is reflected in its original. To create a private mount, type the following at a shell prompt: 
mount --make-private mount_point
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Alternatively, you can change the mount type for the selected mount point and all mount points under it: 
mount --make-rprivate mount_point
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See Example 2.6, “Creating a Private Mount Point” for an example usage.

Example 2.6. Creating a Private Mount Point

Taking into account the scenario in Example 2.4, “Creating a Shared Mount Point”, assume that you have previously created a shared mount point by using the following commands as root: 
~]# mount --bind /media /media
~]# mount --make-shared /media
~]# mount --bind /media /mnt
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To mark the /mnt directory as “private”, type: 
~]# mount --make-private /mnt
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You can now verify that none of the mounts within /media appears in /mnt. For example, if you have non-empty media in your CD-ROM drive and the /media/cdrom/ directory exists, run the following commands: 
~]# mount /dev/cdrom /media/cdrom
~]# ls /media/cdrom
EFI  GPL  isolinux  LiveOS
~]# ls /mnt/cdrom
~]#
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You can also verify that file systems mounted in the /mnt directory are not reflected in /media. For instance, if you have a non-empty USB flash drive that uses the /dev/sdc1 device plugged in and the /mnt/flashdisk/ directory is present, type: 
~]# mount /dev/sdc1 /mnt/flashdisk
~]# ls /media/flashdisk
~]# ls /mnt/flashdisk
en-US  publican.cfg
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Unbindable Mount
An unbindable mount allows you to prevent a given mount point from being duplicated whatsoever. To create an unbindable mount, type the following at a shell prompt: 
mount --make-unbindable mount_point
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Alternatively, you can change the mount type for the selected mount point and all mount points under it: 
mount --make-runbindable mount_point
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See Example 2.7, “Creating an Unbindable Mount Point” for an example usage.

Example 2.7. Creating an Unbindable Mount Point

To prevent the /media directory from being shared, as root, type the following at a shell prompt: 
~]# mount --bind /media /media
~]# mount --make-unbindable /media
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This way, any subsequent attempt to make a duplicate of this mount will fail with an error: 
~]# mount --bind /media /mnt
mount: wrong fs type, bad option, bad superblock on /media/,
       missing code page or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so
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2.2.4. Moving a Mount Point
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To change the directory in which a file system is mounted, use the following command: 
mount --move old_directory new_directory
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See Example 2.8, “Moving an Existing NFS Mount Point” for an example usage.

Example 2.8. Moving an Existing NFS Mount Point

Imagine that you have an NFS storage that contains user directories. Assuming that this storage is already mounted in /mnt/userdirs/, as root, you can move this mount point to /home by using the following command: 
~]# mount --move /mnt/userdirs /home
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To verify the mount point has been moved, list the content of both directories: 
~]# ls /mnt/userdirs
~]# ls /home
jill  joe
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2.3. Unmounting a File System
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To detach a previously mounted file system, use either of the following variants of the umount command: 
umount directory
umount device
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Note that unless you are logged in as root, you must have permissions to unmount the file system (see Section 2.2.2, “Specifying the Mount Options”). See Example 2.9, “Unmounting a CD” for an example usage.

Important

When a file system is in use (for example, when a process is reading a file on this file system), running the umount command will fail with an error. To determine which processes are accessing the file system, use the fuser command in the following form: 
fuser -m directory
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For example, to list the processes that are accessing a file system mounted to the /media/cdrom/ directory, type: 
~]$ fuser -m /media/cdrom
/media/cdrom:         1793  2013  2022  2435 10532c 10672c
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Example 2.9. Unmounting a CD

To unmount a CD that was previously mounted to the /media/cdrom/ directory, type the following at a shell prompt: 
~]$ umount /media/cdrom
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2.4. Additional Resources
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The following resources provide an in-depth documentation on the subject.

2.4.1. Installed Documentation
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  • man 8 mount — The manual page for the mount command that provides a full documentation on its usage.
  • man 8 umount — The manual page for the umount command that provides a full documentation on its usage.
  • man 5 fstab — The manual page providing a thorough description of the /etc/fstab file format.

2.4.2. Useful Websites
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  • Shared subtrees — An LWN article covering the concept of shared subtrees.
  • sharedsubtree.txt — Extensive documentation that is shipped with the shared subtrees patches.

Chapter 3. The ext3 File System
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The default file system is the journaling ext3 file system.

3.1. Features of ext3
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The ext3 file system is essentially an enhanced version of the ext2 file system. These improvements provide the following advantages:
Availability
After an unexpected power failure or system crash (also called an unclean system shutdown), each mounted ext2 file system on the machine must be checked for consistency by the e2fsck program. This is a time-consuming process that can delay system boot time significantly, especially with large volumes containing a large number of files. During this time, any data on the volumes is unreachable.
The journaling provided by the ext3 file system means that this sort of file system check is no longer necessary after an unclean system shutdown. The only time a consistency check occurs using ext3 is in certain rare hardware failure cases, such as hard drive failures. The time to recover an ext3 file system after an unclean system shutdown does not depend on the size of the file system or the number of files; rather, it depends on the size of the journal used to maintain consistency. The default journal size takes about a second to recover, depending on the speed of the hardware.
Data Integrity
The ext3 file system prevents loss of data integrity in the event that an unclean system shutdown occurs. The ext3 file system allows you to choose the type and level of protection that your data receives. By default, the ext3 volumes are configured to keep a high level of data consistency with regard to the state of the file system.
Speed
Despite writing some data more than once, ext3 has a higher throughput in most cases than ext2 because ext3's journaling optimizes hard drive head motion. You can choose from three journaling modes to optimize speed, but doing so means trade-offs in regards to data integrity if the system was to fail.
Easy Transition
It is easy to migrate from ext2 to ext3 and gain the benefits of a robust journaling file system without reformatting. Refer to Section 3.3, “Converting to an ext3 File System” for more on how to perform this task.
The following sections walk you through the steps for creating and tuning ext3 partitions. For ext2 partitions, skip the partitioning and formatting sections below and go directly to Section 3.3, “Converting to an ext3 File System”.

3.2. Creating an ext3 File System
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After installation, it is sometimes necessary to create a new ext3 file system. For example, if you add a new disk drive to the system, you may want to partition the drive and use the ext3 file system.
The steps for creating an ext3 file system are as follows:
  1. Format the partition with the ext3 file system using mkfs.
  2. Label the partition using e2label.

3.3. Converting to an ext3 File System
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The tune2fs allows you to convert an ext2 filesystem to ext3.

Note

Always use the e2fsck utility to check your filesystem before and after using tune2fs. A default installation of Red Hat Enterprise Linux uses ext3 for all file systems.
To convert an ext2 filesystem to ext3, log in as root and type the following command in a terminal: 
tune2fs -j <block_device>
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where <block_device> contains the ext2 filesystem you wish to convert.
A valid block device could be one of two types of entries:
  • A mapped device — A logical volume in a volume group, for example, /dev/mapper/VolGroup00-LogVol02.
  • A static device — A traditional storage volume, for example, /dev/hdbX, where hdb is a storage device name and X is the partition number.
Issue the df command to display mounted file systems.
For the remainder of this section, the sample commands use the following value for the block device: 
/dev/mapper/VolGroup00-LogVol02
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You must recreate the initrd image so that it will contain the ext3 kernel module. To create this, run the mkinitrd program. For information on using the mkinitrd command, type man mkinitrd. Also, make sure your GRUB configuration loads the initrd.
If you fail to make this change, the system still boots, but the file system is mounted as ext2 instead of ext3.

3.4. Reverting to an ext2 File System
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If you wish to revert a partition from ext3 to ext2 for any reason, you must first unmount the partition by logging in as root and typing, 
umount /dev/mapper/VolGroup00-LogVol02
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Next, change the file system type to ext2 by typing the following command as root: 
tune2fs -O ^has_journal /dev/mapper/VolGroup00-LogVol02
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Check the partition for errors by typing the following command as root: 
e2fsck -y /dev/mapper/VolGroup00-LogVol02
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Then mount the partition again as ext2 file system by typing: 
mount -t ext2 /dev/mapper/VolGroup00-LogVol02 /mount/point
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In the above command, replace /mount/point with the mount point of the partition.
Next, remove the .journal file at the root level of the partition by changing to the directory where it is mounted and typing: 
rm -f .journal
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You now have an ext2 partition.
If you want to permanently change the partition to ext2, remember to update the /etc/fstab file.

Chapter 4. The ext4 File System
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4.1. Features of ext4
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The ext4 file system is a scalable extension of the ext3 file system, which is the default file system of Red Hat Enterprise Linux 5. The ext4 file system can support files and file systems of up to 16 terabytes in size. It also supports an unlimited number of sub-directories (the ext3 file system only supports up to 32,000), though once the link count exceeds 65,000 it resets to 1 and is no longer increased. The following are the most important features of ext4:
Main Features
The ext4 file system uses extents (as opposed to the traditional block mapping scheme used by ext2 and ext3), which improves performance when using large files and reduces metadata overhead for large files. In addition, ext4 also labels unallocated block groups and inode table sections accordingly, which allows them to be skipped during a file system check. This makes for quicker file system checks, which becomes more beneficial as the file system grows in size.
Allocation Features
The ext4 file system features the following allocation schemes:
  • Persistent pre-allocation
  • Delayed allocation
  • Multi-block allocation
  • Stripe-aware allocation
Because of delayed allocation and other performance optimizations, ext4's behavior of writing files to disk is different from ext3. In ext4, a program's writes to the file system are not guaranteed to be on-disk unless the program issues an fsync() call afterwards.
By default, ext3 automatically forces newly created files to disk almost immediately even without fsync(). This behavior hid bugs in programs that did not use fsync() to ensure that written data was on-disk. The ext4 file system, on the other hand, often waits several seconds to write out changes to disk, allowing it to combine and reorder writes for better disk performance than ext3.

Warning

Unlike ext3, the ext4 file system does not force data to disk on transaction commit. As such, it takes longer for buffered writes to be flushed to disk. As with any file system, use data integrity calls such as fsync() to ensure that data is written to permanent storage.
Other ext4 Features
The ext4 file system also supports the following:
  • Extended attributes (xattr), which allows the system to associate several additional name/value pairs per file.
  • Quota journaling, which avoids the need for lengthy quota consistency checks after a crash.

    Note

    The only supported journaling mode in ext4 is data=ordered (default).
  • Subsecond timestamps, which allow to specify inode timestamp fields in nanosecond resolution.

4.2. Managing an ext4 File System
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In order to manage ext4 file systems on Red Hat Eterprise Linux 5, it is necessary to install the e4fsprogs package. You can use the Yum utility to install the package: 
~]# yum install e4fsprogs
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The e4fsprogs package contains renamed static binaries from the equivalent upstream e2fsprogs release. This has been done to ensure stability of the e2fsprogs core utilities with all the changes for ext4 included. The most important of these utilities are:
  • mke4fs — A utility used to create an ext4 file system.
  • mkfs.ext4 — Another command used to create an ext4 file system.
  • e4fsck — A utility used to repair inconsistencies of an ext4 file system.
  • tune4fs — A utility used to modify ext4 file system attributes.
  • resize4fs — A utility used to resize an ext4 file system.
  • e4label — A utility used to display or modify the label of the ext4 file system.
  • dumpe4fs — A utility used to display the super block and blocks group information for the ext4 file system.
  • debuge4fs — An interactive file system debugger, used to examine ext4 file systems, manually repair corrupted file systems and create test cases for e4fsck.
The following sections walk you through the steps for creating and tuning ext4 partitions.

4.3. Creating an ext4 File System
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After installation, it is sometimes necessary to create a new ext4 file system. For example, if you add a new disk drive to the system, you may want to partition the drive and use the ext4 file system.
The default options are optimal for most usage scenarios but if you need to set your ext4 file system in a specific way, see manual pages for the mke4fs and mkfs.ext4 commands for available options. Also, you may want to examine and modify the configuration file of mke4fs, /etc/mke4fs.conf, if you plan to create ext4 file systems more often.
The steps for creating an ext4 file system are as follows:
  1. Format the partition with the ext4 file system using the mkfs.ext4 or mke4fs command: 
    ~]# mkfs.ext4 block_device
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    ~]# mke4fs -t ext4 block_device
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    where block_device is a partition which will contain the ext4 filesystem you wish to create.
  2. Label the partition using the e4label command. 
    ~]# e4label <block_device> new-label
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  3. Create a mount point and mount the new file system to that mount point: 
    ~]# mkdir /mount/point
~]# mount block_device /mount/point
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A valid block device could be one of two types of entries:
  • A mapped device — A logical volume in a volume group, for example, /dev/mapper/VolGroup00-LogVol02.
  • A static device — A traditional storage volume, for example, /dev/hdbX, where hdb is a storage device name and X is the partition number.
For striped block devices (for example RAID5 arrays), the stripe geometry can be specified at the time of file system creation. Using proper stripe geometry greatly enhances performance of an ext4 file system.
When creating file systems on lvm or md volumes, mkfs.ext4 chooses an optimal geometry. This may also be true on some hardware RAIDs which export geometry information to the operating system.
To specify stripe geometry, use the -E option of mkfs.ext4 (that is, extended file system options) with the following sub-options:
stride=value
Specifies the RAID chunk size.
stripe-width=value
Specifies the number of data disks in a RAID device, or the number of stripe units in the stripe.
For both sub-options, value must be specified in file system block units. For example, to create a file system with a 64k stride (that is, 16 x 4096) on a 4k-block file system, use the following command: 
~]# mkfs.ext4 -E stride=16,stripe-width=64 block_device
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For more information about creating file systems, refer to man mkfs.ext4.

4.4. Mounting an ext4 File System
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An ext4 file system can be mounted with no extra options, same as any other file system: 
~]# mount block_device /mount/point
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The default mount options are optimal for most users. Options, such as acl, noacl, data, quota, noquota, user_xattr, nouser_xattr, and many others that were already used with the ext2 and ext3 file systems, are backward compatible and have the same usage and functionality. Also, with the ext4 file system, several new ext4-specific mount options have been added, for example:
barrier / nobarrier
By default, ext4 uses write barriers to ensure file system integrity even when power is lost to a device with write caches enabled. For devices without write caches, or with battery-backed write caches, you disable barriers using the nobarrier option: 
~]# mount -o nobarrier block_device /mount/point
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stripe=value
This option allows you to specify the number of file system blocks allocated for a single file operation. For RAID5 this number should be equal the RAID chunk size multiplied by the number of disks.
journal_ioprio=value
This option allows you to set priority of I/O operations submitted during a commit operation. The option can have a value from 7 to 0 (0 is the highest priority), and is set to 3 by default, which is slightly higher priority than the default I/O priority.
Default mount options can be also set in the file system superblock using the tune4fs utility. For example, the following command sets the file system on the /dev/mapper/VolGroup00-LogVol02 device to be mounted by default with debugging disabled and user-specified extended attributes and Posix access control lists enabled: 
~]# tune4fs -o ^debug,user_xattr,acl /dev/mapper/VolGroup00-LogVol02
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For more information on this topic, refer to the tune4fs(8) manual page.
An ext3 file system can also be mounted as ext4 without changing the format, allowing it to be mounted as ext3 again in the future. To do so, run the following command on a block device that contains an ext3 file system: 
~]# mount -t ext4 block_device /mount/point
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Doing so will only allow the ext3 file system to use ext4-specific features that do not require a file format conversion. These features include delayed allocation and multi-block allocation, and exclude features such as extent mapping.

Warning

Using the ext4 driver to mount an ext3 file system has not been fully tested on Red Hat Enterprise Linux 5. Therefore, this action is not supported because Red Hat cannot guarantee consistent performance and predictable behavior for ext3 file systems in this way.
For more information on mount options for the ext4 file system, see Section 2.2.2, “Specifying the Mount Options” and the mount(8) manual page.

Note

If you want to enable persistent mounting of the file system, remember to update the /etc/fstab file accordingly. For example: 
/dev/mapper/VolGroup00-LogVol02    /test    ext4    defaults    0 0
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4.5. Resizing an ext4 File System
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Before growing an ext4 file system, ensure that the underlying block device is of an appropriate size to hold the file system later. Use the appropriate resizing methods for the affected block device.
When grown, the ext4 filesystem can be mounted. When shrunk, the ext4 file system has to be unmounted. You can resize an ext4 file system using the resize4fs command: 
~]# resize4fs block_devicenew_size
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When resizing an ext4 file system, the resize2fs utility reads the size in units of file system block size, unless a suffix indicating a specific unit is used. The following suffixes indicate specific units:
  • s — 512 byte sectors
  • K — kilobytes
  • M — megabytes
  • G — gigabytes
The size parameter is optional (and often redundant) when expanding. The resize4fs automatically expands to fill all available space of the container, usually a logical volume or partition. For more information about resizing an ext4 file system, refer to the resize4fs(8) manual page.

Chapter 5. The proc File System
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The Linux kernel has two primary functions: to control access to physical devices on the computer and to schedule when and how processes interact with these devices. The /proc/ directory — also called the proc file system — contains a hierarchy of special files which represent the current state of the kernel — allowing applications and users to peer into the kernel's view of the system.
Within the /proc/ directory, one can find a wealth of information detailing the system hardware and any processes currently running. In addition, some of the files within the /proc/ directory tree can be manipulated by users and applications to communicate configuration changes to the kernel.

5.1. A Virtual File System
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Under Linux, all data are stored as files. Most users are familiar with the two primary types of files: text and binary. But the /proc/ directory contains another type of file called a virtual file. It is for this reason that /proc/ is often referred to as a virtual file system.
These virtual files have unique qualities. Most of them are listed as zero bytes in size and yet when one is viewed, it can contain a large amount of information. In addition, most of the time and date settings on virtual files reflect the current time and date, indicative of the fact they are constantly updated.
Virtual files such as /proc/interrupts, /proc/meminfo, /proc/mounts, and /proc/partitions provide an up-to-the-moment glimpse of the system's hardware. Others, like the /proc/filesystems file and the /proc/sys/ directory provide system configuration information and interfaces.
For organizational purposes, files containing information on a similar topic are grouped into virtual directories and sub-directories. For instance, /proc/ide/ contains information for all physical IDE devices. Likewise, process directories contain information about each running process on the system.

5.1.1. Viewing Virtual Files
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By using the cat, more, or less commands on files within the /proc/ directory, users can immediately access enormous amounts of information about the system. For example, to display the type of CPU a computer has, type cat /proc/cpuinfo to receive output similar to the following: 
processor	: 0
vendor_id	: AuthenticAMD
cpu family	: 5
model		: 9
model name	: AMD-K6(tm) 3D+
Processor stepping	: 1 cpu
MHz		: 400.919
cache size	: 256 KB
fdiv_bug	: no
hlt_bug		: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 1
wp		: yes
flags		: fpu vme de pse tsc msr mce cx8 pge mmx syscall 3dnow k6_mtrr
bogomips	: 799.53
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When viewing different virtual files in the /proc/ file system, some of the information is easily understandable while some is not human-readable. This is in part why utilities exist to pull data from virtual files and display it in a useful way. Examples of these utilities include lspci, apm, free, and top.

Note

Some of the virtual files in the /proc/ directory are readable only by the root user.

5.1.2. Changing Virtual Files
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As a general rule, most virtual files within the /proc/ directory are read-only. However, some can be used to adjust settings in the kernel. This is especially true for files in the /proc/sys/ subdirectory.
To change the value of a virtual file, use the echo command and a greater than symbol (>) to redirect the new value to the file. For example, to change the hostname on the fly, type: 
echo www.example.com > /proc/sys/kernel/hostname
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Other files act as binary or Boolean switches. Typing cat /proc/sys/net/ipv4/ip_forward returns either a 0 or a 1. A 0 indicates that the kernel is not forwarding network packets. Using the echo command to change the value of the ip_forward file to 1 immediately turns packet forwarding on.

Note

Another command used to alter settings in the /proc/sys/ subdirectory is /sbin/sysctl. For more information on this command, refer to Section 5.4, “Using the sysctl Command”
For a listing of some of the kernel configuration files available in the /proc/sys/ subdirectory, refer to Section 5.3.9, “ /proc/sys/ ”.

5.1.3. Restricting Access to Process Directories
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On multi-user systems, it is often useful to secure the process directories stored in /proc/ so that they can be viewed only by the root user. You can restrict the access to these directories with the use of the hidepid option.
To change the file system parameters, you can use the mount command with the -o remount option. As root, type: 
mount -o remount,hidepid=value /proc
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Here, value passed to hidepid is one of:
  • 0 (default) — every user can read all world-readable files stored in a process directory.
  • 1 — users can access only their own process directories. This protects the sensitive files like cmdline, sched, or status from access by non-root users. This setting does not affect the actual file permissions.
  • 2 — process files are invisible to non-root users. The existence of a process can be learned by other means, but its effective UID and GID is hidden. Hiding these IDs complicates an intruder's task of gathering information about running processes.

Example 5.1. Restricting access to process directories

To make process files accessible only to the root user, type: 
~]# mount -o remount,hidepid=1 /proc
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With hidepid=1, a non-root user cannot access the contents of process directories. An attempt to do so fails with the following message: 
~]$ ls /proc/1/       
ls: /proc/1/: Operation not permitted
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With hidepid=2 enabled, process directories are made invisible to non-root users: 
~]$ ls /proc/1/       
ls: /proc/1/: No such file or directory
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Also, you can specify a user group that will have access to process files even when hidepid is set to 1 or 2. To do this, use the gid option. As root, type: 
mount -o remount,hidepid=value,gid=gid /proc
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Replace gid with the specific group id. For members of selected group, the process files will act as if hidepid was set to 0. However, users which are not supposed to monitor the tasks in the whole system should not be added to the group. For more information on managing users and groups see Chapter 37, Users and Groups.

5.2. Top-level Files within the proc File System
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Below is a list of some of the more useful virtual files in the top-level of the /proc/ directory.

Note

In most cases, the content of the files listed in this section are not the same as those installed on your machine. This is because much of the information is specific to the hardware on which Red Hat Enterprise Linux is running for this documentation effort.

5.2.1.  /proc/apm
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This file provides information about the state of the Advanced Power Management (APM) system and is used by the apm command. If a system with no battery is connected to an AC power source, this virtual file would look similar to the following: 
1.16 1.2 0x07 0x01 0xff 0x80 -1% -1 ?
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Running the apm -v command on such a system results in output similar to the following: 
APM BIOS 1.2 (kernel driver 1.16ac) AC on-line, no system battery
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For systems which do not use a battery as a power source, apm is able do little more than put the machine in standby mode. The apm command is much more useful on laptops. For example, the following output is from the command cat /proc/apm on a laptop while plugged into a power outlet: 
1.16 1.2 0x03 0x01 0x03 0x09 100% -1 ?
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When the same laptop is unplugged from its power source for a few minutes, the content of the apm file changes to something like the following: 
1.16 1.2 0x03 0x00 0x00 0x01 99% 1792 min
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The apm -v command now yields more useful data, such as the following: 
APM BIOS 1.2 (kernel driver 1.16) AC off-line, battery status high: 99% (1 day, 5:52)
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5.2.2.  /proc/buddyinfo
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This file is used primarily for diagnosing memory fragmentation issues. Using the buddy algorithm, each column represents the number of pages of a certain order (a certain size) that are available at any given time. For example, for zone DMA (direct memory access), there are 90 of (2^0)*PAGE_SIZE chunks of memory. Similarly, there are 6 of (2^1)*PAGE_SIZE chunks, and 2 of (2^2)*PAGE_SIZE chunks of memory available.
The DMA row references the first 16 MB on a system, the HighMem row references all memory greater than 4 GB on a system, and the Normal row references all memory in between.
The following is an example of the output typical of /proc/buddyinfo: 
Node 0, zone      DMA     90      6      2      1      1      ...
Node 0, zone   Normal   1650    310      5      0      0      ...
Node 0, zone  HighMem      2      0      0      1      1      ...
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5.2.3.  /proc/cmdline
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This file shows the parameters passed to the kernel at the time it is started. A sample /proc/cmdline file looks like the following: 
ro root=/dev/VolGroup00/LogVol00 rhgb quiet 3
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This output tells us the following:
ro
The root device is mounted read-only at boot time. The presence of ro on the kernel boot line overrides any instances of rw.
root=/dev/VolGroup00/LogVol00
This tells us on which disk device or, in this case, on which logical volume, the root filesystem image is located. With our sample /proc/cmdline output, the root filesystem image is located on the first logical volume (LogVol00) of the first LVM volume group (VolGroup00). On a system not using Logical Volume Management, the root file system might be located on /dev/sda1 or /dev/sda2, meaning on either the first or second partition of the first SCSI or SATA disk drive, depending on whether we have a separate (preceding) boot or swap partition on that drive.
For more information on LVM used in Red Hat Enterprise Linux, refer to http://www.tldp.org/HOWTO/LVM-HOWTO/index.html.
rhgb
A short lowercase acronym that stands for Red Hat Graphical Boot, providing "rhgb" on the kernel command line signals that graphical booting is supported, assuming that /etc/inittab shows that the default runlevel is set to 5 with a line like this: 
id:5:initdefault:
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quiet
Indicates that all verbose kernel messages except those which are extremely serious should be suppressed at boot time.

5.2.4.  /proc/cpuinfo
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This virtual file identifies the type of processor used by your system. The following is an example of the output typical of /proc/cpuinfo: 
processor	: 0
vendor_id	: GenuineIntel
cpu family	: 15
model		: 2
model name	: Intel(R) Xeon(TM) CPU 2.40GHz
stepping	: 7 cpu
MHz		: 2392.371
cache size	: 512 KB
physical id	: 0
siblings	: 2
runqueue	: 0
fdiv_bug	: no
hlt_bug		: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 2
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca  cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm
bogomips	: 4771.02
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  • processor — Provides each processor with an identifying number. On systems that have one processor, only a 0 is present.
  • cpu family — Authoritatively identifies the type of processor in the system. For an Intel-based system, place the number in front of "86" to determine the value. This is particularly helpful for those attempting to identify the architecture of an older system such as a 586, 486, or 386. Because some RPM packages are compiled for each of these particular architectures, this value also helps users determine which packages to install.
  • model name — Displays the common name of the processor, including its project name.
  • cpu MHz — Shows the precise speed in megahertz for the processor to the thousandths decimal place.
  • cache size — Displays the amount of level 2 memory cache available to the processor.
  • siblings — Displays the number of sibling CPUs on the same physical CPU for architectures which use hyper-threading.
  • flags — Defines a number of different qualities about the processor, such as the presence of a floating point unit (FPU) and the ability to process MMX instructions.

5.2.5.  /proc/crypto
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This file lists all installed cryptographic ciphers used by the Linux kernel, including additional details for each. A sample /proc/crypto file looks like the following: 
name         : sha1
module       : kernel
type         : digest
blocksize    : 64
digestsize   : 20
name         : md5
module       : md5
type         : digest
blocksize    : 64
digestsize   : 16
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5.2.6.  /proc/devices
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This file displays the various character and block devices currently configured (not including devices whose modules are not loaded). Below is a sample output from this file: 
Character devices:
  1 mem
  4 /dev/vc/0
  4 tty
  4 ttyS
  5 /dev/tty
  5 /dev/console
  5 /dev/ptmx
  7 vcs
  10 misc
  13 input
  29 fb
  36 netlink
  128 ptm
  136 pts
  180 usb

Block devices:
  1 ramdisk
  3 ide0
  9 md
  22 ide1
  253 device-mapper
  254 mdp
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The output from /proc/devices includes the major number and name of the device, and is broken into two major sections: Character devices and Block devices.
Character devices are similar to block devices, except for two basic differences:
  1. Character devices do not require buffering. Block devices have a buffer available, allowing them to order requests before addressing them. This is important for devices designed to store information — such as hard drives — because the ability to order the information before writing it to the device allows it to be placed in a more efficient order.
  2. Character devices send data with no preconfigured size. Block devices can send and receive information in blocks of a size configured per device.
For more information about devices refer to the following installed documentation: 
/usr/share/doc/kernel-doc-<version>/Documentation/devices.txt
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5.2.7.  /proc/dma
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This file contains a list of the registered ISA DMA channels in use. A sample /proc/dma files looks like the following: 
4: cascade
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5.2.8.  /proc/execdomains
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This file lists the execution domains currently supported by the Linux kernel, along with the range of personalities they support. 
0-0   Linux           [kernel]
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Think of execution domains as the "personality" for an operating system. Because other binary formats, such as Solaris, UnixWare, and FreeBSD, can be used with Linux, programmers can change the way the operating system treats system calls from these binaries by changing the personality of the task. Except for the PER_LINUX execution domain, different personalities can be implemented as dynamically loadable modules.

5.2.9.  /proc/fb
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This file contains a list of frame buffer devices, with the frame buffer device number and the driver that controls it. Typical output of /proc/fb for systems which contain frame buffer devices looks similar to the following: 
0 VESA VGA
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5.2.10.  /proc/filesystems
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This file displays a list of the file system types currently supported by the kernel. Sample output from a generic /proc/filesystems file looks similar to the following: 
nodev   sysfs
nodev   rootfs
nodev   bdev
nodev   proc
nodev   sockfs
nodev   binfmt_misc
nodev   usbfs
nodev   usbdevfs
nodev   futexfs
nodev   tmpfs
nodev   pipefs
nodev   eventpollfs
nodev   devpts
	ext2
nodev   ramfs
nodev   hugetlbfs
	iso9660
nodev   mqueue
	ext3
nodev   rpc_pipefs
nodev   autofs
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The first column signifies whether the file system is mounted on a block device. Those beginning with nodev are not mounted on a device. The second column lists the names of the file systems supported.
The mount command cycles through the file systems listed here when one is not specified as an argument.

5.2.11.  /proc/interrupts
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This file records the number of interrupts per IRQ on the x86 architecture. A standard /proc/interrupts looks similar to the following: 
  CPU0
  0:   80448940          XT-PIC  timer
  1:     174412          XT-PIC  keyboard
  2:          0          XT-PIC  cascade
  8:          1          XT-PIC  rtc
 10:     410964          XT-PIC  eth0
 12:      60330          XT-PIC  PS/2 Mouse
 14:    1314121          XT-PIC  ide0
 15:    5195422          XT-PIC  ide1
NMI:          0
ERR:          0
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For a multi-processor machine, this file may look slightly different: 
	   CPU0       CPU1
  0: 1366814704          0          XT-PIC  timer
  1:        128        340    IO-APIC-edge  keyboard
  2:          0          0          XT-PIC  cascade
  8:          0          1    IO-APIC-edge  rtc
 12:       5323       5793    IO-APIC-edge  PS/2 Mouse
 13:          1          0          XT-PIC  fpu
 16:   11184294   15940594   IO-APIC-level  Intel EtherExpress Pro 10/100 Ethernet
 20:    8450043   11120093   IO-APIC-level  megaraid
 30:      10432      10722   IO-APIC-level  aic7xxx
 31:         23         22   IO-APIC-level  aic7xxx
NMI:          0
ERR:          0
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The first column refers to the IRQ number. Each CPU in the system has its own column and its own number of interrupts per IRQ. The next column reports the type of interrupt, and the last column contains the name of the device that is located at that IRQ.
Each of the types of interrupts seen in this file, which are architecture-specific, mean something different. For x86 machines, the following values are common:
  • XT-PIC — This is the old AT computer interrupts.
  • IO-APIC-edge — The voltage signal on this interrupt transitions from low to high, creating an edge, where the interrupt occurs and is only signaled once. This kind of interrupt, as well as the IO-APIC-level interrupt, are only seen on systems with processors from the 586 family and higher.
  • IO-APIC-level — Generates interrupts when its voltage signal is high until the signal is low again.

5.2.12.  /proc/iomem
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This file shows you the current map of the system's memory for each physical device: 
00000000-0009fbff : System RAM
0009fc00-0009ffff : reserved
000a0000-000bffff : Video RAM area
000c0000-000c7fff : Video ROM
000f0000-000fffff : System ROM
00100000-07ffffff : System RAM
00100000-00291ba8 : Kernel code
00291ba9-002e09cb : Kernel data
e0000000-e3ffffff : VIA Technologies, Inc. VT82C597 [Apollo VP3] e4000000-e7ffffff : PCI Bus #01
e4000000-e4003fff : Matrox Graphics, Inc. MGA G200 AGP
e5000000-e57fffff : Matrox Graphics, Inc. MGA G200 AGP
e8000000-e8ffffff : PCI Bus #01
e8000000-e8ffffff : Matrox Graphics, Inc. MGA G200 AGP
ea000000-ea00007f : Digital Equipment Corporation DECchip 21140 [FasterNet]
ea000000-ea00007f : tulip ffff0000-ffffffff : reserved
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The first column displays the memory registers used by each of the different types of memory. The second column lists the kind of memory located within those registers and displays which memory registers are used by the kernel within the system RAM or, if the network interface card has multiple Ethernet ports, the memory registers assigned for each port.

5.2.13.  /proc/ioports
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The output of /proc/ioports provides a list of currently registered port regions used for input or output communication with a device. This file can be quite long. The following is a partial listing: 
0000-001f : dma1
0020-003f : pic1
0040-005f : timer
0060-006f : keyboard
0070-007f : rtc
0080-008f : dma page reg
00a0-00bf : pic2
00c0-00df : dma2
00f0-00ff : fpu
0170-0177 : ide1
01f0-01f7 : ide0
02f8-02ff : serial(auto)
0376-0376 : ide1
03c0-03df : vga+
03f6-03f6 : ide0
03f8-03ff : serial(auto)
0cf8-0cff : PCI conf1
d000-dfff : PCI Bus #01
e000-e00f : VIA Technologies, Inc. Bus Master IDE
e000-e007 : ide0
e008-e00f : ide1
e800-e87f : Digital Equipment Corporation DECchip 21140 [FasterNet]
e800-e87f : tulip
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The first column gives the I/O port address range reserved for the device listed in the second column.

5.2.14.  /proc/kcore
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This file represents the physical memory of the system and is stored in the core file format. Unlike most /proc/ files, kcore displays a size. This value is given in bytes and is equal to the size of the physical memory (RAM) used plus 4 KB.
The contents of this file are designed to be examined by a debugger, such as gdb, and is not human readable.

Warning

Do not view the /proc/kcore virtual file. The contents of the file scramble text output on the terminal. If this file is accidentally viewed, press Ctrl+C to stop the process and then type reset to bring back the command line prompt.

5.2.15.  /proc/kmsg
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This file is used to hold messages generated by the kernel. These messages are then picked up by other programs, such as /sbin/klogd or /bin/dmesg.

5.2.16.  /proc/loadavg
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This file provides a look at the load average in regard to both the CPU and IO over time, as well as additional data used by uptime and other commands. A sample /proc/loadavg file looks similar to the following: 
0.20 0.18 0.12 1/80 11206
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The first three columns measure CPU and IO utilization of the last one, five, and 15 minute periods. The fourth column shows the number of currently running processes and the total number of processes. The last column displays the last process ID used.
In addition, load average also refers to the number of processes ready to run (i.e. in the run queue, waiting for a CPU share.

5.2.17.  /proc/locks
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This file displays the files currently locked by the kernel. The contents of this file contain internal kernel debugging data and can vary tremendously, depending on the use of the system. A sample /proc/locks file for a lightly loaded system looks similar to the following: 
1: POSIX  ADVISORY  WRITE 3568 fd:00:2531452 0 EOF
2: FLOCK  ADVISORY  WRITE 3517 fd:00:2531448 0 EOF
3: POSIX  ADVISORY  WRITE 3452 fd:00:2531442 0 EOF
4: POSIX  ADVISORY  WRITE 3443 fd:00:2531440 0 EOF
5: POSIX  ADVISORY  WRITE 3326 fd:00:2531430 0 EOF
6: POSIX  ADVISORY  WRITE 3175 fd:00:2531425 0 EOF
7: POSIX  ADVISORY  WRITE 3056 fd:00:2548663 0 EOF
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Each lock has its own line which starts with a unique number. The second column refers to the class of lock used, with FLOCK signifying the older-style UNIX file locks from a flock system call and POSIX representing the newer POSIX locks from the lockf system call.
The third column can have two values: ADVISORY or MANDATORY. ADVISORY means that the lock does not prevent other people from accessing the data; it only prevents other attempts to lock it. MANDATORY means that no other access to the data is permitted while the lock is held. The fourth column reveals whether the lock is allowing the holder READ or WRITE access to the file. The fifth column shows the ID of the process holding the lock. The sixth column shows the ID of the file being locked, in the format of MAJOR-DEVICE:MINOR-DEVICE:INODE-NUMBER . The seventh and eighth column shows the start and end of the file's locked region.

5.2.18.  /proc/mdstat
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This file contains the current information for multiple-disk, RAID configurations. If the system does not contain such a configuration, then /proc/mdstat looks similar to the following: 
Personalities :  read_ahead not set unused devices: <none>
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This file remains in the same state as seen above unless a software RAID or md device is present. In that case, view /proc/mdstat to find the current status of mdX RAID devices.
The /proc/mdstat file below shows a system with its md0 configured as a RAID 1 device, while it is currently re-syncing the disks: 
Personalities : [linear] [raid1] read_ahead 1024 sectors
md0: active raid1 sda2[1] sdb2[0] 9940 blocks [2/2] [UU] resync=1% finish=12.3min algorithm 2 [3/3] [UUU]
unused devices: <none>
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5.2.19.  /proc/meminfo
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This is one of the more commonly used files in the /proc/ directory, as it reports a large amount of valuable information about the systems RAM usage.
The following sample /proc/meminfo virtual file is from a system with 256 MB of RAM and 512 MB of swap space: 
MemTotal:       255908 kB
MemFree:         69936 kB
Buffers:         15812 kB
Cached:         115124 kB
SwapCached:          0 kB
Active:          92700 kB
Inactive:        63792 kB
HighTotal:           0 kB
HighFree:            0 kB
LowTotal:       255908 kB
LowFree:         69936 kB
SwapTotal:      524280 kB
SwapFree:       524280 kB
Dirty:               4 kB
Writeback:           0 kB
Mapped:          42236 kB
Slab:            25912 kB
Committed_AS:   118680 kB
PageTables:       1236 kB
VmallocTotal:  3874808 kB
VmallocUsed:      1416 kB
VmallocChunk:  3872908 kB
HugePages_Total:     0
HugePages_Free:      0
Hugepagesize:     4096 kB
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Much of the information here is used by the free, top, and ps commands. In fact, the output of the free command is similar in appearance to the contents and structure of /proc/meminfo. But by looking directly at /proc/meminfo, more details are revealed:
  • MemTotal — Total amount of physical RAM, in kilobytes.
  • MemFree — The amount of physical RAM, in kilobytes, left unused by the system.
  • Buffers — The amount of physical RAM, in kilobytes, used for file buffers.
  • Cached — The amount of physical RAM, in kilobytes, used as cache memory.
  • SwapCached — The amount of swap, in kilobytes, used as cache memory.
  • Active — The total amount of buffer or page cache memory, in kilobytes, that is in active use. This is memory that has been recently used and is usually not reclaimed for other purposes.
  • Inactive — The total amount of buffer or page cache memory, in kilobytes, that are free and available. This is memory that has not been recently used and can be reclaimed for other purposes.
  • HighTotal and HighFree — The total and free amount of memory, in kilobytes, that is not directly mapped into kernel space. The HighTotal value can vary based on the type of kernel used.
  • LowTotal and LowFree — The total and free amount of memory, in kilobytes, that is directly mapped into kernel space. The LowTotal value can vary based on the type of kernel used.
  • SwapTotal — The total amount of swap available, in kilobytes.
  • SwapFree — The total amount of swap free, in kilobytes.
  • Dirty — The total amount of memory, in kilobytes, waiting to be written back to the disk.
  • Writeback — The total amount of memory, in kilobytes, actively being written back to the disk.
  • Mapped — The total amount of memory, in kilobytes, which have been used to map devices, files, or libraries using the mmap command.
  • Slab — The total amount of memory, in kilobytes, used by the kernel to cache data structures for its own use.
  • Committed_AS — The total amount of memory, in kilobytes, estimated to complete the workload. This value represents the worst case scenario value, and also includes swap memory.
  • PageTables — The total amount of memory, in kilobytes, dedicated to the lowest page table level.
  • VMallocTotal — The total amount of memory, in kilobytes, of total allocated virtual address space.
  • VMallocUsed — The total amount of memory, in kilobytes, of used virtual address space.
  • VMallocChunk — The largest contiguous block of memory, in kilobytes, of available virtual address space.
  • HugePages_Total — The total number of hugepages for the system. The number is derived by dividing Hugepagesize by the megabytes set aside for hugepages specified in /proc/sys/vm/hugetlb_pool. This statistic only appears on the x86, Itanium, and AMD64 architectures.
  • HugePages_Free — The total number of hugepages available for the system. This statistic only appears on the x86, Itanium, and AMD64 architectures.
  • Hugepagesize — The size for each hugepages unit in kilobytes. By default, the value is 4096 KB on uniprocessor kernels for 32 bit architectures. For SMP, hugemem kernels, and AMD64, the default is 2048 KB. For Itanium architectures, the default is 262144 KB. This statistic only appears on the x86, Itanium, and AMD64 architectures.

5.2.20.  /proc/misc
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This file lists miscellaneous drivers registered on the miscellaneous major device, which is device number 10: 
63 device-mapper 175 agpgart 135 rtc 134 apm_bios
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The first column is the minor number of each device, while the second column shows the driver in use.

5.2.21.  /proc/modules
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This file displays a list of all modules loaded into the kernel. Its contents vary based on the configuration and use of your system, but it should be organized in a similar manner to this sample /proc/modules file output:

Note

This example has been reformatted into a readable format. Most of this information can also be viewed via the /sbin/lsmod command. 
nfs      170109  0 -          Live 0x129b0000
lockd    51593   1 nfs,       Live 0x128b0000
nls_utf8 1729    0 -          Live 0x12830000
vfat     12097   0 -          Live 0x12823000
fat      38881   1 vfat,      Live 0x1287b000
autofs4  20293   2 -          Live 0x1284f000
sunrpc   140453  3 nfs,lockd, Live 0x12954000
3c59x    33257   0 -          Live 0x12871000
uhci_hcd 28377   0 -          Live 0x12869000
md5      3777    1 -          Live 0x1282c000
ipv6     211845 16 -          Live 0x128de000
ext3     92585   2 -          Live 0x12886000
jbd      65625   1 ext3,      Live 0x12857000
dm_mod   46677   3 -          Live 0x12833000
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The first column contains the name of the module.
The second column refers to the memory size of the module, in bytes.
The third column lists how many instances of the module are currently loaded. A value of zero represents an unloaded module.
The fourth column states if the module depends upon another module to be present in order to function, and lists those other modules.
The fifth column lists what load state the module is in: Live, Loading, or Unloading are the only possible values.
The sixth column lists the current kernel memory offset for the loaded module. This information can be useful for debugging purposes, or for profiling tools such as oprofile.

5.2.22.  /proc/mounts
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This file provides a list of all mounts in use by the system: 
rootfs / rootfs rw 0 0
/proc /proc proc rw,nodiratime 0 0 none
/dev ramfs rw 0 0
/dev/mapper/VolGroup00-LogVol00 / ext3 rw 0 0
none /dev ramfs rw 0 0
/proc /proc proc rw,nodiratime 0 0
/sys /sys sysfs rw 0 0
none /dev/pts devpts rw 0 0
usbdevfs /proc/bus/usb usbdevfs rw 0 0
/dev/hda1 /boot ext3 rw 0 0
none /dev/shm tmpfs rw 0 0
none /proc/sys/fs/binfmt_misc binfmt_misc rw 0 0
sunrpc /var/lib/nfs/rpc_pipefs rpc_pipefs rw 0 0
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The output found here is similar to the contents of /etc/mtab, except that /proc/mount is more up-to-date.
The first column specifies the device that is mounted, the second column reveals the mount point, and the third column tells the file system type, and the fourth column tells you if it is mounted read-only (ro) or read-write (rw). The fifth and sixth columns are dummy values designed to match the format used in /etc/mtab.

5.2.23.  /proc/mtrr
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This file refers to the current Memory Type Range Registers (MTRRs) in use with the system. If the system architecture supports MTRRs, then the /proc/mtrr file may look similar to the following: 
reg00: base=0x00000000 (   0MB), size= 256MB: write-back, count=1
reg01: base=0xe8000000 (3712MB), size=  32MB: write-combining, count=1
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MTRRs are used with the Intel P6 family of processors (Pentium II and higher) and control processor access to memory ranges. When using a video card on a PCI or AGP bus, a properly configured /proc/mtrr file can increase performance more than 150%.
Most of the time, this value is properly configured by default. More information on manually configuring this file can be found locally at the following location: 
/usr/share/doc/kernel-doc-<version>/Documentation/mtrr.txt
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5.2.24.  /proc/partitions
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This file contains partition block allocation information. A sampling of this file from a basic system looks similar to the following: 
major minor  #blocks  name
  3     0   19531250 hda
  3     1     104391 hda1
  3     2   19422585 hda2
253     0   22708224 dm-0
253     1     524288 dm-1
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Most of the information here is of little importance to the user, except for the following columns:
  • major — The major number of the device with this partition. The major number in the /proc/partitions, (3), corresponds with the block device ide0, in /proc/devices.
  • minor — The minor number of the device with this partition. This serves to separate the partitions into different physical devices and relates to the number at the end of the name of the partition.
  • #blocks — Lists the number of physical disk blocks contained in a particular partition.
  • name — The name of the partition.

5.2.25.  /proc/pci
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This file contains a full listing of every PCI device on the system. Depending on the number of PCI devices, /proc/pci can be rather long. A sampling of this file from a basic system looks similar to the following: 
Bus  0, device 0, function 0: Host bridge: Intel Corporation 440BX/ZX - 82443BX/ZX Host bridge (rev 3). Master Capable. Latency=64. Prefetchable 32 bit memory at 0xe4000000 [0xe7ffffff].
Bus  0, device 1, function 0: PCI bridge: Intel Corporation 440BX/ZX - 82443BX/ZX AGP bridge (rev 3).   Master Capable. Latency=64. Min Gnt=128.
Bus  0, device 4, function 0: ISA bridge: Intel Corporation 82371AB PIIX4 ISA (rev 2).
Bus  0, device 4, function 1: IDE interface: Intel Corporation 82371AB PIIX4 IDE (rev 1). Master Capable. Latency=32. I/O at 0xd800 [0xd80f].
Bus  0, device 4, function 2: USB Controller: Intel Corporation 82371AB PIIX4 USB (rev 1). IRQ 5. Master Capable. Latency=32. I/O at 0xd400 [0xd41f].
Bus  0, device 4, function 3: Bridge: Intel Corporation 82371AB PIIX4 ACPI (rev 2). IRQ 9.
Bus  0, device 9, function 0: Ethernet controller: Lite-On Communications Inc LNE100TX (rev 33). IRQ 5. Master Capable. Latency=32. I/O at 0xd000 [0xd0ff].
Bus  0, device 12, function  0: VGA compatible controller: S3 Inc. ViRGE/DX or /GX (rev 1). IRQ 11. Master Capable. Latency=32. Min Gnt=4.Max Lat=255.
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This output shows a list of all PCI devices, sorted in the order of bus, device, and function. Beyond providing the name and version of the device, this list also gives detailed IRQ information so an administrator can quickly look for conflicts.

Note

To get a more readable version of this information, type: 
lspci -vb
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5.2.26.  /proc/slabinfo
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This file gives full information about memory usage on the slab level. Linux kernels greater than version 2.2 use slab pools to manage memory above the page level. Commonly used objects have their own slab pools.
Instead of parsing the highly verbose /proc/slabinfo file manually, the /usr/bin/slabtop program displays kernel slab cache information in real time. This program allows for custom configurations, including column sorting and screen refreshing.
A sample screen shot of /usr/bin/slabtop usually looks like the following example: 
Active / Total Objects (% used)    : 133629 / 147300 (90.7%)
Active / Total Slabs (% used)      : 11492 / 11493 (100.0%)
Active / Total Caches (% used)     : 77 / 121 (63.6%)
Active / Total Size (% used)       : 41739.83K / 44081.89K (94.7%)
Minimum / Average / Maximum Object : 0.01K / 0.30K / 128.00K
OBJS   ACTIVE USE      OBJ   SIZE     SLABS OBJ/SLAB CACHE SIZE NAME
44814  43159  96%    0.62K   7469      6     29876K ext3_inode_cache
36900  34614  93%    0.05K    492     75      1968K buffer_head
35213  33124  94%    0.16K   1531     23      6124K dentry_cache
7364   6463  87%    0.27K    526      14      2104K radix_tree_node
2585   1781  68%    0.08K     55      47       220K vm_area_struct
2263   2116  93%    0.12K     73      31       292K size-128
1904   1125  59%    0.03K     16      119        64K size-32
1666    768  46%    0.03K     14      119        56K anon_vma
1512   1482  98%    0.44K    168       9       672K inode_cache
1464   1040  71%    0.06K     24      61        96K size-64
1320    820  62%    0.19K     66      20       264K filp
678    587  86%    0.02K      3      226        12K dm_io
678    587  86%    0.02K      3      226        12K dm_tio
576    574  99%    0.47K     72        8       288K proc_inode_cache
528    514  97%    0.50K     66        8       264K size-512
492    372  75%    0.09K     12       41        48K bio
465    314  67%    0.25K     31       15       124K size-256
452    331  73%    0.02K      2      226         8K biovec-1
420    420 100%    0.19K     21       20        84K skbuff_head_cache
305    256  83%    0.06K      5       61        20K biovec-4
290      4   1%    0.01K      1      290         4K revoke_table
264    264 100%    4.00K    264        1      1056K size-4096
260    256  98%    0.19K     13       20        52K biovec-16
260    256  98%    0.75K     52        5       208K biovec-64
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Some of the more commonly used statistics in /proc/slabinfo that are included into /usr/bin/slabtop include:
  • OBJS — The total number of objects (memory blocks), including those in use (allocated), and some spares not in use.
  • ACTIVE — The number of objects (memory blocks) that are in use (allocated).
  • USE — Percentage of total objects that are active. ((ACTIVE/OBJS)(100))
  • OBJ SIZE — The size of the objects.
  • SLABS — The total number of slabs.
  • OBJ/SLAB — The number of objects that fit into a slab.
  • CACHE SIZE — The cache size of the slab.
  • NAME — The name of the slab.
For more information on the /usr/bin/slabtop program, refer to the slabtop man page.

5.2.27.  /proc/stat
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This file keeps track of a variety of different statistics about the system since it was last restarted. The contents of /proc/stat, which can be quite long, usually begins like the following example: 
cpu  259246 7001 60190 34250993 137517 772 0
cpu0 259246 7001 60190 34250993 137517 772 0
intr 354133732 347209999 2272 0 4 4 0 0 3 1 1249247 0 0 80143 0 422626 5169433
ctxt 12547729
btime 1093631447
processes 130523
procs_running 1
procs_blocked 0
preempt 5651840
cpu  209841 1554 21720 118519346 72939 154 27168
cpu0 42536 798 4841 14790880 14778 124 3117
cpu1 24184 569 3875 14794524 30209 29 3130
cpu2 28616 11 2182 14818198 4020 1 3493
cpu3 35350 6 2942 14811519 3045 0 3659
cpu4 18209 135 2263 14820076 12465 0 3373
cpu5 20795 35 1866 14825701 4508 0 3615
cpu6 21607 0 2201 14827053 2325 0 3334
cpu7 18544 0 1550 14831395 1589 0 3447
intr 15239682 14857833 6 0 6 6 0 5 0 1 0 0 0 29 0 2 0 0 0 0 0 0 0 94982 0 286812
ctxt 4209609
btime 1078711415
processes 21905
procs_running 1
procs_blocked 0
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Some of the more commonly used statistics include:
  • cpu — Measures the number of jiffies (1/100 of a second for x86 systems) that the system has been in user mode, user mode with low priority (nice), system mode, idle task, I/O wait, IRQ (hardirq), and softirq respectively. The IRQ (hardirq) is the direct response to a hardware event. The IRQ takes minimal work for queuing the "heavy" work up for the softirq to execute. The softirq runs at a lower priority than the IRQ and therefore may be interrupted more frequently. The total for all CPUs is given at the top, while each individual CPU is listed below with its own statistics. The following example is a 4-way Intel Pentium Xeon configuration with multi-threading enabled, therefore showing four physical processors and four virtual processors totaling eight processors.
  • page — The number of memory pages the system has written in and out to disk.
  • swap — The number of swap pages the system has brought in and out.
  • intr — The number of interrupts the system has experienced.
  • btime — The boot time, measured in the number of seconds since January 1, 1970, otherwise known as the epoch.

5.2.28.  /proc/swaps
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This file measures swap space and its utilization. For a system with only one swap partition, the output of /proc/swaps may look similar to the following: 
Filename                          Type        Size     Used    Priority
/dev/mapper/VolGroup00-LogVol01   partition   524280   0       -1
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While some of this information can be found in other files in the /proc/ directory, /proc/swaps provides a snapshot of every swap file name, the type of swap space, the total size, and the amount of space in use (in kilobytes). The priority column is useful when multiple swap files are in use. The lower the priority, the more likely the swap file is to be used.

5.2.29.  /proc/sysrq-trigger
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Using the echo command to write to this file, a remote root user can execute most System Request Key commands remotely as if at the local terminal. To echo values to this file, the /proc/sys/kernel/sysrq must be set to a value other than 0. For more information about the System Request Key, refer to Section 5.3.9.3, “ /proc/sys/kernel/ ”.
Although it is possible to write to this file, it cannot be read, even by the root user.

5.2.30.  /proc/uptime
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This file contains information detailing how long the system has been on since its last restart. The output of /proc/uptime is quite minimal: 
350735.47 234388.90
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The first number is the total number of seconds the system has been up. The second number is how much of that time the machine has spent idle, in seconds.

5.2.31.  /proc/version
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This file specifies the version of the Linux kernel and gcc in use, as well as the version of Red Hat Enterprise Linux installed on the system: 
Linux version 2.6.8-1.523 (user@foo.redhat.com) (gcc version 3.4.1 20040714 \  (Red Hat Enterprise Linux 3.4.1-7)) #1 Mon Aug 16 13:27:03 EDT 2004
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This information is used for a variety of purposes, including the version data presented when a user logs in.

5.3. Directories within /proc/
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Common groups of information concerning the kernel are grouped into directories and subdirectories within the /proc/ directory.

5.3.1. Process Directories
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Every /proc/ directory contains a number of directories with numerical names. A listing of them may be similar to the following: 
dr-xr-xr-x    3 root     root            0 Feb 13 01:28 1
dr-xr-xr-x    3 root     root            0 Feb 13 01:28 1010
dr-xr-xr-x    3 xfs      xfs             0 Feb 13 01:28 1087
dr-xr-xr-x    3 daemon   daemon          0 Feb 13 01:28 1123
dr-xr-xr-x    3 root     root            0 Feb 13 01:28 11307
dr-xr-xr-x    3 apache   apache          0 Feb 13 01:28 13660
dr-xr-xr-x    3 rpc      rpc             0 Feb 13 01:28 637
dr-xr-xr-x    3 rpcuser  rpcuser         0 Feb 13 01:28 666
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These directories are called process directories, as they are named after a program's process ID and contain information specific to that process. The owner and group of each process directory is set to the user running the process. When the process is terminated, its /proc/ process directory vanishes.
Each process directory contains the following files:
  • cmdline — Contains the command issued when starting the process.
  • cwd — A symbolic link to the current working directory for the process.
  • environ — A list of the environment variables for the process. The environment variable is given in all upper-case characters, and the value is in lower-case characters.
  • exe — A symbolic link to the executable of this process.
  • fd — A directory containing all of the file descriptors for a particular process. These are given in numbered links: 
    total 0
lrwx------    1 root     root           64 May  8 11:31 0 -> /dev/null
lrwx------    1 root     root           64 May  8 11:31 1 -> /dev/null
lrwx------    1 root     root           64 May  8 11:31 2 -> /dev/null
lrwx------    1 root     root           64 May  8 11:31 3 -> /dev/ptmx
lrwx------    1 root     root           64 May  8 11:31 4 -> socket:[7774817]
lrwx------    1 root     root           64 May  8 11:31 5 -> /dev/ptmx
lrwx------    1 root     root           64 May  8 11:31 6 -> socket:[7774829]
lrwx------    1 root     root           64 May  8 11:31 7 -> /dev/ptmx
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  • maps — A list of memory maps to the various executables and library files associated with this process. This file can be rather long, depending upon the complexity of the process, but sample output from the sshd process begins like the following: 
    08048000-08086000 r-xp 00000000 03:03 391479     /usr/sbin/sshd
08086000-08088000 rw-p 0003e000 03:03 391479	/usr/sbin/sshd
08088000-08095000 rwxp 00000000 00:00 0
40000000-40013000 r-xp 0000000 03:03 293205	/lib/ld-2.2.5.so
40013000-40014000 rw-p 00013000 03:03 293205	/lib/ld-2.2.5.so
40031000-40038000 r-xp 00000000 03:03 293282	/lib/libpam.so.0.75
40038000-40039000 rw-p 00006000 03:03 293282	/lib/libpam.so.0.75
40039000-4003a000 rw-p 00000000 00:00 0
4003a000-4003c000 r-xp 00000000 03:03 293218	/lib/libdl-2.2.5.so
4003c000-4003d000 rw-p 00001000 03:03 293218	/lib/libdl-2.2.5.so
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  • mem — The memory held by the process. This file cannot be read by the user.
  • root — A link to the root directory of the process.
  • stat — The status of the process.
  • statm — The status of the memory in use by the process. Below is a sample /proc/statm file: 
    263 210 210 5 0 205 0
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    The seven columns relate to different memory statistics for the process. From left to right, they report the following aspects of the memory used:
    1. Total program size, in kilobytes.
    2. Size of memory portions, in kilobytes.
    3. Number of pages that are shared.
    4. Number of pages that are code.
    5. Number of pages of data/stack.
    6. Number of library pages.
    7. Number of dirty pages.
  • status — The status of the process in a more readable form than stat or statm. Sample output for sshd looks similar to the following: 
    Name:	sshd
State:	S (sleeping)
Tgid:	797
Pid:	797
PPid:	1
TracerPid:	0
Uid:	0	0	0	0
Gid:	0	0	0	0
FDSize:	32
Groups:
VmSize:	    3072 kB
VmLck:	       0 kB
VmRSS:	     840 kB
VmData:	     104 kB
VmStk:	      12 kB
VmExe:	     300 kB
VmLib:	    2528 kB
SigPnd:	0000000000000000
SigBlk:	0000000000000000
SigIgn:	8000000000001000
SigCgt:	0000000000014005
CapInh:	0000000000000000
CapPrm:	00000000fffffeff
CapEff:	00000000fffffeff
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    The information in this output includes the process name and ID, the state (such as S (sleeping) or R (running)), user/group ID running the process, and detailed data regarding memory usage.
5.3.1.1.  /proc/self/
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The /proc/self/ directory is a link to the currently running process. This allows a process to look at itself without having to know its process ID.
Within a shell environment, a listing of the /proc/self/ directory produces the same contents as listing the process directory for that process.

5.3.2.  /proc/bus/
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This directory contains information specific to the various buses available on the system. For example, on a standard system containing PCI and USB buses, current data on each of these buses is available within a subdirectory within /proc/bus/ by the same name, such as /proc/bus/pci/.
The subdirectories and files available within /proc/bus/ vary depending on the devices connected to the system. However, each bus type has at least one directory. Within these bus directories are normally at least one subdirectory with a numerical name, such as 001, which contain binary files.
For example, the /proc/bus/usb/ subdirectory contains files that track the various devices on any USB buses, as well as the drivers required for them. The following is a sample listing of a /proc/bus/usb/ directory: 
total 0 dr-xr-xr-x    1 root     root            0 May  3 16:25 001
-r--r--r--    1 root     root            0 May  3 16:25 devices
-r--r--r--    1 root     root            0 May  3 16:25 drivers
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The /proc/bus/usb/001/ directory contains all devices on the first USB bus and the devices file identifies the USB root hub on the motherboard.
The following is a example of a /proc/bus/usb/devices file: 
T:  Bus=01 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=12  MxCh= 2
B:  Alloc=  0/900 us ( 0%), #Int=  0, #Iso=  0
D:  Ver= 1.00 Cls=09(hub  ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
P:  Vendor=0000 ProdID=0000 Rev= 0.00
S:  Product=USB UHCI Root Hub
S:  SerialNumber=d400
C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr=  0mA
I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
E:  Ad=81(I) Atr=03(Int.) MxPS=   8 Ivl=255ms
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5.3.3.  /proc/driver/
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This directory contains information for specific drivers in use by the kernel.
A common file found here is rtc which provides output from the driver for the system's Real Time Clock (RTC), the device that keeps the time while the system is switched off. Sample output from /proc/driver/rtc looks like the following: 
rtc_time        : 16:21:00
rtc_date        : 2004-08-31
rtc_epoch       : 1900
alarm           : 21:16:27
DST_enable      : no
BCD             : yes
24hr            : yes
square_wave     : no
alarm_IRQ       : no
update_IRQ      : no
periodic_IRQ    : no
periodic_freq   : 1024
batt_status     : okay
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For more information about the RTC, refer to the following installed documentation:
/usr/share/doc/kernel-doc-<version>/Documentation/rtc.txt.

5.3.4.  /proc/fs
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This directory shows which file systems are exported. If running an NFS server, typing cat /proc/fs/nfsd/exports displays the file systems being shared and the permissions granted for those file systems. For more on file system sharing with NFS, refer to Chapter 21, Network File System (NFS).

5.3.5.  /proc/ide/
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This directory contains information about IDE devices on the system. Each IDE channel is represented as a separate directory, such as /proc/ide/ide0 and /proc/ide/ide1. In addition, a drivers file is available, providing the version number of the various drivers used on the IDE channels: 
ide-floppy version 0.99.
newide ide-cdrom version 4.61
ide-disk version 1.18
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Many chipsets also provide a file in this directory with additional data concerning the drives connected through the channels. For example, a generic Intel PIIX4 Ultra 33 chipset produces the /proc/ide/piix file which reveals whether DMA or UDMA is enabled for the devices on the IDE channels: 
Intel PIIX4 Ultra 33 Chipset.
------------- Primary Channel ---------------- Secondary Channel -------------
		enabled                          enabled

------------- drive0 --------- drive1 -------- drive0 ---------- drive1 ------
DMA enabled:    yes              no              yes               no
UDMA enabled:   yes              no              no                no
UDMA enabled:   2                X               X                 X
UDMA DMA PIO
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Navigating into the directory for an IDE channel, such as ide0, provides additional information. The channel file provides the channel number, while the model identifies the bus type for the channel (such as pci).
5.3.5.1. Device Directories
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Within each IDE channel directory is a device directory. The name of the device directory corresponds to the drive letter in the /dev/ directory. For instance, the first IDE drive on ide0 would be hda.

Note

There is a symbolic link to each of these device directories in the /proc/ide/ directory.
Each device directory contains a collection of information and statistics. The contents of these directories vary according to the type of device connected. Some of the more useful files common to many devices include:
  • cache — The device cache.
  • capacity — The capacity of the device, in 512 byte blocks.
  • driver — The driver and version used to control the device.
  • geometry — The physical and logical geometry of the device.
  • media — The type of device, such as a disk.
  • model — The model name or number of the device.
  • settings — A collection of current device parameters. This file usually contains quite a bit of useful, technical information. A sample settings file for a standard IDE hard disk looks similar to the following: 
    name                value          min          max          mode
----                -----          ---          ---          ----
acoustic            0              0            254          rw
address             0              0            2            rw
bios_cyl            38752          0            65535        rw
bios_head           16             0            255          rw
bios_sect           63             0            63           rw
bswap               0              0            1            r
current_speed       68             0            70           rw
failures            0              0            65535        rw
init_speed          68             0            70           rw
io_32bit            0              0            3            rw
keepsettings        0              0            1            rw
lun                 0              0            7            rw
max_failures        1              0            65535        rw
multcount           16             0            16           rw
nice1               1              0            1            rw
nowerr              0              0            1            rw
number              0              0            3            rw
pio_mode            write-only     0            255          w
unmaskirq           0              0            1            rw
using_dma           1              0            1            rw
wcache              1              0            1            rw
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5.3.6.  /proc/irq/
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This directory is used to set IRQ to CPU affinity, which allows the system to connect a particular IRQ to only one CPU. Alternatively, it can exclude a CPU from handling any IRQs.
Each IRQ has its own directory, allowing for the individual configuration of each IRQ. The /proc/irq/prof_cpu_mask file is a bitmask that contains the default values for the smp_affinity file in the IRQ directory. The values in smp_affinity specify which CPUs handle that particular IRQ.
For more information about the /proc/irq/ directory, refer to the following installed documentation: 
/usr/share/doc/kernel-doc-<version>/Documentation/filesystems/proc.txt
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5.3.7.  /proc/net/
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This directory provides a comprehensive look at various networking parameters and statistics. Each directory and virtual file within this directory describes aspects of the system's network configuration. Below is a partial list of the /proc/net/ directory:
  • arp — Lists the kernel's ARP table. This file is particularly useful for connecting a hardware address to an IP address on a system.
  • atm/ directory — The files within this directory contain Asynchronous Transfer Mode (ATM) settings and statistics. This directory is primarily used with ATM networking and ADSL cards.
  • dev — Lists the various network devices configured on the system, complete with transmit and receive statistics. This file displays the number of bytes each interface has sent and received, the number of packets inbound and outbound, the number of errors seen, the number of packets dropped, and more.
  • dev_mcast — Lists Layer2 multicast groups on which each device is listening.
  • igmp — Lists the IP multicast addresses which this system joined.
  • ip_conntrack — Lists tracked network connections for machines that are forwarding IP connections.
  • ip_tables_names — Lists the types of iptables in use. This file is only present if iptables is active on the system and contains one or more of the following values: filter, mangle, or nat.
  • ip_mr_cache — Lists the multicast routing cache.
  • ip_mr_vif — Lists multicast virtual interfaces.
  • netstat — Contains a broad yet detailed collection of networking statistics, including TCP timeouts, SYN cookies sent and received, and much more.
  • psched — Lists global packet scheduler parameters.
  • raw — Lists raw device statistics.
  • route — Lists the kernel's routing table.
  • rt_cache — Contains the current routing cache.
  • snmp — List of Simple Network Management Protocol (SNMP) data for various networking protocols in use.
  • sockstat — Provides socket statistics.
  • tcp — Contains detailed TCP socket information.
  • tr_rif — Lists the token ring RIF routing table.
  • udp — Contains detailed UDP socket information.
  • unix — Lists UNIX domain sockets currently in use.
  • wireless — Lists wireless interface data.

5.3.8.  /proc/scsi/
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This directory is analogous to the /proc/ide/ directory, but it is for connected SCSI devices.
The primary file in this directory is /proc/scsi/scsi, which contains a list of every recognized SCSI device. From this listing, the type of device, as well as the model name, vendor, SCSI channel and ID data is available.
For example, if a system contains a SCSI CD-ROM, a tape drive, a hard drive, and a RAID controller, this file looks similar to the following: 
Attached devices:
Host: scsi1
Channel: 00
Id: 05
Lun: 00
Vendor: NEC
Model: CD-ROM DRIVE:466
Rev: 1.06
Type:   CD-ROM
ANSI SCSI revision: 02
Host: scsi1
Channel: 00
Id: 06
Lun: 00
Vendor: ARCHIVE
Model: Python 04106-XXX
Rev: 7350
Type:   Sequential-Access
ANSI SCSI revision: 02
Host: scsi2
Channel: 00
Id: 06
Lun: 00
Vendor: DELL
Model: 1x6 U2W SCSI BP
Rev: 5.35
Type:   Processor
ANSI SCSI revision: 02
Host: scsi2
Channel: 02
Id: 00
Lun: 00
Vendor: MegaRAID
Model: LD0 RAID5 34556R
Rev: 1.01
Type:   Direct-Access
ANSI SCSI revision: 02
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Each SCSI driver used by the system has its own directory within /proc/scsi/, which contains files specific to each SCSI controller using that driver. From the previous example, aic7xxx/ and megaraid/ directories are present, since two drivers are in use. The files in each of the directories typically contain an I/O address range, IRQ information, and statistics for the SCSI controller using that driver. Each controller can report a different type and amount of information. The Adaptec AIC-7880 Ultra SCSI host adapter's file in this example system produces the following output: 
Adaptec AIC7xxx driver version: 5.1.20/3.2.4
Compile Options:
TCQ Enabled By Default : Disabled
AIC7XXX_PROC_STATS     : Enabled
AIC7XXX_RESET_DELAY    : 5
Adapter Configuration:
SCSI Adapter: Adaptec AIC-7880 Ultra SCSI host adapter
Ultra Narrow Controller     PCI MMAPed
I/O Base: 0xfcffe000
Adapter SEEPROM Config: SEEPROM found and used.
Adaptec SCSI BIOS: Enabled
IRQ: 30
SCBs: Active 0, Max Active 1, Allocated 15, HW 16, Page 255
Interrupts: 33726
BIOS Control Word: 0x18a6
Adapter Control Word: 0x1c5f
Extended Translation: Enabled
Disconnect Enable Flags: 0x00ff
Ultra Enable Flags: 0x0020
Tag Queue Enable Flags: 0x0000
Ordered Queue Tag Flags: 0x0000
Default Tag Queue Depth: 8
Tagged Queue By Device array for aic7xxx
host instance 1:       {255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255}
Actual queue depth per device for aic7xxx host instance 1:       {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1}
Statistics:

(scsi1:0:5:0) Device using Narrow/Sync transfers at 20.0 MByte/sec, offset 15
Transinfo settings: current(12/15/0/0), goal(12/15/0/0), user(12/15/0/0)
Total transfers 0 (0 reads and 0 writes)
		< 2K      2K+     4K+     8K+    16K+    32K+    64K+   128K+
Reads:        0       0       0       0       0       0       0       0
Writes:       0       0       0       0       0       0       0       0

(scsi1:0:6:0) Device using Narrow/Sync transfers at 10.0 MByte/sec, offset 15
Transinfo settings: current(25/15/0/0), goal(12/15/0/0), user(12/15/0/0)
Total transfers 132 (0 reads and 132 writes)
		< 2K      2K+     4K+     8K+    16K+    32K+    64K+   128K+
Reads:        0       0       0       0       0       0       0       0
Writes:       0       0       0       1     131       0       0       0
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This output reveals the transfer speed to the SCSI devices connected to the controller based on channel ID, as well as detailed statistics concerning the amount and sizes of files read or written by that device. For example, this controller is communicating with the CD-ROM at 20 megabytes per second, while the tape drive is only communicating at 10 megabytes per second.

5.3.9.  /proc/sys/
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The /proc/sys/ directory is different from others in /proc/ because it not only provides information about the system but also allows the system administrator to immediately enable and disable kernel features.

Warning

Use caution when changing settings on a production system using the various files in the /proc/sys/ directory. Changing the wrong setting may render the kernel unstable, requiring a system reboot.
For this reason, be sure the options are valid for that file before attempting to change any value in /proc/sys/.
A good way to determine if a particular file can be configured, or if it is only designed to provide information, is to list it with the -l option at the shell prompt. If the file is writable, it may be used to configure the kernel. For example, a partial listing of /proc/sys/fs looks like the following: 
-r--r--r--    1 root     root            0 May 10 16:14 dentry-state
-rw-r--r--    1 root     root            0 May 10 16:14 dir-notify-enable
-r--r--r--    1 root     root            0 May 10 16:14 dquot-nr
-rw-r--r--    1 root     root            0 May 10 16:14 file-max
-r--r--r--    1 root     root            0 May 10 16:14 file-nr
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In this listing, the files dir-notify-enable and file-max can be written to and, therefore, can be used to configure the kernel. The other files only provide feedback on current settings.
Changing a value within a /proc/sys/ file is done by echoing the new value into the file. For example, to enable the System Request Key on a running kernel, type the command: 
echo 1 > /proc/sys/kernel/sysrq
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This changes the value for sysrq from 0 (off) to 1 (on).
A few /proc/sys/ configuration files contain more than one value. To correctly send new values to them, place a space character between each value passed with the echo command, such as is done in this example: 
echo 4 2 45 > /proc/sys/kernel/acct
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Note

Any configuration changes made using the echo command disappear when the system is restarted. To make configuration changes take effect after the system is rebooted, refer to Section 5.4, “Using the sysctl Command”.
The /proc/sys/ directory contains several subdirectories controlling different aspects of a running kernel.
5.3.9.1.  /proc/sys/dev/
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This directory provides parameters for particular devices on the system. Most systems have at least two directories, cdrom/ and raid/. Customized kernels can have other directories, such as parport/, which provides the ability to share one parallel port between multiple device drivers.
The cdrom/ directory contains a file called info, which reveals a number of important CD-ROM parameters: 
CD-ROM information, Id: cdrom.c 3.20 2003/12/17
drive name:             hdc
drive speed:            48
drive # of slots:       1
Can close tray:         1
Can open tray:          1
Can lock tray:          1
Can change speed:       1
Can select disk:        0
Can read multisession:  1
Can read MCN:           1
Reports media changed:  1
Can play audio:         1
Can write CD-R:         0
Can write CD-RW:        0
Can read DVD:           0
Can write DVD-R:        0
Can write DVD-RAM:      0
Can read MRW:           0
Can write MRW:          0
Can write RAM:          0
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This file can be quickly scanned to discover the qualities of an unknown CD-ROM. If multiple CD-ROMs are available on a system, each device is given its own column of information.
Various files in /proc/sys/dev/cdrom, such as autoclose and checkmedia, can be used to control the system's CD-ROM. Use the echo command to enable or disable these features.
If RAID support is compiled into the kernel, a /proc/sys/dev/raid/ directory becomes available with at least two files in it: speed_limit_min and speed_limit_max. These settings determine the acceleration of RAID devices for I/O intensive tasks, such as resyncing the disks.
5.3.9.2.  /proc/sys/fs/
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This directory contains an array of options and information concerning various aspects of the file system, including quota, file handle, inode, and dentry information.
The binfmt_misc/ directory is used to provide kernel support for miscellaneous binary formats.
The important files in /proc/sys/fs/ include:
  • dentry-state — Provides the status of the directory cache. The file looks similar to the following: 
    57411	52939	45	0	0	0
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    The first number reveals the total number of directory cache entries, while the second number displays the number of unused entries. The third number tells the number of seconds between when a directory has been freed and when it can be reclaimed, and the fourth measures the pages currently requested by the system. The last two numbers are not used and display only zeros.
  • dquot-nr — Lists the maximum number of cached disk quota entries.
  • file-max — Lists the maximum number of file handles that the kernel allocates. Raising the value in this file can resolve errors caused by a lack of available file handles.
  • file-nr — Lists the number of allocated file handles, used file handles, and the maximum number of file handles.
  • overflowgid and overflowuid — Defines the fixed group ID and user ID, respectively, for use with file systems that only support 16-bit group and user IDs.
  • super-max — Controls the maximum number of superblocks available.
  • super-nr — Displays the current number of superblocks in use.
5.3.9.3.  /proc/sys/kernel/
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This directory contains a variety of different configuration files that directly affect the operation of the kernel. Some of the most important files include:
  • acct — Controls the suspension of process accounting based on the percentage of free space available on the file system containing the log. By default, the file looks like the following: 
    4	2	30
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    The first value dictates the percentage of free space required for logging to resume, while the second value sets the threshold percentage of free space when logging is suspended. The third value sets the interval, in seconds, that the kernel polls the file system to see if logging should be suspended or resumed.
  • cap-bound — Controls the capability bounding settings, which provides a list of capabilities for any process on the system. If a capability is not listed here, then no process, no matter how privileged, can do it. The idea is to make the system more secure by ensuring that certain things cannot happen, at least beyond a certain point in the boot process.
    For a valid list of values for this virtual file, refer to the following installed documentation:
    /lib/modules/<kernel-version>/build/include/linux/capability.h.
  • ctrl-alt-del — Controls whether Ctrl+Alt+Delete gracefully restarts the computer using init (0) or forces an immediate reboot without syncing the dirty buffers to disk (1).
  • domainname — Configures the system domain name, such as example.com.
  • exec-shield — Configures the Exec Shield feature of the kernel. Exec Shield provides protection against certain types of buffer overflow attacks.
    There are two possible values for this virtual file:
    • 0 — Disables Exec Shield.
    • 1 — Enables Exec Shield. This is the default value.

    Important

    If a system is running security-sensitive applications that were started while Exec Shield was disabled, these applications must be restarted when Exec Shield is enabled in order for Exec Shield to take effect.
  • exec-shield-randomize — Enables location randomization of various items in memory. This helps deter potential attackers from locating programs and daemons in memory. Each time a program or daemon starts, it is put into a different memory location each time, never in a static or absolute memory address.
    There are two possible values for this virtual file:
    • 0 — Disables randomization of Exec Shield. This may be useful for application debugging purposes.
    • 1 — Enables randomization of Exec Shield. This is the default value. Note: The exec-shield file must also be set to 1 for exec-shield-randomize to be effective.
  • hostname — Configures the system hostname, such as www.example.com.
  • hotplug — Configures the utility to be used when a configuration change is detected by the system. This is primarily used with USB and Cardbus PCI. The default value of /sbin/hotplug should not be changed unless testing a new program to fulfill this role.
  • modprobe — Sets the location of the program used to load kernel modules. The default value is /sbin/modprobe which means kmod calls it to load the module when a kernel thread calls kmod.
  • msgmax — Sets the maximum size of any message sent from one process to another and is set to 8192 bytes by default. Be careful when raising this value, as queued messages between processes are stored in non-swappable kernel memory. Any increase in msgmax would increase RAM requirements for the system.
  • msgmnb — Sets the maximum number of bytes in a single message queue. The default is 16384.
  • msgmni — Sets the maximum number of message queue identifiers. The default is 16.
  • osrelease — Lists the Linux kernel release number. This file can only be altered by changing the kernel source and recompiling.
  • ostype — Displays the type of operating system. By default, this file is set to Linux, and this value can only be changed by changing the kernel source and recompiling.
  • overflowgid and overflowuid — Defines the fixed group ID and user ID, respectively, for use with system calls on architectures that only support 16-bit group and user IDs.
  • panic — Defines the number of seconds the kernel postpones rebooting when the system experiences a kernel panic. By default, the value is set to 0, which disables automatic rebooting after a panic.
  • printk — This file controls a variety of settings related to printing or logging error messages. Each error message reported by the kernel has a loglevel associated with it that defines the importance of the message. The loglevel values break down in this order:
    • 0 — Kernel emergency. The system is unusable.
    • 1 — Kernel alert. Action must be taken immediately.
    • 2 — Condition of the kernel is considered critical.
    • 3 — General kernel error condition.
    • 4 — General kernel warning condition.
    • 5 — Kernel notice of a normal but significant condition.
    • 6 — Kernel informational message.
    • 7 — Kernel debug-level messages.
    Four values are found in the printk file: 
    6     4     1     7
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    Each of these values defines a different rule for dealing with error messages. The first value, called the console loglevel, defines the lowest priority of messages printed to the console. (Note that, the lower the priority, the higher the loglevel number.) The second value sets the default loglevel for messages without an explicit loglevel attached to them. The third value sets the lowest possible loglevel configuration for the console loglevel. The last value sets the default value for the console loglevel.
  • random/ directory — Lists a number of values related to generating random numbers for the kernel.
  • rtsig-max — Configures the maximum number of POSIX real-time signals that the system may have queued at any one time. The default value is 1024.
  • rtsig-nr — Lists the current number of POSIX real-time signals queued by the kernel.
  • sem — Configures semaphore settings within the kernel. A semaphore is a System V IPC object that is used to control utilization of a particular process.
  • shmall— Sets the total amount of shared memory pages that can be used at one time, system-wide. By default, this value is 2097152.
  • shmmax — Sets the largest shared memory segment size allowed by the kernel. By default, this value is 33554432. However, the kernel supports much larger values than this.
  • shmmni — Sets the maximum number of shared memory segments for the whole system. By default, this value is 4096.
  • sysrq — Activates the System Request Key, if this value is set to anything other than zero (0), the default.
    The System Request Key allows immediate input to the kernel through simple key combinations. For example, the System Request Key can be used to immediately shut down or restart a system, sync all mounted file systems, or dump important information to the console. To initiate a System Request Key, type Alt+SysRq+ <system request code> . Replace <system request code> with one of the following system request codes:
    • r — Disables raw mode for the keyboard and sets it to XLATE (a limited keyboard mode which does not recognize modifiers such as Alt, Ctrl, or Shift for all keys).
    • k — Kills all processes active in a virtual console. Also called Secure Access Key (SAK), it is often used to verify that the login prompt is spawned from init and not a Trojan copy designed to capture usernames and passwords.
    • b — Reboots the kernel without first unmounting file systems or syncing disks attached to the system.
    • c — Crashes the system without first unmounting file systems or syncing disks attached to the system.
    • o — Shuts off the system.
    • s — Attempts to sync disks attached to the system.
    • u — Attempts to unmount and remount all file systems as read-only.
    • p — Outputs all flags and registers to the console.
    • t — Outputs a list of processes to the console.
    • m — Outputs memory statistics to the console.
    • 0 through 9 — Sets the log level for the console.
    • e — Kills all processes except init using SIGTERM.
    • i — Kills all processes except init using SIGKILL.
    • l — Kills all processes using SIGKILL (including init). The system is unusable after issuing this System Request Key code.
    • h — Displays help text.
    This feature is most beneficial when using a development kernel or when experiencing system freezes.

    Warning

    The System Request Key feature is considered a security risk because an unattended console provides an attacker with access to the system. For this reason, it is turned off by default.
    Refer to /usr/share/doc/kernel-doc-<version>/Documentation/sysrq.txt for more information about the System Request Key.
  • sysrq-key — Defines the key code for the System Request Key (84 is the default).
  • sysrq-sticky — Defines whether the System Request Key is a chorded key combination. The accepted values are as follows:
    • 0 — Alt+SysRq and the system request code must be pressed simultaneously. This is the default value.
    • 1 — Alt+SysRq must be pressed simultaneously, but the system request code can be pressed anytime before the number of seconds specified in /proc/sys/kernel/sysrq-timer elapses.
  • sysrq-timer — Specifies the number of seconds allowed to pass before the system request code must be pressed. The default value is 10.
  • tainted — Indicates whether a non-GPL module is loaded.
    • 0 — No non-GPL modules are loaded.
    • 1 — At least one module without a GPL license (including modules with no license) is loaded.
    • 2 — At least one module was force-loaded with the command insmod -f.
  • threads-max — Sets the maximum number of threads to be used by the kernel, with a default value of 2048.
  • version — Displays the date and time the kernel was last compiled. The first field in this file, such as #3, relates to the number of times a kernel was built from the source base.
5.3.9.4.  /proc/sys/net/
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This directory contains subdirectories concerning various networking topics. Various configurations at the time of kernel compilation make different directories available here, such as ethernet/, ipv4/, ipx/, and ipv6/. By altering the files within these directories, system administrators are able to adjust the network configuration on a running system.
Given the wide variety of possible networking options available with Linux, only the most common /proc/sys/net/ directories are discussed.
The /proc/sys/net/core/ directory contains a variety of settings that control the interaction between the kernel and networking layers. The most important of these files are:
  • message_burst — Sets the amount of time in tenths of a second required to write a new warning message. This setting is used to mitigate Denial of Service (DoS) attacks. The default setting is 50.
  • message_cost — Sets a cost on every warning message. The higher the value of this file (default of 5), the more likely the warning message is ignored. This setting is used to mitigate DoS attacks.
    The idea of a DoS attack is to bombard the targeted system with requests that generate errors and fill up disk partitions with log files or require all of the system's resources to handle the error logging. The settings in message_burst and message_cost are designed to be modified based on the system's acceptable risk versus the need for comprehensive logging.
  • netdev_max_backlog — Sets the maximum number of packets allowed to queue when a particular interface receives packets faster than the kernel can process them. The default value for this file is 300.
  • optmem_max — Configures the maximum ancillary buffer size allowed per socket.
  • rmem_default — Sets the receive socket buffer default size in bytes.
  • rmem_max — Sets the receive socket buffer maximum size in bytes.
  • wmem_default — Sets the send socket buffer default size in bytes.
  • wmem_max — Sets the send socket buffer maximum size in bytes.
The /proc/sys/net/ipv4/ directory contains additional networking settings. Many of these settings, used in conjunction with one another, are useful in preventing attacks on the system or when using the system to act as a router.

Warning

An erroneous change to these files may affect remote connectivity to the system.
The following is a list of some of the more important files within the /proc/sys/net/ipv4/ directory:
  • icmp_destunreach_rate, icmp_echoreply_rate, icmp_paramprob_rate, and icmp_timeexeed_rate — Set the maximum ICMP send packet rate, in 1/100 of a second, to hosts under certain conditions. A setting of 0 removes any delay and is not a good idea.
  • icmp_echo_ignore_all and icmp_echo_ignore_broadcasts — Allows the kernel to ignore ICMP ECHO packets from every host or only those originating from broadcast and multicast addresses, respectively. A value of 0 allows the kernel to respond, while a value of 1 ignores the packets.
  • ip_default_ttl — Sets the default Time To Live (TTL), which limits the number of hops a packet may make before reaching its destination. Increasing this value can diminish system performance.
  • ip_forward — Permits interfaces on the system to forward packets to one other. By default, this file is set to 0. Setting this file to 1 enables network packet forwarding.
  • ip_local_port_range — Specifies the range of ports to be used by TCP or UDP when a local port is needed. The first number is the lowest port to be used and the second number specifies the highest port. Any systems that expect to require more ports than the default 1024 to 4999 should use a range from 32768 to 61000.
  • tcp_syn_retries — Provides a limit on the number of times the system re-transmits a SYN packet when attempting to make a connection.
  • tcp_retries1 — Sets the number of permitted re-transmissions attempting to answer an incoming connection. Default of 3.
  • tcp_retries2 — Sets the number of permitted re-transmissions of TCP packets. Default of 15.
The file called 
/usr/share/doc/kernel-doc-<version>/Documentation/networking/ ip-sysctl.txt
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contains a complete list of files and options available in the /proc/sys/net/ipv4/ directory.
A number of other directories exist within the /proc/sys/net/ipv4/ directory and each covers a different aspect of the network stack. The /proc/sys/net/ipv4/conf/ directory allows each system interface to be configured in different ways, including the use of default settings for unconfigured devices (in the /proc/sys/net/ipv4/conf/default/ subdirectory) and settings that override all special configurations (in the /proc/sys/net/ipv4/conf/all/ subdirectory).
The /proc/sys/net/ipv4/neigh/ directory contains settings for communicating with a host directly connected to the system (called a network neighbor) and also contains different settings for systems more than one hop away.
Routing over IPV4 also has its own directory, /proc/sys/net/ipv4/route/. Unlike conf/ and neigh/, the /proc/sys/net/ipv4/route/ directory contains specifications that apply to routing with any interfaces on the system. Many of these settings, such as max_size, max_delay, and min_delay, relate to controlling the size of the routing cache. To clear the routing cache, write any value to the flush file.
Additional information about these directories and the possible values for their configuration files can be found in: 
/usr/share/doc/kernel-doc-<version>/Documentation/filesystems/proc.txt
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5.3.9.5.  /proc/sys/vm/
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This directory facilitates the configuration of the Linux kernel's virtual memory (VM) subsystem. The kernel makes extensive and intelligent use of virtual memory, which is commonly referred to as swap space.
The following files are commonly found in the /proc/sys/vm/ directory:
  • block_dump — Configures block I/O debugging when enabled. All read/write and block dirtying operations done to files are logged accordingly. This can be useful if diagnosing disk spin up and spin downs for laptop battery conservation. All output when block_dump is enabled can be retrieved via dmesg. The default value is 0.

    Note

    If block_dump is enabled at the same time as kernel debugging, it is prudent to stop the klogd daemon, as it generates erroneous disk activity caused by block_dump.
  • dirty_background_ratio — Starts background writeback of dirty data at this percentage of total memory, via a pdflush daemon. The default value is 10.
  • dirty_expire_centisecs — Defines when dirty in-memory data is old enough to be eligible for writeout. Data which has been dirty in-memory for longer than this interval is written out next time a pdflush daemon wakes up. The default value is 3000, expressed in hundredths of a second.
  • dirty_ratio — Starts active writeback of dirty data at this percentage of total memory for the generator of dirty data, via pdflush. The default value is 40.
  • dirty_writeback_centisecs — Defines the interval between pdflush daemon wakeups, which periodically writes dirty in-memory data out to disk. The default value is 500, expressed in hundredths of a second.
  • laptop_mode — Minimizes the number of times that a hard disk needs to spin up by keeping the disk spun down for as long as possible, therefore conserving battery power on laptops. This increases efficiency by combining all future I/O processes together, reducing the frequency of spin ups. The default value is 0, but is automatically enabled in case a battery on a laptop is used.
    This value is controlled automatically by the acpid daemon once a user is notified battery power is enabled. No user modifications or interactions are necessary if the laptop supports the ACPI (Advanced Configuration and Power Interface) specification.
    For more information, refer to the following installed documentation:
    /usr/share/doc/kernel-doc-<version>/Documentation/laptop-mode.txt
  • lower_zone_protection — Determines how aggressive the kernel is in defending lower memory allocation zones. This is effective when utilized with machines configured with highmem memory space enabled. The default value is 0, no protection at all. All other integer values are in megabytes, and lowmem memory is therefore protected from being allocated by users.
    For more information, refer to the following installed documentation:
    /usr/share/doc/kernel-doc-<version>/Documentation/filesystems/proc.txt
  • max_map_count — Configures the maximum number of memory map areas a process may have. In most cases, the default value of 65536 is appropriate.
  • min_free_kbytes — Forces the Linux VM (virtual memory manager) to keep a minimum number of kilobytes free. The VM uses this number to compute a pages_min value for each lowmem zone in the system. The default value is in respect to the total memory on the machine.
  • nr_hugepages — Indicates the current number of configured hugetlb pages in the kernel.
    For more information, refer to the following installed documentation:
    /usr/share/doc/kernel-doc-<version>/Documentation/vm/hugetlbpage.txt
  • nr_pdflush_threads — Indicates the number of pdflush daemons that are currently running. This file is read-only, and should not be changed by the user. Under heavy I/O loads, the default value of two is increased by the kernel.
  • overcommit_memory — Configures the conditions under which a large memory request is accepted or denied. The following three modes are available:
    • 0 — The kernel performs heuristic memory over commit handling by estimating the amount of memory available and failing requests that are blatantly invalid. Unfortunately, since memory is allocated using a heuristic rather than a precise algorithm, this setting can sometimes allow available memory on the system to be overloaded. This is the default setting.
    • 1 — The kernel performs no memory over commit handling. Under this setting, the potential for memory overload is increased, but so is performance for memory intensive tasks (such as those executed by some scientific software).
    • 2 — The kernel fails requests for memory that add up to all of swap plus the percent of physical RAM specified in /proc/sys/vm/overcommit_ratio. This setting is best for those who desire less risk of memory overcommitment.

      Note

      This setting is only recommended for systems with swap areas larger than physical memory.
  • overcommit_ratio — Specifies the percentage of physical RAM considered when /proc/sys/vm/overcommit_memory is set to 2. The default value is 50.
  • page-cluster — Sets the number of pages read in a single attempt. The default value of 3, which actually relates to 16 pages, is appropriate for most systems.
  • swappiness — Determines how much a machine should swap. The higher the value, the more swapping occurs. The default value, as a percentage, is set to 60.
All kernel-based documentation can be found in the following locally installed location:
/usr/share/doc/kernel-doc-<version>/Documentation/, which contains additional information.

5.3.10.  /proc/sysvipc/
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This directory contains information about System V IPC resources. The files in this directory relate to System V IPC calls for messages (msg), semaphores (sem), and shared memory (shm).

5.3.11.  /proc/tty/
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This directory contains information about the available and currently used tty devices on the system. Originally called teletype devices, any character-based data terminals are called tty devices.
In Linux, there are three different kinds of tty devices. Serial devices are used with serial connections, such as over a modem or using a serial cable. Virtual terminals create the common console connection, such as the virtual consoles available when pressing Alt+<F-key> at the system console. Pseudo terminals create a two-way communication that is used by some higher level applications, such as XFree86. The drivers file is a list of the current tty devices in use, as in the following example: 
serial               /dev/cua        5  64-127 serial:callout
serial               /dev/ttyS       4  64-127 serial
pty_slave            /dev/pts      136   0-255 pty:slave
pty_master           /dev/ptm      128   0-255 pty:master
pty_slave            /dev/ttyp       3   0-255 pty:slave
pty_master           /dev/pty        2   0-255 pty:master
/dev/vc/0            /dev/vc/0       4       0 system:vtmaster
/dev/ptmx            /dev/ptmx       5       2 system
/dev/console         /dev/console    5       1 system:console
/dev/tty             /dev/tty        5       0 system:/dev/tty
unknown              /dev/vc/%d      4    1-63 console
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The /proc/tty/driver/serial file lists the usage statistics and status of each of the serial tty lines.
In order for tty devices to be used as network devices, the Linux kernel enforces line discipline on the device. This allows the driver to place a specific type of header with every block of data transmitted over the device, making it possible for the remote end of the connection to a block of data as just one in a stream of data blocks. SLIP and PPP are common line disciplines, and each are commonly used to connect systems to one other over a serial link.
Registered line disciplines are stored in the ldiscs file, and more detailed information is available within the ldisc/ directory.

5.3.12.  /proc//
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Out of Memory (OOM) refers to a computing state where all available memory, including swap space, has been allocated. When this situation occurs, it will cause the system to panic and stop functioning as expected. There is a switch that controls OOM behavior in /proc/sys/vm/panic_on_oom. When set to 1 the kernel will panic on OOM. A setting of 0 instructs the kernel to call a function named oom_killer on an OOM. Usually, oom_killer can kill rogue processes and the system will survive.
The easiest way to change this is to echo the new value to /proc/sys/vm/panic_on_oom. 
~]# cat /proc/sys/vm/panic_on_oom
1
~]# echo 0 > /proc/sys/vm/panic_on_oom
~]# cat /proc/sys/vm/panic_on_oom
0
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It is also possible to prioritize which processes get killed by adjusting the oom_killer score. In /proc/<PID>/ there are two tools labelled oom_adj and oom_score. Valid scores for oom_adj are in the range -16 to +15. To see the current oom_killer score, view the oom_score for the process. oom_killer will kill processes with the highest scores first.
This example adjusts the oom_score of a process with a PID of 12465 to make it less likely that oom_killer will kill it. 
~]# cat /proc/12465/oom_score
79872
~]# echo -5 > /proc/12465/oom_adj
~]# cat /proc/12465/oom_score
78
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There is also a special value of -17, which disables oom_killer for that process. In the example below, oom_score returns a value of 0, indicating that this process would not be killed. 
~]# cat /proc/12465/oom_score
78
~]# echo -17 > /proc/12465/oom_adj
~]# cat /proc/12465/oom_score
0
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A function called badness() is used to determine the actual score for each process. This is done by adding up 'points' for each examined process. The process scoring is done in the following way:
  1. The basis of each process's score is its memory size.
  2. The memory size of any of the process's children (not including a kernel thread) is also added to the score
  3. The process's score is increased for 'niced' processes and decreased for long running processes.
  4. Processes with the CAP_SYS_ADMIN and CAP_SYS_RAWIO capabilities have their scores reduced.
  5. The final score is then bitshifted by the value saved in the oom_adj file.
Thus, a process with the highest oom_score value will most probably be a non-privileged, recently started process that, along with its children, uses a large amount of memory, has been 'niced', and handles no raw I/O.

5.4. Using the sysctl Command
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The /sbin/sysctl command is used to view, set, and automate kernel settings in the /proc/sys/ directory.
For a quick overview of all settings configurable in the /proc/sys/ directory, type the /sbin/sysctl -a command as root. This creates a large, comprehensive list, a small portion of which looks something like the following: 
net.ipv4.route.min_delay = 2 kernel.sysrq = 0 kernel.sem = 250     32000     32     128
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This is the same information seen if each of the files were viewed individually. The only difference is the file location. For example, the /proc/sys/net/ipv4/route/min_delay file is listed as net.ipv4.route.min_delay, with the directory slashes replaced by dots and the proc.sys portion assumed.
The sysctl command can be used in place of echo to assign values to writable files in the /proc/sys/ directory. For example, instead of using the command 
echo 1 > /proc/sys/kernel/sysrq
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use the equivalent sysctl command as follows: 
~]# sysctl -w kernel.sysrq="1"
kernel.sysrq = 1
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While quickly setting single values like this in /proc/sys/ is helpful during testing, this method does not work as well on a production system as special settings within /proc/sys/ are lost when the machine is rebooted. To preserve custom settings, add them to the /etc/sysctl.conf file.
Each time the system boots, the init program runs the /etc/rc.d/rc.sysinit script. This script contains a command to execute sysctl using /etc/sysctl.conf to determine the values passed to the kernel. Any values added to /etc/sysctl.conf therefore take effect each time the system boots.

5.5. Additional Resources
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Below are additional sources of information about proc file system.

5.5.1. Installed Documentation
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Some of the best documentation about the proc file system is installed on the system by default.
  • /usr/share/doc/kernel-doc-<version>/Documentation/filesystems/proc.txt — Contains assorted, but limited, information about all aspects of the /proc/ directory.
  • /usr/share/doc/kernel-doc-<version>/Documentation/sysrq.txt — An overview of System Request Key options.
  • /usr/share/doc/kernel-doc-<version>/Documentation/sysctl/ — A directory containing a variety of sysctl tips, including modifying values that concern the kernel (kernel.txt), accessing file systems (fs.txt), and virtual memory use (vm.txt).
  • /usr/share/doc/kernel-doc-<version>/Documentation/networking/ip-sysctl.txt — A detailed overview of IP networking options.

5.5.2. Useful Websites
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  • http://www.linuxhq.com/ — This website maintains a complete database of source, patches, and documentation for various versions of the Linux kernel.
The basic idea behind RAID is to combine multiple small, inexpensive disk drives into an array to accomplish performance or redundancy goals not attainable with one large and expensive drive. This array of drives appears to the computer as a single logical storage unit or drive.

6.1. What is RAID?
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RAID allows information to access several disks. RAID uses techniques such as disk striping (RAID Level 0), disk mirroring (RAID Level 1), and disk striping with parity (RAID Level 5) to achieve redundancy, lower latency, increased bandwidth, and maximized ability to recover from hard disk crashes.
RAID consistently distributes data across each drive in the array. RAID then breaks down the data into consistently-sized chunks (commonly 32K or 64k, although other values are acceptable). Each chunk is then written to a hard drive in the RAID array according to the RAID level employed. When the data is read, the process is reversed, giving the illusion that the multiple drives in the array are actually one large drive.

6.1.1. Who Should Use RAID?
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System Administrators and others who manage large amounts of data would benefit from using RAID technology. Primary reasons to deploy RAID include:
  • Enhances speed
  • Increases storage capacity using a single virtual disk
  • Minimizes disk failure

6.1.2. Hardware RAID versus Software RAID
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There are two possible RAID approaches: hardware RAID and software RAID.
Hardware RAID
The hardware-based array manages the RAID subsystem independently from the host. It presents a single disk per RAID array to the host.
A hardware RAID device connects to the SCSI controller and presents the RAID arrays as a single SCSI drive. An external RAID system moves all RAID handling “intelligence” into a controller located in the external disk subsystem. The whole subsystem is connected to the host via a normal SCSI controller and appears to the host as a single disk.
RAID controller cards function like a SCSI controller to the operating system, and handle all the actual drive communications. The user plugs the drives into the RAID controller (just like a normal SCSI controller) and then adds them to the RAID controllers configuration, and the operating system won't know the difference.
Software RAID
Software RAID implements the various RAID levels in the kernel disk (block device) code. It offers the cheapest possible solution, as expensive disk controller cards or hot-swap chassis[1] are not required. Software RAID also works with cheaper IDE disks as well as SCSI disks. With today's faster CPUs, software RAID outperforms hardware RAID.
The Linux kernel contains an MD driver that allows the RAID solution to be completely hardware independent. The performance of a software-based array depends on the server CPU performance and load.
To learn more about software RAID, here are the key features:
  • Threaded rebuild process
  • Kernel-based configuration
  • Portability of arrays between Linux machines without reconstruction
  • Backgrounded array reconstruction using idle system resources
  • Hot-swappable drive support
  • Automatic CPU detection to take advantage of certain CPU optimizations

6.1.3. RAID Levels and Linear Support
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RAID supports various configurations, including levels 0, 1, 4, 5, and linear. These RAID types are defined as follows:
Level 0
RAID level 0, often called “striping”, is a performance-oriented striped data mapping technique. This means the data being written to the array is broken down into strips and written across the member disks of the array, allowing high I/O performance at low inherent cost but provides no redundancy. The storage capacity of a level 0 array is equal to the total capacity of the member disks in a hardware RAID or the total capacity of member partitions in a software RAID.
Level 1
RAID level 1, or “mirroring”, has been used longer than any other form of RAID. Level 1 provides redundancy by writing identical data to each member disk of the array, leaving a “mirrored” copy on each disk. Mirroring remains popular due to its simplicity and high level of data availability. Level 1 operates with two or more disks that may use parallel access for high data-transfer rates when reading but more commonly operate independently to provide high I/O transaction rates. Level 1 provides very good data reliability and improves performance for read-intensive applications but at a relatively high cost. The storage capacity of the level 1 array is equal to the capacity of one of the mirrored hard disks in a hardware RAID or one of the mirrored partitions in a software RAID.

Note

RAID level 1 comes at a high cost because you write the same information to all of the disks in the array, which wastes drive space. For example, if you have RAID level 1 set up so that your root (/) partition exists on two 40G drives, you have 80G total but are only able to access 40G of that 80G. The other 40G acts like a mirror of the first 40G.
Level 4
RAID level 4 uses parity[2] concentrated on a single disk drive to protect data. It is better suited to transaction I/O rather than large file transfers. Because the dedicated parity disk represents an inherent bottleneck, level 4 is seldom used without accompanying technologies such as write-back caching. Although RAID level 4 is an option in some RAID partitioning schemes, it is not an option allowed in Red Hat Enterprise Linux RAID installations. The storage capacity of hardware RAID level 4 is equal to the capacity of member disks, minus the capacity of one member disk. The storage capacity of software RAID level 4 is equal to the capacity of the member partitions, minus the size of one of the partitions if they are of equal size.

Note

RAID level 4 takes up the same amount of space as RAID level 5, but level 5 has more advantages. For this reason, level 4 is not supported.
Level 5
RAID level 5 is the most common type of RAID. By distributing parity across some or all of an array's member disk drives, RAID level 5 eliminates the write bottleneck inherent in level 4. The only performance bottleneck is the parity calculation process. With modern CPUs and software RAID, that usually is not a very big problem. As with level 4, the result is asymmetrical performance, with reads substantially outperforming writes. Level 5 is often used with write-back caching to reduce the asymmetry. The storage capacity of hardware RAID level 5 is equal to the capacity of member disks, minus the capacity of one member disk. The storage capacity of software RAID level 5 is equal to the capacity of the member partitions, minus the size of one of the partitions if they are of equal size.
Linear RAID
Linear RAID is a simple grouping of drives to create a larger virtual drive. In linear RAID, the chunks are allocated sequentially from one member drive, going to the next drive only when the first is completely filled. This grouping provides no performance benefit, as it is unlikely that any I/O operations will be split between member drives. Linear RAID also offers no redundancy and, in fact, decreases reliability — if any one member drive fails, the entire array cannot be used. The capacity is the total of all member disks.

6.2. Configuring Software RAID
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Users can configure software RAID during the graphical installation process, the text-based installation process, or during a kickstart installation. This section discusses software RAID configuration during the installation process using the Disk Druid application, and covers the following steps:
  1. Creating software RAID partitions on physical hard drives.
  2. Creating RAID devices from the software RAID partitions.
  3. (Optional) Configuring LVM from the RAID devices.
  4. Creating file systems from the RAID devices.
To configure software RAID, select Create custom layout from the pulldown list on the Disk Partitioning Setup screen, click the Next button, and follow the instructions in the rest of this section. The example screenshots in this section use two 10 GB disk drives (/dev/hda and /dev/hdb) to illustrate the creation of simple RAID 1 and RAID 0 configurations, and detail how to create a simple RAID configuration by implementing multiple RAID devices.

6.2.1. Creating the RAID Partitions
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In a typical situation, the disk drives are new or are formatted. Both drives are shown as raw devices with no partition configuration in Figure 6.1, “Two Blank Drives, Ready For Configuration”.
Two Blank Drives, Ready For Configuration
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Two Blank Drives, Ready For Configuration

Figure 6.1. Two Blank Drives, Ready For Configuration

  1. In Disk Druid, click the RAID button to enter the software RAID creation screen.
  2. Choose Create a software RAID partition to create a RAID partition as shown in Figure 6.2, “RAID Partition Options”. Note that no other RAID options (such as entering a mount point) are available until RAID partitions, as well as RAID devices, are created. Click OK to confirm the choice.
    RAID Partition Options
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    RAID Partition Options

    Figure 6.2. RAID Partition Options

  3. A software RAID partition must be constrained to one drive. For Allowable Drives, select the drive to use for RAID. If you have multiple drives, by default all drives are selected and you must deselect the drives you do not want.
    Adding a RAID Partition
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    Adding a RAID Partition

    Figure 6.3. Adding a RAID Partition

  4. Edit the Size (MB) field, and enter the size that you want the partition to be (in MB).
  5. Select Fixed Size to specify partition size. Select Fill all space up to (MB) and enter a value (in MB) to specify partition size range. Select Fill to maximum allowable size to allow maximum available space of the hard disk. Note that if you make more than one space growable, they share the available free space on the disk.
  6. Select Force to be a primary partition if you want the partition to be a primary partition. A primary partition is one of the first four partitions on the hard drive. If unselected, the partition is created as a logical partition. If other operating systems are already on the system, unselecting this option should be considered. For more information on primary versus logical/extended partitions, refer to the appendix section of the Red Hat Enterprise Linux Installation Guide.
Repeat these steps to create as many partitions as needed for your RAID setup. Notice that all the partitions do not have to be RAID partitions. For example, you can configure only the /boot partition as a software RAID device, leaving the root partition (/), /home, and swap as regular file systems. Figure 6.4, “RAID 1 Partitions Ready, Pre-Device and Mount Point Creation” shows successfully allocated space for the RAID 1 configuration (for /boot), which is now ready for RAID device and mount point creation:
RAID 1 Partitions Ready, Pre-Device and Mount Point Creation
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RAID 1 Partitions Ready, Pre-Device and Mount Point Creation

Figure 6.4. RAID 1 Partitions Ready, Pre-Device and Mount Point Creation

6.2.2. Creating the RAID Devices and Mount Points
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Once you create all of your partitions as software RAID partitions, you must create the RAID device and mount point.
  1. On the main partitioning screen, click the RAID button. The RAID Options dialog appears as shown in Figure 6.5, “RAID Options”.
    RAID Options
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    RAID Options

    Figure 6.5. RAID Options

  2. Select the Create a RAID device option, and click OK. As shown in Figure 6.6, “Making a RAID Device and Assigning a Mount Point”, the Make RAID Device dialog appears, allowing you to make a RAID device and assign a mount point.
    Making a RAID Device and Assigning a Mount Point
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    Making a RAID Device and Assigning a Mount Point

    Figure 6.6. Making a RAID Device and Assigning a Mount Point

  3. Select a mount point from the Mount Point pulldown list.
  4. Choose the file system type for the partition from the File System Type pulldown list. At this point you can either configure a dynamic LVM file system or a traditional static ext2/ext3 file system. For more information on LVM and its configuration during the installation process, refer to Chapter 11, LVM (Logical Volume Manager). If LVM is not required, continue on with the following instructions.
  5. From the RAID Device pulldown list, select a device name such as md0.
  6. From the RAID Level, choose the required RAID level.

    Note

    If you are making a RAID partition of /boot, you must choose RAID level 1, and it must use one of the first two drives (IDE first, SCSI second). If you are not creating a separate RAID partition of /boot, and you are making a RAID partition for the root file system (that is, /), it must be RAID level 1 and must use one of the first two drives (IDE first, SCSI second).
  7. The RAID partitions created appear in the RAID Members list. Select which of these partitions should be used to create the RAID device.
  8. If configuring RAID 1 or RAID 5, specify the number of spare partitions in the Number of spares field. If a software RAID partition fails, the spare is automatically used as a replacement. For each spare you want to specify, you must create an additional software RAID partition (in addition to the partitions for the RAID device). Select the partitions for the RAID device and the partition(s) for the spare(s).
  9. Click OK to confirm the setup. The RAID device appears in the Drive Summary list.
  10. Repeat this chapter's entire process for configuring additional partitions, devices, and mount points, such as the root partition (/), home partition (/home), or swap.
After completing the entire configuration, the figure as shown in Figure 6.7, “Sample RAID Configuration” resembles the default configuration, except for the use of RAID.
Sample RAID Configuration
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Sample RAID Configuration

Figure 6.7. Sample RAID Configuration

The figure as shown in Figure 6.8, “Sample RAID With LVM Configuration” is an example of a RAID and LVM configuration.
Sample RAID With LVM Configuration
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Sample RAID With LVM Configuration

Figure 6.8. Sample RAID With LVM Configuration

You can proceed with your installation process by clicking Next. Refer to the Red Hat Enterprise Linux Installation Guide for further instructions.

6.3. Managing Software RAID
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This section discusses software RAID configuration and management after the installation, and covers the following topics:
  • Reviewing existing software RAID configuration.
  • Creating a new RAID device.
  • Replacing a faulty device in an array.
  • Adding a new device to an existing array.
  • Deactivating and removing an existing RAID device.
  • Saving the configuration.
All examples in this section use the software RAID configuration from the previous section.

6.3.1. Reviewing RAID Configuration
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When a software RAID is in use, basic information about all presently active RAID devices are stored in the /proc/mdstat special file. To list these devices, display the content of this file by typing the following at a shell prompt: 
cat /proc/mdstat
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To determine whether a certain device is a RAID device or a component device, run the command in the following form as root: 
mdadm --query device…
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In order to examine a RAID device in more detail, use the following command: 
mdadm --detail raid_device…
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Similarly, to examine a component device, type: 
mdadm --examine component_device…
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While the mdadm --detail command displays information about a RAID device, mdadm --examine only relays information about a RAID device as it relates to a given component device. This distinction is particularly important when working with a RAID device that itself is a component of another RAID device.
The mdadm --query command, as well as both mdadm --detail and mdadm --examine commands allow you to specify multiple devices at once.

Example 6.1. Reviewing RAID configuration

Assume the system uses configuration from Figure 6.7, “Sample RAID Configuration”. You can verify that /dev/md0 is a RAID device by typing the following at a shell prompt: 
~]# mdadm --query /dev/md0
/dev/md0: 125.38MiB raid1 2 devices, 0 spares. Use mdadm --detail for more detail.
/dev/md0: No md super block found, not an md component.
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As you can see, the above command produces only a brief overview of the RAID device and its configuration. To display more detailed information, use the following command instead: 
~]# mdadm --detail /dev/md0
/dev/md0:
        Version : 0.90
  Creation Time : Tue Jun 28 16:05:49 2011
     Raid Level : raid1
     Array Size : 128384 (125.40 MiB 131.47 MB)
  Used Dev Size : 128384 (125.40 MiB 131.47 MB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Thu Jun 30 17:06:34 2011
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           UUID : 49c5ac74:c2b79501:5c28cb9c:16a6dd9f
         Events : 0.6

    Number   Major   Minor   RaidDevice State
       0       3        1        0      active sync   /dev/hda1
       1       3       65        1      active sync   /dev/hdb1
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Finally, to list all presently active RAID devices, type: 
~]$ cat /proc/mdstat
Personalities : [raid0] [raid1]
md0 : active raid1 hdb1[1] hda1[0]
      128384 blocks [2/2] [UU]
      
md1 : active raid0 hdb2[1] hda2[0]
      1573888 blocks 256k chunks

md2 : active raid0 hdb3[1] hda3[0]
      19132928 blocks 256k chunks

unused devices: <none>
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6.3.2. Creating a New RAID Device
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To create a new RAID device, use the command in the following form as root: 
mdadm --create raid_device --level=level --raid-devices=number component_device…
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This is the simplest way to create a RAID array. There are many more options that allow you to specify the number of spare devices, the block size of a stripe array, if the array has a write-intent bitmap, and much more. All these options can have a significant impact on the performance, but are beyond the scope of this document. For more detailed information, refer to the CREATE MODE section of the mdadm(8) manual page.

Example 6.2. Creating a new RAID device

Assume that the system has two unused SCSI disk drives available, and that each of these devices has exactly one partition of the same size: 
~]# ls /dev/sd*
/dev/sda  /dev/sda1  /dev/sdb  /dev/sdb1
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To create /dev/md3 as a new RAID level 1 array from /dev/sda1 and /dev/sdb1, run the following command: 
~]# mdadm --create /dev/md3 --level=1 --raid-devices=2 /dev/sda1 /dev/sdb1
mdadm: array /dev/md3 started.
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6.3.3. Replacing a Faulty Device
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To replace a particular device in a software RAID, first make sure it is marked as faulty by running the following command as root: 
mdadm raid_device --fail component_device
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Then remove the faulty device from the array by using the command in the following form: 
mdadm raid_device --remove component_device
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Once the device is operational again, you can re-add it to the array: 
mdadm raid_device --add component_device
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Example 6.3. Replacing a faulty device

Assume the system has an active RAID device, /dev/md3, with the following layout (that is, the RAID device created in Example 6.2, “Creating a new RAID device”): 
~]# mdadm --detail /dev/md3 | tail -n 3
    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
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Imagine the first disk drive fails and needs to be replaced. To do so, first mark the /dev/sdb1 device as faulty: 
~]# mdadm /dev/md3 --fail /dev/sdb1
mdadm: set /dev/sdb1 faulty in /dev/md3
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Then remove it from the RAID device: 
~]# mdadm /dev/md3 --remove /dev/sdb1
mdadm: hot removed /dev/sdb1
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As soon as the hardware is replaced, you can add the device back to the array by using the following command: 
~]# mdadm /dev/md3 --add /dev/sdb1
mdadm: added /dev/sdb1
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6.3.4. Extending a RAID Device
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To add a new device to an existing array, use the command in the following form as root: 
mdadm raid_device --add component_device
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This will add the device as a spare device. To grow the array to use this device actively, type the following at a shell prompt: 
mdadm --grow raid_device --raid-devices=number
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Example 6.4. Extending a RAID device

Assume the system has an active RAID device, /dev/md3, with the following layout (that is, the RAID device created in Example 6.2, “Creating a new RAID device”): 
~]# mdadm --detail /dev/md3 | tail -n 3
    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
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Also assume that a new SCSI disk drive, /dev/sdc, has been added and has exactly one partition. To add it to the /dev/md3 array, type the following at a shell prompt: 
~]# mdadm /dev/md3 --add /dev/sdc1
mdadm: added /dev/sdc1
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This will add /dev/sdc1 as a spare device. To change the size of the array to actually use it, type: 
~]# mdadm --grow /dev/md3 --raid-devices=3
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6.3.5. Removing a RAID Device
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To remove an existing RAID device, first deactivate it by running the following command as root: 
mdadm --stop raid_device
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Once deactivated, remove the RAID device itself: 
mdadm --remove raid_device
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Finally, zero superblocks on all devices that were associated with the particular array: 
mdadm --zero-superblock component_device…
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Example 6.5. Removing a RAID device

Assume the system has an active RAID device, /dev/md3, with the following layout (that is, the RAID device created in Example 6.4, “Extending a RAID device”): 
~]# mdadm --detail /dev/md3 | tail -n 4
    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
       2       8       33        2      active sync   /dev/sdc1
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In order to remove this device, first stop it by typing the following at a shell prompt: 
~]# mdadm --stop /dev/md3
mdadm: stopped /dev/md3
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Once stopped, you can remove the /dev/md3 device by running the following command: 
~]# mdadm --remove /dev/md3
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Finally, to remove the superblocks from all associated devices, type: 
~]# mdadm --zero-superblock /dev/sda1 /dev/sdb1 /dev/sdc1
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6.3.6. Preserving the Configuration
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By default, changes made by the mdadm command only apply to the current session, and will not survive a system restart. At boot time, the mdmonitor service reads the content of the /etc/mdadm.conf configuration file to see which RAID devices to start. If the software RAID was configured during the graphical installation process, this file contains directives listed in Table 6.1, “Common mdadm.conf directives” by default.
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Table 6.1. Common mdadm.conf directives
Option Description
ARRAY
Allows you to identify a particular array.
DEVICE
Allows you to specify a list of devices to scan for a RAID component (for example, “/dev/hda1”). You can also use the keyword partitions to use all partitions listed in /proc/partitions, or containers to specify an array container.
MAILADDR Allows you to specify an email address to use in case of an alert.
To list what ARRAY lines are presently in use regardless of the configuration, run the following command as root: 
mdadm --detail --scan
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Use the output of this command to determine which lines to add to the /etc/mdadm.conf file. You can also display the ARRAY line for a particular device: 
mdadm --detail --brief raid_device
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By redirecting the output of this command, you can add such a line to the configuration file with a single command: 
mdadm --detail --brief raid_device >> /etc/mdadm.conf
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Example 6.6. Preserving the configuration

By default, the /etc/mdadm.conf contains the software RAID configuration created during the system installation: 
# mdadm.conf written out by anaconda
DEVICE partitions
MAILADDR root
ARRAY /dev/md0 level=raid1 num-devices=2 UUID=49c5ac74:c2b79501:5c28cb9c:16a6dd9f
ARRAY /dev/md1 level=raid0 num-devices=2 UUID=76914c11:5bfa2c00:dc6097d1:a1f4506d
ARRAY /dev/md2 level=raid0 num-devices=2 UUID=2b5d38d0:aea898bf:92be20e2:f9d893c5
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Assuming you have created the /dev/md3 device as shown in Example 6.2, “Creating a new RAID device”, you can make it persistent by running the following command: 
~]# mdadm --detail --brief /dev/md3 >> /etc/mdadm.conf
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6.4. Additional Resources
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For more information on RAID, refer to the following resources.

6.4.1. Installed Documentation
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  • mdadm man page — A manual page for the mdadm utility.
  • mdadm.conf man page — A manual page that provides a comprehensive list of available /etc/mdadm.conf configuration options.

[1] A hot-swap chassis allows you to remove a hard drive without having to power-down your system.
[2] Parity information is calculated based on the contents of the rest of the member disks in the array. This information can then be used to reconstruct data when one disk in the array fails. The reconstructed data can then be used to satisfy I/O requests to the failed disk before it is replaced and to repopulate the failed disk after it has been replaced.

Chapter 7. Swap Space
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7.1. What is Swap Space?
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Swap space in Linux is used when the amount of physical memory (RAM) is full. If the system needs more memory resources and the RAM is full, inactive pages in memory are moved to the swap space. While swap space can help machines with a small amount of RAM, it should not be considered a replacement for more RAM. Swap space is located on hard drives, which have a slower access time than physical memory.
Swap space can be a dedicated swap partition (recommended), a swap file, or a combination of swap partitions and swap files.
In years past, the recommended amount of swap space increased linearly with the amount of RAM in the system. But because the amount of memory in modern systems has increased into the hundreds of gigabytes, it is now recognized that the amount of swap space that a system needs is a function of the memory workload running on that system. However, given that swap space is usually designated at install time, and that it can be difficult to determine beforehand the memory workload of a system, we recommend determining system swap using the following table.
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Table 7.1. Recommended System Swap Space
Amount of RAM in the SystemRecommended Amount of Swap Space
4GB of RAM or lessa minimum of 2GB of swap space
4GB to 16GB of RAMa minimum of 4GB of swap space
16GB to 64GB of RAMa minimum of 8GB of swap space
64GB to 256GB of RAMa minimum of 16GB of swap space
256GB to 512GB of RAMa minimum of 32GB of swap space

Important

File systems and LVM2 volumes assigned as swap space cannot be in use when being modified. For example, no system processes can be assigned the swap space, as well as no amount of swap should be allocated and used by the kernel. Use the free and cat /proc/swaps commands to verify how much and where swap is in use.
The best way to achieve swap space modifications is to boot your system in rescue mode, and then follow the instructions (for each scenario) in the remainder of this chapter. Refer to the Red Hat Enterprise Linux Installation Guide for instructions on booting into rescue mode. When prompted to mount the file system, select Skip.

7.2. Adding Swap Space
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Sometimes it is necessary to add more swap space after installation. For example, you may upgrade the amount of RAM in your system from 128 MB to 256 MB, but there is only 256 MB of swap space. It might be advantageous to increase the amount of swap space to 512 MB if you perform memory-intense operations or run applications that require a large amount of memory.
You have three options: create a new swap partition, create a new swap file, or extend swap on an existing LVM2 logical volume. It is recommended that you extend an existing logical volume.

7.2.1. Extending Swap on an LVM2 Logical Volume
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To extend an LVM2 swap logical volume (assuming /dev/VolGroup00/LogVol01 is the volume you want to extend):
  1. Disable swapping for the associated logical volume: 
    swapoff -v /dev/VolGroup00/LogVol01
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  2. Resize the LVM2 logical volume by 256 MB: 
    lvm lvresize /dev/VolGroup00/LogVol01 -L +256M
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  3. Format the new swap space: 
    mkswap /dev/VolGroup00/LogVol01
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  4. Enable the extended logical volume: 
    swapon -va
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  5. Test that the logical volume has been extended properly: 
    cat /proc/swaps
free
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7.2.2. Creating an LVM2 Logical Volume for Swap
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To add a swap volume group (assuming /dev/VolGroup00/LogVol02 is the swap volume you want to add):
  1. Create the LVM2 logical volume of size 256 MB: 
    lvm lvcreate VolGroup00 -n LogVol02 -L 256M
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  2. Format the new swap space: 
    mkswap /dev/VolGroup00/LogVol02
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  3. Add the following entry to the /etc/fstab file: 
    /dev/VolGroup00/LogVol02   swap     swap    defaults     0 0
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  4. Enable the extended logical volume: 
    swapon -va
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  5. Test that the logical volume has been extended properly: 
    cat /proc/swaps
free
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7.2.3. Creating a Swap File
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To add a swap file:
  1. Determine the size of the new swap file in megabytes and multiply by 1024 to determine the number of blocks. For example, the block size of a 64 MB swap file is 65536.
  2. At a shell prompt as root, type the following command with count being equal to the desired block size: 
    dd if=/dev/zero of=/swapfile bs=1024 count=65536
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  3. Change the persmissions of the newly created file: 
    chmod 0600 /swapfile
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  4. Setup the swap file with the command: 
    mkswap /swapfile
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  5. To enable the swap file immediately but not automatically at boot time: 
    swapon /swapfile
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  6. To enable it at boot time, edit /etc/fstab to include the following entry: 
    /swapfile          swap            swap    defaults        0 0
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    The next time the system boots, it enables the new swap file.
  7. After adding the new swap file and enabling it, verify it is enabled by viewing the output of the command cat /proc/swaps or free.

7.3. Removing Swap Space
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Sometimes it can be prudent to reduce swap space after installation. For example, say you downgraded the amount of RAM in your system from 1 GB to 512 MB, but there is 2 GB of swap space still assigned. It might be advantageous to reduce the amount of swap space to 1 GB, since the larger 2 GB could be wasting disk space.
You have three options: remove an entire LVM2 logical volume used for swap, remove a swap file, or reduce swap space on an existing LVM2 logical volume.

7.3.1. Reducing Swap on an LVM2 Logical Volume
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To reduce an LVM2 swap logical volume (assuming /dev/VolGroup00/LogVol01 is the volume you want to reduce):
  1. Disable swapping for the associated logical volume: 
    swapoff -v /dev/VolGroup00/LogVol01
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  2. Reduce the LVM2 logical volume by 512 MB: 
    lvm lvreduce /dev/VolGroup00/LogVol01 -L -512M
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  3. Format the new swap space: 
    mkswap /dev/VolGroup00/LogVol01
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  4. Enable the extended logical volume: 
    swapon -va
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  5. Test that the logical volume has been reduced properly: 
    cat /proc/swaps
free
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7.3.2. Removing an LVM2 Logical Volume for Swap
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The swap logical volume cannot be in use (no system locks or processes on the volume). The easiest way to achieve this is to boot your system in rescue mode. Refer to the Red Hat Enterprise Linux Installation Guide for instructions on booting into rescue mode. When prompted to mount the file system, select Skip.
To remove a swap volume group (assuming /dev/VolGroup00/LogVol02 is the swap volume you want to remove):
  1. Disable swapping for the associated logical volume: 
    swapoff -v /dev/VolGroup00/LogVol02
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  2. Remove the LVM2 logical volume of size 512 MB: 
    lvm lvremove /dev/VolGroup00/LogVol02
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  3. Remove the following entry from the /etc/fstab file: 
    /dev/VolGroup00/LogVol02   swap     swap    defaults     0 0
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  4. Test that the logical volume has been removed: 
    cat /proc/swaps
free
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7.3.3. Removing a Swap File
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To remove a swap file:
  1. At a shell prompt as root, execute the following command to disable the swap file (where /swapfile is the swap file): 
    swapoff -v /swapfile
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  2. Remove its entry from the /etc/fstab file.
  3. Remove the actual file: 
    rm /swapfile
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7.4. Moving Swap Space
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To move swap space from one location to another, follow the steps for removing swap space, and then follow the steps for adding swap space.

Chapter 8. Managing Disk Storage
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8.1. Standard Partitions using parted
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The utility parted allows users to:
  • View the existing partition table
  • Change the size of existing partitions
  • Add partitions from free space or additional hard drives
If you want to view the system's disk space usage or monitor the disk space usage, refer to Section 42.3, “File Systems”.
By default, the parted package is included when installing Red Hat Enterprise Linux. To start parted, log in as root and type the command parted /dev/sda at a shell prompt (where /dev/sda is the device name for the drive you want to configure).
If you want to remove or resize a partition, the device on which that partition resides must not be in use. Creating a new partition on a device which is in use—while possible—is not recommended.
For a device to not be in use, none of the partitions on the device can be mounted, and any swap space on the device must not be enabled.
As well, the partition table should not be modified while it is in use because the kernel may not properly recognize the changes. If the partition table does not match the actual state of the mounted partitions, information could be written to the wrong partition, resulting in lost and overwritten data.
The easiest way to achieve this is to boot your system in rescue mode. When prompted to mount the file system, select Skip.
Alternately, if the drive does not contain any partitions in use (system processes that use or lock the file system from being unmounted), you can unmount them with the umount command and turn off all the swap space on the hard drive with the swapoff command.
Table 8.1, “parted commands” contains a list of commonly used parted commands. The sections that follow explain some of these commands and arguments in more detail.
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Table 8.1. parted commands
Command Description
check minor-num Perform a simple check of the file system
cp from to Copy file system from one partition to another; from and to are the minor numbers of the partitions
help Display list of available commands
mklabel label Create a disk label for the partition table
mkfs minor-num file-system-type Create a file system of type file-system-type
mkpart part-type fs-type start-mb end-mb Make a partition without creating a new file system
mkpartfs part-type fs-type start-mb end-mb Make a partition and create the specified file system
move minor-num start-mb end-mb Move the partition
name minor-num name Name the partition for Mac and PC98 disklabels only
print Display the partition table
quit Quit parted
rescue start-mb end-mb Rescue a lost partition from start-mb to end-mb
resize minor-num start-mb end-mb Resize the partition from start-mb to end-mb
rm minor-num Remove the partition
select device Select a different device to configure
set minor-num flag state Set the flag on a partition; state is either on or off
toggle [NUMBER [FLAG] Toggle the state of FLAG on partition NUMBER
unit UNIT Set the default unit to UNIT

8.1.1. Viewing the Partition Table
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After starting parted, use the command print to view the partition table. A table similar to the following appears: 
Model: ATA ST3160812AS (scsi)
Disk /dev/sda: 160GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End    Size    Type      File system  Flags
 1      32.3kB  107MB  107MB   primary   ext3         boot
 2      107MB   105GB  105GB   primary   ext3
 3      105GB   107GB  2147MB  primary   linux-swap
 4      107GB   160GB  52.9GB  extended		      root
 5      107GB   133GB  26.2GB  logical   ext3
 6      133GB   133GB  107MB   logical   ext3
 7      133GB   160GB  26.6GB  logical                lvm
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The first line contains the disk type, manufacturer, model number and interface, and the second line displays the disk label type. The remaining output below the fourth line shows the partition table.
In the partition table, the Minor number is the partition number. For example, the partition with minor number 1 corresponds to /dev/sda1. The Start and End values are in megabytes. Valid Type are metadata, free, primary, extended, or logical. The Filesystem is the file system type, which can be any of the following:
  • ext2
  • ext3
  • fat16
  • fat32
  • hfs
  • jfs
  • linux-swap
  • ntfs
  • reiserfs
  • hp-ufs
  • sun-ufs
  • xfs
If a Filesystem of a device shows no value, this means that its file system type is unknown.
The Flags column lists the flags set for the partition. Available flags are boot, root, swap, hidden, raid, lvm, or lba.

Note

To select a different device without having to restart parted, use the select command followed by the device name (for example, /dev/sda). Doing so allows you to view or configure the partition table of a device.

8.1.2. Creating a Partition
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Warning

Do not attempt to create a partition on a device that is in use.
Before creating a partition, boot into rescue mode (or unmount any partitions on the device and turn off any swap space on the device).
Start parted, where /dev/sda is the device on which to create the partition: 
parted /dev/sda
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View the current partition table to determine if there is enough free space: 
print
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If there is not enough free space, you can resize an existing partition. Refer to Section 8.1.4, “Resizing a Partition” for details.
8.1.2.1. Making the Partition
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From the partition table, determine the start and end points of the new partition and what partition type it should be. You can only have four primary partitions (with no extended partition) on a device. If you need more than four partitions, you can have three primary partitions, one extended partition, and multiple logical partitions within the extended. For an overview of disk partitions, refer to the appendix An Introduction to Disk Partitions in the Red Hat Enterprise Linux Installation Guide.
For example, to create a primary partition with an ext3 file system from 1024 megabytes until 2048 megabytes on a hard drive type the following command: 
mkpart primary ext3 1024 2048
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Note

If you use the mkpartfs command instead, the file system is created after the partition is created. However, parted does not support creating an ext3 file system. Thus, if you wish to create an ext3 file system, use mkpart and create the file system with the mkfs command as described later.
The changes start taking place as soon as you press Enter, so review the command before executing to it.
After creating the partition, use the print command to confirm that it is in the partition table with the correct partition type, file system type, and size. Also remember the minor number of the new partition so that you can label it. You should also view the output of 
cat /proc/partitions
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to make sure the kernel recognizes the new partition.
8.1.2.2. Formatting the Partition
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The partition still does not have a file system. Create the file system: 
mkfs -t ext3 /dev/sda6
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Warning

Formatting the partition permanently destroys any data that currently exists on the partition.
8.1.2.3. Labeling the Partition
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Next, give the partition a label. For example, if the new partition is /dev/sda6 and you want to label it /work: 
e2label /dev/sda6 /work
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By default, the installation program uses the mount point of the partition as the label to make sure the label is unique. You can use any label you want.
8.1.2.4. Creating the Mount Point
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As root, create the mount point: 
mkdir /work
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8.1.2.5. Add to /etc/fstab
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As root, edit the /etc/fstab file to include the new partition. The new line should look similar to the following: 
LABEL=/work           /work                 ext3    defaults        1 2
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The first column should contain LABEL= followed by the label you gave the partition. The second column should contain the mount point for the new partition, and the next column should be the file system type (for example, ext3 or swap). If you need more information about the format, read the man page with the command man fstab.
If the fourth column is the word defaults, the partition is mounted at boot time. To mount the partition without rebooting, as root, type the command: 
mount /work
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8.1.3. Removing a Partition
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Warning

Do not attempt to remove a partition on a device that is in use.
Before removing a partition, boot into rescue mode (or unmount any partitions on the device and turn off any swap space on the device).
Start parted, where /dev/sda is the device on which to remove the partition: 
parted /dev/sda
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View the current partition table to determine the minor number of the partition to remove: 
print
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Remove the partition with the command rm. For example, to remove the partition with minor number 3: 
rm 3
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The changes start taking place as soon as you press Enter, so review the command before committing to it.
After removing the partition, use the print command to confirm that it is removed from the partition table. You should also view the output of 
cat /proc/partitions
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to make sure the kernel knows the partition is removed.
The last step is to remove it from the /etc/fstab file. Find the line that declares the removed partition, and remove it from the file.

8.1.4. Resizing a Partition
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Warning

Do not attempt to resize a partition on a device that is in use.
Before resizing a partition, boot into rescue mode (or unmount any partitions on the device and turn off any swap space on the device).
Start parted, where /dev/sda is the device on which to resize the partition: 
parted /dev/sda
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View the current partition table to determine the minor number of the partition to resize as well as the start and end points for the partition: 
print
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To resize the partition, use the resize command followed by the minor number for the partition, the starting place in megabytes, and the end place in megabytes. For example: 
resize 3 1024 2048
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Warning

A partition cannot be made larger than the space available on the device
After resizing the partition, use the print command to confirm that the partition has been resized correctly, is the correct partition type, and is the correct file system type.
After rebooting the system into normal mode, use the command df to make sure the partition was mounted and is recognized with the new size.

8.2. LVM Partition Management
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The following commands can be found by issuing lvm help at a command prompt.
Expand
Table 8.2. LVM commands
Command Description
dumpconfig Dump the active configuration
formats List the available metadata formats
help Display the help commands
lvchange Change the attributes of logical volume(s)
lvcreate Create a logical volume
lvdisplay Display information about a logical volume
lvextend Add space to a logical volume
lvmchange Due to use of the device mapper, this command has been deprecated
lvmdiskscan List devices that may be used as physical volumes
lvmsadc Collect activity data
lvmsar Create activity report
lvreduce Reduce the size of a logical volume
lvremove Remove logical volume(s) from the system
lvrename Rename a logical volume
lvresize Resize a logical volume
lvs Display information about logical volumes
lvscan List all logical volumes in all volume groups
pvchange Change attributes of physical volume(s)
pvcreate Initialize physical volume(s) for use by LVM
pvdata Display the on-disk metadata for physical volume(s)
pvdisplay Display various attributes of physical volume(s)
pvmove Move extents from one physical volume to another
pvremove Remove LVM label(s) from physical volume(s)
pvresize Resize a physical volume in use by a volume group
pvs Display information about physical volumes
pvscan List all physical volumes
segtypes List available segment types
vgcfgbackup Backup volume group configuration
vgcfgrestore Restore volume group configuration
vgchange Change volume group attributes
vgck Check the consistency of a volume group
vgconvert Change volume group metadata format
vgcreate Create a volume group
vgdisplay Display volume group information
vgexport Unregister a volume group from the system
vgextend Add physical volumes to a volume group
vgimport Register exported volume group with system
vgmerge Merge volume groups
vgmknodes Create the special files for volume group devices in /dev/
vgreduce Remove a physical volume from a volume group
vgremove Remove a volume group
vgrename Rename a volume group
vgs Display information about volume groups
vgscan Search for all volume groups
vgsplit Move physical volumes into a new volume group
version Display software and driver version information

Chapter 9. Implementing Disk Quotas
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Disk space can be restricted by implementing disk quotas which alert a system administrator before a user consumes too much disk space or a partition becomes full.
Disk quotas can be configured for individual users as well as user groups. This makes it possible to manage the space allocated for user-specific files (such as email) separately from the space allocated to the projects a user works on (assuming the projects are given their own groups).
In addition, quotas can be set not just to control the number of disk blocks consumed but to control the number of inodes (data structures that contain information about files in UNIX file systems). Because inodes are used to contain file-related information, this allows control over the number of files that can be created.
The quota RPM must be installed to implement disk quotas.

Note

For more information on installing RPM packages, refer to Part II, “Package Management”.

9.1. Configuring Disk Quotas
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To implement disk quotas, use the following steps:
  1. Enable quotas per file system by modifying the /etc/fstab file.
  2. Remount the file system(s).
  3. Create the quota database files and generate the disk usage table.
  4. Assign quota policies.
Each of these steps is discussed in detail in the following sections.

9.1.1. Enabling Quotas
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As root, using a text editor, edit the /etc/fstab file. Add the usrquota and/or grpquota options to the file systems that require quotas: 
/dev/VolGroup00/LogVol00 /         ext3    defaults        1 1
LABEL=/boot              /boot     ext3    defaults        1 2
none                     /dev/pts  devpts  gid=5,mode=620  0 0
none                     /dev/shm  tmpfs   defaults        0 0
none                     /proc     proc    defaults        0 0
none                     /sys      sysfs   defaults        0 0
/dev/VolGroup00/LogVol02 /home     ext3    defaults,usrquota,grpquota  1 2
/dev/VolGroup00/LogVol01 swap      swap    defaults        0 0 . . .
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In this example, the /home file system has both user and group quotas enabled.

Note

The following examples assume that a separate /home partition was created during the installation of Red Hat Enterprise Linux. The root (/) partition can be used for setting quota policies in the /etc/fstab file.

9.1.2. Remounting the File Systems
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After adding the usrquota and/or grpquota options, remount each file system whose fstab entry has been modified. If the file system is not in use by any process, use one of the following methods:
  • Issue the umount command followed by the mount command to remount the file system.(See the man page for both umount and mount for the specific syntax for mounting and unmounting various filesystem types.)
  • Issue the mount -o remount <file-system> command (where <file-system> is the name of the file system) to remount the file system. For example, to remount the /home file system, the command to issue is mount -o remount /home.
If the file system is currently in use, the easiest method for remounting the file system is to reboot the system.

9.1.3. Creating the Quota Database Files
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After each quota-enabled file system is remounted, the system is capable of working with disk quotas. However, the file system itself is not yet ready to support quotas. The next step is to run the quotacheck command.
The quotacheck command examines quota-enabled file systems and builds a table of the current disk usage per file system. The table is then used to update the operating system's copy of disk usage. In addition, the file system's disk quota files are updated.
To create the quota files (aquota.user and aquota.group) on the file system, use the -c option of the quotacheck command. For example, if user and group quotas are enabled for the /home file system, create the files in the /home directory: 
quotacheck -cug /home
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The -c option specifies that the quota files should be created for each file system with quotas enabled, the -u option specifies to check for user quotas, and the -g option specifies to check for group quotas.
If neither the -u or -g options are specified, only the user quota file is created. If only -g is specified, only the group quota file is created.
After the files are created, run the following command to generate the table of current disk usage per file system with quotas enabled: 
quotacheck -avug
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The options used are as follows:
  • a — Check all quota-enabled, locally-mounted file systems
  • v — Display verbose status information as the quota check proceeds
  • u — Check user disk quota information
  • g — Check group disk quota information
After quotacheck has finished running, the quota files corresponding to the enabled quotas (user and/or group) are populated with data for each quota-enabled locally-mounted file system such as /home.

9.1.4. Assigning Quotas per User
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The last step is assigning the disk quotas with the edquota command.
To configure the quota for a user, as root in a shell prompt, execute the command: 
edquota username
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Perform this step for each user who needs a quota. For example, if a quota is enabled in /etc/fstab for the /home partition (/dev/VolGroup00/LogVol02 in the example below) and the command edquota testuser is executed, the following is shown in the editor configured as the default for the system: 
Disk quotas for user testuser (uid 501):
Filesystem                blocks     soft     hard    inodes   soft   hard
/dev/VolGroup00/LogVol02  440436        0        0     37418      0      0
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Note

The text editor defined by the EDITOR environment variable is used by edquota. To change the editor, set the EDITOR environment variable in your ~/.bash_profile file to the full path of the editor of your choice.
The first column is the name of the file system that has a quota enabled for it. The second column shows how many blocks the user is currently using. The next two columns are used to set soft and hard block limits for the user on the file system. The inodes column shows how many inodes the user is currently using. The last two columns are used to set the soft and hard inode limits for the user on the file system.
The hard block limit is the absolute maximum amount of disk space that a user or group can use. Once this limit is reached, no further disk space can be used.
The soft block limit defines the maximum amount of disk space that can be used. However, unlike the hard limit, the soft limit can be exceeded for a certain amount of time. That time is known as the grace period. The grace period can be expressed in seconds, minutes, hours, days, weeks, or months.
If any of the values are set to 0, that limit is not set. In the text editor, change the desired limits. For example: 
Disk quotas for user testuser (uid 501):
Filesystem                blocks     soft     hard   inodes   soft   hard
/dev/VolGroup00/LogVol02  440436   500000   550000    37418      0      0
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To verify that the quota for the user has been set, use the command: 
quota testuser
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9.1.5. Assigning Quotas per Group
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Quotas can also be assigned on a per-group basis. For example, to set a group quota for the devel group (the group must exist prior to setting the group quota), use the command: 
edquota -g devel
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This command displays the existing quota for the group in the text editor: 
Disk quotas for group devel (gid 505):
Filesystem                blocks    soft     hard    inodes    soft    hard
/dev/VolGroup00/LogVol02  440400       0        0     37418       0       0
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Modify the limits, then save the file.
To verify that the group quota has been set, use the command: 
quota -g devel
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9.1.6. Setting the Grace Period for Soft Limits
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If soft limits are set for a given quota (whether inode or block and for either users or groups) the grace period, or amount of time a soft limit can be exceeded, should be set with the command: 
edquota -t
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While other edquota commands operate on a particular user's or group's quota, the -t option operates on every filesystem with quotas enabled.

9.2. Managing Disk Quotas
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If quotas are implemented, they need some maintenance — mostly in the form of watching to see if the quotas are exceeded and making sure the quotas are accurate.
Of course, if users repeatedly exceed their quotas or consistently reach their soft limits, a system administrator has a few choices to make depending on what type of users they are and how much disk space impacts their work. The administrator can either help the user determine how to use less disk space or increase the user's disk quota.

9.2.1. Enabling and Disabling
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It is possible to disable quotas without setting them to 0. To turn all user and group quotas off, use the following command: 
quotaoff -vaug
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If neither the -u or -g options are specified, only the user quotas are disabled. If only -g is specified, only group quotas are disabled. The -v switch causes verbose status information to display as the command executes.
To enable quotas again, use the quotaon command with the same options.
For example, to enable user and group quotas for all file systems, use the following command: 
quotaon -vaug
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To enable quotas for a specific file system, such as /home, use the following command: 
quotaon -vug /home
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If neither the -u or -g options are specified, only the user quotas are enabled. If only -g is specified, only group quotas are enabled.

9.2.2. Reporting on Disk Quotas
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Creating a disk usage report entails running the repquota utility. For example, the command repquota /home produces this output: 
*** Report for user quotas on device /dev/mapper/VolGroup00-LogVol02
Block grace time: 7days; Inode grace time: 7days
                        Block limits                File limits
User            used    soft    hard  grace    used  soft  hard  grace
----------------------------------------------------------------------
root      --      36       0       0              4     0     0
kristin   --     540       0       0            125     0     0
testuser  --  440400  500000  550000          37418     0     0
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To view the disk usage report for all (option -a) quota-enabled file systems, use the command: 
repquota -a
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While the report is easy to read, a few points should be explained. The -- displayed after each user is a quick way to determine whether the block or inode limits have been exceeded. If either soft limit is exceeded, a + appears in place of the corresponding -; the first - represents the block limit, and the second represents the inode limit.
The grace columns are normally blank. If a soft limit has been exceeded, the column contains a time specification equal to the amount of time remaining on the grace period. If the grace period has expired, none appears in its place.

9.2.3. Keeping Quotas Accurate
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Whenever a file system is not unmounted cleanly (due to a system crash, for example), it is necessary to run quotacheck. However, quotacheck can be run on a regular basis, even if the system has not crashed. Safe methods for periodically running quotacheck include:
Ensuring quotacheck runs on next reboot

Note

This method works best for (busy) multiuser systems which are periodically rebooted.
As root, place a shell script into the /etc/cron.daily/ or /etc/cron.weekly/ directory—or schedule one using the crontab -e command—that contains the touch /forcequotacheck command. This creates an empty forcequotacheck file in the root directory, which the system init script looks for at boot time. If it is found, the init script runs quotacheck. Afterward, the init script removes the /forcequotacheck file; thus, scheduling this file to be created periodically with cron ensures that quotacheck is run during the next reboot.
Refer to Chapter 39, Automated Tasks for more information about configuring cron.
Running quotacheck in single user mode
An alternative way to safely run quotacheck is to (re-)boot the system into single-user mode to prevent the possibility of data corruption in quota files and run: 
~]# quotaoff -vaug /<file_system>
~]# quotacheck -vaug /<file_system>
~]# quotaon -vaug /<file_system>
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Running quotacheck on a running system
If necessary, it is possible to run quotacheck on a machine during a time when no users are logged in, and thus have no open files on the file system being checked. Run the command quotacheck -vaug <file_system> ; this command will fail if quotacheck cannot remount the given <file_system> as read-only. Note that, following the check, the file system will be remounted read-write.

Important

Running quotacheck on a live file system mounted read-write is not recommended due to the possibility of quota file corruption.
Refer to Chapter 39, Automated Tasks for more information about configuring cron.

9.3. Additional Resources
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For more information on disk quotas, refer to the following resources.

9.3.1. Installed Documentation
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  • The quotacheck, edquota, repquota, quota, quotaon, and quotaoff man pages

Chapter 10. Access Control Lists
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Files and directories have permission sets for the owner of the file, the group associated with the file, and all other users for the system. However, these permission sets have limitations. For example, different permissions cannot be configured for different users. Thus, Access Control Lists (ACLs) were implemented.
The Red Hat Enterprise Linux 5 kernel provides ACL support for the ext3 file system and NFS-exported file systems. ACLs are also recognized on ext3 file systems accessed via Samba.
Along with support in the kernel, the acl package is required to implement ACLs. It contains the utilities used to add, modify, remove, and retrieve ACL information.
The cp and mv commands copy or move any ACLs associated with files and directories.

10.1. Mounting File Systems
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Before using ACLs for a file or directory, the partition for the file or directory must be mounted with ACL support. If it is a local ext3 file system, it can mounted with the following command: 
mount -t ext3 -o acl <device-name> <partition>
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For example: 
mount -t ext3 -o acl /dev/VolGroup00/LogVol02 /work
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Alternatively, if the partition is listed in the /etc/fstab file, the entry for the partition can include the acl option: 
LABEL=/work      /work       ext3    acl        1 2
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If an ext3 file system is accessed via Samba and ACLs have been enabled for it, the ACLs are recognized because Samba has been compiled with the --with-acl-support option. No special flags are required when accessing or mounting a Samba share.

10.1.1. NFS
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By default, if the file system being exported by an NFS server supports ACLs and the NFS client can read ACLs, ACLs are utilized by the client system.
To disable ACLs on NFS shares when configuring the server, include the no_acl option in the /etc/exports file. To disable ACLs on an NFS share when mounting it on a client, mount it with the no_acl option via the command line or the /etc/fstab file.

10.2. Setting Access ACLs
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There are two types of ACLs: access ACLs and default ACLs. An access ACL is the access control list for a specific file or directory. A default ACL can only be associated with a directory; if a file within the directory does not have an access ACL, it uses the rules of the default ACL for the directory. Default ACLs are optional.
ACLs can be configured:
  1. Per user
  2. Per group
  3. Via the effective rights mask
  4. For users not in the user group for the file
The setfacl utility sets ACLs for files and directories. Use the -m option to add or modify the ACL of a file or directory: 
setfacl -m <rules> <files>
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Rules (<rules>) must be specified in the following formats. Multiple rules can be specified in the same command if they are separated by commas.
u:<uid>:<perms>
Sets the access ACL for a user. The user name or UID may be specified. The user may be any valid user on the system.
g:<gid>:<perms>
Sets the access ACL for a group. The group name or GID may be specified. The group may be any valid group on the system.
m:<perms>
Sets the effective rights mask. The mask is the union of all permissions of the owning group and all of the user and group entries.
o:<perms>
Sets the access ACL for users other than the ones in the group for the file.
White space is ignored. Permissions (<perms>) must be a combination of the characters r, w, and x for read, write, and execute.
If a file or directory already has an ACL, and the setfacl command is used, the additional rules are added to the existing ACL or the existing rule is modified.
For example, to give read and write permissions to user andrius: 
setfacl -m u:andrius:rw /project/somefile
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To remove all the permissions for a user, group, or others, use the -x option and do not specify any permissions: 
setfacl -x <rules> <files>
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For example, to remove all permissions from the user with UID 500: 
setfacl -x u:500 /project/somefile
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10.3. Setting Default ACLs
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To set a default ACL, add d: before the rule and specify a directory instead of a file name.
For example, to set the default ACL for the /share/ directory to read and execute for users not in the user group (an access ACL for an individual file can override it): 
setfacl -m d:o:rx /share
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10.4. Retrieving ACLs
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To determine the existing ACLs for a file or directory, use the getfacl command. In the example below, the getfacl is used to determine the existing ACLs for a file. 
getfacl home/john/picture.png
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The above command returns the following output: 
# file: home/john/picture.png
# owner: john
# group: john
user::rw-
group::r--
other::r--
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If a directory with a default ACL is specified, the default ACL is also displayed as illustrated below. 
[john@main /]$ getfacl home/sales/
# file: home/sales/
# owner: john
# group: john
user::rw-
user:barryg:r--
group::r--
mask::r--
other::r--
default:user::rwx
default:user:john:rwx
default:group::r-x
default:mask::rwx
default:other::r-x
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10.5. Archiving File Systems With ACLs
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Warning

The tar and dump commands do not backup ACLs.
The star utility is similar to the tar utility in that it can be used to generate archives of files; however, some of its options are different. Refer to Table 10.1, “Command Line Options for star” for a listing of more commonly used options. For all available options, refer to the star man page. The star package is required to use this utility.
Expand
Table 10.1. Command Line Options for star
Option Description
-c Creates an archive file.
-n Do not extract the files; use in conjunction with -x to show what extracting the files does.
-r Replaces files in the archive. The files are written to the end of the archive file, replacing any files with the same path and file name.
-t Displays the contents of the archive file.
-u Updates the archive file. The files are written to the end of the archive if they do not exist in the archive or if the files are newer than the files of the same name in the archive. This option only work if the archive is a file or an unblocked tape that may backspace.
-x Extracts the files from the archive. If used with -U and a file in the archive is older than the corresponding file on the file system, the file is not extracted.
-help Displays the most important options.
-xhelp Displays the least important options.
-/ Do not strip leading slashes from file names when extracting the files from an archive. By default, they are striped when files are extracted.
-acl When creating or extracting, archive or restore any ACLs associated with the files and directories.

10.6. Compatibility with Older Systems
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If an ACL has been set on any file on a given file system, that file system has the ext_attr attribute. This attribute can be seen using the following command: 
tune2fs -l <filesystem-device>
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A file system that has acquired the ext_attr attribute can be mounted with older kernels, but those kernels do not enforce any ACLs which have been set.
Versions of the e2fsck utility included in version 1.22 and higher of the e2fsprogs package (including the versions in Red Hat Enterprise Linux 2.1 and 4) can check a file system with the ext_attr attribute. Older versions refuse to check it.

10.7. Additional Resources
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Refer to the follow resources for more information.

10.7.1. Installed Documentation
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  • acl man page — Description of ACLs
  • getfacl man page — Discusses how to get file access control lists
  • setfacl man page — Explains how to set file access control lists
  • star man page — Explains more about the star utility and its many options

10.7.2. Useful Websites
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Chapter 11. LVM (Logical Volume Manager)
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11.1. What is LVM?
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LVM is a tool for logical volume management which includes allocating disks, striping, mirroring and resizing logical volumes.
With LVM, a hard drive or set of hard drives is allocated to one or more physical volumes. LVM physical volumes can be placed on other block devices which might span two or more disks.
The physical volumes are combined into logical volumes, with the exception of the /boot partition. The /boot partition cannot be on a logical volume group because the boot loader cannot read it. If the root (/) partition is on a logical volume, create a separate /boot partition which is not a part of a volume group.
Since a physical volume cannot span over multiple drives, to span over more than one drive, create one or more physical volumes per drive.
Logical Volumes
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Logical Volumes

Figure 11.1. Logical Volumes

The volume groups can be divided into logical volumes, which are assigned mount points, such as /home and / and file system types, such as ext2 or ext3. When "partitions" reach their full capacity, free space from the volume group can be added to the logical volume to increase the size of the partition. When a new hard drive is added to the system, it can be added to the volume group, and partitions that are logical volumes can be increased in size.
Logical Volumes
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Logical Volumes

Figure 11.2. Logical Volumes

On the other hand, if a system is partitioned with the ext3 file system, the hard drive is divided into partitions of defined sizes. If a partition becomes full, it is not easy to expand the size of the partition. Even if the partition is moved to another hard drive, the original hard drive space has to be reallocated as a different partition or not used.
To learn how to configure LVM during the installation process, refer to Section 11.2, “LVM Configuration”.

11.1.1. What is LVM2?
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LVM version 2, or LVM2, is the default for Red Hat Enterprise Linux 5, which uses the device mapper driver contained in the 2.6 kernel. LVM2 can be upgraded from versions of Red Hat Enterprise Linux running the 2.4 kernel.

11.2. LVM Configuration
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LVM can be configured during the graphical installation process, the text-based installation process, or during a kickstart installation. You can use the system-config-lvm utility to create your own LVM configuration post-installation. The next two sections focus on using Disk Druid during installation to complete this task. The third section introduces the LVM utility (system-config-lvm) which allows you to manage your LVM volumes in X windows or graphically.
Read Section 11.1, “What is LVM?” first to learn about LVM. An overview of the steps required to configure LVM include:
  • Creating physical volumes from the hard drives.
  • Creating volume groups from the physical volumes.
  • Creating logical volumes from the volume groups and assign the logical volumes mount points.
Two 9.1 GB SCSI drives (/dev/sda and /dev/sdb) are used in the following examples. They detail how to create a simple configuration using a single LVM volume group with associated logical volumes during installation.

11.3. Automatic Partitioning
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On the Disk Partitioning Setup screen, select Remove linux partitions on selected drives and create default layout from the pulldown list.
For Red Hat Enterprise Linux, LVM is the default method for disk partitioning. If you do not wish to have LVM implemented, or if you require RAID partitioning, manual disk partitioning through Disk Druid is required.
The following properties make up the automatically created configuration:
  • The /boot partition resides on its own non-LVM partition. In the following example, it is the first partition on the first drive (/dev/sda1). Bootable partitions cannot reside on LVM logical volumes.
  • A single LVM volume group (VolGroup00) is created, which spans all selected drives and all remaining space available. In the following example, the remainder of the first drive (/dev/sda2), and the entire second drive (/dev/sdb1) are allocated to the volume group.
  • Two LVM logical volumes (LogVol00 and LogVol01) are created from the newly created spanned volume group. In the following example, the recommended swap space is automatically calculated and assigned to LogVol01, and the remainder is allocated to the root file system, LogVol00.
Automatic LVM Configuration With Two SCSI Drives
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Automatic LVM Configuration With Two SCSI Drives

Figure 11.3. Automatic LVM Configuration With Two SCSI Drives

Note

If enabling quotas are of interest to you, it may be best to modify the automatic configuration to include other mount points, such as /home or /var, so that each file system has its own independent quota configuration limits.
In most cases, the default automatic LVM partitioning is sufficient, but advanced implementations could warrant modification or manual configuration of the partition tables.

Note

If you anticipate future memory upgrades, leaving some free space in the volume group would allow for easy future expansion of the swap space logical volume on the system; in which case, the automatic LVM configuration should be modified to leave available space for future growth.

11.4. Manual LVM Partitioning
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The following section explains how to manually configure LVM for Red Hat Enterprise Linux. Because there are numerous ways to manually configure a system with LVM, the following example is similar to the default configuration done in Section 11.3, “Automatic Partitioning”.
On the Disk Partitioning Setup screen, select Create custom layout from the pulldown list and click the Next button in the bottom right corner of the screen.

11.4.1. Creating the /boot Partition
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In a typical situation, the disk drives are new, or formatted clean. The following figure, Figure 11.4, “Two Blank Drives, Ready for Configuration”, shows both drives as raw devices with no partitioning configured.
Two Blank Drives, Ready for Configuration
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Two Blank Drives, Ready for Configuration

Figure 11.4. Two Blank Drives, Ready for Configuration

Warning

The /boot partition cannot reside on an LVM volume because the GRUB boot loader cannot read it.
  1. Select New.
  2. Select /boot from the Mount Point pulldown menu.
  3. Select ext3 from the File System Type pulldown menu.
  4. Select only the sda checkbox from the Allowable Drives area.
  5. Leave 100 (the default) in the Size (MB) menu.
  6. Leave the Fixed size (the default) radio button selected in the Additional Size Options area.
  7. Select Force to be a primary partition to make the partition be a primary partition. A primary partition is one of the first four partitions on the hard drive. If unselected, the partition is created as a logical partition. If other operating systems are already on the system, unselecting this option should be considered. For more information on primary versus logical/extended partitions, refer to the appendix section of the Red Hat Enterprise Linux Installation Guide.
Refer to Figure 11.5, “Creation of the Boot Partition” to verify your inputted values:
Creation of the Boot Partition
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Creation of the Boot Partition

Figure 11.5. Creation of the Boot Partition

Click OK to return to the main screen. The following figure displays the boot partition correctly set:
The /boot Partition Displayed
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The /boot Partition Displayed

Figure 11.6. The /boot Partition Displayed

11.4.2. Creating the LVM Physical Volumes
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Once the boot partition is created, the remainder of all disk space can be allocated to LVM partitions. The first step in creating a successful LVM implementation is the creation of the physical volume(s).
  1. Select New.
  2. Select physical volume (LVM) from the File System Type pulldown menu as shown in Figure 11.7, “Creating a Physical Volume”.
    Creating a Physical Volume
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    Creating a Physical Volume

    Figure 11.7. Creating a Physical Volume

  3. You cannot enter a mount point yet (you can once you have created all your physical volumes and then all volume groups).
  4. A physical volume must be constrained to one drive. For Allowable Drives, select the drive on which the physical volume are created. If you have multiple drives, all drives are selected, and you must deselect all but one drive.
  5. Enter the size that you want the physical volume to be.
  6. Select Fixed size to make the physical volume the specified size, select Fill all space up to (MB) and enter a size in MBs to give range for the physical volume size, or select Fill to maximum allowable size to make it grow to fill all available space on the hard disk. If you make more than one growable, they share the available free space on the disk.
  7. Select Force to be a primary partition if you want the partition to be a primary partition.
  8. Click OK to return to the main screen.
Repeat these steps to create as many physical volumes as needed for your LVM setup. For example, if you want the volume group to span over more than one drive, create a physical volume on each of the drives. The following figure shows both drives completed after the repeated process:
Two Physical Volumes Created
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Two Physical Volumes Created

Figure 11.8. Two Physical Volumes Created

11.4.3. Creating the LVM Volume Groups
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Once all the physical volumes are created, the volume groups can be created:
  1. Click the LVM button to collect the physical volumes into volume groups. A volume group is basically a collection of physical volumes. You can have multiple logical volumes, but a physical volume can only be in one volume group.

    Note

    There is overhead disk space reserved in the volume group. The volume group size is slightly less than the total of physical volume sizes.
    Creating an LVM Volume Group
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    Creating an LVM Volume Group

    Figure 11.9. Creating an LVM Volume Group

  2. Change the Volume Group Name if desired.
  3. All logical volumes inside the volume group must be allocated in physical extent (PE) units. A physical extent is an allocation unit for data.
  4. Select which physical volumes to use for the volume group.

11.4.4. Creating the LVM Logical Volumes
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Create logical volumes with mount points such as /, /home, and swap space. Remember that /boot cannot be a logical volume. To add a logical volume, click the Add button in the Logical Volumes section. A dialog window as shown in Figure 11.10, “Creating a Logical Volume” appears.
Creating a Logical Volume
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Creating a Logical Volume

Figure 11.10. Creating a Logical Volume

Repeat these steps for each volume group you want to create.

Note

You may want to leave some free space in the volume group so you can expand the logical volumes later. The default automatic configuration does not do this, but this manual configuration example does — approximately 1 GB is left as free space for future expansion.
Pending Logical Volumes
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Pending Logical Volumes

Figure 11.11. Pending Logical Volumes

Click OK to apply the volume group and all associated logical volumes.
The following figure shows the final manual configuration:
Final Manual Configuration
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Final Manual Configuration

Figure 11.12. Final Manual Configuration

11.5. Using the LVM utility system-config-lvm
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The LVM utility allows you to manage logical volumes within X windows or graphically. You can access the application by selecting from your menu panel System > Administration > Logical Volume Management. Alternatively you can start the Logical Volume Management utility by typing system-config-lvm from a terminal.
In the example used in this section, the following are the details for the volume group that was created during the installation: 
/boot - (Ext3) file system. Displayed under 'Uninitialized Entities'. (DO NOT initialize this partition).
LogVol00 - (LVM) contains the (/) directory (312 extents).
LogVol02 - (LVM) contains the (/home) directory (128 extents).
LogVol03 - (LVM) swap (28 extents).
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The logical volumes above were created in disk entity /dev/hda2 while /boot was created in /dev/hda1. The system also consists of 'Uninitialized Entities' which are illustrated in Figure 11.17, “Uninitialized Entities”. The figure below illustrates the main window in the LVM utility. The logical and the physical views of the above configuration are illustrated below. The three logical volumes exist on the same physical volume (hda2).
Main LVM Window
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Main LVM Window

Figure 11.13. Main LVM Window

The figure below illustrates the physical view for the volume. In this window, you can select and remove a volume from the volume group or migrate extents from the volume to another volume group. Steps to migrate extents are discussed in Figure 11.22, “Migrate Extents”.
Physical View Window
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Physical View Window

Figure 11.14. Physical View Window

The figure below illustrates the logical view for the selected volume group. The logical volume size is also indicated with the individual logical volume sizes illustrated.
Logical View Window
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Logical View Window

Figure 11.15. Logical View Window

On the left side column, you can select the individual logical volumes in the volume group to view more details about each. In this example the objective is to rename the logical volume name for 'LogVol03' to 'Swap'. To perform this operation select the respective logical volume and click on the Edit Properties button. This will display the Edit Logical Volume window from which you can modify the Logical volume name, size (in extents) and also use the remaining space available in a logical volume group. The figure below illustrates this.
Please note that this logical volume cannot be changed in size as there is currently no free space in the volume group. If there was remaining space, this option would be enabled (see Figure 11.31, “Edit logical volume”). Click on the OK button to save your changes (this will remount the volume). To cancel your changes click on the Cancel button. To revert to the last snapshot settings click on the Revert button. A snapshot can be created by clicking on the Create Snapshot button on the LVM utility window. If the selected logical volume is in use by the system (for example) the / (root) directory, this task will not be successful as the volume cannot be unmounted.
Edit Logical Volume
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Edit Logical Volume

Figure 11.16. Edit Logical Volume

11.5.1. Utilizing uninitialized entities
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'Uninitialized Entities' consist of unpartitioned space and non LVM file systems. In this example partitions 3, 4, 5, 6 and 7 were created during installation and some unpartitioned space was left on the hard disk. Please view each partition and ensure that you read the 'Properties for Disk Entity' on the right column of the window to ensure that you do not delete critical data. In this example partition 1 cannot be initialized as it is /boot. Uninitialized entities are illustrated below.
Uninitialized Entities
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Uninitialized Entities

Figure 11.17. Uninitialized Entities

In this example, partition 3 will be initialized and added to an existing volume group. To initialize a partition or unpartioned space, select the partition and click on the Initialize Entity button. Once initialized, a volume will be listed in the 'Unallocated Volumes' list.
Once initialized, a volume will be listed in the 'Unallocated Volumes' list. The figure below illustrates an unallocated partition (Partition 3). The respective buttons at the bottom of the window allow you to:
  • create a new volume group,
  • add the unallocated volume to an existing volume group,
  • remove the volume from LVM.
To add the volume to an existing volume group, click on the Add to Existing Volume Group button.
Unallocated Volumes
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Unallocated Volumes

Figure 11.18. Unallocated Volumes

Clicking on the Add to Existing Volume Group button will display a pop up window listing the existing volume groups to which you can add the physical volume you are about to initialize. A volume group may span across one or more hard disks. In this example only one volume group exists as illustrated below.
Add physical volume to volume group
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Add physical volume to volume group

Figure 11.19. Add physical volume to volume group

Once added to an existing volume group the new logical volume is automatically added to the unused space of the selected volume group. You can use the unused space to:
  • create a new logical volume (click on the Create New Logical Volume(s) button,
  • select one of the existing logical volumes and increase the extents (see Section 11.5.6, “Extending a volume group”),
  • select an existing logical volume and remove it from the volume group by clicking on the Remove Selected Logical Volume(s) button. Please note that you cannot select unused space to perform this operation.
The figure below illustrates the logical view of 'VolGroup00' after adding the new volume group.
Logical view of volume group
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Logical view of volume group

Figure 11.20. Logical view of volume group

In the figure below, the uninitialized entities (partitions 3, 5, 6 and 7) were added to 'VolGroup00'.
Logical view of volume group
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Logical view of volume group

Figure 11.21. Logical view of volume group

11.5.3. Migrating extents
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To migrate extents from a physical volume, select the volume and click on the Migrate Selected Extent(s) From Volume button. Please note that you need to have a sufficient number of free extents to migrate extents within a volume group. An error message will be displayed if you do not have a sufficient number of free extents. To resolve this problem, please extend your volume group (see Section 11.5.6, “Extending a volume group”). If a sufficient number of free extents is detected in the volume group, a pop up window will be displayed from which you can select the destination for the extents or automatically let LVM choose the physical volumes (PVs) to migrate them to. This is illustrated below.
Migrate Extents
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Migrate Extents

Figure 11.22. Migrate Extents

The figure below illustrates a migration of extents in progress. In this example, the extents were migrated to 'Partition 3'.
Migrating extents in progress
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Migrating extents in progress

Figure 11.23. Migrating extents in progress

Once the extents have been migrated, unused space is left on the physical volume. The figure below illustrates the physical and logical view for the volume group. Please note that the extents of LogVol00 which were initially in hda2 are now in hda3. Migrating extents allows you to move logical volumes in case of hard disk upgrades or to manage your disk space better.
Logical and physical view of volume group
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Logical and physical view of volume group

Figure 11.24. Logical and physical view of volume group

11.5.4. Adding a new hard disk using LVM
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In this example, a new IDE hard disk was added. The figure below illustrates the details for the new hard disk. From the figure below, the disk is uninitialized and not mounted. To initialize a partition, click on the Initialize Entity button. For more details, see Section 11.5.1, “Utilizing uninitialized entities”. Once initialized, LVM will add the new volume to the list of unallocated volumes as illustrated in Figure 11.26, “Create new volume group”.
Uninitialized hard disk
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Uninitialized hard disk

Figure 11.25. Uninitialized hard disk

11.5.5. Adding a new volume group
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Once initialized, LVM will add the new volume to the list of unallocated volumes where you can add it to an existing volume group or create a new volume group. You can also remove the volume from LVM. The volume if removed from LVM will be listed in the list of 'Uninitialized Entities' as illustrated in Figure 11.25, “Uninitialized hard disk”. In this example, a new volume group was created as illustrated below.
Create new volume group
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Create new volume group

Figure 11.26. Create new volume group

Once created a new volume group will be displayed in the list of existing volume groups as illustrated below. The logical view will display the new volume group with unused space as no logical volumes have been created. To create a logical volume, select the volume group and click on the Create New Logical Volume button as illustrated below. Please select the extents you wish to use on the volume group. In this example, all the extents in the volume group were used to create the new logical volume.
Create new logical volume
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Create new logical volume

Figure 11.27. Create new logical volume

The figure below illustrates the physical view of the new volume group. The new logical volume named 'Backups' in this volume group is also listed.
Physical view of new volume group
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Physical view of new volume group

Figure 11.28. Physical view of new volume group

11.5.6. Extending a volume group
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In this example, the objective was to extend the new volume group to include an uninitialized entity (partition). This was to increase the size or number of extents for the volume group. To extend the volume group, click on the Extend Volume Group button. This will display the 'Extend Volume Group' window as illustrated below. On the 'Extend Volume Group' window, you can select disk entities (partitions) to add to the volume group. Please ensure that you check the contents of any 'Uninitialized Disk Entities' (partitions) to avoid deleting any critical data (see Figure 11.25, “Uninitialized hard disk”). In the example, the disk entity (partition) /dev/hda6 was selected as illustrated below.
Select disk entities
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Select disk entities

Figure 11.29. Select disk entities

Once added, the new volume will be added as 'Unused Space' in the volume group. The figure below illustrates the logical and physical view of the volume group after it was extended.
Logical and physical view of an extended volume group
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Logical and physical view of an extended volume group

Figure 11.30. Logical and physical view of an extended volume group

11.5.7. Editing a Logical Volume
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The LVM utility allows you to select a logical volume in the volume group and modify its name, size and specify filesystem options. In this example, the logical volume named 'Backups" was extended onto the remaining space for the volume group.
Clicking on the Edit Properties button will display the 'Edit Logical Volume' popup window from which you can edit the properties of the logical volume. On this window, you can also mount the volume after making the changes and mount it when the system is rebooted. Please note that you should indicate the mount point. If the mount point you specify does not exist, a popup window will be displayed prompting you to create it. The 'Edit Logical Volume' window is illustrated below.
Edit logical volume
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Edit logical volume

Figure 11.31. Edit logical volume

If you wish to mount the volume, select the 'Mount' checkbox indicating the preferred mount point. To mount the volume when the system is rebooted, select the 'Mount when rebooted' checkbox. In this example, the new volume will be mounted in /mnt/backups. This is illustrated in the figure below.
Edit logical volume - specifying mount options
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Edit logical volume - specifying mount options

Figure 11.32. Edit logical volume - specifying mount options

The figure below illustrates the logical and physical view of the volume group after the logical volume was extended to the unused space. Please note in this example that the logical volume named 'Backups' spans across two hard disks. A volume can be striped across two or more physical devices using LVM.
Edit logical volume
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Edit logical volume

Figure 11.33. Edit logical volume

11.6. Additional Resources
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Use these sources to learn more about LVM.

11.6.1. Installed Documentation
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  • rpm -qd lvm2 — This command shows all the documentation available from the lvm package, including man pages.
  • lvm help — This command shows all LVM commands available.

11.6.2. Useful Websites
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Part II. Package Management
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All software on a Red Hat Enterprise Linux system is divided into RPM packages which can be installed, upgraded, or removed. This part describes how to manage the RPM packages on a Red Hat Enterprise Linux system using graphical and command line tools.

Chapter 12. Package Management with RPM
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The RPM Package Manager (RPM) is an open packaging system, which runs on Red Hat Enterprise Linux as well as other Linux and UNIX systems. Red Hat, Inc. encourages other vendors to use RPM for their own products. RPM is distributed under the terms of the GPL.
The utility works only with packages built for processing by the rpm package. For the end user, RPM makes system updates easy. Installing, uninstalling, and upgrading RPM packages can be accomplished with short commands. RPM maintains a database of installed packages and their files, so you can invoke powerful queries and verifications on your system. If you prefer a graphical interface, you can use the Package Management Tool to perform many RPM commands. Refer to Chapter 13, Package Management Tool for details.

Important

When installing a package, please ensure it is compatible with your operating system and architecture. This can usually be determined by checking the package name.
During upgrades, RPM handles configuration files carefully, so that you never lose your customizations — something that you cannot accomplish with regular .tar.gz files.
For the developer, RPM allows you to take software source code and package it into source and binary packages for end users. This process is quite simple and is driven from a single file and optional patches that you create. This clear delineation between pristine sources and your patches along with build instructions eases the maintenance of the package as new versions of the software are released.

Note

Because RPM makes changes to your system, you must be logged in as root to install, remove, or upgrade an RPM package.

12.1. RPM Design Goals
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To understand how to use RPM, it can be helpful to understand the design goals of RPM:
Upgradability
With RPM, you can upgrade individual components of your system without completely reinstalling. When you get a new release of an operating system based on RPM (such as Red Hat Enterprise Linux), you do not need to reinstall on your machine (as you do with operating systems based on other packaging systems). RPM allows intelligent, fully-automated, in-place upgrades of your system. Configuration files in packages are preserved across upgrades, so you do not lose your customizations. There are no special upgrade files needed to upgrade a package because the same RPM file is used to install and upgrade the package on your system.
Powerful Querying
RPM is designed to provide powerful querying options. You can do searches through your entire database for packages or just for certain files. You can also easily find out what package a file belongs to and from where the package came. The files an RPM package contains are in a compressed archive, with a custom binary header containing useful information about the package and its contents, allowing you to query individual packages quickly and easily.
System Verification
Another powerful RPM feature is the ability to verify packages. If you are worried that you deleted an important file for some package, you can verify the package. You are then notified of any anomalies, if any — at which point, you can reinstall the package if necessary. Any configuration files that you modified are preserved during reinstallation.
Pristine Sources
A crucial design goal was to allow the use of pristine software sources, as distributed by the original authors of the software. With RPM, you have the pristine sources along with any patches that were used, plus complete build instructions. This is an important advantage for several reasons. For instance, if a new version of a program is released, you do not necessarily have to start from scratch to get it to compile. You can look at the patch to see what you might need to do. All the compiled-in defaults, and all of the changes that were made to get the software to build properly, are easily visible using this technique.
The goal of keeping sources pristine may seem important only for developers, but it results in higher quality software for end users, too.

12.2. Using RPM
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RPM has five basic modes of operation (not counting package building): installing, uninstalling, upgrading, querying, and verifying. This section contains an overview of each mode. For complete details and options, try rpm --help or man rpm. You can also refer to Section 12.5, “Additional Resources” for more information on RPM.

12.2.1. Finding RPM Packages
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Before using any RPM packages, you must know where to find them. An Internet search returns many RPM repositories, but if you are looking for RPM packages built by Red Hat, they can be found at the following locations:

12.2.2. Installing
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RPM packages typically have file names like foo-1.0-1.i386.rpm. The file name includes the package name (foo), version (1.0), release (1), and architecture (i386). To install a package, log in as root and type the following command at a shell prompt: 
rpm -ivh foo-1.0-1.i386.rpm
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Alternatively, the following command can also be used: 
rpm -Uvh foo-1.0-1.i386.rpm
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If the installation is successful, the following output is displayed: 
Preparing...                ########################################### [100%]
   1:foo                    ########################################### [100%]
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As you can see, RPM prints out the name of the package and then prints a succession of hash marks as a progress meter while the package is installed.
The signature of a package is checked automatically when installing or upgrading a package. The signature confirms that the package was signed by an authorized party. For example, if the verification of the signature fails, an error message such as the following is displayed: 
error: V3 DSA signature: BAD, key ID 0352860f
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If it is a new, header-only, signature, an error message such as the following is displayed: 
error: Header V3 DSA signature: BAD, key ID 0352860f
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If you do not have the appropriate key installed to verify the signature, the message contains the word NOKEY such as: 
warning: V3 DSA signature: NOKEY, key ID 0352860f
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Refer to Section 12.3, “Checking a Package's Signature” for more information on checking a package's signature.

Warning

If you are installing a kernel package, you should use rpm -ivh instead. Refer to Chapter 44, Manually Upgrading the Kernel for details.
12.2.2.1. Package Already Installed
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If a package of the same name and version is already installed, the following output is displayed: 
Preparing...                ########################################### [100%]
package foo-1.0-1 is already installed
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However, if you want to install the package anyway, you can use the --replacepkgs option, which tells RPM to ignore the error: 
rpm -ivh --replacepkgs foo-1.0-1.i386.rpm
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This option is helpful if files installed from the RPM were deleted or if you want the original configuration files from the RPM to be installed.
12.2.2.2. Conflicting Files
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If you attempt to install a package that contains a file which has already been installed by another package, the following is displayed: 
Preparing...                ########################################### [100%]
file /usr/bin/foo from install of foo-1.0-1 conflicts with file from package bar-2.0.20
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To make RPM ignore this error, use the --replacefiles option: 
rpm -ivh --replacefiles foo-1.0-1.i386.rpm
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12.2.2.3. Unresolved Dependency
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RPM packages may sometimes depend on other packages, which means that they require other packages to be installed to run properly. If you try to install a package which has an unresolved dependency, output similar to the following is displayed: 
error: Failed dependencies:
        bar.so.2 is needed by foo-1.0-1
Suggested resolutions:
	bar-2.0.20-3.i386.rpm
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If you are installing a package from the Red Hat Enterprise Linux CD-ROM set, it usually suggest the package(s) needed to resolve the dependency. Find the suggested package(s) on the Red Hat Enterprise Linux CD-ROMs or from Red Hat Network , and add it to the command: 
rpm -ivh foo-1.0-1.i386.rpm bar-2.0.20-3.i386.rpm
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If installation of both packages is successful, output similar to the following is displayed: 
Preparing...                ########################################### [100%]
   1:foo                    ########################################### [ 50%]
   2:bar                    ########################################### [100%]
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If it does not suggest a package to resolve the dependency, you can try the -q --whatprovides option combination to determine which package contains the required file. 
rpm -q --whatprovides bar.so.2
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To force the installation anyway (which is not recommended since the package may not run correctly), use the --nodeps option.

12.2.3. Uninstalling
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Uninstalling a package is just as simple as installing one. Type the following command at a shell prompt: 
rpm -e foo
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Note

Notice that we used the package name foo, not the name of the original package file foo-1.0-1.i386.rpm. To uninstall a package, replace foo with the actual package name of the original package.
You can encounter a dependency error when uninstalling a package if another installed package depends on the one you are trying to remove. For example: 
error: Failed dependencies:
	foo is needed by (installed) bar-2.0.20-3.i386.rpm
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To make RPM ignore this error and uninstall the package anyway (which may break the package dependent on it) use the --nodeps option.

12.2.4. Upgrading
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Upgrading a package is similar to installing one. Type the following command at a shell prompt: 
rpm -Uvh foo-2.0-1.i386.rpm
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As part of upgrading a package, RPM automatically uninstalls any old versions of the foo package. Note that -U will also install a package even when there are no previous versions of the package installed.

Note

It is not advisable to use the -U option for installing kernel packages, because RPM replaces the previous kernel package. This does not affect a running system, but if the new kernel is unable to boot during your next restart, there would be no other kernel to boot instead.
Using the -i option adds the kernel to your GRUB boot menu (/etc/grub.conf). Similarly, removing an old, unneeded kernel removes the kernel from GRUB.
Because RPM performs intelligent upgrading of packages with configuration files, you may see a message like the following: 
saving /etc/foo.conf as /etc/foo.conf.rpmsave
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This message means that changes you made to the configuration file may not be forward compatible with the new configuration file in the package, so RPM saved your original file and installed a new one. You should investigate the differences between the two configuration files and resolve them as soon as possible, to ensure that your system continues to function properly.
If you attempt to upgrade to a package with an older version number (that is, if a more updated version of the package is already installed), the output is similar to the following: 
package foo-2.0-1 (which is newer than foo-1.0-1) is already installed
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To force RPM to upgrade anyway, use the --oldpackage option: 
rpm -Uvh --oldpackage foo-1.0-1.i386.rpm
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12.2.5. Freshening
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Freshening is similar to upgrading, except that only existent packages are upgraded. Type the following command at a shell prompt: 
rpm -Fvh foo-1.2-1.i386.rpm
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RPM's freshen option checks the versions of the packages specified on the command line against the versions of packages that have already been installed on your system. When a newer version of an already-installed package is processed by RPM's freshen option, it is upgraded to the newer version. However, RPM's freshen option does not install a package if no previously-installed package of the same name exists. This differs from RPM's upgrade option, as an upgrade does install packages whether or not an older version of the package was already installed.
Freshening works for single packages or package groups. If you have just downloaded a large number of different packages, and you only want to upgrade those packages that are already installed on your system, freshening does the job. Thus, you do not have to delete any unwanted packages from the group that you downloaded before using RPM.
In this case, issue the following command: 
rpm -Fvh *.rpm
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RPM automatically upgrades only those packages that are already installed.

12.2.6. Querying
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The RPM database stores information about all RPM packages installed in your system. It is stored in the directory /var/lib/rpm/, and is used to query what packages are installed, what versions each package is, and any changes to any files in the package since installation, among others.
To query this database, use the -q option. The rpm -q package name command displays the package name, version, and release number of the installed package package name . For example, using rpm -q foo to query installed package foo might generate the following output: 
foo-2.0-1
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You can also use the following Package Selection Options with -q to further refine or qualify your query:
  • -a — queries all currently installed packages.
  • -f <filename> — queries the RPM database for which package owns f<filename> . When specifying a file, specify the absolute path of the file (for example, rpm -qf /bin/ls ).
  • -p <packagefile> — queries the uninstalled package <packagefile> .
There are a number of ways to specify what information to display about queried packages. The following options are used to select the type of information for which you are searching. These are called Package Query Options.
  • -i displays package information including name, description, release, size, build date, install date, vendor, and other miscellaneous information.
  • -l displays the list of files that the package contains.
  • -s displays the state of all the files in the package.
  • -d displays a list of files marked as documentation (man pages, info pages, READMEs, etc.).
  • -c displays a list of files marked as configuration files. These are the files you edit after installation to adapt and customize the package to your system (for example, sendmail.cf, passwd, inittab, etc.).
For options that display lists of files, add -v to the command to display the lists in a familiar ls -l format.

12.2.7. Verifying
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Verifying a package compares information about files installed from a package with the same information from the original package. Among other things, verifying compares the size, MD5 sum, permissions, type, owner, and group of each file.
The command rpm -V verifies a package. You can use any of the Verify Options listed for querying to specify the packages you wish to verify. A simple use of verifying is rpm -V foo, which verifies that all the files in the foo package are as they were when they were originally installed. For example:
  • To verify a package containing a particular file: 
    rpm -Vf /usr/bin/foo
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    In this example, /usr/bin/foo is the absolute path to the file used to query a package.
  • To verify ALL installed packages throughout the system: 
    rpm -Va
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  • To verify an installed package against an RPM package file: 
    rpm -Vp foo-1.0-1.i386.rpm
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    This command can be useful if you suspect that your RPM databases are corrupt.
If everything verified properly, there is no output. If there are any discrepancies, they are displayed. The format of the output is a string of eight characters (a c denotes a configuration file) and then the file name. Each of the eight characters denotes the result of a comparison of one attribute of the file to the value of that attribute recorded in the RPM database. A single period (.) means the test passed. The following characters denote specific discrepancies:
  • 5 — MD5 checksum
  • S — file size
  • L — symbolic link
  • T — file modification time
  • D — device
  • U — user
  • G — group
  • M — mode (includes permissions and file type)
  • ? — unreadable file
If you see any output, use your best judgment to determine if you should remove the package, reinstall it, or fix the problem in another way.

12.3. Checking a Package's Signature
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If you wish to verify that a package has not been corrupted or tampered with, examine only the md5sum by typing the following command at a shell prompt (where <rpm-file> is the file name of the RPM package): 
rpm -K --nosignature <rpm-file>
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The message <rpm-file>: md5 OK is displayed. This brief message means that the file was not corrupted by the download. To see a more verbose message, replace -K with -Kvv in the command.
On the other hand, how trustworthy is the developer who created the package? If the package is signed with the developer's GnuPG key, you know that the developer really is who they say they are.
An RPM package can be signed using Gnu Privacy Guard (or GnuPG), to help you make certain your downloaded package is trustworthy.
GnuPG is a tool for secure communication; it is a complete and free replacement for the encryption technology of PGP, an electronic privacy program. With GnuPG, you can authenticate the validity of documents and encrypt/decrypt data to and from other recipients. GnuPG is capable of decrypting and verifying PGP 5.x files as well.
During installation, GnuPG is installed by default. That way you can immediately start using GnuPG to verify any packages that you receive from Red Hat. Before doing so, you must first import Red Hat's public key.

12.3.1. Importing Keys
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To verify Red Hat packages, you must import the Red Hat GPG key. To do so, execute the following command at a shell prompt: 
rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release
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To display a list of all keys installed for RPM verification, execute the command: 
rpm -qa gpg-pubkey*
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For the Red Hat key, the output includes: 
gpg-pubkey-37017186-45761324
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To display details about a specific key, use rpm -qi followed by the output from the previous command: 
rpm -qi gpg-pubkey-37017186-45761324
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12.3.2. Verifying Signature of Packages
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To check the GnuPG signature of an RPM file after importing the builder's GnuPG key, use the following command (replace <rpm-file> with the filename of the RPM package): 
rpm -K <rpm-file>
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If all goes well, the following message is displayed: md5 gpg OK. This means that the signature of the package has been verified, and that it is not corrupt.

12.4. Practical and Common Examples of RPM Usage
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RPM is a useful tool for both managing your system and diagnosing and fixing problems. The best way to make sense of all of its options is to look at some examples.
  • Perhaps you have deleted some files by accident, but you are not sure what you deleted. To verify your entire system and see what might be missing, you could try the following command: 
    rpm -Va
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    If some files are missing or appear to have been corrupted, you should probably either re-install the package or uninstall and then re-install the package.
  • At some point, you might see a file that you do not recognize. To find out which package owns it, enter: 
    rpm -qf /usr/bin/ggv
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    The output would look like the following: 
    ggv-2.6.0-2
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  • We can combine the above two examples in the following scenario. Say you are having problems with /usr/bin/paste. You would like to verify the package that owns that program, but you do not know which package owns paste. Enter the following command, 
    rpm -Vf /usr/bin/paste
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    and the appropriate package is verified.
  • Do you want to find out more information about a particular program? You can try the following command to locate the documentation which came with the package that owns that program: 
    rpm -qdf /usr/bin/free
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    The output would be similar to the following: 
    /usr/share/doc/procps-3.2.3/BUGS
/usr/share/doc/procps-3.2.3/FAQ
/usr/share/doc/procps-3.2.3/NEWS
/usr/share/doc/procps-3.2.3/TODO
/usr/share/man/man1/free.1.gz
/usr/share/man/man1/pgrep.1.gz
/usr/share/man/man1/pkill.1.gz
/usr/share/man/man1/pmap.1.gz
/usr/share/man/man1/ps.1.gz
/usr/share/man/man1/skill.1.gz
/usr/share/man/man1/slabtop.1.gz
/usr/share/man/man1/snice.1.gz
/usr/share/man/man1/tload.1.gz
/usr/share/man/man1/top.1.gz
/usr/share/man/man1/uptime.1.gz
/usr/share/man/man1/w.1.gz
/usr/share/man/man1/watch.1.gz
/usr/share/man/man5/sysctl.conf.5.gz
/usr/share/man/man8/sysctl.8.gz
/usr/share/man/man8/vmstat.8.gz
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  • You may find a new RPM, but you do not know what it does. To find information about it, use the following command: 
    rpm -qip crontabs-1.10-7.noarch.rpm
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    The output would be similar to the following: 
    Name        : crontabs                     Relocations: (not relocatable)
Version     : 1.10                              Vendor: Red Hat, Inc.
Release     : 7                             Build Date: Mon 20 Sep 2004 05:58:10 PM EDT
Install Date: (not installed)               Build Host: tweety.build.redhat.com
Group       : System Environment/Base       Source RPM: crontabs-1.10-7.src.rpm
Size        : 1004                             License: Public Domain
Signature   : DSA/SHA1, Wed 05 Jan 2005 06:05:25 PM EST, Key ID 219180cddb42a60e
Packager    : Red Hat, Inc. <http://bugzilla.redhat.com/bugzilla>
Summary     : Root crontab files used to schedule the execution of programs.
Description : The crontabs package contains root crontab files. Crontab is the
program used to install, uninstall, or list the tables used to drive the
cron daemon. The cron daemon checks the crontab files to see when
particular commands are scheduled to be executed. If commands are
scheduled, then it executes them.
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  • Perhaps you now want to see what files the crontabs RPM installs. You would enter the following: 
    rpm -qlp crontabs-1.10-5.noarch.rpm
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    The output is similar to the following: 
    /etc/cron.daily
/etc/cron.hourly
/etc/cron.monthly
/etc/cron.weekly
/etc/crontab
/usr/bin/run-parts
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These are just a few examples. As you use RPM, you may find more uses for it.

12.5. Additional Resources
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RPM is an extremely complex utility with many options and methods for querying, installing, upgrading, and removing packages. Refer to the following resources to learn more about RPM.

12.5.1. Installed Documentation
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  • rpm --help — This command displays a quick reference of RPM parameters.
  • man rpm — The RPM man page gives more detail about RPM parameters than the rpm --help command.

12.5.2. Useful Websites
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Chapter 13. Package Management Tool
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If you prefer to use a graphical interface to view and manage packages in your system, you can use the Package Management Tool, better known as pirut. This tool allows you to perform basic package management of your system through an easy-to-use interface to remove installed packages or download (and install) packages compatible to your system. It also allows you to view what packages are installed in your system and which ones are available for download from Red Hat Network. In addition, the Package Management Tool also automatically resolves any critical dependencies when you install or remove packages in the same way that the rpm command does.

Note

While the Package Management Tool can automatically resolve dependencies during package installation and removal, it cannot perform a forced install / remove the same way that rpm -e --nodeps or rpm -U --nodeps can.
The X Window System is required to run the Package Management Tool. To start the application, go to Applications (the main menu on the panel) > Add/Remove Software. Alternatively, you can type the commands system-config-packages or pirut at shell prompt.
Package Management Tool
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Package Management Tool

Figure 13.1. Package Management Tool

13.1. Listing and Analyzing Packages
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You can use the Package Management Tool to search and list all packages installed in your system, as well as any packages available for you to download. The Browse, Search, and List tabs present different options in viewing, analyzing, installing or removing packages.
The Browse tab allows you to view packages by group. In Figure 13.1, “Package Management Tool”, the left window shows the different package group types you can choose from (for example, Desktop Environments, Applications, Development and more). When a package group type is selected, the right window displays the different package groups of that type.
To view what packages are included in a package group, click Optional packages. Installed packages are checked.
Optional Packages
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Optional Packages

Figure 13.2. Optional Packages

The List tab displays a list of packages installed or available for download. Packages already installed in your system are marked with a green check ( ).
By default, the All packages option above the main window is selected; this specifies that all packages be displayed. Use the Installed packages option to display only packages that are already installed in your system, and the Available packages option to view what packages you can download and install.
The Search tab allows you to use keywords to search for particular packages. This tab also allows you to view a short description of a package. To do so, simply select a package and click the Package Details button below the main window.

13.2. Installing and Removing Packages
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To install a package available for download, click the checkbox beside the package name. When you do so, an installation icon ( ) appears beside its checkbox. This indicates that the package is queued for download and installation. You can select multiple packages to download and install; once you have made your selection, click the Apply button.
Package installation
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Package installation

Figure 13.3. Package installation

If there are any package dependencies for your selected downloads, the Package Management Tool will notify you accordingly. Click Details to view what additional packages are needed. To proceed with downloading and installing the package (along with all other dependent packages) click Continue.
Package dependencies: installation
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Package dependencies: installation

Figure 13.4. Package dependencies: installation

Removing a package can be done in a similar manner. To remove a package installed in your system, click the checkbox beside the package name. The green check appearing beside the package name will be replaced by a package removal icon ( ). This indicates that the package is queued for removal; you can also select multiple packages to be removed at the same time. Once you have selected the packages you want to remove, click the Apply button.
Package removal
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Package removal

Figure 13.5. Package removal

Note that if any other installed packages are dependent on the package you are removing, they will be removed as well. The Package Management Tool will notify you if there are any such dependencies. Click Details to view what packages are dependent on the one you are removing. To proceed with removing your selected package/s (along with all other dependent packages) click Continue.
Package dependencies: removal
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Package dependencies: removal

Figure 13.6. Package dependencies: removal

You can install and remove multiple packages by selecting packages to be installed / removed and then clicking Apply. The Package selections window displays the number of packages to be installed and removed.
Installing and removing packages simultaneously
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Installing and removing packages simultaneously

Figure 13.7. Installing and removing packages simultaneously

Chapter 14. YUM (Yellowdog Updater Modified)
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Yellowdog Update, Modified (YUM) is a package manager that was developed by Duke University to improve the installation of RPMs. yum searches numerous repositories for packages and their dependencies so they may be installed together in an effort to alleviate dependency issues. Red Hat Enterprise Linux 5.10 uses yum to fetch packages and install RPMs.
up2date is now deprecated in favor of yum (Yellowdog Updater Modified). The entire stack of tools which installs and updates software in Red Hat Enterprise Linux 5.10 is now based on yum. This includes everything, from the initial installation via Anaconda to host software management tools like pirut.
yum also allows system administrators to configure a local (i.e. available over a local network) repository to supplement packages provided by Red Hat. This is useful for user groups that use applications and packages that are not officially supported by Red Hat.
Aside from being able to supplement available packages for local users, using a local yum repository also saves bandwidth for the entire network. Further, clients that use local yum repositories do not need to be registered individually to install or update the latest packages from Red Hat Network.

14.1. Setting Up a Yum Repository
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To set up a repository for Red Hat Enterprise Linux packages, follow these steps:
  1. Install the createrepo package: 
    ~]# yum install createrepo
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  2. Copy all the packages you want to provide in the repository into one directory (/mnt/local_repo for example).
  3. Run createrepo on that directory (for example, createrepo /mnt/local_repo). This will create the necessary metadata for your Yum repository.

14.2.  yum Commands
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yum commands are typically run as yum <command> <package name/s> . By default, yum will automatically attempt to check all configured repositories to resolve all package dependencies during an installation/upgrade.
The following is a list of the most commonly-used yum commands. For a complete list of available yum commands, refer to man yum.
yum install <package name/s>
Used to install the latest version of a package or group of packages. If no package matches the specified package name(s), they are assumed to be a shell glob, and any matches are then installed.
yum update <package name/s>
Used to update the specified packages to the latest available version. If no package name/s are specified, then yum will attempt to update all installed packages.
If the --obsoletes option is used (i.e. yum --obsoletes <package name/s> , yum will process obsolete packages. As such, packages that are obsoleted across updates will be removed and replaced accordingly.
yum check-update
This command allows you to determine whether any updates are available for your installed packages. yum returns a list of all package updates from all repositories if any are available.
yum remove <package name/s>
Used to remove specified packages, along with any other packages dependent on the packages being removed.
yum provides <file name>
Used to determine which packages provide a specific file or feature.
yum search <keyword>
This command is used to find any packages containing the specified keyword in the description, summary, packager and package name fields of RPMs in all repositories.
yum localinstall <absolute path to package name/s>
Used when using yum to install a package located locally in the machine.

14.3.  yum Options
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yum options are typically stated before specific yum commands; i.e. yum <options> <command> <package name/s> . Most of these options can be set as default using the configuration file.
The following is a list of the most commonly-used yum options. For a complete list of available yum options, refer to man yum.
-y
Answer "yes" to every question in the transaction.
-t
Sets yum to be "tolerant" of errors with regard to packages specified in the transaction. For example, if you run yum update package1 package2 and package2 is already installed, yum will continue to install package1.
--exclude=<package name>
Excludes a specific package by name or glob in a specific transaction.

14.4. Configuring yum
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By default, yum is configured through /etc/yum.conf. The following is an example of a typical /etc/yum.conf file: 
[main]
cachedir=/var/cache/yum
keepcache=0
debuglevel=2
logfile=/var/log/yum.log
distroverpkg=redhat-release
tolerant=1
exactarch=1
obsoletes=1
gpgcheck=1
plugins=1
metadata_expire=1800
[myrepo]
name=RHEL 5 $releasever - $basearch
baseurl=http://local/path/to/yum/repository/
enabled=1
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A typical /etc/yum.conf file is made up of two types of sections: a [main] section, and a repository section. There can only be one [main] section, but you can specify multiple repositories in a single /etc/yum.conf.

14.4.1.  [main] Options
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The [main] section is mandatory, and there must only be one. For a complete list of options you can use in the [main] section, refer to man yum.conf.
The following is a list of the most commonly-used options in the [main] section.
cachedir
This option specifies the directory where yum should store its cache and database files. By default, the cache directory of yum is /var/cache/yum.
keepcache=<1 or 0>
Setting keepcache=1 instructs yum to keep the cache of headers and packages after a successful installation. keepcache=1 is the default.
reposdir=<absolute path to directory of .repo files>
This option allows you to specify a directory where .repo files are located. .repo files contain repository information (similar to the [repository] section of /etc/yum.conf).
yum collects all repository information from .repo files and the [repository] section of the /etc/yum.conf file to create a master list of repositories to use for each transaction. Refer to Section 14.4.2, “ [repository] Options” for more information about options you can use for both the [repository] section and .repo files.
If reposdir is not set, yum uses the default directory /etc/yum.repos.d.
gpgcheck=<1 or 0>
This disables/enables GPG signature checking on packages on all repositories, including local package installation. The default is gpgcheck=0, which disables GPG checking.
If this option is set in the [main] section of the /etc/yum.conf file, it sets the GPG checking rule for all repositories. However, you can also set this on individual repositories instead; i.e., you can enable GPG checking on one repository while disabling it on another.
assumeyes=<1 or 0>
This determines whether or not yum should prompt for confirmation of critical actions. The default if assumeyes=0, which means yum will prompt you for confirmation.
If assumeyes=1 is set, yum behaves in the same way that the command line option -y does.
tolerant=<1 or 0>
When enabled (tolerant=1), yum will be tolerant of errors on the command line with regard to packages. This is similar to the yum command line option -t.
The default value for this is tolerant=0 (not tolerant).
exclude=<package name/s>
This option allows you to exclude packages by keyword during installation/updates. If you are specifying multiple packages, this is a space-delimited list. Shell globs using wildcards (for example, * and ?) are allowed.
retries=<number of retries>
This sets the number of times yum should attempt to retrieve a file before returning an error. Setting this to 0 makes yum retry forever. The default value is 6.

14.4.2.  [repository] Options
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The [repository] section of the /etc/yum.conf file contains information about a repository yum can use to find packages during package installation, updating and dependency resolution. A repository entry takes the following form: 
[repository ID]
name=repository name
baseurl=url, file or ftp://path to repository
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You can also specify repository information in a separate .repo files (for example, rhel5.repo). The format of repository information placed in .repo files is identical with the [repository] of /etc/yum.conf.
.repo files are typically placed in /etc/yum.repos.d, unless you specify a different repository path in the [main] section of /etc/yum.conf with reposdir=. .repo files and the /etc/yum.conf file can contain multiple repository entries.
Each repository entry consists of the following mandatory parts:
[repository ID]
The repository ID is a unique, one-word string that serves as a repository identifier.
name=repository name
This is a human-readable string describing the repository.
baseurl=http, file or ftp://path
This is a URL to the directory where the repodatadirectory of a repository is located. If the repository is local to the machine, use baseurl=file://path to local repository . If the repository is located online using HTTP, use baseurl=http://link . If the repository is online and uses FTP, use baseurl=ftp://link .
If a specific online repository requires basic HTTP authentication, you can specify your username and password in the baseurl line by prepending it as username:password@link. For example, if a repository on http://www.example.com/repo/ requires a username of "user" and a password os "password", then the baseurl link can be specified as baseurl=http://user:password@www.example.com/repo/.
The following is a list of options most commonly used in repository entries. For a complete list of repository entries, refer to man yum.conf.
gpgcheck=<1 or 0>
This disables/enables GPG signature checking a specific repository. The default is gpgcheck=0, which disables GPG checking.
gpgkey=URL
This option allows you to point to a URL of the ASCII-armoured GPG key file for a repository. This option is normally used if yum needs a public key to verify a package and the required key was not imported into the RPM database.
If this option is set, yum will automatically import the key from the specified URL. You will be prompted before the key is installed unless you set assumeyes=1 (in the [main] section of /etc/yum.conf) or -y (in a yum transaction).
exclude=<package name/s>
This option is similar to the exclude option in the [main] section of /etc/yum.conf. However, it only applies to the repository in which it is specified.
includepkgs=<package name/s>
This option is the opposite of exclude. When this option is set on a repository, yum will only be able to see the specified packages in that repository. By default, all packages in a repository are visible to yum.
For systems that are disconnected from the Internet or Red Hat Network, using the yum update command with the Red Hat Enterprise Linux installation ISO image is an easy and quick way to upgrade systems to the latest minor version. The following steps illustrate the upgrading process:
  1. Create a target directory to mount your ISO image. This directory is not automatically created when mounting, so create it before proceeding to the next step, as root, type: 
    mkdir mount_dir
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    Replace mount_dir with a path to the mount directory. Typicaly, users create it as a subdirectory in the /media/ directory.
  2. Mount the Red Hat Enterprise Linux 5 installation ISO image to the previously created target directory. As root, type: 
    mount -o loop iso_name mount_dir
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    Replace iso_name with a path to your ISO image and mount_dir with a path to the target directory. Here, the -o loop option is required to mount the file as a block device.
  3. Check the numeric value found on the first line of the .discinfo file from the mount directory: 
    head -n1 mount_dir/.discinfo
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    The output of this command is an identification number of the ISO image, you need to know it to perform the following step.
  4. Create a new file in the /etc/yum.repos.d/ directory, named for instance new.repo, and add a content in the following form. Note that configuration files in this directory must have the .repo extension to function properly. 
    [repository] 
mediaid=media_id 
name=repository_name
baseurl=repository_url
gpgkey=gpg_key 
enabled=1 
gpgcheck=1
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    Replace media_id with the numeric value found in mount_dir/.discinfo. Set the repository name instead of repository_name, replace repository_url with a path to a repository directory in the mount point and gpg_key with a path to the GPG key.
    For example, the repository settings for Red Hat Enterprise Linux 5 Server ISO can look as follows: 
    [rhel5-Server] 
mediaid=1354216429.587870 
name=RHEL5-Server
baseurl=file:///media/rhel5/Server 
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release 
enabled=1 
gpgcheck=1
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  5. Update all yum repositories including /etc/yum.repos.d/new.repo created in previous steps. As root, type: 
    yum update
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    This upgrades your system to the version provided by the mounted ISO image.
  6. After successful upgrade, you can unmount the ISO image, with the root privileges: 
    umount mount_dir
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    where mount_dir is a path to your mount directory. Also, you can remove the mount directory created in the first step. As root, type: 
    rmdir mount_dir
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  7. If you will not use the previously created configuration file for another installation or update, you can remove it. As root, type: 
    rm /etc/yum.repos.d/new.repo
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Example 14.1. Upgrading from Red Hat Enterprise Linux 5.8 to 5.9

Imagine you need to upgrade your system without access to the Internet connection. To do so, you want to use an ISO image with the newer version of the system, called for instance RHEL5.9-Server-20121129.0-x86_64-DVD1.iso. You have crated a target directory /media/rhel5/. As root, change into the directory with your ISO image and type: 
~]# mount -o loop RHEL5.9-Server-20121129.0-x86_64-DVD1.iso /media/rhel5/
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To find the identification number of the mounted image, run: 
~]# head -n1 /media/rhel5/.discinfo 
1354216429.587870
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You need this number to configure your mount point as a yum repository. Create the/etc/yum.repos.d/rhel5.repo file and insert the following text into it: 
[rhel5-Server] 
mediaid=1354216429.587870 
name=RHEL5-Server
baseurl=file:///media/rhel5/Server 
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release 
enabled=1 
gpgcheck=1
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Update the yum repository, which effectively upgrades your system to a version provided by RHEL5.9-Server-20121129.0-x86_64-DVD1.iso. As root, execute: 
~]# yum update
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When your system is successfully upgraded, unmount the image, remove the target directory and the configuration file: 
~]# umount /media/rhel5/
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~]# rmdir /media/rhel5/
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~]# rm /etc/yum.repos.d/rhel5.repo
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14.6. Useful yum Variables
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The following is a list of variables you can use for both yum commands and yum configuration files (i.e. /etc/yum.conf and .repo files).
$releasever
This is replaced with the package's version, as listed in distroverpkg. This defaults to the version of the redhat-release package.
$arch
This is replaced with your system's architecture, as listed by os.uname() in Python.
$basearch
This is replaced with your base architecture. For example, if $arch=i686 then $basearch=i386.
$YUM0-9
This is replaced with the value of the shell environment variable of the same name. If the shell environment variable does not exist, then the configuration file variable will not be replaced.
Effective asset management requires a mechanism to handle the software inventory — both the type of products and the number of systems that the software is installed on. The subscription service provides that mechanism and gives transparency into both global allocations of subscriptions for an entire organization and the specific subscriptions assigned to a single system.
Red Hat Subscription Manager works with yum to unite content delivery with subscription management. The Subscription Manager handles only the subscription-system associations. yum or other package management tools handle the actual content delivery. Chapter 14, YUM (Yellowdog Updater Modified) describes how to use yum.

15.1. Using Red Hat Subscription Manager Tools
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Both registration and subscriptions are managed on the local system through GUI and CLI tools called Red Hat Subscription Manager.

Note

The Red Hat Subscription Manager tools are always run as root because of the nature of the changes to the system. However, Red Hat Subscription Manager connects to the subscription service as a user account for the subscription service.
Red Hat Subscription Manager is listed as one of the administrative tools in the System > Administration menu in the top management bar.
Red Hat Subscription Manager Menu Option
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Red Hat Subscription Manager Menu Option

Figure 15.1. Red Hat Subscription Manager Menu Option

Alternatively, the Red Hat Subscription Manager GUI can be opened from the command line with a single command: 
[root@server1 ~]# subscription-manager-gui
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Any of the operations that can be performed through the Red Hat Subscription Manager UI can also be performed by running the subscription-manager tool. This tool has the following format: 
[root@server1 ~]# subscription-manager command [options]
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Each command has its own set of options that are used with it. The subscription-manager help and manpage have more information.
Expand
Table 15.1. Common subscription-manager Commands
Command Description
register Registers or identifies a new system to the subscription service.
unregister Unregisters a machine, which strips its subscriptions and removes the machine from the subscription service.
subscribe Attaches a specific subscription to the machine.
redeem Auto-attaches a machine to a pre-specified subscription that was purchased from a vendor, based on its hardware and BIOS information.
unsubscribe Removes a specific subscription or all subscriptions from the machine.
list Lists all of the subscriptions that are compatible with a machine, either subscriptions that are actually attached to the machine or unused subscriptions that are available to the machine.

15.2. Registering and Unregistering a System
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Systems can be registered with a subscription service during the firstboot process or as part of the kickstart setup (both described in the Installation Guide). Systems can also be registered after they have been configured or removed from the subscription service inventory (unregistered) if they will no longer be managed within that subscription service.

15.2.1. Registering from the GUI
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  1. Launch Subscription Manager. For example: 
    [root@server ~]# subscription-manager-gui
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  2. If the system is not already registered, then there will be a Register button at the top of the window in the top right corner of the My Installed Products tab.
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  3. To identify which subscription server to use for registration, enter the hostname of the service. The default service is Customer Portal Subscription Management, with the hostname subscription.rhn.redhat.com. To use a different subscription service, such as Subscription Asset Manager, enter the hostname of the local server.
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    There are seveal different subscription services which use and recognize certificate-based subscriptions, and a system can be registered with any of them in firstboot:
    • Customer Portal Subscription Management, hosted services from Red Hat (the default)
    • Subscription Asset Manager, an on-premise subscription server which proxies content delivery back to the Customer Portal's services
    • CloudForms System Engine, an on-premise service which handles both subscription services and content delivery
  4. Enter the user credentials for the given subscription service to log in.
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    The user credentials to use depend on the subscription service. When registering with the Customer Portal, use the Red Hat Network credentials for the administrator or company account.
    However, for Subscription Asset Manager or CloudForms System engine, the user account to use is created within the on-premise service and probably is not the same as the Customer Portal user account.
  5. Optionally, select the Manually assign subscriptions after registration checkbox.
    By default, the registration process automatically attaches the best-matched subscription to the system. This can be turned off so that the subscriptions can be selected manually, as in Section 15.3, “Attaching and Removing Subscriptions”.
  6. When registration begins, Subscription Manager scans for organizations and environments (sub-domains within the organization) to which to register the system.
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    IT environments that use Customer Portal Subscription Management have only a single organization, so no further configuration is necessary. IT infrastructures that use a local subscription service like Subscription Asset Manager might have multiple organizations configured, and those organizations may have multiple environments configured within them.
    If multiple organizations are detected, Subscription Manager prompts to select the one to join.
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  7. With the default setting, subscriptions are automatically selected and attached to the system. Review and confirm the subscriptions to attach to the system.
    1. If prompted, select the service level to use for the discovered subscriptions.
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    2. Subscription Manager lists the selected subscription. This subscription selection must be confirmed by clicking the Subscribe button for the wizard to complete.
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15.2.2. Registering from the Command Line
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The simplest way to register a machine is to pass the register command with the user account information required to authenticate to Customer Portal Subscription Management. When the system is successfully authenticated, it echoes back the newly-assigned system inventory ID and the user account name which registered it.
The register options are listed in Table 15.2, “register Options”.

Example 15.1. Registering a System to the Customer Portal

[root@server1 ~]# subscription-manager register --username admin-example --password secret

The system has been registered with id: 7d133d55-876f-4f47-83eb-0ee931cb0a97
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Example 15.2. Automatically Subscribing While Registering

The register command has an option, --autosubscribe, which allows the system to be registered to the subscription service and immediately attaches the subscription which best matches the system's architecture, in a single step. 
[root@server1 ~]# subscription-manager register --username admin-example --password secret --autosubscribe
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This is the same behavior as when registering with the default settings in the Subscription Manager UI.

Example 15.3. Registering a System with Subscription Asset Manager

With Subscription Asset Managr or CloudForms System Engine, an account can have multiple, independent subdivisions called organizationst is required that you specify which organization (essentially an independent group or unit within the main account) to join the system to. This is done by using the --org option in addition to the username and password. The given user must also have the access permissions to add systems to that organization.
To register with a subscription service other than Customer Portal Subscription Management, several additional options must be used to identify the environment and organizational divisions that the system is being registered to:
  • The username and password for the user account withint the subscription service itself
  • --serverurl to give the hostname of the subscription service
  • --baseurl to give the hostname of the content delivery service (for CloudForms System Engine only)
  • --org to give the name of the organization under which to register the system
  • --environment to give the name of an environment (group) within the organization to which to add the system; this is optional, since a default environment is set for any organization
    A system can only be added to an environment during registration. 
[root@server1 ~]# subscription-manager register --username=admin-example --password=secret --org="IT Department" --environment="dev" --serverurl=sam-server.example.com

The system has been registered with id: 7d133d55-876f-4f47-83eb-0ee931cb0a97
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Note

If the system is in a multi-org environment and no organization is given, the register command returns a Remote Server error.
Expand
Table 15.2. register Options
Options Description Required
--username=name Gives the content server user account name. Required
--password=password Gives the password for the user account. Required
--serverurl=hostname Gives the hostname of the subscription service to use. The default is for Customer Portal Subcription Management, subscription.rhn.redhat.com. If this option is not used, the system is registered with Customer Portal Subscription Management. Required for Subscription Asset Manager or CloudForms System Engine
--baseurl=URL Gives the hostname of the content delivery server to use to receive updates. Both Customer Portal Subscription Management and Subscription Asset Manager use Red Hat's hosted content delivery services, with the URL https://cdn.redhat.com. Since CloudForms System Engine hosts its own content, the URL must be used for systems registered with System Engine. Required for CloudForms System Engine
--org=name Gives the organization to which to join the system. Required, except for hosted environments
--environment=name Registers the system to an environment within an organization. Optional
--name=machine_name Sets the name of the system to register. This defaults to be the same as the hostname. Optional
--autosubscribe Automatically ataches the best-matched compatible subscription. This is good for automated setup operations, since the system can be configured in a single step. Optional
--activationkey=key Attaches existing subscriptions as part of the registration process. The subscriptions are pre-assigned by a vendor or by a systems administrator using Subscription Asset Manager. Optional
--servicelevel=None|Standard|Premium Sets the service level to use for subscriptions on that machine. This is only used with the --autosubscribe option. Optional
--release=NUMBER Sets the operating system minor release to use for subscriptions for the system. Products and updates are limited to that specific minor release version. This is used only used with the --autosubscribe option. Optional
--force Registers the system even if it is already registered. Normally, any register operations will fail if the machine is already registered. Optional

15.2.3. Unregistering
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The only thing required to unregister a machine is to run the unregister command. This removes the system's entry from the subscription service, removes any subscriptions, and, locally, deletes its identity and subscription certificates.
From the command line, this requires only the unregister command.

Example 15.4. Unregistering a System

[root@server1 ~]# subscription-manager unregister
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To unregister from the Subscription Manager GUI:
  1. Open the Subscription Manager UI. 
    [root@server ~]# subscription-manager-gui
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  2. Open the System menu, and select the Unregister item.
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  3. Confirm that the system should be unregistered.

15.3. Attaching and Removing Subscriptions
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Assigning a subscription to a system gives the system the ability to install and update any Red Hat product in that subscription. A subscription is a list of all of the products, in all variations, that were purchased at one time, and it defines both the products and the number of times that subscription can be used. When one of those licenses is associated with a system, that subscription is attached to the system.
15.3.1.1. Attaching a Subscription
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  1. Launch Subscription Manager. For example: 
    [root@server ~]# subscription-manager-gui
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  2. Open the All Available Subscriptions tab.
  3. Optionally, set the date range and click the Filters button to set the filters to use to search for available subscriptions.
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    Subscriptions can be filtered by their active date and by their name. The checkboxes provide more fine-grained filtering:
    • match my system shows only subscriptions which match the system architecture.
    • match my installed products shows subscriptions which work with currently installed products on the system.
    • have no overlap with existing subscriptions excludes subscriptions with duplicate products. If a subscription is already attached to the system for a specific product or if multiple subscriptions supply the same product, then the subscription service filters those subscriptions and shows only the best fit.
    • contain the text searches for strings, such as the product name, within the subscription or pool.
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    After setting the date and filters, click the Update button to apply them.
  4. Select one of the available subscriptions.
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  5. Click the Subscribe button.
15.3.1.2. Removing Subscriptions
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  1. Launch Subscription Manager. For example: 
    [root@server ~]# subscription-manager-gui
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  2. Open the My Subscriptions tab.
    All of the active subscriptions to which the system is currently attached are listed. (The products available through the subscription may or may not be installed.)
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  3. Select the subscription to remove.
  4. Click the Unsubscribe button in the bottom right of the window.
15.3.2.1. Attaching Subscriptions
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Attaching subscriptions to a system requires specifying the individual product or subscription to attach, using the --pool option. 
[root@server1 ~]# subscription-manager subscribe --pool=XYZ01234567
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The options for the subscribe command are listed in Table 15.3, “subscribe Options”.
The ID of the subscription pool for the purchased product must be specified. The pool ID is listed with the product subscription information, which is available from running the list command: 
[root@server1 ~]# subscription-manager list --available

+-------------------------------------------+
    Available Subscriptions
+-------------------------------------------+
ProductName:            RHEL for Physical Servers
ProductId:              MKT-rhel-server
PoolId:                 ff8080812bc382e3012bc3845ca000cb
Quantity:               10
Expires:                2011-09-20
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Alternatively,the best-fitting subscriptions, as identified by the subscription service, can be attached to the system by using the --auto option (which is analogous to the --autosubscribe option with the register command). 
[root@server1 ~]# subscription-manager subscribe --auto
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Expand
Table 15.3. subscribe Options
Options Description Required
--pool=pool-id Gives the ID for the subscription to attach to the system. Required, unless --auto is used
--auto Automatically attaches the system to the best-match subscription or subscriptions. Optional
--quantity=number Attaches multiple counts of a subscription to the system. This is used to cover subscriptions that define a count limit, like using two 2-socket server subscriptions to cover a 4-socket machine. Optional
--servicelevel=None|Standard|Premium Sets the service level to use for subscriptions on that machine. This is only used with the --auto option. Optional
A system can be attached to multiple subscriptions and products. Similarly, a single subscription or all subscriptions can be removed from the system.
Running the unsubscribe command with the --all option removes every product subscription and subscription pool that is currently attached to the system. 
[root@server1 ~]# subscription-manager unsubscribe --all
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It is also possible to remove a single product subscription. Each product has an identifying X.509 certificate installed with it. The product subscription to remove is identified in the unsubscribe command by referencing the ID number of that X.509 certificate.
  1. Get the serial number for the product certificate, if you are removing a single product subscription. The serial number can be obtained from the subscription#.pem file (for example, 392729555585697907.pem) or by using the list command. For example: 
    [root@server1 ~]# subscription-manager list --consumed

+-------------------------------------------+
    Consumed Product Subscriptions
+-------------------------------------------+


ProductName:         High availability (cluster suite)
ContractNumber:      0
SerialNumber:        11287514358600162
Active:              True
Begins:              2010-09-18
Expires:             2011-11-18
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  2. Run the subscription-manager tool with the --serial option to specify the certificate. 
    [root@server1 ~]# subscription-manager unsubscribe --serial=11287514358600162
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15.4. Redeeming Vendor Subscriptions
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Systems can be set up with pre-existing subscriptions already available to that system. For some systems which were purchased through third-party vendors, a subscription to Red Hat products is included with the purchase of the machine.
Red Hat Subscription Manager pulls information about the system hardware and the BIOS into the system facts to recognize the hardware vendor. If the vendor and BIOS information matches a certain configuration, then the subscription can be redeemed, which will allow subscriptions to be automatically attached to the system.

15.4.1. Redeeming Subscriptions through the GUI
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Note

If the machine does not have any subscriptions to be redeemed, then the Redeem menu item is not there.
  1. Launch Subscription Manager. For example: 
    [root@server ~]# subscription-manager-gui
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  2. If necessary, register the system, as described in Section 15.2.1, “Registering from the GUI”.
  3. Open the System menu in the top left of the window, and click the Redeem item.
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  4. In the dialog window, enter the email address to send the notification to when the redemption is complete. Because the redemption process can take several minutes to contact the vendor and receive information about the pre-configured subscriptions, the notification message is sent through email rather than through the Subscription Manager dialog window.
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  5. Click the Redeem button.
It can take up to ten minutes for the confirmation email to arrive.

Note

The machine must be registered first so that the subscription service can properly identify the system and its subscriptions.
The machine subscriptions are redeemed by running the redeem command, with an email address to send the redemption email to when the process is complete. 
# subscription-manager redeem --email=jsmith@example.com
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A local Subscription Asset Manager can pre-configure subscriptions to use for a system, and that pre-configured set of subscriptions is identified by an activation key. That key can then be used to attach those subscriptions on a local system.
The Subscription Asset Manager activation key can be used as part of the registration process for the new system: 
# subscription-manager register --username=jsmith --password=secret --org="IT Dept" --activationkey=abcd1234
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If there are multiple organizations, it is still necessary to specify the organization for the system. That information is not defined in the activation key.

15.6. Setting Preferences for Systems
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Auto-attaching and healing (updating) subscriptions selects what subscriptions to attach to a system based on a variety of criteria, including current installed products, hardware, and architecture. It is possible to set two additional preferences for Subscription Manager to use:
  • Service levels for subscriptions
  • The operating system minor version (X.Y) to use
This is especially useful for healing, which runs daily to ensure that all installed products and current subscriptions remain active.

15.6.1. Setting Preferences in the UI
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Both a service level preference and an operating system release version preference are set in the System Preferences dialog box in Subscription Manager.
  1. Open the Subscription Manager.
  2. Open the System menu.
  3. Select the System Preferences menu item.
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  4. Select the desired service level agreement preference from the drop-down menu. Only service levels available to the Red Hat account, based on all of its active subscriptions, are listed.
  5. Select the operating system release preference in the Release version drop-down menu. The only versions listed are Red Hat Enterprise Linux versions for which the account has an active subscription.
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  6. The preferences are saved and applied to future subscription operations when they are set. To close the dialog, click Close.
A general service level preference can be set using the service-level --set command.

Example 15.5. Setting a Service Level Preference

First, list the available service levels for the system, using the --list option with the service-level command. 
[root@server ~]# subscription-manager service-level --list
+-------------------------------------------+
          Available Service Levels
+-------------------------------------------+
Standard
None
Premium
Self-Support
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Then, set the desired level for the system. 
[root@server ~]# subscription-manager service-level --set=self-support
Service level set to: self-support
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The current setting for the local system is shown with the --show option: 
[root#server ~]# subscription-manager service-level --show
Current service level: self-support
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A service level preference can be defined when a subscription operation is being run (such as registering a system or attaching subscriptions after registration). This can be used to override a system preference. Both the register and subscribe commands have the --servicelevel option to set a preference for that action.

Example 15.6. Autoattaching Subscriptions with a Premium Service Level

[root#server ~]# subscription-manager subscribe --auto --servicelevel Premium
Service level set to: Premium
Installed Product Current Status:
ProductName:            Red Hat Enterprise Linux 5 Server
Status:                 Subscribed
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Note

The --servicelevel option requires the --autosubscribe option (for register) or --auto option (for subscribe). It cannot be used when attaching a specified pool or when importing a subscription.
Many IT environments have to be certified to meet a certain level of security or other criteria. In that case, major upgrades must be carefully planned and controlled — so administrators cannot simply run yum update and move from version to version.
Setting a release version preference limits the system access to content repositories associated with that operating system version instead of automatically using the newest or latest version repositories.
For example, if the preferred operating system version is 5.9, then 5.9 content repositories will be preferred for all installed products and attached subscriptions for the system, even as other repositories become available.

Example 15.7. Setting an Operating System Release During Registration

A preference for a release version can be set when the system is registered by using --release option with the register. This applies the release preference to any subscriptions selected and auto-attached to the system at registration time.
Setting a preference requires the --autosubscribe option, because it is one of the criteria used to select subscriptions to auto-attach. 
[root#server ~]# subscription-manager register --autosubscribe --release=5.9 --username=admin@example.com...
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Note

Unlike setting a service level preference, a release preference can only be used during registration or set as a preference. It cannot be specified with the subscribe command.

Example 15.8. Setting an Operating System Release Preference

The release command can display the available operating system releases, based on the available, purchased (not only attached) subscriptions for the organization. 
[root#server ~]# subscription-manager release --list
+-------------------------------------------+
          Available Releases
+-------------------------------------------+
5.8
5.9
5Server
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The --set then sets the preference to one of the available release versions: 
[root#server ~]# subscription-manager release --set=5.9
Release version set to: 5.9
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15.6.4. Removing a Preference
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To remove a preference through the command line, use the --unset with the appropriate command. For example, to unset a release version preference: 
[root#server ~]# subscription-manager release --unset
Release version set to:
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To remove or unset a preference in the UI:
  1. Open the Subscription Manager.
  2. Open the System menu.
  3. Select the System Preferences menu item.
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  4. Set the service level or release version value to the blank line in the corresponding drop-down menu.
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  5. Click Close.
Subscriptions are active for a certain period of time, called the validity period. When a subscription is purchased, the start and end dates for the contract are set.
On a system, there can be multiple subscriptions attached. Each product requires its own subscription. Additionally, some products may require multiple quantities for it to be fully subscribed. For example, a 16 socket machine may require four 4-socket operating system subscriptions to cover the socket count.
The My Installed Software tab shows the subscription status for the entire system. It also shows a date; that is the first date that a product subscription goes from valid to invalid (meaning it expires).
Valid Until...
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Valid Until...

Figure 15.2. Valid Until...

The Red Hat Subscription Manager provides a series of log and UI messages that indicate any changes to the valid certificates of any installed products for a system. In the Subscription Manager GUI, the status of the system subscriptions is color-coded, where green means all products are fully subscribed, yellow means that some products may not be subscribed but updates are still in effect, and red means that updates are disabled.
Color-Coded Status Views
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Color-Coded Status Views

Figure 15.3. Color-Coded Status Views

The command-line tools also indicate that status of the machine. The green, yellow, and red codes translate to text status messages of subscribed, partially subscribed, and expired/not subscribed, respectively. 
[root@server ~]# subscription-manager list
+-------------------------------------------+
    Installed Product Status
+-------------------------------------------+

ProductName:            Red Hat Enterprise Linux Server
Status: Not Subscribed
Expires:                                         
SerialNumber:                                    
ContractNumber:                                  
AccountNumber:
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Whenever there is a warning about subscription changes, a small icon appears in the top menu bar, similar to a fuel gauge.
Subscription Notification Icon
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Subscription Notification Icon

Figure 15.4. Subscription Notification Icon

As any installed product nears the expiration date of the subscription, the Subscription Manager daemon will issue a warning. A similar message is given when the system has products without a valid certificate, meaning either a subscription is not atached that covers that product or the product is installed past the expiration of the subscription. Clicking the Manage My Subscriptions... button in the subscription notification window opens the Red Hat Subscription Manager GUI to view and update subscriptions.
Subscription Warning Message
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Subscription Warning Message

Figure 15.5. Subscription Warning Message

When the Subscription Manager UI opens, whether it was opened through a notification or just opened normally, there is an icon in the upper left corner that shows whether products lack a valid certificate. The easiest way to attach subscriptions which match invalidated products is to click the Autosubscribe button.
Autosubscribe Button
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Autosubscribe Button

Figure 15.6. Autosubscribe Button

The Subscribe System dialog shows a targeted list of available subscriptions that apply to the specific products that do not have valid certificates (assuming subscriptions are available).
Subscribe System
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Subscribe System

Figure 15.7. Subscribe System

Part IV. System Configuration
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Part of a system administrator's job is configuring the system for various tasks, types of users, and hardware configurations. This section explains how to configure a Red Hat Enterprise Linux system.

Chapter 31. Console Access
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When normal (non-root) users log into a computer locally, they are given two types of special permissions:
  1. They can run certain programs that they would otherwise be unable to run.
  2. They can access certain files (normally special device files used to access diskettes, CD-ROMs, and so on) that they would otherwise be unable to access.
Since there are multiple consoles on a single computer and multiple users can be logged into the computer locally at the same time, one of the users has to essentially win the race to access the files. The first user to log in at the console owns those files. Once the first user logs out, the next user who logs in owns the files.
In contrast, every user who logs in at the console is allowed to run programs that accomplish tasks normally restricted to the root user. If X is running, these actions can be included as menu items in a graphical user interface. As shipped, these console-accessible programs include halt, poweroff, and reboot.

31.1. Disabling Shutdown Via Ctrl+Alt+Del
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By default, /etc/inittab specifies that your system is set to shutdown and reboot in response to a Ctrl+Alt+Del key combination used at the console. To completely disable this ability, comment out the following line in /etc/inittab by putting a hash mark (#) in front of it: 
ca::ctrlaltdel:/sbin/shutdown -t3 -r now
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Alternatively, you may want to allow certain non-root users the right to shutdown or reboot the system from the console using Ctrl+Alt+Del . You can restrict this privilege to certain users, by taking the following steps:
  1. Add the -a option to the /etc/inittab line shown above, so that it reads: 
    ca::ctrlaltdel:/sbin/shutdown -a -t3 -r now
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    The -a flag tells shutdown to look for the /etc/shutdown.allow file.
  2. Create a file named shutdown.allow in /etc. The shutdown.allow file should list the usernames of any users who are allowed to shutdown the system using Ctrl+Alt+Del . The format of the shutdown.allow file is a list of usernames, one per line, like the following: 
    stephen
jack
sophie
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According to this example shutdown.allow file, the users stephen, jack, and sophie are allowed to shutdown the system from the console using Ctrl+Alt+Del . When that key combination is used, the shutdown -a command in /etc/inittab checks to see if any of the users in /etc/shutdown.allow (or root) are logged in on a virtual console. If one of them is, the shutdown of the system continues; if not, an error message is written to the system console instead.
For more information on shutdown.allow, refer to the shutdown man page.

31.2. Disabling Console Program Access
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To disable access by users to console programs, run the following command as root: 
rm -f /etc/security/console.apps/*
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In environments where the console is otherwise secured (BIOS and boot loader passwords are set, Ctrl+Alt+Delete is disabled, the power and reset switches are disabled, and so forth), you may not want to allow any user at the console to run poweroff, halt, and reboot, which are accessible from the console by default.
To disable these abilities, run the following commands as root: 
rm -f /etc/security/console.apps/poweroff
rm -f /etc/security/console.apps/halt
rm -f /etc/security/console.apps/reboot
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31.3. Defining the Console
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The pam_console.so module uses the /etc/security/console.perms file to determine the permissions for users at the system console. The syntax of the file is very flexible; you can edit the file so that these instructions no longer apply. However, the default file has a line that looks like this: 
<console>=tty[0-9][0-9]* vc/[0-9][0-9]* :[0-9]\.[0-9] :[0-9]
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When users log in, they are attached to some sort of named terminal, which can be either an X server with a name like :0 or mymachine.example.com:1.0, or a device like /dev/ttyS0 or /dev/pts/2. The default is to define that local virtual consoles and local X servers are considered local, but if you want to consider the serial terminal next to you on port /dev/ttyS1 to also be local, you can change that line to read: 
<console>=tty[0-9][0-9]* vc/[0-9][0-9]* :[0-9]\.[0-9] :[0-9] /dev/ttyS1
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31.4. Making Files Accessible From the Console
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The default settings for individual device classes and permission definitions are defined in /etc/security/console.perms.d/50-default.perms. To edit file and device permissions, it is advisable to create a new default file in /etc/security/console.perms.d/ containing your preferred settings for a specified set of files or devices. The name of the new default file must begin with a number higher than 50 (for example, 51-default.perms) in order to override 50-default.perms.
To do this, create a new file named 51-default.perms in /etc/security/console.perms.d/: 
touch /etc/security/console.perms.d/51-default.perms
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Open the original default perms file, 50-default.perms. The first section defines device classes, with lines similar to the following: 
<floppy>=/dev/fd[0-1]* \
          /dev/floppy/* /mnt/floppy*
<sound>=/dev/dsp* /dev/audio* /dev/midi* \ 
	  /dev/mixer* /dev/sequencer \ 
	  /dev/sound/* /dev/beep \ 
	  /dev/snd/*
<cdrom>=/dev/cdrom* /dev/cdroms/* /dev/cdwriter* /mnt/cdrom*
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Items enclosed in brackets name the device; in the above example, <cdrom> refers to the CD-ROM drive. To add a new device, do not define it in the default 50-default.perms file; instead, define it in 51-default.perms. For example, to define a scanner, add the following line to 51-default.perms: 
<scanner>=/dev/scanner /dev/usb/scanner*
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Of course, you must use the appropriate name for the device. Ensure that /dev/scanner is really your scanner and not some other device, such as your hard drive.
Once you have properly defined a device or file, the second step is to specify its permission definitions. The second section of /etc/security/console.perms.d/50-default.perms defines this, with lines similar to the following: 
<console> 0660 <floppy> 0660 root.floppy
<console> 0600 <sound>  0640 root
<console> 0600 <cdrom>  0600 root.disk
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To define permissions for a scanner, add a line similar to the following in 51-default.perms: 
<console> 0600 <scanner> 0600 root
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Then, when you log in at the console, you are given ownership of the /dev/scanner device with the permissions of 0600 (readable and writable by you only). When you log out, the device is owned by root, and still has the permissions 0600 (now readable and writable by root only).

Warning

You must never edit the default 50-default.perms file. To edit permissions for a device already defined in 50-default.perms, add the desired permission definition for that device in 51-default.perms. This will override whatever permissions are defined in 50-default.perms.
To make other applications accessible to console users, a bit more work is required.
First of all, console access only works for applications which reside in /sbin/ or /usr/sbin/, so the application that you wish to run must be there. After verifying that, perform the following steps:
  1. Create a link from the name of your application, such as our sample foo program, to the /usr/bin/consolehelper application: 
    cd /usr/bin
ln -s consolehelper foo
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  2. Create the file /etc/security/console.apps/foo: 
    touch /etc/security/console.apps/foo
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  3. Create a PAM configuration file for the foo service in /etc/pam.d/. An easy way to do this is to copy the PAM configuration file of the halt service, and then modify the copy if you want to change the behavior: 
    cp /etc/pam.d/halt /etc/pam.d/foo
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Now, when /usr/bin/foo is executed, consolehelper is called, which authenticates the user with the help of /usr/sbin/userhelper. To authenticate the user, consolehelper asks for the user's password if /etc/pam.d/foo is a copy of /etc/pam.d/halt (otherwise, it does precisely what is specified in /etc/pam.d/foo) and then runs /usr/sbin/foo with root permissions.
In the PAM configuration file, an application can be configured to use the pam_timestamp module to remember (or cache) a successful authentication attempt. When an application is started and proper authentication is provided (the root password), a timestamp file is created. By default, a successful authentication is cached for five minutes. During this time, any other application that is configured to use pam_timestamp and run from the same session is automatically authenticated for the user — the user does not have to enter the root password again.
This module is included in the pam package. To enable this feature, add the following lines to your PAM configuration file in etc/pam.d/: 
auth            include         config-util
account         include         config-util
session         include         config-util
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These lines can be copied from any of the /etc/pam.d/system-config-* configuration files. Note that these lines must be added below any other auth sufficient session optional lines in your PAM configuration file.
If an application configured to use pam_timestamp is successfully authenticated from the Applications (the main menu on the panel), the icon is displayed in the notification area of the panel if you are running the GNOME or KDE desktop environment. After the authentication expires (the default is five minutes), the icon disappears.
The user can select to forget the cached authentication by clicking on the icon and selecting the option to forget authentication.

31.6. The floppy Group
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If, for whatever reason, console access is not appropriate for you and your non-root users require access to your system's diskette drive, this can be done using the floppy group. Add the user(s) to the floppy group using the tool of your choice. For example, the gpasswd command can be used to add user fred to the floppy group: 
gpasswd -a fred floppy
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Now, user fred is able to access the system's diskette drive from the console.

Chapter 32. The sysconfig Directory
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The /etc/sysconfig/ directory contains a variety of system configuration files for Red Hat Enterprise Linux.
This chapter outlines some of the files found in the /etc/sysconfig/ directory, their function, and their contents. The information in this chapter is not intended to be complete, as many of these files have a variety of options that are only used in very specific or rare circumstances.

32.1. Files in the /etc/sysconfig/ Directory
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The following sections offer descriptions of files normally found in the /etc/sysconfig/ directory. Files not listed here, as well as extra file options, are found in the /usr/share/doc/initscripts-<version-number>/sysconfig.txt file (replace <version-number> with the version of the initscripts package). Alternatively, looking through the initscripts in the /etc/rc.d/ directory can prove helpful.

Note

If some of the files listed here are not present in the /etc/sysconfig/ directory, then the corresponding program may not be installed.

32.1.1. /etc/sysconfig/amd
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The /etc/sysconfig/amd file contains various parameters used by amd; these parameters allow for the automatic mounting and unmounting of file systems.

32.1.2. /etc/sysconfig/apmd
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The /etc/sysconfig/apmd file is used by apmd to configure what power settings to start/stop/change on suspend or resume. This file configures how apmd functions at boot time, depending on whether the hardware supports Advanced Power Management (APM) or whether the user has configured the system to use it. The apm daemon is a monitoring program that works with power management code within the Linux kernel. It is capable of alerting users to low battery power on laptops and other power-related settings.

32.1.3. /etc/sysconfig/arpwatch
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The /etc/sysconfig/arpwatch file is used to pass arguments to the arpwatch daemon at boot time. The arpwatch daemon maintains a table of Ethernet MAC addresses and their IP address pairings. By default, this file sets the owner of the arpwatch process to the user pcap and sends any messages to the root mail queue. For more information regarding available parameters for this file, refer to the arpwatch man page.

32.1.4. /etc/sysconfig/authconfig
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The /etc/sysconfig/authconfig file sets the authorization to be used on the host. It contains one or more of the following lines:
  • USEMD5=<value>, where <value> is one of the following:
    • yes — MD5 is used for authentication.
    • no — MD5 is not used for authentication.
  • USEKERBEROS=<value>, where <value> is one of the following:
    • yes — Kerberos is used for authentication.
    • no — Kerberos is not used for authentication.
  • USELDAPAUTH=<value>, where <value> is one of the following:
    • yes — LDAP is used for authentication.
    • no — LDAP is not used for authentication.

32.1.5. /etc/sysconfig/autofs
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The /etc/sysconfig/autofs file defines custom options for the automatic mounting of devices. This file controls the operation of the automount daemons, which automatically mount file systems when you use them and unmount them after a period of inactivity. File systems can include network file systems, CD-ROMs, diskettes, and other media.
The /etc/sysconfig/autofs file may contain the following:
  • LOCALOPTIONS="<value>", where <value> is a string for defining machine-specific automount rules. The default value is an empty string ("").
  • DAEMONOPTIONS="<value>", where <value> is the timeout length in seconds before unmounting the device. The default value is 60 seconds ("--timeout=60").
  • UNDERSCORETODOT=<value>, where <value> is a binary value that controls whether to convert underscores in file names into dots. For example, auto_home to auto.home and auto_mnt to auto.mnt. The default value is 1 (true).
  • DISABLE_DIRECT=<value>, where <value> is a binary value that controls whether to disable direct mount support, as the Linux implementation does not conform to the Sun Microsystems' automounter behavior. The default value is 1 (true), and allows for compatibility with the Sun automounter options specification syntax.

32.1.6. /etc/sysconfig/clock
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The /etc/sysconfig/clock file controls the interpretation of values read from the system hardware clock.
The correct values are:
  • UTC=<value>, where <value> is one of the following boolean values:
    • true or yes — The hardware clock is set to Universal Time.
    • false or no — The hardware clock is set to local time.
  • ARC=<value>, where <value> is the following:
    • false or no — This value indicates that the normal UNIX epoch is in use. Other values are used by systems not supported by Red Hat Enterprise Linux.
  • SRM=<value>, where <value> is the following:
    • false or no — This value indicates that the normal UNIX epoch is in use. Other values are used by systems not supported by Red Hat Enterprise Linux.
  • ZONE=<filename> — The time zone file under /usr/share/zoneinfo that /etc/localtime is a copy of. The file contains information such as: 
    ZONE="America/New York"
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    Note that the ZONE parameter is read by the Time and Date Properties Tool (system-config-date), and manually editing it does not change the system timezone.
Earlier releases of Red Hat Enterprise Linux used the following values (which are deprecated):
  • CLOCKMODE=<value>, where <value> is one of the following:
    • GMT — The clock is set to Universal Time (Greenwich Mean Time).
    • ARC — The ARC console's 42-year time offset is in effect (for Alpha-based systems only).

32.1.7. /etc/sysconfig/desktop
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The /etc/sysconfig/desktop file specifies the desktop for new users and the display manager to run when entering runlevel 5.
Correct values are:
  • DESKTOP="<value>", where "<value>" is one of the following:
    • GNOME — Selects the GNOME desktop environment.
    • KDE — Selects the KDE desktop environment.
  • DISPLAYMANAGER="<value>", where "<value>" is one of the following:
    • GNOME — Selects the GNOME Display Manager.
    • KDE — Selects the KDE Display Manager.
    • XDM — Selects the X Display Manager.
For more information, refer to Chapter 35, The X Window System.

32.1.8. /etc/sysconfig/dhcpd
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The /etc/sysconfig/dhcpd file is used to pass arguments to the dhcpd daemon at boot time. The dhcpd daemon implements the Dynamic Host Configuration Protocol (DHCP) and the Internet Bootstrap Protocol (BOOTP). DHCP and BOOTP assign hostnames to machines on the network. For more information about what parameters are available in this file, refer to the dhcpd man page.

32.1.9. /etc/sysconfig/exim
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The /etc/sysconfig/exim file allows messages to be sent to one or more clients, routing the messages over whatever networks are necessary. The file sets the default values for exim to run. Its default values are set to run as a background daemon and to check its queue each hour in case something has backed up.
The values include:
  • DAEMON=<value>, where <value> is one of the following:
    • yes — exim should be configured to listen to port 25 for incoming mail. yes implies the use of the Exim's -bd options.
    • no — exim should not be configured to listen to port 25 for incoming mail.
  • QUEUE=1h which is given to exim as -q$QUEUE. The -q option is not given to exim if /etc/sysconfig/exim exists and QUEUE is empty or undefined.

32.1.10. /etc/sysconfig/firstboot
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The first time the system boots, the /sbin/init program calls the etc/rc.d/init.d/firstboot script, which in turn launches the Setup Agent. This application allows the user to install the latest updates as well as additional applications and documentation.
The /etc/sysconfig/firstboot file tells the Setup Agent application not to run on subsequent reboots. To run it the next time the system boots, remove /etc/sysconfig/firstboot and execute chkconfig --level 5 firstboot on.

32.1.11. /etc/sysconfig/gpm
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The /etc/sysconfig/gpm file is used to pass arguments to the gpm daemon at boot time. The gpm daemon is the mouse server which allows mouse acceleration and middle-click pasting. For more information about what parameters are available for this file, refer to the gpm man page. By default, the DEVICE directive is set to /dev/input/mice.

32.1.12. /etc/sysconfig/hwconf
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The /etc/sysconfig/hwconf file lists all the hardware that kudzu detected on the system, as well as the drivers used, vendor ID, and device ID information. The kudzu program detects and configures new and/or changed hardware on a system. The /etc/sysconfig/hwconf file is not meant to be manually edited. If edited, devices could suddenly show up as being added or removed.

32.1.13. /etc/sysconfig/i18n
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The /etc/sysconfig/i18n file sets the default language, any supported languages, and the default system font. For example: 
LANG="en_US.UTF-8"
SUPPORTED="en_US.UTF-8:en_US:en"
SYSFONT="latarcyrheb-sun16"
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32.1.14. /etc/sysconfig/init
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The /etc/sysconfig/init file controls how the system appears and functions during the boot process.
The following values may be used:
  • BOOTUP=<value>, where <value> is one of the following:
    • color — The standard color boot display, where the success or failure of devices and services starting up is shown in different colors.
    • verbose — An old style display which provides more information than purely a message of success or failure.
    • Anything else means a new display, but without ANSI-formatting.
  • RES_COL=<value>, where <value> is the number of the column of the screen to start status labels. The default is set to 60.
  • MOVE_TO_COL=<value>, where <value> moves the cursor to the value in the RES_COL line via the echo -en command.
  • SETCOLOR_SUCCESS=<value>, where <value> sets the success color via the echo -en command. The default color is set to green.
  • SETCOLOR_FAILURE=<value>, where <value> sets the failure color via the echo -en command. The default color is set to red.
  • SETCOLOR_WARNING=<value>, where <value> sets the warning color via the echo -en command. The default color is set to yellow.
  • SETCOLOR_NORMAL=<value>, where <value> resets the color to "normal" via the echo -en.
  • LOGLEVEL=<value>, where <value> sets the initial console logging level for the kernel. The default is 3; 8 means everything (including debugging), while 1 means only kernel panics. The syslogd daemon overrides this setting once started.
  • PROMPT=<value>, where <value> is one of the following boolean values:
    • yes — Enables the key check for interactive mode.
    • no — Disables the key check for interactive mode.

32.1.15. /etc/sysconfig/ip6tables-config
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The /etc/sysconfig/ip6tables-config file stores information used by the kernel to set up IPv6 packet filtering at boot time or whenever the ip6tables service is started.
Do not modify this file by hand unless familiar with how to construct ip6tables rules. Rules also can be created manually using the /sbin/ip6tables command. Once created, add the rules to the /etc/sysconfig/ip6tables file by typing the following command: 
service ip6tables save
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Once this file exists, any firewall rules saved in it persists through a system reboot or a service restart.
For more information on ip6tables, refer to Section 48.9, “IPTables”.

32.1.16. /etc/sysconfig/iptables-config
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The /etc/sysconfig/iptables-config file stores information used by the kernel to set up packet filtering services at boot time or whenever the service is started.
Do not modify this file by hand unless you are familiar with constructing iptables rules. The easiest way to add rules is to use the Security Level Configuration Tool (system-config-securitylevel) application to create a firewall. These applications automatically edit this file at the end of the process.
Rules can also be created manually using the /sbin/iptables command. Once created, add the rule(s) to the /etc/sysconfig/iptables file by typing the following command: 
service iptables save
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Once this file exists, any firewall rules saved in it persists through a system reboot or a service restart.
For more information on iptables, refer to Section 48.9, “IPTables”.

32.1.17. /etc/sysconfig/irda
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The /etc/sysconfig/irda file controls how infrared devices on the system are configured at startup.
The following values may be used:
  • IRDA=<value>, where <value> is one of the following boolean values:
    • yes — irattach runs and periodically checks to see if anything is trying to connect to the infrared port, such as another notebook computer trying to make a network connection. For infrared devices to work on the system, this line must be set to yes.
    • no — irattach does not run, preventing infrared device communication.
  • DEVICE=<value>, where <value> is the device (usually a serial port) that handles infrared connections. A sample serial device entry could be /dev/ttyS2.
  • DONGLE=<value>, where <value> specifies the type of dongle being used for infrared communication. This setting exists for people who use serial dongles rather than real infrared ports. A dongle is a device that is attached to a traditional serial port to communicate via infrared. This line is commented out by default because notebooks with real infrared ports are far more common than computers with add-on dongles. A sample dongle entry could be actisys+.
  • DISCOVERY=<value>, where <value> is one of the following boolean values:
    • yes — Starts irattach in discovery mode, meaning it actively checks for other infrared devices. This must be turned on for the machine to actively look for an infrared connection (meaning the peer that does not initiate the connection).
    • no — Does not start irattach in discovery mode.

32.1.18. /etc/sysconfig/kernel
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The /etc/sysconfig/kernel configuration file controls the kernel selection at boot. It has two options with the following default values:
UPDATEDEFAULT=yes
This option makes a newly installed kernel as the default in the boot entry selection.
DEFAULTKERNEL=kernel
This option specifies what package type will be used as the default.
To keep an old kernel version as the default in the boot entry selection:
  • Comment out the UPDATEDEFAULT option in /etc/sysconfig/kernel as follows: 
                                    # UPDATEDEFAULT=yes
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To set kernel debugger as the default kernel in boot entry selection:
  • Edit the /etc/sysconfig/kernel configuration file as follows: 
                                    DEFAULTKERNEL=kernel-debug
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32.1.19. /etc/sysconfig/keyboard
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The /etc/sysconfig/keyboard file controls the behavior of the keyboard. The following values may be used:
  • KEYBOARDTYPE="sun|pc" where sun means a Sun keyboard is attached on /dev/kbd, or pc means a PS/2 keyboard connected to a PS/2 port.
  • KEYTABLE="<file>", where <file> is the name of a keytable file.
    For example: KEYTABLE="us". The files that can be used as keytables start in /lib/kbd/keymaps/i386 and branch into different keyboard layouts from there, all labeled <file>.kmap.gz. The first file found beneath /lib/kbd/keymaps/i386 that matches the KEYTABLE setting is used.

32.1.20. /etc/sysconfig/kudzu
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The /etc/sysconfig/kuzdu file triggers a safe probe of the system hardware by kudzu at boot time. A safe probe is one that disables serial port probing.
  • SAFE=<value>, where <value> is one of the following:
    • yes — kuzdu does a safe probe.
    • no — kuzdu does a normal probe.

32.1.21. /etc/sysconfig/named
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The /etc/sysconfig/named file is used to pass arguments to the named daemon at boot time. The named daemon is a Domain Name System (DNS) server which implements the Berkeley Internet Name Domain (BIND) version 9 distribution. This server maintains a table of which hostnames are associated with IP addresses on the network.
Currently, only the following values may be used:
  • ROOTDIR="</some/where>", where </some/where> refers to the full directory path of a configured chroot environment under which named runs. This chroot environment must first be configured. Type info chroot for more information.
  • OPTIONS="<value>", where <value> is any option listed in the man page for named except -t. In place of -t, use the ROOTDIR line above.
For more information about available parameters for this file, refer to the named man page. For detailed information on how to configure a BIND DNS server, refer to Chapter 19, Berkeley Internet Name Domain (BIND). By default, the file contains no parameters.

32.1.22. /etc/sysconfig/network
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The /etc/sysconfig/network file is used to specify information about the desired network configuration. The following values may be used:
  • NETWORKING=<value>, where <value> is one of the following boolean values:
    • yes — Networking should be configured.
    • no — Networking should not be configured.
  • HOSTNAME=<value>, where <value> should be the Fully Qualified Domain Name (FQDN), such as hostname.expample.com, but can be whatever hostname is necessary.
  • GATEWAY=<value>, where <value> is the IP address of the network's gateway.
  • GATEWAYDEV=<value>, where <value> is the gateway device, such as eth0. Configure this option if you have multiple interfaces on the same subnet, and require one of those interfaces to be the preferred route to the default gateway.
  • NISDOMAIN=<value>, where <value> is the NIS domain name.
  • NOZEROCONF=<value>, where setting <value> to true disables the zeroconf route.
    By default, the zeroconf route (169.254.0.0) is enabled when the system boots. For more information about zeroconf, refer to http://www.zeroconf.org/.

Warning

Do not use custom initscripts to configure network settings. When performing a post-boot network service restart, custom initscripts configuring network settings that are run outside of the network init script lead to unpredictable results.

32.1.23. /etc/sysconfig/nfs
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NFS requires portmap, which dynamically assigns ports for RPC services. This causes problems for configuring firewall rules. To overcome this problem, use the /etc/sysconfig/nfs file to control which ports the required RPC services run on.
The /etc/sysconfig/nfs may not exist by default on all systems. If it does not exist, create it and add the following variables (alternatively, if the file exists, un-comment and change the default entries as required):
MOUNTD_PORT=x
control which TCP and UDP port mountd (rpc.mountd) uses. Replace x with an unused port number.
STATD_PORT=x
control which TCP and UDP port status (rpc.statd) uses. Replace x with an unused port number.
LOCKD_TCPPORT=x
control which TCP port nlockmgr (rpc.lockd) uses. Replace x with an unused port number.
LOCKD_UDPPORT=x
control which UDP port nlockmgr (rpc.lockd) uses. Replace x with an unused port number.
If NFS fails to start, check /var/log/messages. Normally, NFS will fail to start if you specify a port number that is already in use. After editing /etc/sysconfig/nfs restart the NFS service by running the service nfs restart command. Run the rpcinfo -p command to confirm the changes.
To configure a firewall to allow NFS:
  1. Allow TCP and UDP port 2049 for NFS.
  2. Allow TCP and UDP port 111 (portmap/sunrpc).
  3. Allow the TCP and UDP port specified with MOUNTD_PORT="x"
  4. Allow the TCP and UDP port specified with STATD_PORT="x"
  5. Allow the TCP port specified with LOCKD_TCPPORT="x"
  6. Allow the UDP port specified with LOCKD_UDPPORT="x"

32.1.24. /etc/sysconfig/ntpd
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The /etc/sysconfig/ntpd file is used to pass arguments to the ntpd daemon at boot time. The ntpd daemon sets and maintains the system clock to synchronize with an Internet standard time server. It implements version 4 of the Network Time Protocol (NTP). For more information about what parameters are available for this file, use a Web browser to view the following file: /usr/share/doc/ntp-<version>/ntpd.htm (where <version> is the version number of ntpd). By default, this file sets the owner of the ntpd process to the user ntp.

32.1.25. /etc/sysconfig/radvd
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The /etc/sysconfig/radvd file is used to pass arguments to the radvd daemon at boot time. The radvd daemon listens for router requests and sends router advertisements for the IP version 6 protocol. This service allows hosts on a network to dynamically change their default routers based on these router advertisements. For more information about available parameters for this file, refer to the radvd man page. By default, this file sets the owner of the radvd process to the user radvd.

32.1.26. /etc/sysconfig/samba
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The /etc/sysconfig/samba file is used to pass arguments to the smbd and the nmbd daemons at boot time. The smbd daemon offers file sharing connectivity for Windows clients on the network. The nmbd daemon offers NetBIOS over IP naming services. For more information about what parameters are available for this file, refer to the smbd man page. By default, this file sets smbd and nmbd to run in daemon mode.

32.1.27. /etc/sysconfig/selinux
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The /etc/sysconfig/selinux file contains the basic configuration options for SELinux. This file is a symbolic link to /etc/selinux/config.

32.1.28. /etc/sysconfig/sendmail
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The /etc/sysconfig/sendmail file allows messages to be sent to one or more clients, routing the messages over whatever networks are necessary. The file sets the default values for the Sendmail application to run. Its default values are set to run as a background daemon and to check its queue each hour in case something has backed up.
Values include:
  • DAEMON=<value>, where <value> is one of the following:
    • yes — Sendmail should be configured to listen to port 25 for incoming mail. yes implies the use of Sendmail's -bd options.
    • no — Sendmail should not be configured to listen to port 25 for incoming mail.
  • QUEUE=1h which is given to Sendmail as -q$QUEUE. The -q option is not given to Sendmail if /etc/sysconfig/sendmail exists and QUEUE is empty or undefined.

32.1.29. /etc/sysconfig/spamassassin
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The /etc/sysconfig/spamassassin file is used to pass arguments to the spamd daemon (a daemonized version of Spamassassin) at boot time. Spamassassin is an email spam filter application. For a list of available options, refer to the spamd man page. By default, it configures spamd to run in daemon mode, create user preferences, and auto-create whitelists (allowed bulk senders).
For more information about Spamassassin, refer to Section 27.5.2.6, “Spam Filters”.

32.1.30. /etc/sysconfig/squid
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The /etc/sysconfig/squid file is used to pass arguments to the squid daemon at boot time. The squid daemon is a proxy caching server for Web client applications. For more information on configuring a squid proxy server, use a Web browser to open the /usr/share/doc/squid-<version>/ directory (replace <version> with the squid version number installed on the system). By default, this file sets squid to start in daemon mode and sets the amount of time before it shuts itself down.
The /etc/sysconfig/system-config-securitylevel file contains all options chosen by the user the last time the Security Level Configuration Tool (system-config-securitylevel) was run. Users should not modify this file by hand. For more information about the Security Level Configuration Tool, refer to Section 48.8.2, “Basic Firewall Configuration”.

32.1.32. /etc/sysconfig/system-config-selinux
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The /etc/sysconfig/system-config-selinux file contains all options chosen by the user the last time the SELinux Administration Tool (system-config-selinux) was run. Users should not modify this file by hand. For more information about the SELinux Administration Tool and SELinux in general, refer to Section 49.2, “Introduction to SELinux”.

32.1.33. /etc/sysconfig/system-config-users
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The /etc/sysconfig/system-config-users file is the configuration file for the graphical application, User Manager. This file is used to filter out system users such as root, daemon, or lp. This file is edited by the Preferences > Filter system users and groups pull-down menu in the User Manager application and should never be edited by hand. For more information on using this application, refer to Section 37.1, “User and Group Configuration”.

32.1.34. /etc/sysconfig/system-logviewer
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The /etc/sysconfig/system-logviewer file is the configuration file for the graphical, interactive log viewing application, Log Viewer. This file is edited by the Edit > Preferences pull-down menu in the Log Viewer application and should not be edited by hand. For more information on using this application, refer to Chapter 40, Log Files.

32.1.35. /etc/sysconfig/tux
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The /etc/sysconfig/tux file is the configuration file for the Red Hat Content Accelerator (formerly known as TUX), the kernel-based Web server. For more information on configuring the Red Hat Content Accelerator, use a Web browser to open the /usr/share/doc/tux-<version>/tux/index.html file (replace <version> with the version number of TUX installed on the system). The parameters available for this file are listed in /usr/share/doc/tux-<version>/tux/parameters.html.

32.1.36. /etc/sysconfig/vncservers
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The /etc/sysconfig/vncservers file configures the way the Virtual Network Computing (VNC) server starts up.
VNC is a remote display system which allows users to view the desktop environment not only on the machine where it is running but across different networks on a variety of architectures.
It may contain the following:
  • VNCSERVERS=<value>, where <value> is set to something like "1:fred", to indicate that a VNC server should be started for user fred on display :1. User fred must have set a VNC password using the vncpasswd command before attempting to connect to the remote VNC server.

32.1.37. /etc/sysconfig/xinetd
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The /etc/sysconfig/xinetd file is used to pass arguments to the xinetd daemon at boot time. The xinetd daemon starts programs that provide Internet services when a request to the port for that service is received. For more information about available parameters for this file, refer to the xinetd man page. For more information on the xinetd service, refer to Section 48.5.3, “xinetd”.

32.2. Directories in the /etc/sysconfig/ Directory
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The following directories are normally found in /etc/sysconfig/.
apm-scripts/
This directory contains the APM suspend/resume script. Do not edit the files directly. If customization is necessary, create a file called /etc/sysconfig/apm-scripts/apmcontinue which is called at the end of the script. It is also possible to control the script by editing /etc/sysconfig/apmd.
cbq/
This directory contains the configuration files needed to do Class Based Queuing for bandwidth management on network interfaces. CBQ divides user traffic into a hierarchy of classes based on any combination of IP addresses, protocols, and application types.
networking/
This directory is used by the Network Administration Tool (system-config-network), and its contents should not be edited manually. For more information about configuring network interfaces using the Network Administration Tool, refer to Chapter 17, Network Configuration.
network-scripts/
This directory contains the following network-related configuration files:
  • Network configuration files for each configured network interface, such as ifcfg-eth0 for the eth0 Ethernet interface.
  • Scripts used to bring network interfaces up and down, such as ifup and ifdown.
  • Scripts used to bring ISDN interfaces up and down, such as ifup-isdn and ifdown-isdn.
  • Various shared network function scripts which should not be edited directly.
For more information on the network-scripts directory, refer to Chapter 16, Network Interfaces.
rhn/
Deprecated. This directory contains the configuration files and GPG keys used by the RHN Classic content service. No files in this directory should be edited by hand.
This directory is available for legacy systems which are still managed by RHN Classic. Systems which are registered against the Certificate-Based Red Hat Network do not use this directory.

32.3. Additional Resources
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This chapter is only intended as an introduction to the files in the /etc/sysconfig/ directory. The following source contains more comprehensive information.

32.3.1. Installed Documentation
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  • /usr/share/doc/initscripts-<version-number>/sysconfig.txt — This file contains a more authoritative listing of the files found in the /etc/sysconfig/ directory and the configuration options available for them. The <version-number> in the path to this file corresponds to the version of the initscripts package installed.

Chapter 33. Date and Time Configuration
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The Time and Date Properties Tool allows the user to change the system date and time, to configure the time zone used by the system, and to setup the Network Time Protocol (NTP) daemon to synchronize the system clock with a time server.
You must be running the X Window System and have root privileges to use the tool. There are three ways to start the application:
  • From the desktop, go to Applications (the main menu on the panel) > System Settings > Date & Time
  • From the desktop, right-click on the time in the toolbar and select Adjust Date and Time.
  • Type the command system-config-date, system-config-time, or dateconfig at a shell prompt (for example, in an XTerm or a GNOME terminal).

33.1. Time and Date Properties
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As shown in Figure 33.1, “Time and Date Properties”, the first tabbed window that appears is for configuring the system date and time.
Time and Date Properties
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Time and Date Properties

Figure 33.1. Time and Date Properties

To change the date, use the arrows to the left and right of the month to change the month, use the arrows to the left and right of the year to change the year, and click on the day of the week to change the day of the week.
To change the time, use the up and down arrow buttons beside the Hour, Minute, and Second in the Time section.
Clicking the OK button applies any changes made to the date and time, the NTP daemon settings, and the time zone settings. It also exits the program.

33.2. Network Time Protocol (NTP) Properties
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As shown in Figure 33.2, “NTP Properties”, the second tabbed window that appears is for configuring NTP.
NTP Properties
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NTP Properties

Figure 33.2. NTP Properties

The Network Time Protocol (NTP) daemon synchronizes the system clock with a remote time server or time source. The application allows you to configure an NTP daemon to synchronize your system clock with a remote server. To enable this feature, select Enable Network Time Protocol. This enables the NTP Servers list and other options. You can choose one of the predefined servers, edit a predefined server by clicking the Edit or add a new server name by clicking Add. Your system does not start synchronizing with the NTP server until you click OK. After clicking OK, the configuration is saved and the NTP daemon is started (or restarted if it is already running).
Clicking the OK button applies any changes made to the date and time, the NTP daemon settings, and the time zone settings. It also exits the program.

33.3. Time Zone Configuration
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The third tabbed window that appears is for configuring the system time zone.
To configure the system time zone, click the Time Zone tab. The time zone can be changed by either using the interactive map or by choosing the desired time zone from the list below the map. To use the map, click on the desired region. The map zooms into the region selected, after which you may choose the city specific to your time zone. A red X appears and the time zone selection changes in the list below the map.
Alternatively, you can also use the list below the map. In the same way that the map lets you choose a region before choosing a city, the list of time zones is now a treelist, with cities and countries grouped within their specific continents. Non-geographic time zones have also been added to address needs in the scientific community.
Click OK to apply the changes and exit the program.
If your system clock is set to use UTC, select the System clock uses UTC option. UTC stands for the Universal Time, Coordinated, also known as Greenwich Mean Time (GMT). Other time zones are determined by adding or subtracting from the UTC time.

Chapter 34. Keyboard Configuration
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The installation program allows you to configure a keyboard layout for your system. To configure a different keyboard layout after installation, use the Keyboard Configuration Tool.
To start the Keyboard Configuration Tool, select System (on the panel) > Administration > Keyboard, or type the command system-config-keyboard at a shell prompt.
Keyboard Configuration Tool
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Keyboard Configuration Tool

Figure 34.1. Keyboard Configuration Tool

Select a keyboard layout from the list (for example, U.S. English) and click OK.
Changes take effect immediately.

Chapter 35. The X Window System
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While the heart of Red Hat Enterprise Linux is the kernel, for many users, the face of the operating system is the graphical environment provided by the X Window System, also called X.
Other windowing environments have existed in the UNIX world, including some that predate the release of the X Window System in June 1984. Nonetheless, X has been the default graphical environment for most UNIX-like operating systems, including Red Hat Enterprise Linux, for many years.
The graphical environment for Red Hat Enterprise Linux is supplied by the X.Org Foundation, an open source organization created to manage development and strategy for the X Window System and related technologies. X.Org is a large-scale, rapidly developing project with hundreds of developers around the world. It features a wide degree of support for a variety of hardware devices and architectures, and can run on a variety of different operating systems and platforms. This release for Red Hat Enterprise Linux specifically includes the X11R7.1 release of the X Window System.
The X Window System uses a client-server architecture. The X server (the Xorg binary) listens for connections from X client applications via a network or local loopback interface. The server communicates with the hardware, such as the video card, monitor, keyboard, and mouse. X client applications exist in the user-space, creating a graphical user interface (GUI) for the user and passing user requests to the X server.

35.1. The X11R7.1 Release
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Red Hat Enterprise Linux 5.10 now uses the X11R7.1 release as the base X Window System, which includes several video driver, EXA, and platform support enhancements over the previous release, among others. In addition, this release also includes several automatic configuration features for the X server.
X11R7.1 is the first release to take specific advantage of the modularization of the X Window System. This modularization, which splits X into logically distinct modules, makes it easier for open source developers to contribute code to the system.

Important

Red Hat Enterprise Linux no longer provides the XFree86™ server packages. Before upgrading a system to the latest version of Red Hat Enterprise Linux, be sure that the system's video card is compatible with the X11R7.1 release by checking the Red Hat Hardware Compatibility List located online at http://hardware.redhat.com/.
In the X11R7.1 release, all libraries, headers, and binaries now live under /usr/ instead of /usr/X11R6. The /etc/X11/ directory contains configuration files for X client and server applications. This includes configuration files for the X server itself, the xfs font server, the X display managers, and many other base components.
The configuration file for the newer Fontconfig-based font architecture is still /etc/fonts/fonts.conf. For more on configuring and adding fonts, refer to Section 35.4, “Fonts”.
Because the X server performs advanced tasks on a wide array of hardware, it requires detailed information about the hardware it works on. The X server automatically detects some of this information; other details must be configured.
The installation program installs and configures X automatically, unless the X11R7.1 release packages are not selected for installation. However, if there are any changes to the monitor, video card or other devices managed by the X server, X must be reconfigured. The best way to do this is to use the X Configuration Tool (system-config-display), particularly for devices that are not detected manually.
In Red Hat Enterprise Linux's default graphical environment, the X Configuration Tool is available at System (on the panel) > Administration > Display.
Changes made with the X Configuration Tool take effect after logging out and logging back in.
For more information about X Configuration Tool, refer to Chapter 36, X Window System Configuration.
In some situations, reconfiguring the X server may require manually editing its configuration file, /etc/X11/xorg.conf. For information about the structure of this file, refer to Section 35.3, “X Server Configuration Files”.

35.2. Desktop Environments and Window Managers
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Once an X server is running, X client applications can connect to it and create a GUI for the user. A range of GUIs are possible with Red Hat Enterprise Linux, from the rudimentary Tab Window Manager to the highly developed and interactive GNOME desktop environment that most Red Hat Enterprise Linux users are familiar with.
To create the latter, more comprehensive GUI, two main classes of X client application must connect to the X server: a desktop environment and a window manager.

35.2.1. Desktop Environments
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A desktop environment integrates various X clients to create a common graphical user environment and development platform.
Desktop environments have advanced features allowing X clients and other running processes to communicate with one another, while also allowing all applications written to work in that environment to perform advanced tasks, such as drag and drop operations.
Red Hat Enterprise Linux provides two desktop environments:
  • GNOME — The default desktop environment for Red Hat Enterprise Linux based on the GTK+ 2 graphical toolkit.
  • KDE — An alternative desktop environment based on the Qt 3 graphical toolkit.
Both GNOME and KDE have advanced productivity applications, such as word processors, spreadsheets, and Web browsers; both also provide tools to customize the look and feel of the GUI. Additionally, if both the GTK+ 2 and the Qt libraries are present, KDE applications can run in GNOME and vice-versa.

35.2.2. Window Managers
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Window managers are X client programs which are either part of a desktop environment or, in some cases, stand-alone. Their primary purpose is to control the way graphical windows are positioned, resized, or moved. Window managers also control title bars, window focus behavior, and user-specified key and mouse button bindings.
Four window managers are included with Red Hat Enterprise Linux:
kwin
The KWin window manager is the default window manager for KDE. It is an efficient window manager which supports custom themes.
metacity
The Metacity window manager is the default window manager for GNOME. It is a simple and efficient window manager which also supports custom themes. To run this window manager, you need to install the metacity package.
mwm
The Motif Window Manager (mwm) is a basic, stand-alone window manager. Since it is designed to be a stand-alone window manager, it should not be used in conjunction with GNOME or KDE. To run this window manager, you need to install the openmotif package.
twm
The minimalist Tab Window Manager (twm, which provides the most basic tool set of any of the window managers, can be used either as a stand-alone or with a desktop environment. It is installed as part of the X11R7.1 release.
To run any of the aforementioned window managers, you will first need to boot into Runlevel 3. For instructions on how to do this, refer to Section 18.1, “Runlevels”.
Once you are logged in to Runlevel 3, you will be presented with a terminal prompt, not a graphical environment. To start a window manager, type xinit -e <path-to-window-manager> at the prompt.
<path-to-window-manager> is the location of the window manager binary file. The binary file can be located by typing which window-manager-name, where window-manager-name is the name of the window manager you want to run.
For example: 
~]# which twm
/usr/bin/twm
~]# xinit -e /usr/bin/twm
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The first command above returns the absolute path to the twm window manager, the second command starts twm.
To exit a window manager, close the last window or press Ctrl+Alt+Backspace. Once you have exited the window manager, you can log back into Runlevel 5 by typing startx at the prompt.

35.3. X Server Configuration Files
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The X server is a single binary executable (/usr/bin/Xorg). Associated configuration files are stored in the /etc/X11/ directory (as is a symbolic link — X — which points to /usr/bin/Xorg). The configuration file for the X server is /etc/X11/xorg.conf.
The directory /usr/lib/xorg/modules/ contains X server modules that can be loaded dynamically at runtime. By default, only some modules in /usr/lib/xorg/modules/ are automatically loaded by the X server.
To load optional modules, they must be specified in the X server configuration file, /etc/X11/xorg.conf. For more information about loading modules, refer to Section 35.3.1.5, “Module”.
When Red Hat Enterprise Linux 5.10 is installed, the configuration files for X are created using information gathered about the system hardware during the installation process.

35.3.1. xorg.conf
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While there is rarely a need to manually edit the /etc/X11/xorg.conf file, it is useful to understand the various sections and optional parameters available, especially when troubleshooting.
35.3.1.1. The Structure
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The /etc/X11/xorg.conf file is comprised of many different sections which address specific aspects of the system hardware.
Each section begins with a Section "<section-name>" line (where <section-name> is the title for the section) and ends with an EndSection line. Each section contains lines that include option names and one or more option values. These are sometimes enclosed in double quotes (").
Lines beginning with a hash mark (#) are not read by the X server and are used for human-readable comments.
Some options within the /etc/X11/xorg.conf file accept a boolean switch which turns the feature on or off. Acceptable boolean values are:
  • 1, on, true, or yes — Turns the option on.
  • 0, off, false, or no — Turns the option off.
The following are some of the more important sections in the order in which they appear in a typical /etc/X11/xorg.conf file. More detailed information about the X server configuration file can be found in the xorg.conf man page.
35.3.1.2. ServerFlags
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The optional ServerFlags section contains miscellaneous global X server settings. Any settings in this section may be overridden by options placed in the ServerLayout section (refer to Section 35.3.1.3, “ServerLayout” for details).
Each entry within the ServerFlags section is on its own line and begins with the term Option followed by an option enclosed in double quotation marks (").
The following is a sample ServerFlags section: 
Section "ServerFlags"
	Option "DontZap" "true"
EndSection
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The following lists some of the most useful options:
  • "DontZap" "<boolean>" — When the value of <boolean> is set to true, this setting prevents the use of the Ctrl+Alt+Backspace key combination to immediately terminate the X server.
  • "DontZoom" "<boolean>" — When the value of <boolean> is set to true, this setting prevents cycling through configured video resolutions using the Ctrl+Alt+Keypad-Plus and Ctrl+Alt+Keypad-Minus key combinations.
35.3.1.3. ServerLayout
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The ServerLayout section binds together the input and output devices controlled by the X server. At a minimum, this section must specify one output device and one input device. By default, a monitor (output device) and keyboard (input device) are specified.
The following example illustrates a typical ServerLayout section: 
Section  "ServerLayout"
	Identifier     "Default Layout"
	Screen      0  "Screen0" 0 0
	InputDevice    "Mouse0" "CorePointer"
	InputDevice    "Keyboard0" "CoreKeyboard"
EndSection
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The following entries are commonly used in the ServerLayout section:
  • Identifier — Specifies a unique name for this ServerLayout section.
  • Screen — Specifies the name of a Screen section to be used with the X server. More than one Screen option may be present.
    The following is an example of a typical Screen entry: 
    Screen      0  "Screen0" 0 0
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    The first number in this example Screen entry (0) indicates that the first monitor connector or head on the video card uses the configuration specified in the Screen section with the identifier "Screen0".
    An example of a Screen section with the identifier "Screen0" can be found in Section 35.3.1.9, “Screen”.
    If the video card has more than one head, another Screen entry with a different number and a different Screen section identifier is necessary .
    The numbers to the right of "Screen0" give the absolute X and Y coordinates for the upper-left corner of the screen (0 0 by default).
  • InputDevice — Specifies the name of an InputDevice section to be used with the X server.
    It is advisable that there be at least two InputDevice entries: one for the default mouse and one for the default keyboard. The options CorePointer and CoreKeyboard indicate that these are the primary mouse and keyboard.
  • Option "<option-name>" — An optional entry which specifies extra parameters for the section. Any options listed here override those listed in the ServerFlags section.
    Replace <option-name> with a valid option listed for this section in the xorg.conf man page.
It is possible to put more than one ServerLayout section in the /etc/X11/xorg.conf file. By default, the server only reads the first one it encounters, however.
If there is an alternative ServerLayout section, it can be specified as a command line argument when starting an X session.
35.3.1.4. Files
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The Files section sets paths for services vital to the X server, such as the font path. This is an optional section, these paths are normally detected automatically. This section may be used to override any automatically detected defaults.
The following example illustrates a typical Files section: 
Section "Files"
	RgbPath      "/usr/share/X11/rgb.txt"
	FontPath     "unix/:7100"
EndSection
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The following entries are commonly used in the Files section:
  • RgbPath — Specifies the location of the RGB color database. This database defines all valid color names in X and ties them to specific RGB values.
  • FontPath — Specifies where the X server must connect to obtain fonts from the xfs font server.
    By default, the FontPath is unix/:7100. This tells the X server to obtain font information using UNIX-domain sockets for inter-process communication (IPC) on port 7100.
    Refer to Section 35.4, “Fonts” for more information concerning X and fonts.
  • ModulePath — An optional parameter which specifies alternate directories which store X server modules.
35.3.1.5. Module
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By default, the X server automatically loads the following modules from the /usr/lib/xorg/modules/ directory:
  • extmod
  • dbe
  • glx
  • freetype
  • type1
  • record
  • dri
The default directory for loading these modules can be changed by specifying a different directory with the optional ModulePath parameter in the Files section. Refer to Section 35.3.1.4, “Files” for more information on this section.
Adding a Module section to /etc/X11/xorg.conf instructs the X server to load the modules listed in this section instead of the default modules.
For example, the following typical Module section: 
Section "Module"
	Load  "fbdevhw"
EndSection
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instructs the X server to load the fbdevhw instead of the default modules.
As such, if you add a Module section to /etc/X11/xorg.conf, you will need to specify any default modules you want to load as well as any extra modules.
35.3.1.6. InputDevice
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Each InputDevice section configures one input device for the X server. Systems typically have at least one InputDevice section for the keyboard. It is perfectly normal to have no entry for a mouse, as most mouse settings are automatically detected.
The following example illustrates a typical InputDevice section for a keyboard: 
Section "InputDevice"
        Identifier  "Keyboard0"
        Driver      "kbd"
        Option      "XkbModel" "pc105"
        Option      "XkbLayout" "us"
EndSection
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The following entries are commonly used in the InputDevice section:
  • Identifier — Specifies a unique name for this InputDevice section. This is a required entry.
  • Driver — Specifies the name of the device driver X must load for the device.
  • Option — Specifies necessary options pertaining to the device.
    A mouse may also be specified to override any autodetected defaults for the device. The following options are typically included when adding a mouse in the xorg.conf:
    • Protocol — Specifies the protocol used by the mouse, such as IMPS/2.
    • Device — Specifies the location of the physical device.
    • Emulate3Buttons — Specifies whether to allow a two-button mouse to act like a three-button mouse when both mouse buttons are pressed simultaneously.
    Consult the xorg.conf man page for a list of valid options for this section.
35.3.1.7. Monitor
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Each Monitor section configures one type of monitor used by the system. This is an optional entry as well, as most monitors are now automatically detected.
The easiest way to configure a monitor is to configure X during the installation process or by using the X Configuration Tool. For more information about using the X Configuration Tool, refer to Chapter 36, X Window System Configuration.
This example illustrates a typical Monitor section for a monitor: 
Section "Monitor"
	Identifier   "Monitor0"
	VendorName   "Monitor Vendor"
	ModelName    "DDC Probed Monitor - ViewSonic G773-2"
	DisplaySize  320	240
	HorizSync    30.0 - 70.0
	VertRefresh  50.0 - 180.0
EndSection
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Warning

Be careful when manually editing values in the Monitor section of /etc/X11/xorg.conf. Inappropriate values can damage or destroy a monitor. Consult the monitor's documentation for a listing of safe operating parameters.
The following are commonly entries used in the Monitor section:
  • Identifier — Specifies a unique name for this Monitor section. This is a required entry.
  • VendorName — An optional parameter which specifies the vendor of the monitor.
  • ModelName — An optional parameter which specifies the monitor's model name.
  • DisplaySize — An optional parameter which specifies, in millimeters, the physical size of the monitor's picture area.
  • HorizSync — Specifies the range of horizontal sync frequencies compatible with the monitor in kHz. These values help the X server determine the validity of built-in or specified Modeline entries for the monitor.
  • VertRefresh — Specifies the range of vertical refresh frequencies supported by the monitor, in kHz. These values help the X server determine the validity of built in or specified Modeline entries for the monitor.
  • Modeline — An optional parameter which specifies additional video modes for the monitor at particular resolutions, with certain horizontal sync and vertical refresh resolutions. Refer to the xorg.conf man page for a more detailed explanation of Modeline entries.
  • Option "<option-name>" — An optional entry which specifies extra parameters for the section. Replace <option-name> with a valid option listed for this section in the xorg.conf man page.
35.3.1.8. Device
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Each Device section configures one video card on the system. While one Device section is the minimum, additional instances may occur for each video card installed on the machine.
The best way to configure a video card is to configure X during the installation process or by using the X Configuration Tool. For more about using the X Configuration Tool, refer to Chapter 36, X Window System Configuration.
The following example illustrates a typical Device section for a video card: 
Section "Device"
	Identifier  "Videocard0"
	Driver      "mga"
	VendorName  "Videocard vendor"
	BoardName   "Matrox Millennium G200"
	VideoRam    8192
	Option      "dpms"
EndSection
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The following entries are commonly used in the Device section:
  • Identifier — Specifies a unique name for this Device section. This is a required entry.
  • Driver — Specifies which driver the X server must load to utilize the video card. A list of drivers can be found in /usr/share/hwdata/videodrivers, which is installed with the hwdata package.
  • VendorName — An optional parameter which specifies the vendor of the video card.
  • BoardName — An optional parameter which specifies the name of the video card.
  • VideoRam — An optional parameter which specifies the amount of RAM available on the video card in kilobytes. This setting is only necessary for video cards the X server cannot probe to detect the amount of video RAM.
  • BusID — An entry which specifies the bus location of the video card. On systems with only one video card a BusID entry is optional and may not even be present in the default /etc/X11/xorg.conf file. On systems with more than one video card, however, a BusID entry must be present.
  • Screen — An optional entry which specifies which monitor connector or head on the video card the Device section configures. This option is only useful for video cards with multiple heads.
    If multiple monitors are connected to different heads on the same video card, separate Device sections must exist and each of these sections must have a different Screen value.
    Values for the Screen entry must be an integer. The first head on the video card has a value of 0. The value for each additional head increments this value by one.
  • Option "<option-name>" — An optional entry which specifies extra parameters for the section. Replace <option-name> with a valid option listed for this section in the xorg.conf man page.
    One of the more common options is "dpms" (for Display Power Management Signaling, a VESA standard), which activates the Service Star energy compliance setting for the monitor.
35.3.1.9. Screen
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Each Screen section binds one video card (or video card head) to one monitor by referencing the Device section and the Monitor section for each. While one Screen section is the minimum, additional instances may occur for each video card and monitor combination present on the machine.
The following example illustrates a typical Screen section: 
Section "Screen"
	Identifier "Screen0"
	Device     "Videocard0"
	Monitor    "Monitor0"
	DefaultDepth     16
	SubSection "Display"
		Depth     24
		Modes    "1280x1024" "1280x960" "1152x864" "1024x768" "800x600" "640x480"
	EndSubSection
	SubSection "Display"
		Depth     16
		Modes    "1152x864" "1024x768" "800x600" "640x480"
	EndSubSection
EndSection
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The following entries are commonly used in the Screen section:
  • Identifier — Specifies a unique name for this Screen section. This is a required entry.
  • Device — Specifies the unique name of a Device section. This is a required entry.
  • Monitor — Specifies the unique name of a Monitor section. This is only required if a specific Monitor section is defined in the xorg.conf file. Normally, monitors are automatically detected.
  • DefaultDepth — Specifies the default color depth in bits. In the previous example, 16 (which provides thousands of colors) is the default. Only one DefaultDepth is permitted, although this can be overridden with the Xorg command line option -depth <n>,where <n> is any additional depth specified.
  • SubSection "Display" — Specifies the screen modes available at a particular color depth. The Screen section can have multiple Display subsections, which are entirely optional since screen modes are automatically detected.
    This subsection is normally used to override autodetected modes.
  • Option "<option-name>" — An optional entry which specifies extra parameters for the section. Replace <option-name> with a valid option listed for this section in the xorg.conf man page.
35.3.1.10. DRI
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The optional DRI section specifies parameters for the Direct Rendering Infrastructure (DRI). DRI is an interface which allows 3D software applications to take advantage of 3D hardware acceleration capabilities built into most modern video hardware. In addition, DRI can improve 2D performance via hardware acceleration, if supported by the video card driver.
This section rarely appears, as the DRI Group and Mode are automatically initialized to default values. If a different Group or Mode is desired, then adding this section to the xorg.conf file will override those defaults.
The following example illustrates a typical DRI section: 
Section "DRI"
	Group        0
	Mode         0666
EndSection
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Since different video cards use DRI in different ways, do not add to this section without first referring to http://dri.sourceforge.net/.

35.4. Fonts
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Red Hat Enterprise Linux uses two subsystems to manage and display fonts under X: Fontconfig and xfs.
The newer Fontconfig font subsystem simplifies font management and provides advanced display features, such as anti-aliasing. This system is used automatically for applications programmed using the Qt 3 or GTK+ 2 graphical toolkit.
For compatibility, Red Hat Enterprise Linux includes the original font subsystem, called the core X font subsystem. This system, which is over 15 years old, is based around the X Font Server (xfs).
This section discusses how to configure fonts for X using both systems.

35.4.1. Fontconfig
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The Fontconfig font subsystem allows applications to directly access fonts on the system and use Xft or other rendering mechanisms to render Fontconfig fonts with advanced anti-aliasing. Graphical applications can use the Xft library with Fontconfig to draw text to the screen.
Over time, the Fontconfig/Xft font subsystem replaces the core X font subsystem.

Important

The Fontconfig font subsystem does not yet work for OpenOffice.org, which uses its own font rendering technology.
It is important to note that Fontconfig uses the /etc/fonts/fonts.conf configuration file, which should not be edited by hand.

Note

Due to the transition to the new font system, GTK+ 1.2 applications are not affected by any changes made via the Font Preferences dialog (accessed by selecting System (on the panel) > Preferences > Fonts). For these applications, a font can be configured by adding the following lines to the file ~/.gtkrc.mine: 
style "user-font" {
	fontset = "<font-specification>"
}

widget_class "*" style "user-font"
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Replace <font-specification> with a font specification in the style used by traditional X applications, such as -adobe-helvetica-medium-r-normal--*-120-*-*-*-*-*-*. A full list of core fonts can be obtained by running xlsfonts or created interactively using the xfontsel command.
35.4.1.1. Adding Fonts to Fontconfig
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Adding new fonts to the Fontconfig subsystem is a straightforward process.
  1. To add fonts system-wide, copy the new fonts into the /usr/share/fonts/ directory. It is a good idea to create a new subdirectory, such as local/ or similar, to help distinguish between user-installed and default fonts.
    To add fonts for an individual user, copy the new fonts into the .fonts/ directory in the user's home directory.
  2. Use the fc-cache command to update the font information cache, as in the following example: 
    fc-cache <path-to-font-directory>
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    In this command, replace <path-to-font-directory> with the directory containing the new fonts (either /usr/share/fonts/local/ or /home/<user>/.fonts/).

Note

Individual users may also install fonts graphically, by typing fonts:/// into the Nautilus address bar, and dragging the new font files there.

Important

If the font file name ends with a .gz extension, it is compressed and cannot be used until uncompressed. To do this, use the gunzip command or double-click the file and drag the font to a directory in Nautilus.

35.4.2. Core X Font System
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For compatibility, Red Hat Enterprise Linux provides the core X font subsystem, which uses the X Font Server (xfs) to provide fonts to X client applications.
The X server looks for a font server specified in the FontPath directive within the Files section of the /etc/X11/xorg.conf configuration file. Refer to Section 35.3.1.4, “Files” for more information about the FontPath entry.
The X server connects to the xfs server on a specified port to acquire font information. For this reason, the xfs service must be running for X to start. For more about configuring services for a particular runlevel, refer to Chapter 18, Controlling Access to Services.
35.4.2.1. xfs Configuration
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The /etc/rc.d/init.d/xfs script starts the xfs server. Several options can be configured within its configuration file, /etc/X11/fs/config.
The following lists common options:
  • alternate-servers — Specifies a list of alternate font servers to be used if this font server is not available. A comma must separate each font server in a list.
  • catalogue — Specifies an ordered list of font paths to use. A comma must separate each font path in a list.
    Use the string :unscaled immediately after the font path to make the unscaled fonts in that path load first. Then specify the entire path again, so that other scaled fonts are also loaded.
  • client-limit — Specifies the maximum number of clients the font server services. The default is 10.
  • clone-self — Allows the font server to clone a new version of itself when the client-limit is hit. By default, this option is on.
  • default-point-size — Specifies the default point size for any font that does not specify this value. The value for this option is set in decipoints. The default of 120 corresponds to a 12 point font.
  • default-resolutions — Specifies a list of resolutions supported by the X server. Each resolution in the list must be separated by a comma.
  • deferglyphs — Specifies whether to defer loading glyphs (the graphic used to visually represent a font). To disable this feature use none, to enable this feature for all fonts use all, or to turn this feature on only for 16-bit fonts use 16.
  • error-file — Specifies the path and file name of a location where xfs errors are logged.
  • no-listen — Prevents xfs from listening to particular protocols. By default, this option is set to tcp to prevent xfs from listening on TCP ports for security reasons.

    Note

    If xfs is used to serve fonts over the network, remove this line.
  • port — Specifies the TCP port that xfs listens on if no-listen does not exist or is commented out.
  • use-syslog — Specifies whether to use the system error log.
35.4.2.2. Adding Fonts to xfs
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To add fonts to the core X font subsystem (xfs), follow these steps:
  1. If it does not already exist, create a directory called /usr/share/fonts/local/ using the following command as root: 
    mkdir /usr/share/fonts/local/
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    If creating the /usr/share/fonts/local/ directory is necessary, it must be added to the xfs path using the following command as root: 
    chkfontpath --add /usr/share/fonts/local/
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  2. Copy the new font file into the /usr/share/fonts/local/ directory
  3. Update the font information by issuing the following command as root: 
    ttmkfdir -d /usr/share/fonts/local/ -o /usr/share/fonts/local/fonts.scale
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  4. Reload the xfs font server configuration file by issuing the following command as root: 
    service xfs reload
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35.5. Runlevels and X
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In most cases, the Red Hat Enterprise Linux installer configures a machine to boot into a graphical login environment, known as Runlevel 5. It is possible, however, to boot into a text-only multi-user mode called Runlevel 3 and begin an X session from there.
For more information about runlevels, refer to Section 18.1, “Runlevels”.
The following subsections review how X starts up in both runlevel 3 and runlevel 5.

35.5.1. Runlevel 3
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When in runlevel 3, the best way to start an X session is to log in and type startx. The startx command is a front-end to the xinit command, which launches the X server (Xorg) and connects X client applications to it. Because the user is already logged into the system at runlevel 3, startx does not launch a display manager or authenticate users. Refer to Section 35.5.2, “Runlevel 5” for more information about display managers.
When the startx command is executed, it searches for the .xinitrc file in the user's home directory to define the desktop environment and possibly other X client applications to run. If no .xinitrc file is present, it uses the system default /etc/X11/xinit/xinitrc file instead.
The default xinitrc script then searches for user-defined files and default system files, including .Xresources, .Xmodmap, and .Xkbmap in the user's home directory, and Xresources, Xmodmap, and Xkbmap in the /etc/X11/ directory. The Xmodmap and Xkbmap files, if they exist, are used by the xmodmap utility to configure the keyboard. The Xresources file is read to assign specific preference values to applications.
After setting these options, the xinitrc script executes all scripts located in the /etc/X11/xinit/xinitrc.d/ directory. One important script in this directory is xinput.sh, which configures settings such as the default language.
Next, the xinitrc script attempts to execute .Xclients in the user's home directory and turns to /etc/X11/xinit/Xclients if it cannot be found. The purpose of the Xclients file is to start the desktop environment or, possibly, just a basic window manager. The .Xclients script in the user's home directory starts the user-specified desktop environment in the .Xclients-default file. If .Xclients does not exist in the user's home directory, the standard /etc/X11/xinit/Xclients script attempts to start another desktop environment, trying GNOME first and then KDE followed by twm.
When in runlevel 3, the user is returned to a text mode user session after ending an X session.

35.5.2. Runlevel 5
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When the system boots into runlevel 5, a special X client application called a display manager is launched. A user must authenticate using the display manager before any desktop environment or window managers are launched.
Depending on the desktop environments installed on the system, three different display managers are available to handle user authentication.
  • GNOME — The default display manager for Red Hat Enterprise Linux, GNOME allows the user to configure language settings, shutdown, restart or log in to the system.
  • KDE — KDE's display manager which allows the user to shutdown, restart or log in to the system.
  • xdm — A very basic display manager which only lets the user log in to the system.
When booting into runlevel 5, the prefdm script determines the preferred display manager by referencing the /etc/sysconfig/desktop file. A list of options for this file is available in this file: 
/usr/share/doc/initscripts-<version-number>/sysconfig.txt
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where <version-number> is the version number of the initscripts package.
Each of the display managers reference the /etc/X11/xdm/Xsetup_0 file to set up the login screen. Once the user logs into the system, the /etc/X11/xdm/GiveConsole script runs to assign ownership of the console to the user. Then, the /etc/X11/xdm/Xsession script runs to accomplish many of the tasks normally performed by the xinitrc script when starting X from runlevel 3, including setting system and user resources, as well as running the scripts in the /etc/X11/xinit/xinitrc.d/ directory.
Users can specify which desktop environment they want to utilize when they authenticate using the GNOME or KDE display managers by selecting it from the Sessions menu item (accessed by selecting System (on the panel) > Preferences > More Preferences > Sessions). If the desktop environment is not specified in the display manager, the /etc/X11/xdm/Xsession script checks the .xsession and .Xclients files in the user's home directory to decide which desktop environment to load. As a last resort, the /etc/X11/xinit/Xclients file is used to select a desktop environment or window manager to use in the same way as runlevel 3.
When the user finishes an X session on the default display (:0) and logs out, the /etc/X11/xdm/TakeConsole script runs and reassigns ownership of the console to the root user. The original display manager, which continues running after the user logged in, takes control by spawning a new display manager. This restarts the X server, displays a new login window, and starts the entire process over again.
The user is returned to the display manager after logging out of X from runlevel 5.
For more information on how display managers control user authentication, refer to the /usr/share/doc/gdm-<version-number>/README (where <version-number> is the version number for the gdm package installed) and the xdm man page.

35.6. Additional Resources
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There is a large amount of detailed information available about the X server, the clients that connect to it, and the assorted desktop environments and window managers.

35.6.1. Installed Documentation
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  • /usr/share/X11/doc/ — contains detailed documentation on the X Window System architecture, as well as how to get additional information about the Xorg project as a new user.
  • man xorg.conf — Contains information about the xorg.conf configuration files, including the meaning and syntax for the different sections within the files.
  • man Xorg — Describes the Xorg display server.

35.6.2. Useful Websites
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  • http://www.X.org/ — Home page of the X.Org Foundation, which produces the X11R7.1 release of the X Window System. The X11R7.1 release is bundled with Red Hat Enterprise Linux to control the necessary hardware and provide a GUI environment.
  • http://dri.sourceforge.net/ — Home page of the DRI (Direct Rendering Infrastructure) project. The DRI is the core hardware 3D acceleration component of X.
  • http://www.gnome.org/ — Home of the GNOME project.
  • http://www.kde.org/ — Home of the KDE desktop environment.

Chapter 36. X Window System Configuration
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During installation, the system's monitor, video card, and display settings are configured. To change any of these settings after installation, use the X Configuration Tool.
To start the X Configuration Tool, go to System (on the panel) > Administration > Display, or type the command system-config-display at a shell prompt (for example, in an XTerm or GNOME terminal). If the X Window System is not running, a small version of X is started to run the program.
After changing any of the settings, log out of the graphical desktop and log back in to enable the changes.

36.1. Display Settings
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The Settings tab allows users to change the resolution and color depth. The display of a monitor consists of tiny dots called pixels. The number of pixels displayed at one time is called the resolution. For example, the resolution 1024x768 means that 1024 horizontal pixels and 768 vertical pixels are used. The higher the resolution values, the more images the monitor can display at one time.
The color depth of the display determines how many possible colors are displayed. A higher color depth means more contrast between colors.
Display Settings
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Display Settings

Figure 36.1. Display Settings

36.2. Display Hardware Settings
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When the X Configuration Tool is started, it probes the monitor and video card. If the hardware is probed properly, the information for it is shown on the Hardware tab as shown in Figure 36.2, “Display Hardware Settings”.
Display Hardware Settings
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Display Hardware Settings

Figure 36.2. Display Hardware Settings

To change the monitor type or any of its settings, click the corresponding Configure button. To change the video card type or any of its settings, click the Configure button beside its settings.

36.3. Dual Head Display Settings
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If multiple video cards are installed on the system, dual head monitor support is available and is configured via the Dual head tab, as shown in Figure 36.3, “Dual Head Display Settings”.
Dual Head Display Settings
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Dual Head Display Settings

Figure 36.3. Dual Head Display Settings

To enable use of Dual head, check the Use dual head checkbox.
To configure the second monitor type, click the corresponding Configure button. You can also configure the other Dual head settings by using the corresponding drop-down list.
For the Desktop layout option, selecting Spanning Desktops allows both monitors to use an enlarged usable workspace. Selecting Individual Desktops shares the mouse and keyboard among the displays, but restricts windows to a single display.

Chapter 37. Users and Groups
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The control of users and groups is a core element of Red Hat Enterprise Linux system administration.
Users can be either people (meaning accounts tied to physical users) or accounts which exist for specific applications to use.
Groups are logical expressions of organization, tying users together for a common purpose. Users within a group can read, write, or execute files owned by that group.
Each user and group has a unique numerical identification number called a userid (UID) and a groupid (GID), respectively.
A user who creates a file is also the owner and group owner of that file. The file is assigned separate read, write, and execute permissions for the owner, the group, and everyone else. The file owner can be changed only by the root user, and access permissions can be changed by both the root user and file owner.
Red Hat Enterprise Linux also supports access control lists (ACLs) for files and directories which allow permissions for specific users outside of the owner to be set. For more information about ACLs, refer to Chapter 10, Access Control Lists.

37.1. User and Group Configuration
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The User Manager allows you to view, modify, add, and delete local users and groups.
To use the User Manager, you must be running the X Window System, have root privileges, and have the system-config-users RPM package installed. To start the User Manager from the desktop, go to System (on the panel) > Administration > Users & Groups. You can also type the command system-config-users at a shell prompt (for example, in an XTerm or a GNOME terminal).
User Manager
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User Manager

Figure 37.1. User Manager

To view a list of local users on the system, click the Users tab. To view a list of local groups on the system, click the Groups tab.
To find a specific user or group, type the first few letters of the name in the Search filter field. Press Enter or click the Apply filter button. The filtered list is displayed.
To sort the users or groups, click on the column name. The users or groups are sorted according to the value of that column.
Red Hat Enterprise Linux reserves user IDs below 500 for system users. By default, User Manager does not display system users. To view all users, including the system users, go to Edit > Preferences and uncheck Hide system users and groups from the dialog box.

37.1.1. Adding a New User
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To add a new user, click the Add User button. A window as shown in Figure 37.2, “New User” appears. Type the username and full name for the new user in the appropriate fields. Type the user's password in the Password and Confirm Password fields. The password must be at least six characters.

Note

It is advisable to use a much longer password, as this makes it more difficult for an intruder to guess it and access the account without permission. It is also recommended that the password not be based on a dictionary term; use a combination of letters, numbers and special characters.
Select a login shell. If you are not sure which shell to select, accept the default value of /bin/bash. The default home directory is /home/<username>/. You can change the home directory that is created for the user, or you can choose not to create the home directory by unselecting Create home directory.
If you select to create the home directory, default configuration files are copied from the /etc/skel/ directory into the new home directory.
Red Hat Enterprise Linux uses a user private group (UPG) scheme. The UPG scheme does not add or change anything in the standard UNIX way of handling groups; it offers a new convention. Whenever you create a new user, by default, a unique group with the same name as the user is created. If you do not want to create this group, unselect Create a private group for the user.
To specify a user ID for the user, select Specify user ID manually. If the option is not selected, the next available user ID above 500 is assigned to the new user. Because Red Hat Enterprise Linux reserves user IDs below 500 for system users, it is not advisable to manually assign user IDs 1-499.
Click OK to create the user.
New User
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New User

Figure 37.2. New User

To configure more advanced user properties, such as password expiration, modify the user's properties after adding the user. Refer to Section 37.1.2, “Modifying User Properties” for more information.

37.1.2. Modifying User Properties
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To view the properties of an existing user, click on the Users tab, select the user from the user list, and click Properties from the menu (or choose File > Properties from the pulldown menu). A window similar to Figure 37.3, “User Properties” appears.
User Properties
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User Properties

Figure 37.3. User Properties

The User Properties window is divided into multiple tabbed pages:
  • User Data — Shows the basic user information configured when you added the user. Use this tab to change the user's full name, password, home directory, or login shell.
  • Account Info — Select Enable account expiration if you want the account to expire on a certain date. Enter the date in the provided fields. Select Local password is locked to lock the user account and prevent the user from logging into the system.
  • Password Info — Displays the date that the user's password last changed. To force the user to change passwords after a certain number of days, select Enable password expiration and enter a desired value in the Days before change required: field. The number of days before the user's password expires, the number of days before the user is warned to change passwords, and days before the account becomes inactive can also be changed.
  • Groups — Allows you to view and configure the Primary Group of the user, as well as other groups that you want the user to be a member of.

37.1.3. Adding a New Group
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To add a new user group, click the Add Group button. A window similar to Figure 37.4, “New Group” appears. Type the name of the new group to create. To specify a group ID for the new group, select Specify group ID manually and select the GID. Note that Red Hat Enterprise Linux also reserves group IDs lower than 500 for system groups.
New Group
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New Group

Figure 37.4. New Group

Click OK to create the group. The new group appears in the group list.

37.1.4. Modifying Group Properties
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To view the properties of an existing group, select the group from the group list and click Properties from the menu (or choose File > Properties from the pulldown menu). A window similar to Figure 37.5, “Group Properties” appears.
Group Properties
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Group Properties

Figure 37.5. Group Properties

The Group Users tab displays which users are members of the group. Use this tab to add or remove users from the group. Click OK to save your changes.

37.2. User and Group Management Tools
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Managing users and groups can be a tedious task; this is why Red Hat Enterprise Linux provides tools and conventions to make them easier to manage.
The easiest way to manage users and groups is through the graphical application, User Manager (system-config-users). For more information on User Manager, refer to Section 37.1, “User and Group Configuration”.
The following command line tools can also be used to manage users and groups:
  • useradd, usermod, and userdel — Industry-standard methods of adding, deleting and modifying user accounts
  • groupadd, groupmod, and groupdel — Industry-standard methods of adding, deleting, and modifying user groups
  • gpasswd — Industry-standard method of administering the /etc/group file
  • pwck, grpck — Tools used for the verification of the password, group, and associated shadow files
  • pwconv, pwunconv — Tools used for the conversion of passwords to shadow passwords and back to standard passwords

37.2.1. Command Line Configuration
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If you prefer command line tools or do not have the X Window System installed, use this section to configure users and groups.

37.2.2. Adding a User
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To add a user to the system:
  1. Issue the useradd command to create a locked user account: 
    useradd <username>
    Copy to Clipboard Toggle word wrap
  2. Unlock the account by issuing the passwd command to assign a password and set password aging guidelines: 
    passwd <username>
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Command line options for useradd are detailed in Table 37.1, “useradd Command Line Options”.
Expand
Table 37.1. useradd Command Line Options
Option Description
-c '<comment>' <comment> can be replaced with any string. This option is generally used to specify the full name of a user.
-d <home-dir> Home directory to be used instead of default /home/<username>/
-e <date> Date for the account to be disabled in the format YYYY-MM-DD
-f <days> Number of days after the password expires until the account is disabled. If 0 is specified, the account is disabled immediately after the password expires. If -1 is specified, the account is not be disabled after the password expires.
-g <group-name> Group name or group number for the user's default group. The group must exist prior to being specified here.
-G <group-list> List of additional (other than default) group names or group numbers, separated by commas, of which the user is a member. The groups must exist prior to being specified here.
-m Create the home directory if it does not exist.
-M Do not create the home directory.
-n Do not create a user private group for the user.
-r Create a system account with a UID less than 500 and without a home directory
-p <password> The password encrypted with crypt
-s User's login shell, which defaults to /bin/bash
-u <uid> User ID for the user, which must be unique and greater than 499

37.2.3. Adding a Group
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To add a group to the system, use the command groupadd: 
groupadd <group-name>
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Command line options for groupadd are detailed in Table 37.2, “groupadd Command Line Options”.
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Table 37.2. groupadd Command Line Options
Option Description
-g <gid> Group ID for the group, which must be unique and greater than 499
-r Create a system group with a GID less than 500
-f When used with -g <gid> and <gid> already exists, groupadd will choose another unique <gid> for the group.

37.2.4. Password Aging
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For security reasons, it is advisable to require users to change their passwords periodically. This can be done when adding or editing a user on the Password Info tab of the User Manager.
To configure password expiration for a user from a shell prompt, use the chage command with an option from Table 37.3, “chage Command Line Options”, followed by the username.

Important

Shadow passwords must be enabled to use the chage command. For more information, see Section 37.6, “Shadow Passwords”.
Expand
Table 37.3. chage Command Line Options
Option Description
-m <days> Specifies the minimum number of days between which the user must change passwords. If the value is 0, the password does not expire.
-M <days> Specifies the maximum number of days for which the password is valid. When the number of days specified by this option plus the number of days specified with the -d option is less than the current day, the user must change passwords before using the account.
-d <days> Specifies the number of days since January 1, 1970 the password was changed
-I <days> Specifies the number of inactive days after the password expiration before locking the account. If the value is 0, the account is not locked after the password expires.
-E <date> Specifies the date on which the account is locked, in the format YYYY-MM-DD. Instead of the date, the number of days since January 1, 1970 can also be used.
-W <days> Specifies the number of days before the password expiration date to warn the user.
-l Lists current account aging settings.

Note

If the chage command is followed directly by a username (with no options), it displays the current password aging values and allows them to be changed interactively.
You can configure a password to expire the first time a user logs in. This forces users to change passwords immediately.
  1. Set up an initial password — There are two common approaches to this step. The administrator can assign a default password or assign a null password.
    To assign a default password, use the following steps:
    • Start the command line Python interpreter with the python command. It displays the following: 
      Python 2.4.3 (#1, Jul 21 2006, 08:46:09)
[GCC 4.1.1 20060718 (Red Hat 4.1.1-9)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
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    • At the prompt, type the following commands. Replace <password> with the password to encrypt and <salt> with a random combination of at least 2 of the following: any alphanumeric character, the slash (/) character or a dot (.): 
      import crypt
print crypt.crypt("<password>","<salt>")
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      The output is the encrypted password, similar to '12CsGd8FRcMSM'.
    • Press Ctrl-D to exit the Python interpreter.
    • At the shell, enter the following command (replacing <encrypted-password> with the encrypted output of the Python interpreter): 
      usermod -p "<encrypted-password>" <username>
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    Alternatively, you can assign a null password instead of an initial password. To do this, use the following command: 
    usermod -p "" username
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    Warning

    Using a null password, while convenient, is a highly unsecure practice, as any third party can log in first an access the system using the unsecure username. Always make sure that the user is ready to log in before unlocking an account with a null password.
  2. Force immediate password expiration — Type the following command: 
    chage -d 0 username
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    This command sets the value for the date the password was last changed to the epoch (January 1, 1970). This value forces immediate password expiration no matter what password aging policy, if any, is in place.
Upon the initial log in, the user is now prompted for a new password.

37.2.5. Explaining the Process
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The following steps illustrate what happens if the command useradd juan is issued on a system that has shadow passwords enabled:
  1. A new line for juan is created in /etc/passwd. The line has the following characteristics:
    • It begins with the username juan.
    • There is an x for the password field indicating that the system is using shadow passwords.
    • A UID greater than 499 is created. (Under Red Hat Enterprise Linux, UIDs and GIDs below 500 are reserved for system use.)
    • A GID greater than 499 is created.
    • The optional GECOS information is left blank.
    • The home directory for juan is set to /home/juan/.
    • The default shell is set to /bin/bash.
  2. A new line for juan is created in /etc/shadow. The line has the following characteristics:
    • It begins with the username juan.
    • Two exclamation points (!!) appear in the password field of the /etc/shadow file, which locks the account.

      Note

      If an encrypted password is passed using the -p flag, it is placed in the /etc/shadow file on the new line for the user.
    • The password is set to never expire.
  3. A new line for a group named juan is created in /etc/group. A group with the same name as a user is called a user private group. For more information on user private groups, refer to Section 37.1.1, “Adding a New User”.
    The line created in /etc/group has the following characteristics:
    • It begins with the group name juan.
    • An x appears in the password field indicating that the system is using shadow group passwords.
    • The GID matches the one listed for user juan in /etc/passwd.
  4. A new line for a group named juan is created in /etc/gshadow. The line has the following characteristics:
    • It begins with the group name juan.
    • An exclamation point (!) appears in the password field of the /etc/gshadow file, which locks the group.
    • All other fields are blank.
  5. A directory for user juan is created in the /home/ directory. This directory is owned by user juan and group juan. However, it has read, write, and execute privileges only for the user juan. All other permissions are denied.
  6. The files within the /etc/skel/ directory (which contain default user settings) are copied into the new /home/juan/ directory.
At this point, a locked account called juan exists on the system. To activate it, the administrator must next assign a password to the account using the passwd command and, optionally, set password aging guidelines.

37.3. Standard Users
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Table 37.4, “Standard Users” lists the standard users configured in the /etc/passwd file by an Everything installation. The groupid (GID) in this table is the primary group for the user. See Section 37.4, “Standard Groups” for a listing of standard groups.
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Table 37.4. Standard Users
User UID GID Home Directory Shell
root 0 0 /root /bin/bash
bin 1 1 /bin /sbin/nologin
daemon 2 2 /sbin /sbin/nologin
adm 3 4 /var/adm /sbin/nologin
lp 4 7 /var/spool/lpd /sbin/nologin
sync 5 0 /sbin /bin/sync
shutdown 6 0 /sbin /sbin/shutdown
halt 7 0 /sbin /sbin/halt
mail 8 12 /var/spool/mail /sbin/nologin
news 9 13 /etc/news  
uucp 10 14 /var/spool/uucp /sbin/nologin
operator 11 0 /root /sbin/nologin
games 12 100 /usr/games /sbin/nologin
gopher 13 30 /var/gopher /sbin/nologin
ftp 14 50 /var/ftp /sbin/nologin
nobody 99 99 / /sbin/nologin
rpm 37 37 /var/lib/rpm /sbin/nologin
vcsa 69 69 /dev /sbin/nologin
dbus 81 81 / /sbin/nologin
ntp 38 38 /etc/ntp /sbin/nologin
canna 39 39 /var/lib/canna /sbin/nologin
nscd 28 28 / /sbin/nologin
rpc 32 32 / /sbin/nologin
postfix 89 89 /var/spool/postfix /sbin/nologin
mailman 41 41 /var/mailman /sbin/nologin
named 25 25 /var/named /bin/false
amanda 33 6 var/lib/amanda/ /bin/bash
postgres 26 26 /var/lib/pgsql /bin/bash
exim 93 93 /var/spool/exim /sbin/nologin
sshd 74 74 /var/empty/sshd /sbin/nologin
rpcuser 29 29 /var/lib/nfs /sbin/nologin
nsfnobody 65534 65534 /var/lib/nfs /sbin/nologin
pvm 24 24 /usr/share/pvm3 /bin/bash
apache 48 48 /var/www /sbin/nologin
xfs 43 43 /etc/X11/fs /sbin/nologin
gdm 42 42 /var/gdm /sbin/nologin
htt 100 101 /usr/lib/im /sbin/nologin
mysql 27 27 /var/lib/mysql /bin/bash
webalizer 67 67 /var/www/usage /sbin/nologin
mailnull 47 47 /var/spool/mqueue /sbin/nologin
smmsp 51 51 /var/spool/mqueue /sbin/nologin
squid 23 23 /var/spool/squid /sbin/nologin
ldap 55 55 /var/lib/ldap /bin/false
netdump 34 34 /var/crash /bin/bash
pcap 77 77 /var/arpwatch /sbin/nologin
radiusd 95 95 / /bin/false
radvd 75 75 / /sbin/nologin
quagga 92 92 /var/run/quagga /sbin/login
wnn 49 49 /var/lib/wnn /sbin/nologin
dovecot 97 97 /usr/libexec/dovecot /sbin/nologin

37.4. Standard Groups
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Table 37.5, “Standard Groups” lists the standard groups configured by an Everything installation. Groups are stored in the /etc/group file.
Expand
Table 37.5. Standard Groups
Group GID Members
root 0 root
bin 1 root, bin, daemon
daemon 2 root, bin, daemon
sys 3 root, bin, adm
adm 4 root, adm, daemon
tty 5  
disk 6 root
lp 7 daemon, lp
mem 8  
kmem 9  
wheel 10 root
mail 12 mail, postfix, exim
news 13 news
uucp 14 uucp
man 15  
games 20  
gopher 30  
dip 40  
ftp 50  
lock 54  
nobody 99  
users 100  
rpm 37  
utmp 22  
floppy 19  
vcsa 69  
dbus 81  
ntp 38  
canna 39  
nscd 28  
rpc 32  
postdrop 90  
postfix 89  
mailman 41  
exim 93  
named 25  
postgres 26  
sshd 74  
rpcuser 29  
nfsnobody 65534  
pvm 24  
apache 48  
xfs 43  
gdm 42  
htt 101  
mysql 27  
webalizer 67  
mailnull 47  
smmsp 51  
squid 23  
ldap 55  
netdump 34  
pcap 77  
quaggavt 102  
quagga 92  
radvd 75  
slocate 21  
wnn 49  
dovecot 97  
radiusd 95  

37.5. User Private Groups
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Red Hat Enterprise Linux uses a user private group (UPG) scheme, which makes UNIX groups easier to manage.
A UPG is created whenever a new user is added to the system. A UPG has the same name as the user for which it was created and that user is the only member of the UPG.
UPGs make it safe to set default permissions for a newly created file or directory, allowing both the user and the group of that user to make modifications to the file or directory.
The setting which determines what permissions are applied to a newly created file or directory is called a umask and is configured in the /etc/bashrc file. Traditionally on UNIX systems, the umask is set to 022, which allows only the user who created the file or directory to make modifications. Under this scheme, all other users, including members of the creator's group, are not allowed to make any modifications. However, under the UPG scheme, this "group protection" is not necessary since every user has their own private group.

37.5.1. Group Directories
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Many IT organizations like to create a group for each major project and then assign people to the group if they need to access that project's files. Using this traditional scheme, managing files has been difficult; when someone creates a file, it is associated with the primary group to which they belong. When a single person works on multiple projects, it is difficult to associate the right files with the right group. Using the UPG scheme, however, groups are automatically assigned to files created within a directory with the setgid bit set. The setgid bit makes managing group projects that share a common directory very simple because any files a user creates within the directory are owned by the group which owns the directory.
Let us say, for example, that a group of people need to work on files in the /usr/share/emacs/site-lisp/ directory. Some people are trusted to modify the directory, but certainly not everyone is trusted. First create an emacs group, as in the following command: 
groupadd emacs
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To associate the contents of the directory with the emacs group, type: 
chown -R root.emacs /usr/share/emacs/site-lisp
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Now, it is possible to add the proper users to the group with the gpasswd command: 
gpasswd -a <username> emacs
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To allow users to create files within the directory, use the following command: 
chmod 775 /usr/share/emacs/site-lisp
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When a user creates a new file, it is assigned the group of the user's default private group. Next, set the setgid bit, which assigns everything created in the directory the same group permission as the directory itself (emacs). Use the following command: 
chmod 2775 /usr/share/emacs/site-lisp
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At this point, because the default umask of each user is 002, all members of the emacs group can create and edit files in the /usr/share/emacs/site-lisp/ directory without the administrator having to change file permissions every time users write new files.

37.6. Shadow Passwords
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In multiuser environments it is very important to use shadow passwords (provided by the shadow-utils package). Doing so enhances the security of system authentication files. For this reason, the installation program enables shadow passwords by default.
The following lists the advantages pf shadow passwords have over the traditional way of storing passwords on UNIX-based systems:
  • Improves system security by moving encrypted password hashes from the world-readable /etc/passwd file to /etc/shadow, which is readable only by the root user.
  • Stores information about password aging.
  • Allows the use the /etc/login.defs file to enforce security policies.
Most utilities provided by the shadow-utils package work properly whether or not shadow passwords are enabled. However, since password aging information is stored exclusively in the /etc/shadow file, any commands which create or modify password aging information do not work.
The following is a list of commands which do not work without first enabling shadow passwords:
  • chage
  • gpasswd
  • /usr/sbin/usermod -e or -f options
  • /usr/sbin/useradd -e or -f options

37.7. Additional Resources
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For more information about users and groups, and tools to manage them, refer to the following resources.

37.7.1. Installed Documentation
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  • Related man pages — There are a number of man pages for the various applications and configuration files involved with managing users and groups. Some of the more important man pages have been listed here:
    User and Group Administrative Applications
    • man chage — A command to modify password aging policies and account expiration.
    • man gpasswd — A command to administer the /etc/group file.
    • man groupadd — A command to add groups.
    • man grpck — A command to verify the /etc/group file.
    • man groupdel — A command to remove groups.
    • man groupmod — A command to modify group membership.
    • man pwck — A command to verify the /etc/passwd and /etc/shadow files.
    • man pwconv — A tool to convert standard passwords to shadow passwords.
    • man pwunconv — A tool to convert shadow passwords to standard passwords.
    • man useradd — A command to add users.
    • man userdel — A command to remove users.
    • man usermod — A command to modify users.
    Configuration Files
    • man 5 group — The file containing group information for the system.
    • man 5 passwd — The file containing user information for the system.
    • man 5 shadow — The file containing passwords and account expiration information for the system.

Chapter 38. Printer Configuration
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Printer Configuration Tool allows users to configure a printer. This tool helps maintain the printer configuration file, print spool directories, print filters, and printer classes.
Red Hat Enterprise Linux 5.10 uses the Common Unix Printing System (CUPS). If a system was upgraded from a previous Red Hat Enterprise Linux version that used CUPS, the upgrade process preserves the configured queues.

Important

The cupsd.conf man page documents configuration of a CUPS server. It includes directives for enabling SSL support. However, CUPS does not allow control of the protocol versions used. Due to the vulnerability described in Resolution for POODLE SSLv3.0 vulnerability (CVE-2014-3566) for components that do not allow SSLv3 to be disabled via configuration settings, Red Hat recommends that you do not rely on this for security. It is recommend that you use stunnel to provide a secure tunnel and disable SSLv3.
For ad-hoc secure connections to a remote system's Print Settings tool, use X11 forwarding over SSH as described in Section 20.7.1, “X11 Forwarding”.
Using Printer Configuration Tool requires root privileges. To start the application, select System (on the panel) > Administration > Printing, or type the command system-config-printer at a shell prompt.
Printer Configuration Tool
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Printer Configuration Tool

Figure 38.1. Printer Configuration Tool

The following types of print queues can be configured:
  • AppSocket/HP JetDirect — a printer connected directly to the network through HP JetDirect or Appsocket interface instead of a computer.
  • Internet Printing Protocol (IPP) — a printer that can be accessed over a TCP/IP network via the Internet Printing Protocol (for example, a printer attached to another Red Hat Enterprise Linux system running CUPS on the network).
  • LPD/LPR Host or Printer — a printer attached to a different UNIX system that can be accessed over a TCP/IP network (for example, a printer attached to another Red Hat Enterprise Linux system running LPD on the network).
  • Networked Windows (SMB) — a printer attached to a different system which is sharing a printer over an SMB network (for example, a printer attached to a Microsoft Windows™ machine).
  • Networked JetDirect — a printer connected directly to the network through HP JetDirect instead of a computer.

Important

If you add a new print queue or modify an existing one, you must apply the changes for them to take effect.
Clicking the Apply button prompts the printer daemon to restart with the changes you have configured.
Clicking the Revert button discards unapplied changes.

38.1. Adding a Local Printer
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To add a local printer, such as one attached through a parallel port or USB port on your computer, click the New Printer button in the main Printer Configuration Tool window to display the window in Figure 38.2, “Adding a Printer”.
Adding a Printer
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Adding a Printer

Figure 38.2. Adding a Printer

Click Forward to proceed.
Enter a unique name for the printer in the Printer Name field. The printer name can contain letters, numbers, dashes (-), and underscores (_); it must not contain any spaces.
You can also use the Description and Location fields to further distinguish this printer from others that may be configured on your system. Both of these fields are optional, and may contain spaces.
Click Forward to open the New Printer dialogue (refer to Figure 38.3, “Adding a Local Printer”). If the printer has been automatically detected, the printer model appears in Select Connection. Select the printer model and click Forward to continue.
If the device does not automatically appear, select the device to which the printer is connected (such as LPT #1 or Serial Port #1) in Select Connection.
Adding a Local Printer
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Adding a Local Printer

Figure 38.3. Adding a Local Printer

Next, select the printer type. Refer to Section 38.5, “Selecting the Printer Model and Finishing” for details.

38.2. Adding an IPP Printer
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An IPP printer is a printer attached to a different system on the same TCP/IP network. The system this printer is attached to may either be running CUPS or simply configured to use IPP.
If a firewall is enabled on the printer server, then the firewall should be configured to allow send / receive connections on the incoming UDP port 631. If a firewall is enabled on the client (the system sending the print request) then the firewall should be configured to allow accept and create connections through port 631.
You can add a networked IPP printer by clicking the New Printer button in the main Printer Configuration Tool window to display the window in Figure 38.2, “Adding a Printer”. Enter the Printer Name (printer names cannot contain spaces and may contain letters, numbers, dashes (-), and underscores (_)), Description, and Location to distinguish this printer from others that you may configure on your system. Click Forward to proceed.
In the window shown in Figure 38.4, “Adding an IPP Printer”, enter the hostname of the IPP printer in the Hostname field as well as a unique name for the printer in the Printername field.
Adding an IPP Printer
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Adding an IPP Printer

Figure 38.4. Adding an IPP Printer

Click Forward to continue.
Next, select the printer type. Refer to Section 38.5, “Selecting the Printer Model and Finishing” for details.

38.3. Adding a Samba (SMB) Printer
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You can add a Samba (SMB) based printer share by clicking the New Printer button in the main Printer Configuration Tool window to display the window in Figure 38.2, “Adding a Printer”. Enter a unique name for the printer in the Printer Name field. The printer name can contain letters, numbers, dashes (-), and underscores (_); it must not contain any spaces.
You can also use the Description and Location fields to further distinguish this printer from others that may be configured on your system. Both of these fields are optional, and may contain spaces.
Adding a SMB Printer
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Adding a SMB Printer

Figure 38.5. Adding a SMB Printer

As shown in Figure 38.5, “Adding a SMB Printer”, available SMB shares are automatically detected and listed in the Share column. Click the arrow ( ) beside a Workgroup to expand it. From the expanded list, select a printer.
If the printer you are looking for does not appear in the list, enter the SMB address in the smb:// field. Use the format computer name/printer share. In Figure 38.5, “Adding a SMB Printer”, the computer name is dellbox, while the printer share is r2.
In the Username field, enter the username to access the printer. This user must exist on the SMB system, and the user must have permission to access the printer. The default user name is typically guest for Windows servers, or nobody for Samba servers.
Enter the Password (if required) for the user specified in the Username field.
You can then test the connection by clicking Verify. Upon successful verification, a dialog box appears confirming printer share accessibility.
Next, select the printer type. Refer to Section 38.5, “Selecting the Printer Model and Finishing” for details.

Warning

Samba printer usernames and passwords are stored in the printer server as unencrypted files readable by root and lpd. Thus, other users that have root access to the printer server can view the username and password you use to access the Samba printer.
As such, when you choose a username and password to access a Samba printer, it is advisable that you choose a password that is different from what you use to access your local Red Hat Enterprise Linux system.
If there are files shared on the Samba print server, it is recommended that they also use a password different from what is used by the print queue.

38.4. Adding a JetDirect Printer
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To add a JetDirect or AppSocket connected printer share, click the New Printer button in the main Printer Configuration Tool window to display the window in Figure 38.2, “Adding a Printer”. Enter a unique name for the printer in the Printer Name field. The printer name can contain letters, numbers, dashes (-), and underscores (_); it must not contain any spaces.
You can also use the Description and Location fields to further distinguish this printer from others that may be configured on your system. Both of these fields are optional, and may contain spaces.
Adding a JetDirect Printer
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Adding a JetDirect Printer

Figure 38.6. Adding a JetDirect Printer

Click Forward to continue.
Text fields for the following options appear:
  • Hostname — The hostname or IP address of the JetDirect printer.
  • Port Number — The port on the JetDirect printer that is listening for print jobs. The default port is 9100.
Next, select the printer type. Refer to Section 38.5, “Selecting the Printer Model and Finishing” for details.

38.5. Selecting the Printer Model and Finishing
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Once you have properly selected a printer queue type, you can choose either option:
  • Select a Printer from database - If you select this option, choose the make of your printer from the list of Makes. If your printer make is not listed, choose Generic.
  • Provide PPD file - A PostScript Printer Description (PPD) file may also be provided with your printer. This file is normally provided by the manufacturer. If you are provided with a PPD file, you can choose this option and use the browser bar below the option description to select the PPD file.
Refer to Figure 38.7, “Selecting a Printer Model”.
Selecting a Printer Model
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Selecting a Printer Model

Figure 38.7. Selecting a Printer Model

After choosing an option, click Forward to continue. Figure 38.7, “Selecting a Printer Model” appears. You now have to choose the corresponding model and driver for the printer.
The recommended printed driver is automatically selected based on the printer model you chose. The print driver processes the data that you want to print into a format the printer can understand. Since a local printer is attached directly to your computer, you need a printer driver to process the data that is sent to the printer.
If you have a PPD file for the device (usually provided by the manufacturer), you can select it by choosing Provide PPD file. You can then browse the filesystem for the PPD file by clicking Browse.

38.5.1. Confirming Printer Configuration
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The last step is to confirm your printer configuration. Click Apply to add the print queue if the settings are correct. Click Back to modify the printer configuration.
After applying the changes, print a test page to ensure the configuration is correct. Refer to Section 38.6, “Printing a Test Page” for details.

38.6. Printing a Test Page
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After you have configured your printer, you should print a test page to make sure the printer is functioning properly. To print a test page, select the printer that you want to try out from the printer list, then click Print Test Page from the printer's Settings tab.
If you change the print driver or modify the driver options, you should print a test page to test the different configuration.

38.7. Modifying Existing Printers
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To delete an existing printer, select the printer and click the Delete button on the toolbar. The printer is removed from the printer list once you confirm deletion of the printer configuration.
To set the default printer, select the printer from the printer list and click the Make Default Printer button in the Settings tab.

38.7.1. The Settings Tab
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To change printer driver configuration, click the corresponding name in the Printer list and click the Settings tab.
You can modify printer settings such as make and model, make a printer the default, print a test page, change the device location (URI), and more.
Settings Tab
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Settings Tab

Figure 38.8. Settings Tab

38.7.2. The Policies Tab
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To change settings in print output, click the Policies tab.
For example, to create a banner page (a page that describes aspects of the print job such as the originating printer, the username from the which the job originated, and the security status of the document being printed) click the Starting Banner or Ending Banner drop-menu and choose the option that best describes the nature of the print jobs (such as topsecret, classified, or confidential).
Policies Tab
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Policies Tab

Figure 38.9. Policies Tab

You can also configure the Error Policy of the printer, by choosing an option from the drop-down menu. You can choose to abort the print job, retry, or stop it.

38.7.3. The Access Control Tab
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You can change user-level access to the configured printer by clicking the Access Control tab.
Add users using the text box and click the Add button beside it. You can then choose to only allow use of the printer to that subset of users or deny use to those users.
Access Control Tab
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Access Control Tab

Figure 38.10. Access Control Tab

38.7.4. The Printer and Job OptionsTab
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The Printer Options tab contains various configuration options for the printer media and output.
Printer Options Tab
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Printer Options Tab

Figure 38.11. Printer Options Tab

  • Page Size — Allows the paper size to be selected. The options include US Letter, US Legal, A3, and A4
  • Media Source — set to Automatic by default. Change this option to use paper from a different tray.
  • Media Type — Allows you to change paper type. Options include: Plain, thick, bond, and transparency.
  • Resolution — Configure the quality and detail of the printout. Default is 300 dots per inch (dpi).
  • Toner Saving — Choose whether the printer uses less toner to conserve resources.
You can also configure printer job options using the Job Options tab. Use the drop-menu and choose the job options you wish to use, such as Landscape modes (horizontal or vertical printout), copies, or scaling (increase or decrease the size of the printable area, which can be used to fit an oversize print area onto a smaller physical sheet of print medium).

38.8. Managing Print Jobs
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When you send a print job to the printer daemon, such as printing a text file from Emacs or printing an image from The GIMP, the print job is added to the print spool queue. The print spool queue is a list of print jobs that have been sent to the printer and information about each print request, such as the status of the request, the job number, and more.
During the printing process, the Printer Status icon appears in the Notification Area on the panel. To check the status of a print job, double click the Printer Status, which displays a window similar to Figure 38.12, “GNOME Print Status”.
GNOME Print Status
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GNOME Print Status

Figure 38.12. GNOME Print Status

To cancel a specific print job listed in the GNOME Print Status, select it from the list and select Edit > Cancel Documents from the pulldown menu.
To view the list of print jobs in the print spool from a shell prompt, type the command lpq. The last few lines look similar to the following:

Example 38.1. Example of lpq output

Rank   Owner/ID              Class  Job Files       Size Time 
active user@localhost+902    A      902 sample.txt  2050 01:20:46
Copy to Clipboard Toggle word wrap
If you want to cancel a print job, find the job number of the request with the command lpq and then use the command lprm job number. For example, lprm 902 would cancel the print job in Example 38.1, “Example of lpq output”. You must have proper permissions to cancel a print job. You can not cancel print jobs that were started by other users unless you are logged in as root on the machine to which the printer is attached.
You can also print a file directly from a shell prompt. For example, the command lpr sample.txt prints the text file sample.txt. The print filter determines what type of file it is and converts it into a format the printer can understand.

38.9. Additional Resources
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To learn more about printing on Red Hat Enterprise Linux, refer to the following resources.

38.9.1. Installed Documentation
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  • map lpr — The manual page for the lpr command that allows you to print files from the command line.
  • man lprm — The manual page for the command line utility to remove print jobs from the print queue.
  • man mpage — The manual page for the command line utility to print multiple pages on one sheet of paper.
  • man cupsd — The manual page for the CUPS printer daemon.
  • man cupsd.conf — The manual page for the CUPS printer daemon configuration file.
  • man classes.conf — The manual page for the class configuration file for CUPS.

38.9.2. Useful Websites
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Chapter 39. Automated Tasks
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In Linux, tasks can be configured to run automatically within a specified period of time, on a specified date, or when the system load average is below a specified number. Red Hat Enterprise Linux is pre-configured to run important system tasks to keep the system updated. For example, the slocate database used by the locate command is updated daily. A system administrator can use automated tasks to perform periodic backups, monitor the system, run custom scripts, and more.
Red Hat Enterprise Linux comes with several automated tasks utilities: cron, at, and batch.

39.1. Cron
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Cron is a daemon that can be used to schedule the execution of recurring jobs according to a combination of the time, day of the month, month, day of the week, and week.
Cron assumes that the system is on continuously. If the system is not on when a job is scheduled, it is not executed. To schedule one-time jobs, refer to Section 39.2, “At and Batch”.
To use the cron service, the vixie-cron RPM package must be installed and the crond service must be running. To determine if the package is installed, use the rpm -q vixie-cron command. To determine if the service is running, use the command /sbin/service crond status.

39.1.1. Configuring Cron Jobs
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The main configuration file for cron, /etc/crontab, contains the following lines: 
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/

# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly
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The first four lines are variables used to configure the environment in which the cron jobs are run. The SHELL variable tells the system which shell environment to use (in this example the bash shell), while the PATH variable defines the path used to execute commands. The output of the cron jobs are emailed to the username defined with the MAILTO variable. If the MAILTO variable is defined as an empty string (MAILTO=""), email is not sent. The HOME variable can be used to set the home directory to use when executing commands or scripts.
Each line in the /etc/crontab file represents a job and has the following format: 
minute   hour   day   month   dayofweek   command
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  • minute — any integer from 0 to 59
  • hour — any integer from 0 to 23
  • day — any integer from 1 to 31 (must be a valid day if a month is specified)
  • month — any integer from 1 to 12 (or the short name of the month such as jan or feb)
  • dayofweek — any integer from 0 to 7, where 0 or 7 represents Sunday (or the short name of the week such as sun or mon)
  • command — the command to execute (the command can either be a command such as ls /proc >> /tmp/proc or the command to execute a custom script)
For any of the above values, an asterisk (*) can be used to specify all valid values. For example, an asterisk for the month value means execute the command every month within the constraints of the other values.
A hyphen (-) between integers specifies a range of integers. For example, 1-4 means the integers 1, 2, 3, and 4.
A list of values separated by commas (,) specifies a list. For example, 3, 4, 6, 8 indicates those four specific integers.
The forward slash (/) can be used to specify step values. The value of an integer can be skipped within a range by following the range with /<integer>. For example, 0-59/2 can be used to define every other minute in the minute field. Step values can also be used with an asterisk. For instance, the value */3 can be used in the month field to run the job every third month.
Any lines that begin with a hash mark (#) are comments and are not processed.
As shown in the /etc/crontab file, the run-parts script executes the scripts in the /etc/cron.hourly/, /etc/cron.daily/, /etc/cron.weekly/, and /etc/cron.monthly/ directories on an hourly, daily, weekly, or monthly basis respectively. The files in these directories should be shell scripts.
If a cron job is required to be executed on a schedule other than hourly, daily, weekly, or monthly, it can be added to the /etc/cron.d/ directory. All files in this directory use the same syntax as /etc/crontab. Refer to Example 39.1, “Sample of /etc/crontab” for examples.

Example 39.1. Sample of /etc/crontab

# record the memory usage of the system every monday
# at 3:30AM in the file /tmp/meminfo
30 3 * * mon cat /proc/meminfo >> /tmp/meminfo
# run custom script the first day of every month at 4:10AM
10 4 1 * * /root/scripts/backup.sh
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Users other than root can configure cron jobs by using the crontab utility. All user-defined crontabs are stored in the /var/spool/cron/ directory and are executed using the usernames of the users that created them. To create a crontab as a user, login as that user and type the command crontab -e to edit the user's crontab using the editor specified by the VISUAL or EDITOR environment variable. The file uses the same format as /etc/crontab. When the changes to the crontab are saved, the crontab is stored according to username and written to the file /var/spool/cron/username.
The cron daemon checks the /etc/crontab file, the /etc/cron.d/ directory, and the /var/spool/cron/ directory every minute for any changes. If any changes are found, they are loaded into memory. Thus, the daemon does not need to be restarted if a crontab file is changed.
Cron jobs can be run at random intervals, which is useful for highly loaded shared networks in order to avoid overloading the network. Job randomization is disabled by default but it can be configured in the /etc/sysconfig/run-parts file by specifying the following parameters:
  • RANDOMIZE — When set to 1, it enables randomize functionality. When set to 0, cron job randomization is disabled.
  • RANDOM — Specifies the initial random seed. It has to be set to an integer value greater than or equal to 1.
  • RANDOMTIME — When set to an integer value greater than or equal to 1, it provides an additional level of randomization.

Example 39.2. Sample of /etc/sysconfig/run-parts - Job Randomization Setting

RANDOMIZE=1
RANDOM=4
RANDOMTIME=8
Copy to Clipboard Toggle word wrap

39.1.2. Controlling Access to Cron
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The /etc/cron.allow and /etc/cron.deny files are used to restrict access to cron. The format of both access control files is one username on each line. Whitespace is not permitted in either file. The cron daemon (crond) does not have to be restarted if the access control files are modified. The access control files are read each time a user tries to add or delete a cron job.
The root user can always use cron, regardless of the usernames listed in the access control files.
If the file cron.allow exists, only users listed in it are allowed to use cron, and the cron.deny file is ignored.
If cron.allow does not exist, users listed in cron.deny are not allowed to use cron.

39.1.3. Starting and Stopping the Service
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To start the cron service, use the command /sbin/service crond start. To stop the service, use the command /sbin/service crond stop. It is recommended that you start the service at boot time. Refer to Chapter 18, Controlling Access to Services for details on starting the cron service automatically at boot time.

39.2. At and Batch
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While cron is used to schedule recurring jobs, the at command is used to schedule a one-time job at a specific time and the batch command is used to schedule a one-time job to be executed when the systems load average drops below 0.8.
To use at or batch, the at RPM package must be installed, and the atd service must be running. To determine if the package is installed, use the rpm -q at command. To determine if the service is running, use the command /sbin/service atd status.

39.2.1. Configuring At Jobs
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To schedule a one-time job at a specific time, type the command at time, where time is the time to execute the command.
The argument time can be one of the following:
  • HH:MM format — For example, 04:00 specifies 4:00 a.m. If the time is already past, it is executed at the specified time the next day.
  • midnight — Specifies 12:00 a.m.
  • noon — Specifies 12:00 p.m.
  • teatime — Specifies 4:00 p.m.
  • month-name day year format — For example, January 15 2002 specifies the 15th day of January in the year 2002. The year is optional.
  • MMDDYY, MM/DD/YY, or MM.DD.YY formats — For example, 011502 for the 15th day of January in the year 2002.
  • now + time — time is in minutes, hours, days, or weeks. For example, now + 5 days specifies that the command should be executed at the same time five days from now.
The time must be specified first, followed by the optional date. For more information about the time format, read the /usr/share/doc/at-<version>/timespec text file.
After typing the at command with the time argument, the at> prompt is displayed. Type the command to execute, press Enter, and type Ctrl+D . Multiple commands can be specified by typing each command followed by the Enter key. After typing all the commands, press Enter to go to a blank line and type Ctrl+D . Alternatively, a shell script can be entered at the prompt, pressing Enter after each line in the script, and typing Ctrl+D on a blank line to exit. If a script is entered, the shell used is the shell set in the user's SHELL environment, the user's login shell, or /bin/sh (whichever is found first).
If the set of commands or script tries to display information to standard out, the output is emailed to the user.
Use the command atq to view pending jobs. Refer to Section 39.2.3, “Viewing Pending Jobs” for more information.
Usage of the at command can be restricted. For more information, refer to Section 39.2.5, “Controlling Access to At and Batch” for details.

39.2.2. Configuring Batch Jobs
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To execute a one-time job when the load average is below 0.8, use the batch command.
After typing the batch command, the at> prompt is displayed. Type the command to execute, press Enter, and type Ctrl+D . Multiple commands can be specified by typing each command followed by the Enter key. After typing all the commands, press Enter to go to a blank line and type Ctrl+D . Alternatively, a shell script can be entered at the prompt, pressing Enter after each line in the script, and typing Ctrl+D on a blank line to exit. If a script is entered, the shell used is the shell set in the user's SHELL environment, the user's login shell, or /bin/sh (whichever is found first). As soon as the load average is below 0.8, the set of commands or script is executed.
If the set of commands or script tries to display information to standard out, the output is emailed to the user.
Use the command atq to view pending jobs. Refer to Section 39.2.3, “Viewing Pending Jobs” for more information.
Usage of the batch command can be restricted. For more information, refer to Section 39.2.5, “Controlling Access to At and Batch” for details.

39.2.3. Viewing Pending Jobs
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To view pending at and batch jobs, use the atq command. The atq command displays a list of pending jobs, with each job on a line. Each line follows the job number, date, hour, job class, and username format. Users can only view their own jobs. If the root user executes the atq command, all jobs for all users are displayed.

39.2.4. Additional Command Line Options
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Additional command line options for at and batch include:
Expand
Table 39.1. at and batch Command Line Options
Option Description
-f Read the commands or shell script from a file instead of specifying them at the prompt.
-m Send email to the user when the job has been completed.
-v Display the time that the job is executed.

39.2.5. Controlling Access to At and Batch
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The /etc/at.allow and /etc/at.deny files can be used to restrict access to the at and batch commands. The format of both access control files is one username on each line. Whitespace is not permitted in either file. The at daemon (atd) does not have to be restarted if the access control files are modified. The access control files are read each time a user tries to execute the at or batch commands.
The root user can always execute at and batch commands, regardless of the access control files.
If the file at.allow exists, only users listed in it are allowed to use at or batch, and the at.deny file is ignored.
If at.allow does not exist, users listed in at.deny are not allowed to use at or batch.

39.2.6. Starting and Stopping the Service
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To start the at service, use the command /sbin/service atd start. To stop the service, use the command /sbin/service atd stop. It is recommended that you start the service at boot time. Refer to Chapter 18, Controlling Access to Services for details on starting the cron service automatically at boot time.

39.3. Additional Resources
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To learn more about configuring automated tasks, refer to the following resources.

39.3.1. Installed Documentation
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  • cron man page — overview of cron.
  • crontab man pages in sections 1 and 5 — The man page in section 1 contains an overview of the crontab file. The man page in section 5 contains the format for the file and some example entries.
  • /usr/share/doc/at-<version>/timespec contains more detailed information about the times that can be specified for cron jobs.
  • at man page — description of at and batch and their command line options.

Chapter 40. Log Files
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Log files are files that contain messages about the system, including the kernel, services, and applications running on it. There are different log files for different information. For example, there is a default system log file, a log file just for security messages, and a log file for cron tasks.
Log files can be very useful when trying to troubleshoot a problem with the system such as trying to load a kernel driver or when looking for unauthorized log in attempts to the system. This chapter discusses where to find log files, how to view log files, and what to look for in log files.
Some log files are controlled by a daemon called syslogd. A list of log messages maintained by syslogd can be found in the /etc/syslog.conf configuration file.

40.1. Locating Log Files
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Most log files are located in the /var/log/ directory. Some applications such as httpd and samba have a directory within /var/log/ for their log files.
You may notice multiple files in the log file directory with numbers after them. These are created when the log files are rotated. Log files are rotated so their file sizes do not become too large. The logrotate package contains a cron task that automatically rotates log files according to the /etc/logrotate.conf configuration file and the configuration files in the /etc/logrotate.d/ directory. By default, it is configured to rotate every week and keep four weeks worth of previous log files.

40.2. Viewing Log Files
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Most log files are in plain text format. You can view them with any text editor such as Vi or Emacs. Some log files are readable by all users on the system; however, root privileges are required to read most log files.
To view system log files in an interactive, real-time application, use the System Log Viewer. To start the application, go to Applications (the main menu on the panel) > System > System Logs, or type the command gnome-system-log at a shell prompt.
The application only displays log files that exist; thus, the list might differ from the one shown in Figure 40.1, “System Log Viewer”.
System Log Viewer
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System Log Viewer

Figure 40.1. System Log Viewer

To filter the contents of the selected log file, click on View from the menu and select Filter as illustrated below.
System Log Viewer - View Menu
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System Log Viewer - View Menu

Figure 40.2. System Log Viewer - View Menu

Selecting the Filter menu item will display the Filter text field where you can type the keywords you wish to use for your filter. To clear your filter click on the Clear button.The figure below illustrates a sample filter.
System Log Viewer - Filter
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System Log Viewer - Filter

Figure 40.3. System Log Viewer - Filter

40.3. Adding a Log File
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To add a log file you wish to view in the list, select File > Open. This will display the Open Log window where you can select the directory and filename of the log file you wish to view.The figure below illustrates the Open Log window.
Adding a Log File
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Adding a Log File

Figure 40.4. Adding a Log File

Click on the Open button to open the file. The file is immediately added to the viewing list where you can select it and view the contents.
Please also note that the System Log Viewer also allows you to open zipped logs whose filenames end in ".gz".

40.4. Monitoring Log Files
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System Log Viewer monitors all opened logs by default. If a new line is added to a monitored log file, the log name appears in bold in the log list. If the log file is selected or displayed, the new lines appear in bold at the bottom of the log file and after five seconds are displayed in normal format. This is illustrated in the figures below. The figure below illustrates a new alert in the messages log file. The log file is listed in bold text.
Log File Alert
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Log File Alert

Figure 40.5. Log File Alert

Clicking on the messages log file displays the logs in the file with the new lines in bold as illustrated below.
Log file contents
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Log file contents

Figure 40.6. Log file contents

The new lines are displayed in bold for five seconds after which they are displayed in normal font.
Log file contents after five seconds
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Log file contents after five seconds

Figure 40.7. Log file contents after five seconds

Part V. System Monitoring
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System administrators also monitor system performance. Red Hat Enterprise Linux contains tools to assist administrators with these tasks.

Chapter 41. SystemTap
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41.1. Introduction
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SystemTap provides a simple command line interface and scripting language to simplify the gathering of information about the running Linux kernel so that it can be further analyzed. Data may be extracted, filtered, and summarized quickly and safely, to enable diagnoses of complex performance or functional problems.
SystemTap allows scripts to be written in the SystemTap scripting language, which are then compiled to C-code kernel modules and inserted into the kernel.
The essential idea behind a systemtap script is to name events, and to give them handlers. Whenever a specified event occurs, the Linux kernel runs the handler as if it were a quick subroutine, then resumes. There are several kind of events, such as entering or exiting a function, a timer expiring, or the entire systemtap session starting or stopping. A handler is a series of script language statements that specify the work to be done whenever the event occurs. This work normally includes extracting data from the event context, storing them into internal variables, or printing results.

41.2. Implementation
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SystemTap takes a compiler-oriented approach to generating instrumentation. Refer to Figure 41.1, “Flow of Data in SystemTap” "Flow of data in SystemTap" for an overall diagram of SystemTap used in this discussion. In the upper right hand corner of the diagram is the probe.stp, the probe script the developer has written. This is parsed by the translator into parse trees. During this time the input is checked for syntax errors. The translator then performs elaboration, pulling in additional code from the script library and determining locations of probe points and variables from the debug information. After the elaboration is complete the translator can generate the probe.c, the kernel module in C.
The probe.c file is compiled into a regular kernel module, probe.ko, using the GCC compiler. The compilation may pull in support code from the runtime libraries. After GCC has generated the probe.ko, the SystemTap daemon is started to collect the output of the instrumentation module. The instrumentation module is loaded into the kernel, and data collection is started. Data from the instrumentation module is transferred to user-space via relayfs and displayed by the daemon. When the user hits Control-C the daemon unloads the module, which also shuts down the data collection process.
Flow of Data in SystemTap
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Flow of Data in SystemTap

Figure 41.1. Flow of Data in SystemTap

41.3. Using SystemTap
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Systemtap works by translating a SystemTap script to C, running the system C compiler to create a kernel module from that. When the module is loaded, it activates all the probed events by hooking into the kernel. Then, as events occur on any processor, the compiled handlers run. Eventually, the session stops, the hooks are disconnected, and the module removed. This entire process is driven from a single command-line program, stap.

41.3.1.  Tracing
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The simplest kind of probe is simply to trace an event. This is the effect of inserting strategically located print statements into a program. This is often the first step of problem solving: explore by seeing a history of what has happened.
This style of instrumentation is the simplest. It just asks systemtap to print something at each event. To express this in the script language, you need to say where to probe and what to print there.
41.3.1.1. Where to Probe
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Systemtap supports a number of built-in events. The library of scripts that comes with systemtap, each called a "tapset", may define additional ones defined in terms of the built-in family. See the stapprobes man page for details. All these events are named using a unified syntax that looks like dot-separated parameterized identifiers:
Expand
Table 41.1. SystemTap Events
Event Description
begin The startup of the systemtap session.
end The end of the systemtap session.
kernel.function("sys_open") The entry to the function named sys_open in the kernel.
syscall.close.return The return from the close system call..
module("ext3").statement(0xdeadbeef) The addressed instruction in the ext3 filesystem driver.
timer.ms(200) A timer that fires every 200 milliseconds.
We will use as a demonstration case that you would like to trace all function entries and exits in a source file, for example net/socket.c in the kernel. The kernel.function probe point lets you express that easily, since systemtap examines the kernel's debugging information to relate object code to source code. It works like a debugger: if you can name or place it, you can probe it. Use kernel.function("*@net/socket.c") for the function entries, and kernel.function("*@net/socket.c").return for the exits. Note the use of wildcards in the function name part, and the subsequent @FILENAME part. You can also put wildcards into the file name, and even add a colon (:) and a line number, if you want to restrict the search that precisely. Since systemtap will put a separate probe in every place that matches a probe point, a few wildcards can expand to hundreds or thousands of probes, so be careful what you ask for.
Once you identify the probe points, the skeleton of the systemtap script appears. The probe keyword introduces a probe point, or a comma-separated list of them. The following { and } braces enclose the handler for all listed probe points.
You can run this script as is, though with empty handlers there will be no output. Put the two lines into a new file. Run stap -v FILE. Terminate it any time with ^C. (The -v option tells systemtap to print more verbose messages during its processing. Try the -h option to see more options.)
41.3.1.2. What to Print
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Since you are interested in each function that was entered and exited, a line should be printed for each, containing the function name. In order to make that list easy to read, systemtap should indent the lines so that functions called by other traced functions are nested deeper. To tell each single process apart from any others that may be running concurrently, systemtap should also print the process ID in the line.

Chapter 42. Gathering System Information
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Before you learn how to configure your system, you should learn how to gather essential system information. For example, you should know how to find the amount of free memory, the amount of available hard drive space, how your hard drive is partitioned, and what processes are running. This chapter discusses how to retrieve this type of information from your Red Hat Enterprise Linux system using simple commands and a few simple programs.

42.1. System Processes
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The ps ax command displays a list of current system processes, including processes owned by other users. To display the owner alongside each process, use the ps aux command. This list is a static list; in other words, it is a snapshot of what was running when you invoked the command. If you want a constantly updated list of running processes, use top as described below.
The ps output can be long. To prevent it from scrolling off the screen, you can pipe it through less: 
ps aux | less
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You can use the ps command in combination with the grep command to see if a process is running. For example, to determine if Emacs is running, use the following command: 
ps ax | grep emacs
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The top command displays currently running processes and important information about them including their memory and CPU usage. The list is both real-time and interactive. An example of output from the top command is provided as follows: 
top - 15:02:46 up 35 min,  4 users,  load average: 0.17, 0.65, 1.00
Tasks: 110 total,   1 running, 107 sleeping,   0 stopped,   2 zombie
Cpu(s): 41.1% us,  2.0% sy,  0.0% ni, 56.6% id,  0.0% wa,  0.3% hi,  0.0% si
Mem:    775024k total,   772028k used,     2996k free,    68468k buffers
Swap:  1048568k total,      176k used,  1048392k free,   441172k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 4624 root      15   0 40192  18m 7228 S 28.4  2.4   1:23.21 X
 4926 mhideo    15   0 55564  33m 9784 S 13.5  4.4   0:25.96 gnome-terminal
 6475 mhideo    16   0  3612  968  760 R  0.7  0.1   0:00.11 top
 4920 mhideo    15   0 20872  10m 7808 S  0.3  1.4   0:01.61 wnck-applet
    1 root      16   0  1732  548  472 S  0.0  0.1   0:00.23 init
    2 root      34  19     0    0    0 S  0.0  0.0   0:00.00 ksoftirqd/0
    3 root       5 -10     0    0    0 S  0.0  0.0   0:00.03 events/0
    4 root       6 -10     0    0    0 S  0.0  0.0   0:00.02 khelper
    5 root       5 -10     0    0    0 S  0.0  0.0   0:00.00 kacpid
   29 root       5 -10     0    0    0 S  0.0  0.0   0:00.00 kblockd/0
   47 root      16   0     0    0    0 S  0.0  0.0   0:01.74 pdflush
   50 root      11 -10     0    0    0 S  0.0  0.0   0:00.00 aio/0
   30 root      15   0     0    0    0 S  0.0  0.0   0:00.05 khubd
   49 root      16   0     0    0    0 S  0.0  0.0   0:01.44 kswapd0
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To exit top, press the q key.
Table 42.1, “Interactive top commands” contains useful interactive commands that you can use with top. For more information, refer to the top(1) manual page.
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Table 42.1. Interactive top commands
Command Description
Space Immediately refresh the display
h Display a help screen
k Kill a process. You are prompted for the process ID and the signal to send to it.
n Change the number of processes displayed. You are prompted to enter the number.
u Sort by user.
M Sort by memory usage.
P Sort by CPU usage.
If you prefer a graphical interface for top, you can use the GNOME System Monitor. To start it from the desktop, select System > Administration > System Monitor or type gnome-system-monitor at a shell prompt (such as an XTerm). Select the Process Listing tab.
The GNOME System Monitor allows you to search for a process in the list of running processes. Using the Gnome System Monitor, you can also view all processes, your processes, or active processes.
The Edit menu item allows you to:
  • Stop a process.
  • Continue or start a process.
  • End a processes.
  • Kill a process.
  • Change the priority of a selected process.
  • Edit the System Monitor preferences. These include changing the interval seconds to refresh the list and selecting process fields to display in the System Monitor window.
The View menu item allows you to:
  • View only active processes.
  • View all processes.
  • View my processes.
  • View process dependencies.
  • Hide a process.
  • View hidden processes.
  • View memory maps.
  • View the files opened by the selected process.
To stop a process, select it and click End Process. Alternatively you can also stop a process by selecting it, clicking Edit on your menu and selecting Stop Process.
To sort the information by a specific column, click on the name of the column. This sorts the information by the selected column in ascending order. Click on the name of the column again to toggle the sort between ascending and descending order.
GNOME System Monitor
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GNOME System Monitor

Figure 42.1. GNOME System Monitor

42.2. Memory Usage
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The free command displays the total amount of physical memory and swap space for the system as well as the amount of memory that is used, free, shared, in kernel buffers, and cached. 
             total       used       free     shared    buffers     cached
 Mem:        645712     549720      95992          0     176248     224452
 -/+ buffers/cache:     149020     496692
 Swap:      1310712          0    1310712
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The command free -m shows the same information in megabytes, which are easier to read. 
             total       used       free     shared    buffers     cached
Mem:           630        536         93          0        172        219
-/+ buffers/cache:        145        485
Swap:         1279          0       1279
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If you prefer a graphical interface for free, you can use the GNOME System Monitor. To start it from the desktop, go to System > Administration > System Monitor or type gnome-system-monitor at a shell prompt (such as an XTerm). Click on the Resources tab.
GNOME System Monitor - Resources tab
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GNOME System Monitor - Resources tab

Figure 42.2. GNOME System Monitor - Resources tab

42.3. File Systems
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The df command reports the system's disk space usage. If you type the command df at a shell prompt, the output looks similar to the following: 
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
                       11675568   6272120   4810348  57% / /dev/sda1
	                 100691      9281     86211  10% /boot
none                     322856         0    322856   0% /dev/shm
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By default, this utility shows the partition size in 1 kilobyte blocks and the amount of used and available disk space in kilobytes. To view the information in megabytes and gigabytes, use the command df -h. The -h argument stands for human-readable format. The output looks similar to the following: 
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
                        12G  6.0G  4.6G  57% / /dev/sda1
			99M  9.1M   85M  10% /boot
none 			316M     0  316M   0% /dev/shm
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In the list of mounted partitions, there is an entry for /dev/shm. This entry represents the system's virtual memory file system.
The du command displays the estimated amount of space being used by files in a directory. If you type du at a shell prompt, the disk usage for each of the subdirectories is displayed in a list. The grand total for the current directory and subdirectories are also shown as the last line in the list. If you do not want to see the totals for all the subdirectories, use the command du -hs to see only the grand total for the directory in human-readable format. Use the du --help command to see more options.
To view the system's partitions and disk space usage in a graphical format, use the Gnome System Monitor by clicking on System > Administration > System Monitor or type gnome-system-monitor at a shell prompt (such as an XTerm). Select the File Systems tab to view the system's partitions. The figure below illustrates the File Systems tab.
GNOME System Monitor - File Systems
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GNOME System Monitor - File Systems

Figure 42.3. GNOME System Monitor - File Systems

42.4. Hardware
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If you are having trouble configuring your hardware or just want to know what hardware is in your system, you can use the Hardware Browser application to display the hardware that can be probed. To start the program from the desktop, select System (the main menu on the panel) > Administration > Hardware or type hwbrowser at a shell prompt. As shown in Figure 42.4, “Hardware Browser”, it displays your CD-ROM devices, diskette drives, hard drives and their partitions, network devices, pointing devices, system devices, and video cards. Click on the category name in the left menu, and the information is displayed.
Hardware Browser
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Hardware Browser

Figure 42.4. Hardware Browser

The Device Manager application can also be used to display your system hardware. This application can be started by selecting System (the main menu on the panel) > Administration > Hardware like the Hardware Browser. To start the application from a terminal, type hal-device-manager. Depending on your installation preferences, the graphical menu above may start this application or the Hardware Browser when clicked. The figure below illustrates the Device Manager window.
Device Manager
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Device Manager

Figure 42.5. Device Manager

You can also use the lspci command to list all PCI devices. Use the command lspci -v for more verbose information or lspci -vv for very verbose output.
For example, lspci can be used to determine the manufacturer, model, and memory size of a system's video card: 
00:00.0 Host bridge: ServerWorks CNB20LE Host Bridge (rev 06)
00:00.1 Host bridge: ServerWorks CNB20LE Host Bridge (rev 06)
00:01.0 VGA compatible controller: S3 Inc. Savage 4 (rev 04)
00:02.0 Ethernet controller: Intel Corp. 82557/8/9 [Ethernet Pro 100] (rev 08)
00:0f.0 ISA bridge: ServerWorks OSB4 South Bridge (rev 50)
00:0f.1 IDE interface: ServerWorks OSB4 IDE Controller
00:0f.2 USB Controller: ServerWorks OSB4/CSB5 OHCI USB Controller (rev 04)
01:03.0 SCSI storage controller: Adaptec AIC-7892P U160/m (rev 02)
01:05.0 RAID bus controller: IBM ServeRAID Controller
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The lspci is also useful to determine the network card in your system if you do not know the manufacturer or model number.

42.5. Additional Resources
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To learn more about gathering system information, refer to the following resources.

42.5.1. Installed Documentation
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  • ps --help — Displays a list of options that can be used with ps.
  • top manual page — Type man top to learn more about top and its many options.
  • free manual page — type man free to learn more about free and its many options.
  • df manual page — Type man df to learn more about the df command and its many options.
  • du manual page — Type man du to learn more about the du command and its many options.
  • lspci manual page — Type man lspci to learn more about the lspci command and its many options.
  • /proc/ directory — The contents of the /proc/ directory can also be used to gather more detailed system information.

Chapter 43. OProfile
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OProfile is a low overhead, system-wide performance monitoring tool. It uses the performance monitoring hardware on the processor to retrieve information about the kernel and executables on the system, such as when memory is referenced, the number of L2 cache requests, and the number of hardware interrupts received. On a Red Hat Enterprise Linux system, the oprofile RPM package must be installed to use this tool.
Many processors include dedicated performance monitoring hardware. This hardware makes it possible to detect when certain events happen (such as the requested data not being in cache). The hardware normally takes the form of one or more counters that are incremented each time an event takes place. When the counter value, essentially rolls over, an interrupt is generated, making it possible to control the amount of detail (and therefore, overhead) produced by performance monitoring.
OProfile uses this hardware (or a timer-based substitute in cases where performance monitoring hardware is not present) to collect samples of performance-related data each time a counter generates an interrupt. These samples are periodically written out to disk; later, the data contained in these samples can then be used to generate reports on system-level and application-level performance.
OProfile is a useful tool, but be aware of some limitations when using it:
  • Use of shared libraries — Samples for code in shared libraries are not attributed to the particular application unless the --separate=library option is used.
  • Performance monitoring samples are inexact — When a performance monitoring register triggers a sample, the interrupt handling is not precise like a divide by zero exception. Due to the out-of-order execution of instructions by the processor, the sample may be recorded on a nearby instruction.
  • opreport does not associate samples for inline functions' properly — opreport uses a simple address range mechanism to determine which function an address is in. Inline function samples are not attributed to the inline function but rather to the function the inline function was inserted into.
  • OProfile accumulates data from multiple runs — OProfile is a system-wide profiler and expects processes to start up and shut down multiple times. Thus, samples from multiple runs accumulate. Use the command opcontrol --reset to clear out the samples from previous runs.
  • Non-CPU-limited performance problems — OProfile is oriented to finding problems with CPU-limited processes. OProfile does not identify processes that are asleep because they are waiting on locks or for some other event to occur (for example an I/O device to finish an operation).

43.1. Overview of Tools
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Table 43.1, “OProfile Commands” provides a brief overview of the tools provided with the oprofile package.
Expand
Table 43.1. OProfile Commands
Command Description
ophelp
Displays available events for the system's processor along with a brief description of each.
opimport
Converts sample database files from a foreign binary format to the native format for the system. Only use this option when analyzing a sample database from a different architecture.
opannotate Creates annotated source for an executable if the application was compiled with debugging symbols. Refer to Section 43.5.4, “Using opannotate” for details.
opcontrol
Configures what data is collected. Refer to Section 43.2, “Configuring OProfile” for details.
opreport
Retrieves profile data. Refer to Section 43.5.1, “Using opreport” for details.
oprofiled
Runs as a daemon to periodically write sample data to disk.

43.2. Configuring OProfile
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Before OProfile can be run, it must be configured. At a minimum, selecting to monitor the kernel (or selecting not to monitor the kernel) is required. The following sections describe how to use the opcontrol utility to configure OProfile. As the opcontrol commands are executed, the setup options are saved to the /root/.oprofile/daemonrc file.

43.2.1. Specifying the Kernel
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First, configure whether OProfile should monitor the kernel. This is the only configuration option that is required before starting OProfile. All others are optional.
To monitor the kernel, execute the following command as root: 
opcontrol --setup --vmlinux=/usr/lib/debug/lib/modules/`uname -r`/vmlinux
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Note

The debuginfo package must be installed (which contains the uncompressed kernel) in order to monitor the kernel.
To configure OProfile not to monitor the kernel, execute the following command as root: 
opcontrol --setup --no-vmlinux
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This command also loads the oprofile kernel module, if it is not already loaded, and creates the /dev/oprofile/ directory, if it does not already exist. Refer to Section 43.6, “Understanding /dev/oprofile/” for details about this directory.

Note

Even if OProfile is configured not to profile the kernel, the SMP kernel still must be running so that the oprofile module can be loaded from it.
Setting whether samples should be collected within the kernel only changes what data is collected, not how or where the collected data is stored. To generate different sample files for the kernel and application libraries, refer to Section 43.2.3, “Separating Kernel and User-space Profiles”.

43.2.2. Setting Events to Monitor
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Most processors contain counters, which are used by OProfile to monitor specific events. As shown in Table 43.2, “OProfile Processors and Counters”, the number of counters available depends on the processor.
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Table 43.2. OProfile Processors and Counters
Processor cpu_type Number of Counters
Pentium Pro i386/ppro 2
Pentium II i386/pii 2
Pentium III i386/piii 2
Pentium 4 (non-hyper-threaded) i386/p4 8
Pentium 4 (hyper-threaded) i386/p4-ht 4
Athlon i386/athlon 4
AMD64 x86-64/hammer 4
Itanium ia64/itanium 4
Itanium 2 ia64/itanium2 4
TIMER_INT timer 1
IBM eServer iSeries and pSeries timer 1
  ppc64/power4 8
  ppc64/power5 6
  ppc64/970 8
IBM eServer S/390 and S/390x timer 1
IBM eServer zSeries timer 1
Use Table 43.2, “OProfile Processors and Counters” to verify that the correct processor type was detected and to determine the number of events that can be monitored simultaneously. timer is used as the processor type if the processor does not have supported performance monitoring hardware.
If timer is used, events cannot be set for any processor because the hardware does not have support for hardware performance counters. Instead, the timer interrupt is used for profiling.
If timer is not used as the processor type, the events monitored can be changed, and counter 0 for the processor is set to a time-based event by default. If more than one counter exists on the processor, the counters other than counter 0 are not set to an event by default. The default events monitored are shown in Table 43.3, “Default Events”.
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Table 43.3. Default Events
Processor Default Event for Counter Description
Pentium Pro, Pentium II, Pentium III, Athlon, AMD64 CPU_CLK_UNHALTED The processor's clock is not halted
Pentium 4 (HT and non-HT) GLOBAL_POWER_EVENTS The time during which the processor is not stopped
Itanium 2 CPU_CYCLES CPU Cycles
TIMER_INT (none) Sample for each timer interrupt
ppc64/power4 CYCLES Processor Cycles
ppc64/power5 CYCLES Processor Cycles
ppc64/970 CYCLES Processor Cycles
The number of events that can be monitored at one time is determined by the number of counters for the processor. However, it is not a one-to-one correlation; on some processors, certain events must be mapped to specific counters. To determine the number of counters available, execute the following command: 
ls -d /dev/oprofile/[0-9]*
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The events available vary depending on the processor type. To determine the events available for profiling, execute the following command as root (the list is specific to the system's processor type): 
ophelp
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The events for each counter can be configured via the command line or with a graphical interface. For more information on the graphical interface, refer to Section 43.8, “Graphical Interface”. If the counter cannot be set to a specific event, an error message is displayed.
To set the event for each configurable counter via the command line, use opcontrol: 
opcontrol --event=<event-name>:<sample-rate>
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Replace <event-name> with the exact name of the event from ophelp, and replace <sample-rate> with the number of events between samples.
43.2.2.1. Sampling Rate
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By default, a time-based event set is selected. It creates a sample every 100,000 clock cycles per processor. If the timer interrupt is used, the timer is set to whatever the jiffy rate is and is not user-settable. If the cpu_type is not timer, each event can have a sampling rate set for it. The sampling rate is the number of events between each sample snapshot.
When setting the event for the counter, a sample rate can also be specified: 
opcontrol --event=<event-name>:<sample-rate>
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Replace <sample-rate> with the number of events to wait before sampling again. The smaller the count, the more frequent the samples. For events that do not happen frequently, a lower count may be needed to capture the event instances.

Warning

Be extremely careful when setting sampling rates. Sampling too frequently can overload the system, causing the system to appear as if it is frozen or causing the system to actually freeze.
43.2.2.2. Unit Masks
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Some user performance monitoring events may also require unit masks to further define the event.
Unit masks for each event are listed with the ophelp command. The values for each unit mask are listed in hexadecimal format. To specify more than one unit mask, the hexadecimal values must be combined using a bitwise or operation. 
opcontrol --event=<event-name>:<sample-rate>:<unit-mask>
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43.2.3. Separating Kernel and User-space Profiles
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By default, kernel mode and user mode information is gathered for each event. To configure OProfile to ignore events in kernel mode for a specific counter, execute the following command: 
opcontrol --event=<event-name>:<sample-rate>:<unit-mask>:0
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Execute the following command to start profiling kernel mode for the counter again: 
opcontrol --event=<event-name>:<sample-rate>:<unit-mask>:1
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To configure OProfile to ignore events in user mode for a specific counter, execute the following command: 
opcontrol --event=<event-name>:<sample-rate>:<unit-mask>:<kernel>:0
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Execute the following command to start profiling user mode for the counter again: 
opcontrol --event=<event-name>:<sample-rate>:<unit-mask>:<kernel>:1
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When the OProfile daemon writes the profile data to sample files, it can separate the kernel and library profile data into separate sample files. To configure how the daemon writes to sample files, execute the following command as root: 
opcontrol --separate=<choice>
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<choice> can be one of the following:
  • none — do not separate the profiles (default)
  • library — generate per-application profiles for libraries
  • kernel — generate per-application profiles for the kernel and kernel modules
  • all — generate per-application profiles for libraries and per-application profiles for the kernel and kernel modules
If --separate=library is used, the sample file name includes the name of the executable as well as the name of the library.

Note

These configuration changes will take effect when oprofile is restarted.

43.3. Starting and Stopping OProfile
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To start monitoring the system with OProfile, execute the following command as root: 
opcontrol --start
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Output similar to the following is displayed: 
Using log file /var/lib/oprofile/oprofiled.log Daemon started. Profiler running.
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The settings in /root/.oprofile/daemonrc are used.
The OProfile daemon, oprofiled, is started; it periodically writes the sample data to the /var/lib/oprofile/samples/ directory. The log file for the daemon is located at /var/lib/oprofile/oprofiled.log.
To stop the profiler, execute the following command as root: 
opcontrol --shutdown
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43.4. Saving Data
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Sometimes it is useful to save samples at a specific time. For example, when profiling an executable, it may be useful to gather different samples based on different input data sets. If the number of events to be monitored exceeds the number of counters available for the processor, multiple runs of OProfile can be used to collect data, saving the sample data to different files each time.
To save the current set of sample files, execute the following command, replacing <name> with a unique descriptive name for the current session. 
opcontrol --save=<name>
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The directory /var/lib/oprofile/samples/name/ is created and the current sample files are copied to it.

43.5. Analyzing the Data
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Periodically, the OProfile daemon, oprofiled, collects the samples and writes them to the /var/lib/oprofile/samples/ directory. Before reading the data, make sure all data has been written to this directory by executing the following command as root: 
opcontrol --dump
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Each sample file name is based on the name of the executable. For example, the samples for the default event on a Pentium III processor for /bin/bash becomes: 
\{root\}/bin/bash/\{dep\}/\{root\}/bin/bash/CPU_CLK_UNHALTED.100000
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The following tools are available to profile the sample data once it has been collected:
  • opreport
  • opannotate
Use these tools, along with the binaries profiled, to generate reports that can be further analyzed.

Warning

The executable being profiled must be used with these tools to analyze the data. If it must change after the data is collected, backup the executable used to create the samples as well as the sample files. Please note that the sample file and the binary have to agree. Making a backup isn't going to work if they do not match. oparchive can be used to address this problem.
Samples for each executable are written to a single sample file. Samples from each dynamically linked library are also written to a single sample file. While OProfile is running, if the executable being monitored changes and a sample file for the executable exists, the existing sample file is automatically deleted. Thus, if the existing sample file is needed, it must be backed up, along with the executable used to create it before replacing the executable with a new version. The oprofile analysis tools use the executable file that created the samples during analysis. If the executable changes the analysis tools will be unable to analyze the associated samples. Refer to Section 43.4, “Saving Data” for details on how to backup the sample file.

43.5.1. Using opreport
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The opreport tool provides an overview of all the executables being profiled.
The following is part of a sample output: 
Profiling through timer interrupt
TIMER:0|
samples|      %|
------------------
25926 97.5212 no-vmlinux
359  1.3504 pi
65  0.2445 Xorg
62  0.2332 libvte.so.4.4.0
56  0.2106 libc-2.3.4.so
34  0.1279 libglib-2.0.so.0.400.7
19  0.0715 libXft.so.2.1.2
17  0.0639 bash
8  0.0301 ld-2.3.4.so
8  0.0301 libgdk-x11-2.0.so.0.400.13
6  0.0226 libgobject-2.0.so.0.400.7
5  0.0188 oprofiled
4  0.0150 libpthread-2.3.4.so
4  0.0150 libgtk-x11-2.0.so.0.400.13
3  0.0113 libXrender.so.1.2.2
3  0.0113 du
1  0.0038 libcrypto.so.0.9.7a
1  0.0038 libpam.so.0.77
1  0.0038 libtermcap.so.2.0.8
1  0.0038 libX11.so.6.2
1  0.0038 libgthread-2.0.so.0.400.7
1  0.0038 libwnck-1.so.4.9.0
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Each executable is listed on its own line. The first column is the number of samples recorded for the executable. The second column is the percentage of samples relative to the total number of samples. The third column is the name of the executable.
Refer to the opreport man page for a list of available command line options, such as the -r option used to sort the output from the executable with the smallest number of samples to the one with the largest number of samples.

43.5.2. Using opreport on a Single Executable
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To retrieve more detailed profiled information about a specific executable, use opreport: 
opreport <mode> <executable>
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<executable> must be the full path to the executable to be analyzed. <mode> must be one of the following:
-l
List sample data by symbols. For example, the following is part of the output from running the command opreport -l /lib/tls/libc-<version>.so: 
samples  %        symbol name
12       21.4286  __gconv_transform_utf8_internal
5         8.9286  _int_malloc
4         7.1429  malloc
3         5.3571  __i686.get_pc_thunk.bx
3         5.3571  _dl_mcount_wrapper_check
3         5.3571  mbrtowc
3         5.3571  memcpy
2         3.5714  _int_realloc
2         3.5714  _nl_intern_locale_data
2         3.5714  free
2         3.5714  strcmp
1         1.7857  __ctype_get_mb_cur_max
1         1.7857  __unregister_atfork
1         1.7857  __write_nocancel
1         1.7857  _dl_addr
1         1.7857  _int_free
1         1.7857  _itoa_word
1         1.7857  calc_eclosure_iter
1         1.7857  fopen@@GLIBC_2.1
1         1.7857  getpid
1         1.7857  memmove
1         1.7857  msort_with_tmp
1         1.7857  strcpy
1         1.7857  strlen
1         1.7857  vfprintf
1         1.7857  write
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The first column is the number of samples for the symbol, the second column is the percentage of samples for this symbol relative to the overall samples for the executable, and the third column is the symbol name.
To sort the output from the largest number of samples to the smallest (reverse order), use -r in conjunction with the -l option.
-i <symbol-name>
List sample data specific to a symbol name. For example, the following output is from the command opreport -l -i __gconv_transform_utf8_internal /lib/tls/libc-<version>.so: 
samples  %        symbol name
12       100.000  __gconv_transform_utf8_internal
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The first line is a summary for the symbol/executable combination.
The first column is the number of samples for the memory symbol. The second column is the percentage of samples for the memory address relative to the total number of samples for the symbol. The third column is the symbol name.
-d
List sample data by symbols with more detail than -l. For example, the following output is from the command opreport -l -d __gconv_transform_utf8_internal /lib/tls/libc-<version>.so: 
vma      samples  %        symbol name
00a98640 12       100.000  __gconv_transform_utf8_internal
00a98640 1         8.3333
00a9868c 2        16.6667
00a9869a 1         8.3333
00a986c1 1         8.3333
00a98720 1         8.3333
00a98749 1         8.3333
00a98753 1         8.3333
00a98789 1         8.3333
00a98864 1         8.3333
00a98869 1         8.3333
00a98b08 1         8.3333
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The data is the same as the -l option except that for each symbol, each virtual memory address used is shown. For each virtual memory address, the number of samples and percentage of samples relative to the number of samples for the symbol is displayed.
-x<symbol-name>
Exclude the comma-separated list of symbols from the output.
session:<name>
Specify the full path to the session or a directory relative to the /var/lib/oprofile/samples/ directory.
OProfile collects data on a system-wide basis for kernel- and user-space code running on the machine. However, once a module is loaded into the kernel, the information about the origin of the kernel module is lost. The module could have come from the initrd file on boot up, the directory with the various kernel modules, or a locally created kernel module. As a result when OProfile records sample for a module, it just lists the samples for the modules for an executable in the root directory, but this is unlikely to be the place with the actual code for the module. You will need to take some steps to make sure that analysis tools get the executable.
For example on an AMD64 machine the sampling is set up to record "Data cache accesses" and "Data cache misses" and assuming you would like to see the data for the ext3 module: 
~]$ opreport /ext3
CPU: AMD64 processors, speed 797.948 MHz (estimated)
Counted DATA_CACHE_ACCESSES events (Data cache accesses) with a unit mask of 0x00 (No unit mask) count 500000
Counted DATA_CACHE_MISSES events (Data cache misses) with a unit mask of 0x00 (No unit mask) count 500000
DATA_CACHE_ACC...|DATA_CACHE_MIS...|
samples|      %|  samples|      %|
------------------------------------
148721 100.000      1493 100.000 ext3
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To get a more detailed view of the actions of the module, you will need to either have the module unstripped (e.g. installed from a custom build) or have the debuginfo RPM installed for the kernel.
Find out which kernel is running, "uname -a", get the appropriate debuginfo rpm, and install on the machine.
Then make a symbolic link so oprofile finds the code for the module in the correct place: 
~]# ln -s /lib/modules/`uname -r`/kernel/fs/ext3/ext3.ko /ext3
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Then the detailed information can be obtained with: 
~]# opreport image:/ext3 -l|more
warning: could not check that the binary file /ext3 has not been modified since the profile was taken. Results may be inaccurate.
CPU: AMD64 processors, speed 797.948 MHz (estimated)
Counted DATA_CACHE_ACCESSES events (Data cache accesses) with a unit mask of 0x00 (No unit mask) count 500000
Counted DATA_CACHE_MISSES events (Data cache misses) with a unit mask of 0x00 (No unit mask) count 500000
samples  %        samples  %        symbol name
16728    11.2479  7         0.4689  ext3_group_sparse
16454    11.0637  4         0.2679  ext3_count_free_blocks
14583     9.8056  51        3.4159  ext3_fill_super
8281      5.5681  129       8.6403  ext3_ioctl
7810      5.2514  62        4.1527  ext3_write_info
7286      4.8991  67        4.4876  ext3_ordered_writepage
6509      4.3767  130       8.7073  ext3_new_inode
6378      4.2886  156      10.4488  ext3_new_block
5932      3.9887  87        5.8272  ext3_xattr_block_list
...
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43.5.4. Using opannotate
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The opannotate tool tries to match the samples for particular instructions to the corresponding lines in the source code. The resulting files generated should have the samples for the lines at the left. It also puts in a comment at the beginning of each function listing the total samples for the function.
For this utility to work, the executable must be compiled with GCC's -g option. By default, Red Hat Enterprise Linux packages are not compiled with this option.
The general syntax for opannotate is as follows: 
opannotate --search-dirs <src-dir> --source <executable>
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The directory containing the source code and the executable to be analyzed must be specified. Refer to the opannotate man page for a list of additional command line options.

43.6. Understanding /dev/oprofile/
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The /dev/oprofile/ directory contains the file system for OProfile. Use the cat command to display the values of the virtual files in this file system. For example, the following command displays the type of processor OProfile detected: 
cat /dev/oprofile/cpu_type
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A directory exists in /dev/oprofile/ for each counter. For example, if there are 2 counters, the directories /dev/oprofile/0/ and dev/oprofile/1/ exist.
Each directory for a counter contains the following files:
  • count — The interval between samples.
  • enabled — If 0, the counter is off and no samples are collected for it; if 1, the counter is on and samples are being collected for it.
  • event — The event to monitor.
  • kernel — If 0, samples are not collected for this counter event when the processor is in kernel-space; if 1, samples are collected even if the processor is in kernel-space.
  • unit_mask — Defines which unit masks are enabled for the counter.
  • user — If 0, samples are not collected for the counter event when the processor is in user-space; if 1, samples are collected even if the processor is in user-space.
The values of these files can be retrieved with the cat command. For example: 
cat /dev/oprofile/0/count
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43.7. Example Usage
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While OProfile can be used by developers to analyze application performance, it can also be used by system administrators to perform system analysis. For example:
  • Determine which applications and services are used the most on a system — opreport can be used to determine how much processor time an application or service uses. If the system is used for multiple services but is under performing, the services consuming the most processor time can be moved to dedicated systems.
  • Determine processor usage — The CPU_CLK_UNHALTED event can be monitored to determine the processor load over a given period of time. This data can then be used to determine if additional processors or a faster processor might improve system performance.

43.8. Graphical Interface
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Some OProfile preferences can be set with a graphical interface. To start it, execute the oprof_start command as root at a shell prompt. To use the graphical interface, you will need to have the oprofile-gui package installed.
After changing any of the options, save them by clicking the Save and quit button. The preferences are written to /root/.oprofile/daemonrc, and the application exits. Exiting the application does not stop OProfile from sampling.
On the Setup tab, to set events for the processor counters as discussed in Section 43.2.2, “Setting Events to Monitor”, select the counter from the pulldown menu and select the event from the list. A brief description of the event appears in the text box below the list. Only events available for the specific counter and the specific architecture are displayed. The interface also displays whether the profiler is running and some brief statistics about it.
OProfile Setup
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OProfile Setup

Figure 43.1. OProfile Setup

On the right side of the tab, select the Profile kernel option to count events in kernel mode for the currently selected event, as discussed in Section 43.2.3, “Separating Kernel and User-space Profiles”. If this option is unselected, no samples are collected for the kernel.
Select the Profile user binaries option to count events in user mode for the currently selected event, as discussed in Section 43.2.3, “Separating Kernel and User-space Profiles”. If this option is unselected, no samples are collected for user applications.
Use the Count text field to set the sampling rate for the currently selected event as discussed in Section 43.2.2.1, “Sampling Rate”.
If any unit masks are available for the currently selected event, as discussed in Section 43.2.2.2, “Unit Masks”, they are displayed in the Unit Masks area on the right side of the Setup tab. Select the checkbox beside the unit mask to enable it for the event.
On the Configuration tab, to profile the kernel, enter the name and location of the vmlinux file for the kernel to monitor in the Kernel image file text field. To configure OProfile not to monitor the kernel, select No kernel image.
OProfile Configuration
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OProfile Configuration

Figure 43.2. OProfile Configuration

If the Verbose option is selected, the oprofiled daemon log includes more information.
If Per-application kernel samples files is selected, OProfile generates per-application profiles for the kernel and kernel modules as discussed in Section 43.2.3, “Separating Kernel and User-space Profiles”. This is equivalent to the opcontrol --separate=kernel command. If Per-application shared libs samples files is selected, OProfile generates per-application profiles for libraries. This is equivalent to the opcontrol --separate=library command.
To force data to be written to samples files as discussed in Section 43.5, “Analyzing the Data”, click the Flush profiler data button. This is equivalent to the opcontrol --dump command.
To start OProfile from the graphical interface, click Start profiler. To stop the profiler, click Stop profiler. Exiting the application does not stop OProfile from sampling.

43.9. Additional Resources
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This chapter only highlights OProfile and how to configure and use it. To learn more, refer to the following resources.

43.9.1. Installed Docs
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  • /usr/share/doc/oprofile-<version>/oprofile.html — OProfile Manual
  • oprofile man page — Discusses opcontrol, opreport, opannotate, and ophelp

43.9.2. Useful Websites
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Part VI. Kernel and Driver Configuration
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System administrators can learn about and customize their kernels. Red Hat Enterprise Linux contains kernel tools to assist administrators with their customizations.

Chapter 44. Manually Upgrading the Kernel
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The Red Hat Enterprise Linux kernel is custom built by the Red Hat Enterprise Linux kernel team to ensure its integrity and compatibility with supported hardware. Before Red Hat releases a kernel, it must first pass a rigorous set of quality assurance tests.
Red Hat Enterprise Linux kernels are packaged in RPM format so that they are easy to upgrade and verify using the Package Management Tool, or the yum command. The Package Management Tool automatically queries the Red Hat Enterprise Linux servers and determines which packages need to be updated on your machine, including the kernel. This chapter is only useful for those individuals that require manual updating of kernel packages, without using the yum command.

Warning

Building a custom kernel is not supported by the Red Hat Global Services Support team, and therefore is not explored in this manual.

Note

The use of yum is highly recommended by Red Hat for installing upgraded kernels.
For more information on Red Hat Network, the Package Management Tool, and yum, refer to Chapter 15, Registering a System and Managing Subscriptions.

44.1. Overview of Kernel Packages
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Red Hat Enterprise Linux contains the following kernel packages (some may not apply to your architecture):
  • kernel — Contains the kernel for multi-processor systems. For x86 system, only the first 4GB of RAM is used. As such, x86 systems with over 4GB of RAM should use the kernel-PAE.
  • kernel-devel — Contains the kernel headers and makefiles sufficient to build modules against the kernel package.
  • kernel-PAE (only for i686 systems) — This package offers the following key configuration option (in addition to the options already enabled for the kernel package):
    • PAE (Physical Address Extension) support for systems with more than 4GB of RAM, and reliably up to 16GB.

      Important

      Physical Address Extension allows x86 processors to address up to 64GB of physical RAM, but due to differences between the Red Hat Enterprise Linux 4 and 5 kernels, only Red Hat Enterprise Linux 4 (with the kernel-hugemem package) is able to reliably address all 64GB of memory. Additionally, the Red Hat Enterprise Linux 5 PAE variant does not allow 4GB of addressable memory per-process like the Red Hat Enterprise Linux 4 kernel-hugemem variant does. However, the x86_64 kernel does not suffer from any of these limitations, and is the suggested Red Hat Enterprise Linux 5 architecture to use with large-memory systems.
  • kernel-PAE-devel — Contains the kernel headers and makefiles required to build modules against the kernel-PAE package.
  • kernel-doc — Contains documentation files from the kernel source. Various portions of the Linux kernel and the device drivers shipped with it are documented in these files. Installation of this package provides a reference to the options that can be passed to Linux kernel modules at load time.
    By default, these files are placed in the /usr/share/doc/kernel-doc-<version>/ directory.
  • kernel-headers — Includes the C header files that specify the interface between the Linux kernel and userspace libraries and programs. The header files define structures and constants that are needed for building most standard programs.
  • kernel-xen — Includes a version of the Linux kernel which is needed to run Virtualization.
  • kernel-xen-devel — Contains the kernel headers and makefiles required to build modules against the kernel-xen package

Note

The kernel-source package has been removed and replaced with an RPM that can only be retrieved from Red Hat Network. This *.src.rpm package must then be rebuilt locally using the rpmbuild command. For more information on obtaining and installing the kernel source package, refer to the latest updated Release Notes (including all updates) at http://www.redhat.com/docs/manuals/enterprise/

44.2. Preparing to Upgrade
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Before upgrading the kernel, it is recommended that you take some precautionary steps. The first step is to make sure working boot media exists for the system in case a problem occurs. If the boot loader is not configured properly to boot the new kernel, the system cannot be booted into Red Hat Enterprise Linux without working boot media.
To create a boot diskette, login as root, and run the command /sbin/mkbootdisk `uname -r` at a shell prompt.

Note

Refer to the mkbootdisk man page for more options. You can create bootable media via CD-Rs, CD-RWs, and USB flash drives, provided that your system BIOS also supports it.
Reboot the machine with the boot media and verify that it works before continuing.
To determine which kernel packages are installed, execute the command rpm -qa | grep kernel at a shell prompt:
The output contains some or all of the following packages, depending on the system's architecture (the version numbers and packages may differ): 
kernel-2.6.9-5.EL
kernel-devel-2.6.9-5.EL
kernel-utils-2.6.9-5.EL
kernel-doc-2.6.9-5.EL
kernel-smp-2.6.9-5.EL
kernel-smp-devel-2.6.9-5.EL
kernel-hugemem-devel-2.6.9-5.EL
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From the output, determine which packages need to be download for the kernel upgrade. For a single processor system, the only required package is the kernel package. Refer to Section 44.1, “Overview of Kernel Packages” for descriptions of the different packages.
In the file name, each kernel package contains the architecture for which the package was built. The format is kernel-<variant>-<version>.<arch>.rpm, where <variant> is one of either PAE, xen, and so forth. The <arch> is one of the following:
  • x86_64 for the AMD64 and Intel EM64T architectures
  • ia64 for the Intel® Itanium™ architecture
  • ppc64 for the IBM® eServer™ pSeries™ architecture
  • s390 for the IBM® S/390® architecture
  • s390x for the IBM® eServer™ System z® architecture
  • i686 for Intel® Pentium® II, Intel® Pentium® III, Intel® Pentium® 4, AMD Athlon®, and AMD Duron® systems

44.3. Downloading the Upgraded Kernel
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There are several ways to determine if an updated kernel is available for the system.
  • Security Errata — Refer to http://www.redhat.com/security/updates/ for information on security errata, including kernel upgrades that fix security issues.
  • Via Red Hat Network — Download and install the kernel RPM packages. Red Hat Network can download the latest kernel, upgrade the kernel on the system, create an initial RAM disk image if needed, and configure the boot loader to boot the new kernel. For more information, refer to http://www.redhat.com/docs/manuals/RHNetwork/.
If Red Hat Network was used to download and install the updated kernel, follow the instructions in Section 44.5, “Verifying the Initial RAM Disk Image” and Section 44.6, “Verifying the Boot Loader”, only do not change the kernel to boot by default. Red Hat Network automatically changes the default kernel to the latest version. To install the kernel manually, continue to Section 44.4, “Performing the Upgrade”.

44.4. Performing the Upgrade
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After retrieving all of the necessary packages, it is time to upgrade the existing kernel.

Important

It is strongly recommended that you keep the old kernel in case there are problems with the new kernel.
At a shell prompt, change to the directory that contains the kernel RPM packages. Use -i argument with the rpm command to keep the old kernel. Do not use the -U option, since it overwrites the currently installed kernel, which creates boot loader problems. For example:
rpm -ivh kernel-<kernel version>.<arch>.rpm
The next step is to verify that the initial RAM disk image has been created. Refer to Section 44.5, “Verifying the Initial RAM Disk Image” for details.

44.5. Verifying the Initial RAM Disk Image
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If the system uses the ext3 file system, a SCSI controller, or labels to reference partitions in /etc/fstab, an initial RAM disk is needed. The initial RAM disk allows a modular kernel to have access to modules that it might need to boot from before the kernel has access to the device where the modules normally reside.
On architectures other than IBM eServer iSeries, the initial RAM disk can be created with the mkinitrd command. However, this step is performed automatically if the kernel and its associated packages are installed or upgraded from the RPM packages distributed by Red Hat; in such cases, you do not need to create the initial RAM disk manually. To verify that an initial RAM disk already exists, use the command ls -l /boot to make sure the initrd-<version>.img file was created (the version should match the version of the kernel just installed).
On iSeries systems, the initial RAM disk file and vmlinux file are combined into one file, which is created with the addRamDisk command. This step is performed automatically if the kernel and its associated packages are installed or upgraded from the RPM packages distributed by Red Hat, Inc.; thus, it does not need to be executed manually. To verify that it was created, use the command ls -l /boot to make sure the /boot/vmlinitrd-<kernel-version> file already exists (the <kernel-version> should match the version of the kernel just installed).
The next step is to verify that the boot loader has been configured to boot the new kernel. Refer to Section 44.6, “Verifying the Boot Loader” for details.

44.6. Verifying the Boot Loader
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The kernel RPM package configures the boot loader to boot the newly installed kernel (except for IBM eServer iSeries systems). However, it does not configure the boot loader to boot the new kernel by default.
It is always a good idea to confirm that the boot loader has been configured correctly. This is a crucial step. If the boot loader is configured incorrectly, the system will not boot into Red Hat Enterprise Linux properly. If this happens, boot the system with the boot media created earlier and try configuring the boot loader again.

44.6.1. x86 Systems
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All x86 systems (including all AMD64 systems) use GRUB as the boot loader.
44.6.1.1. GRUB
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Confirm that the file /boot/grub/grub.conf contains a title section with the same version as the kernel package just installed 
# Note that you do not have to rerun grub after making changes to this file
# NOTICE:  You have a /boot partition.  This means that
#          all kernel and initrd paths are relative to /boot/, eg.
#          root (hd0,0)
#          kernel /vmlinuz-version ro root=/dev/hda2
#          initrd /initrd-version.img
#boot=/dev/hda
default=1 timeout=10
splashimage=(hd0,0)/grub/splash.xpm.gz
title Red Hat Enterprise Linux (2.6.9-5.EL)
         root (hd0,0)
	 kernel /vmlinuz-2.6.9-5.EL ro root=LABEL=/
	 initrd /initrd-2.6.9-5.EL.img
title Red Hat Enterprise Linux (2.6.9-1.906_EL)
         root (hd0,0)
	 kernel /vmlinuz-2.6.9-1.906_EL ro root=LABEL=/
	 initrd /initrd-2.6.9-1.906_EL.img
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If a separate /boot/ partition was created, the paths to the kernel and initrd image are relative to /boot/.
Notice that the default is not set to the new kernel. To configure GRUB to boot the new kernel by default, change the value of the default variable to the title section number for the title section that contains the new kernel. The count starts with 0. For example, if the new kernel is the first title section, set default to 0.
Begin testing the new kernel by rebooting the computer and watching the messages to ensure that the hardware is detected properly.

44.6.2. Itanium Systems
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Itanium systems use ELILO as the boot loader, which uses /boot/efi/EFI/redhat/elilo.conf as the configuration file. Confirm that this file contains an image section with the same version as the kernel package just installed: 
prompt timeout=50 default=old  image=vmlinuz-2.6.9-5.EL
         label=linux
	 initrd=initrd-2.6.9-5.EL.img         read-only
	 append="root=LABEL=/" image=vmlinuz-2.6.9-1.906_EL
	 label=old
	 initrd=initrd-2.6.9-1.906.img         read-only
	 append="root=LABEL=/"
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Notice that the default is not set to the new kernel. To configure ELILO to boot the new kernel, change the value of the default variable to the value of the label for the image section that contains the new kernel.
Begin testing the new kernel by rebooting the computer and watching the messages to ensure that the hardware is detected properly.

44.6.3. IBM S/390 and IBM System z Systems
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The IBM S/390 and IBM System z systems use z/IPL as the boot loader, which uses /etc/zipl.conf as the configuration file. Confirm that the file contains a section with the same version as the kernel package just installed: 
[defaultboot] default=old target=/boot/
[linux]
         image=/boot/vmlinuz-2.6.9-5.EL
	 ramdisk=/boot/initrd-2.6.9-5.EL.img
	 parameters="root=LABEL=/"
[old]
         image=/boot/vmlinuz-2.6.9-1.906_EL
	 ramdisk=/boot/initrd-2.6.9-1.906_EL.img
	 parameters="root=LABEL=/"
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Notice that the default is not set to the new kernel. To configure z/IPL to boot the new kernel by default, change the value of the default variable to the name of the section that contains the new kernel. The first line of each section contains the name in brackets.
After modifying the configuration file, run /sbin/zipl as root to enable the changes.
Begin testing the new kernel by rebooting the computer and watching the messages to ensure that the hardware is detected properly.

44.6.4. IBM eServer iSeries Systems
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The /boot/vmlinitrd-<kernel-version> file is installed when you upgrade the kernel. However, you must use the dd command to configure the system to boot the new kernel:
  1. As root, issue the command cat /proc/iSeries/mf/side to determine the default side (either A, B, or C).
  2. As root, issue the following command, where <kernel-version> is the version of the new kernel and <side> is the side from the previous command:
    dd if=/boot/vmlinitrd-<kernel-version> of=/proc/iSeries/mf/<side>/vmlinux bs=8k
Begin testing the new kernel by rebooting the computer and watching the messages to ensure that the hardware is detected properly.

44.6.5. IBM eServer pSeries Systems
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IBM eServer pSeries systems use YABOOT as the boot loader, which uses /etc/aboot.conf as the configuration file. Confirm that the file contains an image section with the same version as the kernel package just installed: 
boot=/dev/sda1 init-message=Welcome to Red Hat Enterprise Linux! Hit <TAB> for boot options
partition=2 timeout=30 install=/usr/lib/yaboot/yaboot delay=10 nonvram
image=/vmlinux--2.6.9-5.EL
         label=old
	 read-only
	 initrd=/initrd--2.6.9-5.EL.img
	 append="root=LABEL=/"
image=/vmlinux-2.6.9-5.EL
	 label=linux
	 read-only
	 initrd=/initrd-2.6.9-5.EL.img
	 append="root=LABEL=/"
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Notice that the default is not set to the new kernel. The kernel in the first image is booted by default. To change the default kernel to boot either move its image stanza so that it is the first one listed or add the directive default and set it to the label of the image stanza that contains the new kernel.
Begin testing the new kernel by rebooting the computer and watching the messages to ensure that the hardware is detected properly.

Chapter 45. General Parameters and Modules
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This chapter is provided to illustrate some of the possible parameters available for common hardware device drivers [9], which under Red Hat Enterprise Linux are called kernel modules. In most cases, the default parameters do work. However, there may be times when extra module parameters are necessary for a device to function properly or to override the module's default parameters for the device.
During installation, Red Hat Enterprise Linux uses a limited subset of device drivers to create a stable installation environment. Although the installation program supports installation on many different types of hardware, some drivers (including those for SCSI adapters and network adapters) are not included in the installation kernel. Rather, they must be loaded as modules by the user at boot time.
Once installation is completed, support exists for a large number of devices through kernel modules.

Important

Red Hat provides a large number of unsupported device drivers in groups of packages called kernel-smp-unsupported-<kernel-version> and kernel-hugemem-unsupported-<kernel-version> . Replace <kernel-version> with the version of the kernel installed on the system. These packages are not installed by the Red Hat Enterprise Linux installation program, and the modules provided are not supported by Red Hat, Inc.

45.1. Kernel Module Utilities
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A group of commands for managing kernel modules is available if the module-init-tools package is installed. Use these commands to determine if a module has been loaded successfully or when trying different modules for a piece of new hardware.
The command /sbin/lsmod displays a list of currently loaded modules. For example: 
Module                  Size  Used by
tun                    11585  1
autofs4                21573  1
hidp                   16193  2
rfcomm                 37849  0
l2cap                  23873  10 hidp,rfcomm
bluetooth              50085  5 hidp,rfcomm,l2cap
sunrpc                153725  1
dm_mirror              29073  0
dm_mod                 57433  1 dm_mirror
video                  17221  0
sbs                    16257  0
i2c_ec                  5569  1 sbs
container               4801  0
button                  7249  0
battery                10565  0
asus_acpi              16857  0
ac                      5701  0
ipv6                  246113  12
lp                     13065  0
parport_pc             27493  1
parport                37001  2 lp,parport_pc
uhci_hcd               23885  0
floppy                 57317  1
sg                     34653  0
snd_ens1371            26721  1
gameport               16073  1 snd_ens1371
snd_rawmidi            24897  1 snd_ens1371
snd_ac97_codec         91360  1 snd_ens1371
snd_ac97_bus            2753  1 snd_ac97_codec
snd_seq_dummy           4293  0
snd_seq_oss            32705  0
serio_raw               7493  0
snd_seq_midi_event      8001  1 snd_seq_oss
snd_seq                51633  5 snd_seq_dummy,snd_seq_oss,snd_seq_midi_event
snd_seq_device          8781  4 snd_rawmidi,snd_seq_dummy,snd_seq_oss,snd_seq
snd_pcm_oss            42849  0
snd_mixer_oss          16833  1 snd_pcm_oss
snd_pcm                76485  3 snd_ens1371,snd_ac97_codec,snd_pcm_oss
snd_timer              23237  2 snd_seq,snd_pcm
snd                    52933  12 snd_ens1371,snd_rawmidi,snd_ac97_codec,snd_seq_oss,snd_seq,snd_seq_device,snd_pcm_oss,snd_mixer_oss,snd_pcm,snd_timer
soundcore              10145  1 snd
i2c_piix4               8909  0
ide_cd                 38625  3
snd_page_alloc         10569  1 snd_pcm
i2c_core               21697  2 i2c_ec,i2c_piix4
pcnet32                34117  0
cdrom                  34913  1 ide_cd
mii                     5825  1 pcnet32
pcspkr                  3521  0
ext3                  129737  2
jbd                    58473  1 ext3
mptspi                 17353  3
scsi_transport_spi     25025  1 mptspi
mptscsih               23361  1 mptspi
sd_mod                 20929  16
scsi_mod              134121  5 sg,mptspi,scsi_transport_spi,mptscsih,sd_mod
mptbase                52193  2 mptspi,mptscsih
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For each line, the first column is the name of the module, the second column is the size of the module, and the third column is the use count.
The /sbin/lsmod output is less verbose and easier to read than the output from viewing /proc/modules.
To load a kernel module, use the /sbin/modprobe command followed by the kernel module name. By default, modprobe attempts to load the module from the /lib/modules/<kernel-version>/kernel/drivers/ subdirectories. There is a subdirectory for each type of module, such as the net/ subdirectory for network interface drivers. Some kernel modules have module dependencies, meaning that other modules must be loaded first for it to load. The /sbin/modprobe command checks for these dependencies and loads the module dependencies before loading the specified module.
For example, the command 
modprobe e100
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loads any module dependencies and then the e100 module.
To print to the screen all commands as /sbin/modprobe executes them, use the -v option. For example: 
modprobe -v e100
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Output similar to the following is displayed: 
insmod /lib/modules/2.6.9-5.EL/kernel/drivers/net/e100.ko
Using /lib/modules/2.6.9-5.EL/kernel/drivers/net/e100.ko
Symbol version prefix 'smp_'
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The /sbin/insmod command also exists to load kernel modules; however, it does not resolve dependencies. Thus, it is recommended that the /sbin/modprobe command be used.
To unload kernel modules, use the /sbin/rmmod command followed by the module name. The rmmod utility only unloads modules that are not in use and that are not a dependency of other modules in use.
For example, the command 
rmmod e100
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unloads the e100 kernel module.
Another useful kernel module utility is modinfo. Use the command /sbin/modinfo to display information about a kernel module. The general syntax is: 
modinfo [options] <module>
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Options include -d, which displays a brief description of the module, and -p, which lists the parameters the module supports. For a complete list of options, refer to the modinfo man page (man modinfo).

45.2. Persistent Module Loading
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Kernel modules are usually loaded directly by the facility that requires them, which is given correct settings in the /etc/modprobe.conf file. However, it is sometimes necessary to explicitly force the loading of a module at boot time.
Red Hat Enterprise Linux checks for the existence of the /etc/rc.modules file at boot time, which contains various commands to load modules. The rc.modules should be used, and not rc.local because rc.modules is executed earlier in the boot process.
For example, the following commands configure loading of the foo module at boot time (as root): 
echo modprobe foo >> /etc/rc.modules
chmod +x /etc/rc.modules
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Note

This approach is not necessary for network and SCSI interfaces because they have their own specific mechanisms.

45.3. Specifying Module Parameters
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In some situations, it may be necessary to supply parameters to a module as it is loaded for it to function properly.
For instance, to enable full duplex at 100Mbps connection speed for an Intel Ether Express/100 card, load the e100 driver with the e100_speed_duplex=4 option.

Warning

When a parameter has commas, be sure not to put a space after a comma.

Note

The modinfo command is also useful for listing various information about a kernel module, such as version, dependencies, parameter options, and aliases.

45.4. Storage parameters
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Expand
Table 45.1. Storage Module Parameters
Hardware Module Parameters
3ware Storage Controller and 9000 series 3w-xxxx.ko, 3w-9xxx.ko  
Adaptec Advanced Raid Products, Dell PERC2, 2/Si, 3/Si, 3/Di, HP NetRAID-4M, IBM ServeRAID, and ICP SCSI driver aacraid.ko
nondasd — Control scanning of hba for nondasd devices. 0=off, 1=on
dacmode — Control whether dma addressing is using 64 bit DAC. 0=off, 1=on
commit — Control whether a COMMIT_CONFIG is issued to the adapter for foreign arrays. This is typically needed in systems that do not have a BIOS. 0=off, 1=on
startup_timeout — The duration of time in seconds to wait for adapter to have it's kernel up and running. This is typically adjusted for large systems that do not have a BIOS
aif_timeout — The duration of time in seconds to wait for applications to pick up AIFs before deregistering them. This is typically adjusted for heavily burdened systems.
numacb — Request a limit to the number of adapter control blocks (FIB) allocated. Valid values are 512 and down. Default is to use suggestion from Firmware.
acbsize — Request a specific adapter control block (FIB) size. Valid values are 512, 2048, 4096 and 8192. Default is to use suggestion from Firmware.
Adaptec 28xx, R9xx, 39xx AHA-284x, AHA-29xx, AHA-394x, AHA-398x, AHA-274x, AHA-274xT, AHA-2842, AHA-2910B, AHA-2920C, AHA-2930/U/U2, AHA-2940/W/U/UW/AU/, U2W/U2/U2B/, U2BOEM, AHA-2944D/WD/UD/UWD, AHA-2950U2/W/B, AHA-3940/U/W/UW/, AUW/U2W/U2B, AHA-3950U2D, AHA-3985/U/W/UW, AIC-777x, AIC-785x, AIC-786x, AIC-787x, AIC-788x , AIC-789x, AIC-3860 aic7xxx.ko
verbose — Enable verbose/diagnostic logging
allow_memio — Allow device registers to be memory mapped
debug — Bitmask of debug values to enable
no_probe — Toggle EISA/VLB controller probing
probe_eisa_vl — Toggle EISA/VLB controller probing
no_reset — Supress initial bus resets
extended — Enable extended geometry on all controllers
periodic_otag — Send an ordered tagged transaction periodically to prevent tag starvation. This may be required by some older disk drives or RAID arrays.
tag_info:<tag_str> — Set per-target tag depth
global_tag_depth:<int> — Global tag depth for every target on every bus
seltime:<int> — Selection Timeout (0/256ms,1/128ms,2/64ms,3/32ms)
IBM ServeRAID ips.ko  
LSI Logic MegaRAID Mailbox Driver megaraid_mbox.ko
unconf_disks — Set to expose unconfigured disks to kernel (default=0)
busy_wait — Max wait for mailbox in microseconds if busy (default=10)
max_sectors — Maximum number of sectors per IO command (default=128)
cmd_per_lun — Maximum number of commands per logical unit (default=64)
fast_load — Faster loading of the driver, skips physical devices! (default=0)
debug_level — Debug level for driver (default=0)
Emulex LightPulse Fibre Channel SCSI driver lpfc.ko
lpfc_poll — FCP ring polling mode control: 0 - none, 1 - poll with interrupts enabled 3 - poll and disable FCP ring interrupts
lpfc_log_verbose — Verbose logging bit-mask
lpfc_lun_queue_depth — Max number of FCP commands we can queue to a specific LUN
lpfc_hba_queue_depth — Max number of FCP commands we can queue to a lpfc HBA
lpfc_scan_down — Start scanning for devices from highest ALPA to lowest
lpfc_nodev_tmo — Seconds driver will hold I/O waiting for a device to come back
lpfc_topology — Select Fibre Channel topology
lpfc_link_speed — Select link speed
lpfc_fcp_class — Select Fibre Channel class of service for FCP sequences
lpfc_use_adisc — Use ADISC on rediscovery to authenticate FCP devices
lpfc_ack0 — Enable ACK0 support
lpfc_cr_delay — A count of milliseconds after which an interrupt response is generated
lpfc_cr_count — A count of I/O completions after which an interrupt response is generated
lpfc_multi_ring_support — Determines number of primary SLI rings to spread IOCB entries across
lpfc_fdmi_on — Enable FDMI support
lpfc_discovery_threads — Maximum number of ELS commands during discovery
lpfc_max_luns — Maximum allowed LUN
lpfc_poll_tmo — Milliseconds driver will wait between polling FCP ring
HP Smart Array cciss.ko  
LSI Logic MPT Fusion mptbase.ko mptctl.ko mptfc.ko mptlan.ko mptsas.ko mptscsih.ko mptspi.ko
mpt_msi_enable — MSI Support Enable
mptfc_dev_loss_tmo — Initial time the driver programs the transport to wait for an rport to return following a device loss event.
mpt_pt_clear — Clear persistency table
mpt_saf_te — Force enabling SEP Processor
QLogic Fibre Channel Driver qla2xxx.ko
ql2xlogintimeout — Login timeout value in seconds.
qlport_down_retry — Maximum number of command retries to a port that returns a PORT-DOWN status
ql2xplogiabsentdevice — Option to enable PLOGI to devices that are not present after a Fabric scan.
ql2xloginretrycount — Specify an alternate value for the NVRAM login retry count.
ql2xallocfwdump — Option to enable allocation of memory for a firmware dump during HBA initialization. Default is 1 - allocate memory.
extended_error_logging — Option to enable extended error logging.
ql2xfdmienable — Enables FDMI registrations.
NCR, Symbios and LSI 8xx and 1010 sym53c8xx
cmd_per_lun — The maximum number of tags to use by default
tag_ctrl — More detailed control over tags per LUN
burst — Maximum burst. 0 to disable, 255 to read from registers
led — Set to 1 to enable LED support
diff — 0 for no differential mode, 1 for BIOS, 2 for always, 3 for not GPIO3
irqm — 0 for open drain, 1 to leave alone, 2 for totem pole
buschk — 0 to not check, 1 for detach on error, 2 for warn on error
hostid — The SCSI ID to use for the host adapters
verb — 0 for minimal verbosity, 1 for normal, 2 for excessive
debug — Set bits to enable debugging
settle — Settle delay in seconds. Default 3
nvram — Option currently not used
excl — List ioport addresses here to prevent controllers from being attached
safe — Set other settings to a "safe mode"

45.5. Ethernet Parameters
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Important

Most modern Ethernet-based network interface cards (NICs), do not require module parameters to alter settings. Instead, they can be configured using ethtool or mii-tool. Only after these tools fail to work should module parameters be adjusted. Module parameters can be viewed using the modinfo command.

Note

For information about using these tools, consult the man pages for ethtool, mii-tool, and modinfo.
Expand
Table 45.2. Ethernet Module Parameters
Hardware Module Parameters
3Com EtherLink PCI III/XL Vortex (3c590, 3c592, 3c595, 3c597) Boomerang (3c900, 3c905, 3c595) 3c59x.ko
debug — 3c59x debug level (0-6)
options — 3c59x: Bits 0-3: media type, bit 4: bus mastering, bit 9: full duplex
global_options — 3c59x: same as options, but applies to all NICs if options is unset
full_duplex — 3c59x full duplex setting(s) (1)
global_full_duplex — 3c59x: same as full_duplex, but applies to all NICs if full_duplex is unset
hw_checksums — 3c59x Hardware checksum checking by adapter(s) (0-1)
flow_ctrl — 3c59x 802.3x flow control usage (PAUSE only) (0-1)
enable_wol — 3c59x: Turn on Wake-on-LAN for adapter(s) (0-1)
global_enable_wol — 3c59x: same as enable_wol, but applies to all NICs if enable_wol is unset
rx_copybreak — 3c59x copy breakpoint for copy-only-tiny-frames
max_interrupt_work — 3c59x maximum events handled per interrupt
compaq_ioaddr — 3c59x PCI I/O base address (Compaq BIOS problem workaround)
compaq_irq — 3c59x PCI IRQ number (Compaq BIOS problem workaround)
compaq_device_id — 3c59x PCI device ID (Compaq BIOS problem workaround)
watchdog — 3c59x transmit timeout in milliseconds
global_use_mmio — 3c59x: same as use_mmio, but applies to all NICs if options is unset
use_mmio — 3c59x: use memory-mapped PCI I/O resource (0-1)
RTL8139, SMC EZ Card Fast Ethernet, RealTek cards using RTL8129, or RTL8139 Fast Ethernet chipsets 8139too.ko  
Broadcom 4400 10/100 PCI ethernet driver b44.ko
b44_debug — B44 bitmapped debugging message enable value
Broadcom NetXtreme II BCM5706/5708 Driver bnx2.ko
disable_msi — Disable Message Signaled Interrupt (MSI)
Intel Ether Express/100 driver e100.ko
debug — Debug level (0=none,...,16=all)
eeprom_bad_csum_allow — Allow bad eeprom checksums
Intel EtherExpress/1000 Gigabit e1000.ko
TxDescriptors — Number of transmit descriptors
RxDescriptors — Number of receive descriptors
Speed — Speed setting
Duplex — Duplex setting
AutoNeg — Advertised auto-negotiation setting
FlowControl — Flow Control setting
XsumRX — Disable or enable Receive Checksum offload
TxIntDelay — Transmit Interrupt Delay
TxAbsIntDelay — Transmit Absolute Interrupt Delay
RxIntDelay — Receive Interrupt Delay
RxAbsIntDelay — Receive Absolute Interrupt Delay
InterruptThrottleRate — Interrupt Throttling Rate
SmartPowerDownEnable — Enable PHY smart power down
KumeranLockLoss — Enable Kumeran lock loss workaround
Myricom 10G driver (10GbE) myri10ge.ko
myri10ge_fw_name — Firmware image name
myri10ge_ecrc_enable — Enable Extended CRC on PCI-E
myri10ge_max_intr_slots — Interrupt queue slots
myri10ge_small_bytes — Threshold of small packets
myri10ge_msi — Enable Message Signalled Interrupts
myri10ge_intr_coal_delay — Interrupt coalescing delay
myri10ge_flow_control — Pause parameter
myri10ge_deassert_wait — Wait when deasserting legacy interrupts
myri10ge_force_firmware — Force firmware to assume aligned completions
myri10ge_skb_cross_4k — Can a small skb cross a 4KB boundary?
myri10ge_initial_mtu — Initial MTU
myri10ge_napi_weight — Set NAPI weight
myri10ge_watchdog_timeout — Set watchdog timeout
myri10ge_max_irq_loops — Set stuck legacy IRQ detection threshold
NatSemi DP83815 Fast Ethernet natsemi.ko
mtu — DP8381x MTU (all boards)
debug — DP8381x default debug level
rx_copybreak — DP8381x copy breakpoint for copy-only-tiny-frames
options — DP8381x: Bits 0-3: media type, bit 17: full duplex
full_duplex — DP8381x full duplex setting(s) (1)
AMD PCnet32 and AMD PCnetPCI pcnet32.ko  
PCnet32 and PCnetPCI pcnet32.ko
debug — pcnet32 debug level
max_interrupt_work — pcnet32 maximum events handled per interrupt
rx_copybreak — pcnet32 copy breakpoint for copy-only-tiny-frames
tx_start_pt — pcnet32 transmit start point (0-3)
pcnet32vlb — pcnet32 Vesa local bus (VLB) support (0/1)
options — pcnet32 initial option setting(s) (0-15)
full_duplex — pcnet32 full duplex setting(s) (1)
homepna — pcnet32 mode for 79C978 cards (1 for HomePNA, 0 for Ethernet, default Ethernet
RealTek RTL-8169 Gigabit Ethernet driver r8169.ko
media — force phy operation. Deprecated by ethtool (8).
rx_copybreak — Copy breakpoint for copy-only-tiny-frames
use_dac — Enable PCI DAC. Unsafe on 32 bit PCI slot.
debug — Debug verbosity level (0=none, ..., 16=all)
Neterion Xframe 10GbE Server Adapter s2io.ko  
SIS 900/701G PCI Fast Ethernet sis900.ko
multicast_filter_limit — SiS 900/7016 maximum number of filtered multicast addresses
max_interrupt_work — SiS 900/7016 maximum events handled per interrupt
sis900_debug — SiS 900/7016 bitmapped debugging message level
Adaptec Starfire Ethernet driver starfire.ko
max_interrupt_work — Maximum events handled per interrupt
mtu — MTU (all boards)
debug — Debug level (0-6)
rx_copybreak — Copy breakpoint for copy-only-tiny-frames
intr_latency — Maximum interrupt latency, in microseconds
small_frames — Maximum size of receive frames that bypass interrupt latency (0,64,128,256,512)
options — Deprecated: Bits 0-3: media type, bit 17: full duplex
full_duplex — Deprecated: Forced full-duplex setting (0/1)
enable_hw_cksum — Enable/disable hardware cksum support (0/1)
Broadcom Tigon3 tg3.ko
tg3_debug — Tigon3 bitmapped debugging message enable value
ThunderLAN PCI tlan.ko
aui — ThunderLAN use AUI port(s) (0-1)
duplex — ThunderLAN duplex setting(s) (0-default, 1-half, 2-full)
speed — ThunderLAN port speen setting(s) (0,10,100)
debug — ThunderLAN debug mask
bbuf — ThunderLAN use big buffer (0-1)
Digital 21x4x Tulip PCI Ethernet cards SMC EtherPower 10 PCI(8432T/8432BT) SMC EtherPower 10/100 PCI(9332DST) DEC EtherWorks 100/10 PCI(DE500-XA) DEC EtherWorks 10 PCI(DE450) DEC QSILVER's, Znyx 312 etherarray Allied Telesis LA100PCI-T Danpex EN-9400, Cogent EM110 tulip.ko io io_port
VIA Rhine PCI Fast Ethernet cards with either the VIA VT86c100A Rhine-II PCI or 3043 Rhine-I D-Link DFE-930-TX PCI 10/100 via-rhine.ko
max_interrupt_work — VIA Rhine maximum events handled per interrupt
debug — VIA Rhine debug level (0-7)
rx_copybreak — VIA Rhine copy breakpoint for copy-only-tiny-frames
avoid_D3 — Avoid power state D3 (work-around for broken BIOSes)

45.5.1. The Channel Bonding Module
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Red Hat Enterprise Linux allows administrators to bind NICs together into a single channel using the bonding kernel module and a special network interface, called a channel bonding interface. Channel bonding enables two or more network interfaces to act as one, simultaneously increasing the bandwidth and providing redundancy.
To channel bond multiple network interfaces, the administrator must perform the following steps:
  1. Add the following line to /etc/modprobe.conf: 
    alias bond<N> bonding
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    Replace <N> with the interface number, such as 0. For each configured channel bonding interface, there must be a corresponding entry in /etc/modprobe.conf.
  2. Configure a channel bonding interface as outlined in Section 16.2.3, “Channel Bonding Interfaces”.
  3. To enhance performance, adjust available module options to ascertain what combination works best. Pay particular attention to the miimon or arp_interval and the arp_ip_target parameters. Refer to Section 45.5.1.1, “bonding Module Directives” for a list of available options and how to quickly determine the best ones for your bonded interface.
45.5.1.1. bonding Module Directives
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It is a good idea to test which channel bonding module parameters work best for your bonded interfaces before adding them to the BONDING_OPTS="<bonding parameters>" directive in your bonding interface configuration file (ifcfg-bond0 for example). Parameters to bonded interfaces can be configured without unloading (and reloading) the bonding module by manipulating files in the sysfs file system.
sysfs is a virtual file system that represents kernel objects as directories, files and symbolic links. sysfs can be used to query for information about kernel objects, and can also manipulate those objects through the use of normal file system commands. The sysfs virtual file system has a line in /etc/fstab, and is mounted under /sys. All bonded interfaces can be configured dynamically by interacting with and manipulating files under the /sys/class/net/ directory.
After you have created a channel bonding interface file such as ifcfg-bond0 and inserted SLAVE=yes and MASTER=bond0 directives in the bonded interfaces following the instructions in Section 16.2.3, “Channel Bonding Interfaces”, you can proceed to testing and determining the best parameters for your bonded interface.
First, bring up the bond you created by running ifconfig bond<N>  up as root: 
ifconfig bond0 up
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If you have correctly created the ifcfg-bond0 bonding interface file, you will be able to see bond0 listed in the output of running ifconfig (without any options): 
~]# ifconfig
bond0     Link encap:Ethernet  HWaddr 00:00:00:00:00:00
          UP BROADCAST RUNNING MASTER MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)
eth0      Link encap:Ethernet  HWaddr 52:54:00:26:9E:F1
          inet addr:192.168.122.251  Bcast:192.168.122.255  Mask:255.255.255.0
          inet6 addr: fe80::5054:ff:fe26:9ef1/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:207 errors:0 dropped:0 overruns:0 frame:0
          TX packets:205 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:70374 (68.7 KiB)  TX bytes:25298 (24.7 KiB)
[output truncated]
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To view all existing bonds, even if they are not up, run: 
~]# cat /sys/class/net/bonding_masters
bond0
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You can configure each bond individually by manipulating the files located in the /sys/class/net/bond<N>/bonding/ directory. First, the bond you are configuring must be taken down: 
ifconfig bond0 down
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As an example, to enable MII monitoring on bond0 with a 1 second interval, you could run (as root): 
echo 1000 > /sys/class/net/bond0/bonding/miimon
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To configure bond0 for balance-alb mode, you could run either: 
echo 6 > /sys/class/net/bond0/bonding/mode
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...or, using the name of the mode: 
echo balance-alb > /sys/class/net/bond0/bonding/mode
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After configuring some options for the bond in question, you can bring it up and test it by running ifconfig bond<N> up . If you decide to change the options, take the interface down, modify its parameters using sysfs, bring it back up, and re-test.
Once you have determined the best set of parameters for your bond, add those parameters as a space-separated list to the BONDING_OPTS= directive of the /etc/sysconfig/network-scripts/ifcfg-bond<N> file for the bonded interface you are configuring. Whenever that bond is brought up (for example, by the system during the boot sequence if the ONBOOT=yes directive is set), the bonding options specified in the BONDING_OPTS will take effect for that bond. For more information on configuring bonded interfaces (and BONDING_OPTS), refer to Section 16.2.3, “Channel Bonding Interfaces”.
The following is a list of available channel bonding module parameters for the bonding module. For more in-depth information on configuring channel bonding and the exhaustive list of bonding module parameters, install the kernel-doc package and then locating and opening the included bonding.txt file: 
yum -y install kernel-doc
nano -w $(rpm -ql kernel-doc | grep bonding.txt)
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Bonding Interface Parameters

arp_interval=<time_in_milliseconds>
Specifies (in milliseconds) how often ARP monitoring occurs.

Important

It is essential that both arp_interval and arp_ip_target parameters are specified, or, alternatively, the miimon parameter is specified. Failure to do so can cause degradation of network performance in the event that a link fails.
If using this setting while in mode=0 or mode=1 (the two load-balancing modes), the network switch must be configured to distribute packets evenly across the NICs. For more information on how to accomplish this, refer to /usr/share/doc/kernel-doc-<kernel_version>/Documentation/networking/bonding.txt
The value is set to 0 by default, which disables it.
arp_ip_target=<ip_address> [,<ip_address_2>,...<ip_address_16> ]
Specifies the target IP address of ARP requests when the arp_interval parameter is enabled. Up to 16 IP addresses can be specified in a comma separated list.
arp_validate=<value>
Validate source/distribution of ARP probes; default is none. Other valid values are active, backup, and all.
debug=<number>
Enables debug messages. Possible values are:
  • 0 — Debug messages are disabled. This is the default.
  • 1 — Debug messages are enabled.
downdelay=<time_in_milliseconds>
Specifies (in milliseconds) how long to wait after link failure before disabling the link. The value must be a multiple of the value specified in the miimon parameter. The value is set to 0 by default, which disables it.
lacp_rate=<value>
Specifies the rate at which link partners should transmit LACPDU packets in 802.3ad mode. Possible values are:
  • slow or 0 — Default setting. This specifies that partners should transmit LACPDUs every 30 seconds.
  • fast or 1 — Specifies that partners should transmit LACPDUs every 1 second.
miimon=<time_in_milliseconds>
Specifies (in milliseconds) how often MII link monitoring occurs. This is useful if high availability is required because MII is used to verify that the NIC is active. To verify that the driver for a particular NIC supports the MII tool, type the following command as root: 
ethtool <interface_name> | grep "Link detected:"
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In this command, replace <interface_name> with the name of the device interface, such as eth0, not the bond interface. If MII is supported, the command returns: 
Link detected: yes
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If using a bonded interface for high availability, the module for each NIC must support MII. Setting the value to 0 (the default), turns this feature off. When configuring this setting, a good starting point for this parameter is 100.

Important

It is essential that both arp_interval and arp_ip_target parameters are specified, or, alternatively, the miimon parameter is specified. Failure to do so can cause degradation of network performance in the event that a link fails.
mode=<value>
...where <value> is one of:
  • balance-rr or 0 — Sets a round-robin policy for fault tolerance and load balancing. Transmissions are received and sent out sequentially on each bonded slave interface beginning with the first one available.
  • active-backup or 1 — Sets an active-backup policy for fault tolerance. Transmissions are received and sent out via the first available bonded slave interface. Another bonded slave interface is only used if the active bonded slave interface fails.
  • balance-xor or 2 — Sets an XOR (exclusive-or) policy for fault tolerance and load balancing. Using this method, the interface matches up the incoming request's MAC address with the MAC address for one of the slave NICs. Once this link is established, transmissions are sent out sequentially beginning with the first available interface.
  • broadcast or 3 — Sets a broadcast policy for fault tolerance. All transmissions are sent on all slave interfaces.
  • 802.3ad or 4 — Sets an IEEE 802.3ad dynamic link aggregation policy. Creates aggregation groups that share the same speed and duplex settings. Transmits and receives on all slaves in the active aggregator. Requires a switch that is 802.3ad compliant.
  • balance-tlb or 5 — Sets a Transmit Load Balancing (TLB) policy for fault tolerance and load balancing. The outgoing traffic is distributed according to the current load on each slave interface. Incoming traffic is received by the current slave. If the receiving slave fails, another slave takes over the MAC address of the failed slave.
  • balance-alb or 6 — Sets an Active Load Balancing (ALB) policy for fault tolerance and load balancing. Includes transmit and receive load balancing for IPV4 traffic. Receive load balancing is achieved through ARP negotiation.
num_unsol_na=<number>
Specifies the number of unsolicited IPv6 Neighbor Advertisements to be issued after a failover event. One unsolicited NA is issued immediately after the failover.
The valid range is 0 - 255; the default value is 1. This option affects only the active-backup mode.
primary=<interface_name>
Specifies the interface name, such as eth0, of the primary device. The primary device is the first of the bonding interfaces to be used and is not abandoned unless it fails. This setting is particularly useful when one NIC in the bonding interface is faster and, therefore, able to handle a bigger load.
This setting is only valid when the bonding interface is in active-backup mode. Refer to /usr/share/doc/kernel-doc-<kernel-version>/Documentation/networking/bonding.txt for more information.
primary_reselect=<value>
Specifies the reselection policy for the primary slave. This affects how the primary slave is chosen to become the active slave when failure of the active slave or recovery of the primary slave occurs. This option is designed to prevent flip-flopping between the primary slave and other slaves. Possible values are:
  • always or 0 (default) — The primary slave becomes the active slave whenever it comes back up.
  • better or 1 — The primary slave becomes the active slave when it comes back up, if the speed and duplex of the primary slave is better than the speed and duplex of the current active slave.
  • failure or 2 — The primary slave becomes the active slave only if the current active slave fails and the primary slave is up.
The primary_reselect setting is ignored in two cases:
  • If no slaves are active, the first slave to recover is made the active slave.
  • When initially enslaved, the primary slave is always made the active slave.
Changing the primary_reselect policy via sysfs will cause an immediate selection of the best active slave according to the new policy. This may or may not result in a change of the active slave, depending upon the circumstances
updelay=<time_in_milliseconds>
Specifies (in milliseconds) how long to wait before enabling a link. The value must be a multiple of the value specified in the miimon parameter. The value is set to 0 by default, which disables it.
use_carrier=<number>
Specifies whether or not miimon should use MII/ETHTOOL ioctls or netif_carrier_ok() to determine the link state. The netif_carrier_ok() function relies on the device driver to maintains its state with netif_carrier_on/off ; most device drivers support this function.
The MII/ETHROOL ioctls tools utilize a deprecated calling sequence within the kernel. However, this is still configurable in case your device driver does not support netif_carrier_on/off .
Valid values are:
  • 1 — Default setting. Enables the use of netif_carrier_ok().
  • 0 — Enables the use of MII/ETHTOOL ioctls.

Note

If the bonding interface insists that the link is up when it should not be, it is possible that your network device driver does not support netif_carrier_on/off .
xmit_hash_policy=<value>
Selects the transmit hash policy used for slave selection in balance-xor and 802.3ad modes. Possible values are:
  • 0 or layer2 — Default setting. This option uses the XOR of hardware MAC addresses to generate the hash. The formula used is: 
    (<source_MAC_address> XOR <destination_MAC>) MODULO <slave_count>
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    This algorithm will place all traffic to a particular network peer on the same slave, and is 802.3ad compliant.
  • 1 or layer3+4 — Uses upper layer protocol information (when available) to generate the hash. This allows for traffic to a particular network peer to span multiple slaves, although a single connection will not span multiple slaves.
    The formula for unfragmented TCP and UDP packets used is: 
    ((<source_port> XOR <dest_port>) XOR
  ((<source_IP> XOR <dest_IP>) AND 0xffff)
    MODULO <slave_count>
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    For fragmented TCP or UDP packets and all other IP protocol traffic, the source and destination port information is omitted. For non-IP traffic, the formula is the same as the layer2 transmit hash policy.
    This policy intends to mimic the behavior of certain switches; particularly, Cisco switches with PFC2 as well as some Foundry and IBM products.
    The algorithm used by this policy is not 802.3ad compliant.
  • 2 or layer2+3 — Uses a combination of layer2 and layer3 protocol information to generate the hash.
    Uses XOR of hardware MAC addresses and IP addresses to generate the hash. The formula is: 
    (((<source_IP> XOR <dest_IP>) AND 0xffff) XOR
  ( <source_MAC> XOR <destination_MAC> ))
    MODULO <slave_count>
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    This algorithm will place all traffic to a particular network peer on the same slave. For non-IP traffic, the formula is the same as for the layer2 transmit hash policy.
    This policy is intended to provide a more balanced distribution of traffic than layer2 alone, especially in environments where a layer3 gateway device is required to reach most destinations.
    This algorithm is 802.3ad compliant.

45.6. Additional Resources
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For more information on kernel modules and their utilities, refer to the following resources.

45.6.1. Installed Documentation
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  • lsmod man page — description and explanation of its output.
  • insmod man page — description and list of command line options.
  • modprobe man page — description and list of command line options.
  • rmmod man page — description and list of command line options.
  • modinfo man page — description and list of command line options.
  • /usr/share/doc/kernel-doc-<version>/Documentation/kbuild/modules.txt — how to compile and use kernel modules. Note you must have the kernel-doc package installed to read this file.

45.6.2. Useful Websites
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[9] A driver is software which enables Linux to use a particular hardware device. Without a driver, the kernel cannot communicate with attached devices.

Chapter 46. The kdump Crash Recovery Service
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kdump is an advanced crash dumping mechanism. When enabled, the system is booted from the context of another kernel. This second kernel reserves a small amount of memory and its only purpose is to capture the core dump image in case the system crashes. The ability to analyze the core dump significantly helps to determine the exact cause of the system failure, and as a consequence, it is strongly recommended to have this feature enabled.
This chapter explains how to configure, test, and use the kdump service in Red Hat Enterprise Linux, and provides a brief overview of how to analyze the resulting core dump using the crash debugging utility.

46.1. Installing the kdump Service
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In order to use the kdump service on your system, make sure you have the kexec-tools package installed. To do so, type the following at a shell prompt as root: 
~]# yum install kexec-tools
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For more information on how to install new packages in Red Hat Enterprise Linux, refer to Part II, “Package Management”.

46.2. Configuring the kdump Service
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There are three common means of configuring the kdump service: you can enable and configure it at the first boot, use the Kernel Dump Configuration utility for the graphical user interface, or do so manually on the command line.

Important

A limitation in the current implementation of the Intel IOMMU driver can occasionally prevent the kdump service from capturing the core dump image. To use kdump on Intel architectures reliably, it is advised that the IOMMU support is disabled.

Warning

It is known that the kdump service does not work reliably on certain combinations of HP Smart Array devices and system boards from the same vendor. Consequent to this, users are strongly advised to test the configuration before using it in production environment, and if necessary, configure kdump to store the kernel crash dump to a remote machine over a network. For more information on how to test the kdump configuration, refer to Section 46.2.4, “Testing the Configuration”.

46.2.1. Configuring kdump at First Boot
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When the system boots for the first time, the firstboot application is launched to guide the user through the initial configuration of the freshly installed system. To configure kdump, navigate to the Kdump page and follow the instructions below.

Important

Unless the system has enough memory, the Kdump page will not be available. For information on minimum memory requirements, refer to the Red Hat Enterprise Linux comparison chart. When the kdump crash recovery is enabled, the minimum memory requirements increase by the amount of memory reserved for it. This value is determined by the user and on x86, AMD64, and Intel 64 architectures, it defaults to 128 MB plus 64 MB for each TB of physical memory (that is, a total of 192 MB for a system with 1 TB of physical memory).
The kdump configuration screen
View larger image
The kdump configuration screen

Figure 46.1. The kdump configuration screen

46.2.1.1. Enabling the Service
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To start the kdump daemon at boot time, select the Enable kdump? checkbox. This will enable the service for runlevels 2, 3, 4, and 5, and start it for the current session. Similarly, clearing the checkbox will disable it for all runlevels and stop the service immediately.
46.2.1.2. Configuring the Memory Usage
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To configure the amount of memory that is reserved for the kdump kernel, click the up and down arrow buttons next to the Kdump Memory field to increase or decrease the value. Notice that the Usable System Memory field changes accordingly showing you the remaining memory that will be available to the system.
To start the Kernel Dump Configuration utility, select Applications → System Tools → Kdump from the panel, or type system-config-kdump at a shell prompt. Unless you are already authenticated, you will be prompted to enter the root password.
The Kernel Dump Configuration utility
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The Kernel Dump Configuration utility

Figure 46.2. The Kernel Dump Configuration utility

The utility allows you to configure kdump as well as to enable or disable starting the service at boot time. When you are done, click OK to save the changes. The system reboot will be requested.

Important

Unless the system has enough memory, the Kernel Dump Configuration utility will not start and you will be presented with an error message. For information on minimum memory requirements, refer to the Red Hat Enterprise Linux comparison chart. When the kdump crash recovery is enabled, the minimum memory requirements increase by the amount of memory reserved for it. This value is determined by the user and on x86, AMD64, and Intel 64 architectures, it defaults to 128 MB plus 64 MB for each TB of physical memory (that is, a total of 192 MB for a system with 1 TB of physical memory).
46.2.2.1. Enabling the Service
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To start the kdump daemon at boot time, select the Enable kdump checkbox. This will enable the service for runlevels 2, 3, 4, and 5, and start it for the current session. Similarly, clearing the checkbox will disable it for all runlevels and stop the service immediately.
46.2.2.2. Configuring the Memory Usage
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To configure the amount of memory that is reserved for the kdump kernel, click the up and down arrow buttons next to the New kdump Memory field to increase or decrease the value. Notice that the Usable Memory field changes accordingly showing you the remaining memory that will be available to the system.
46.2.2.3. Configuring the Target Type
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When a kernel crash is captured, the core dump can be either stored as a file in a local file system, written directly to a device, or sent over a network using the NFS (Network File System) or SSH (Secure Shell) protocol. To change this, click the Edit Location button, and select a location type as described below.
The Edit Location dialog
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The Edit Location dialog

Figure 46.3. The Edit Location dialog

To save the dump to the local file system, select file from the pulldown list. Optionally, if you wish to write the file to a different partition, select ext3 or ext2 from the pulldown list according to the file system you are using, and enter a valid device name to the Enter location field. Note that after clicking OK, you can then customize the destination directory by changing the value in the Path field at the bottom.
To write the dump directly to a device, select raw from the pulldown list, and enter a valid device name (for example, /dev/sdb1). When you are done, click OK to confirm your choice.
To store the dump to a remote machine using the NFS protocol, select nfs from the pulldown list, and enter a valid target in the hostname:directory form (for example, penguin.example.com:/export). Clicking the OK button will confirm your changes. Finally, edit the value of the Path field to customize the destination directory (for instance, cores).
To store the dump to a remote machine using the SSH protocol, select ssh from the pulldown list, and enter a valid username and hostname in the username@hostname form (for example, john@penguin.example.com). Clicking the OK button will confirm your changes. Finally, edit the value of the Path field to customize the destination directory (for instance, /export/cores).
Refer to Chapter 20, OpenSSH for information on how to configure an SSH server, and how to set up a key-based authentication.
46.2.2.4. Configuring the Core Collector
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To reduce the size of the vmcore dump file, kdump allows you to specify an external application (that is, a core collector) to compress the data, and optionally leave out all irrelevant information. Currently, the only fully supported core collector is makedumpfile.
To enable the dump file compression, make sure the -c parameter is listed after the makedumpfile command in the Core Collector field (for example, makedumpfile -c).
To remove certain pages from the dump, add the -d value parameter after the makedumpfile command in the Core Collector field. The value is a sum of values of pages you want to omit as described in Table 46.1, “Supported filtering levels”. For example, to remove both zero and free pages, use makedumpfile -d 17.
Refer to the manual page for makedumpfile for a complete list of available options.
46.2.2.5. Changing the Default Action
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To choose what action to perform when kdump fails to create a core dump, select the appropriate option from the Default Action pulldown list. Available options are mount rootfs and run /sbin/init (the default action), reboot (to reboot the system), shell (to present a user with an interactive shell prompt), and halt (to halt the system).

46.2.3. Configuring kdump on the Command Line
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46.2.3.1. Configuring the Memory Usage
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To configure the amount of memory that is reserved for the kdump kernel on x86, AMD64, and Intel 64 architectures, open the /boot/grub/grub.conf file as root and add the crashkernel=<size>M@16M parameter to the list of kernel options as shown in Example 46.1, “Sample /boot/grub/grub.conf file”.

Important

Unless the system has enough memory, the kdump crash recovery service will not be operational. For information on minimum memory requirements, refer to the Red Hat Enterprise Linux comparison chart. When kdump is enabled, the minimum memory requirements increase by the amount of memory reserved for it. This value is determined by the user and on x86, AMD64, and Intel 64 architectures, it defaults to 128 MB plus 64 MB for each TB of physical memory (that is, a total of 192 MB for a system with 1 TB of physical memory).

Example 46.1. Sample /boot/grub/grub.conf file

# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE:  You have a /boot partition.  This means that
#          all kernel and initrd paths are relative to /boot/, eg.
#          root (hd0,0)
#          kernel /vmlinuz-version ro root=/dev/sda3
#          initrd /initrd-version.img
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title Red Hat Enterprise Linux Server (2.6.18-274.3.1.el5)
        root (hd0,0)
        kernel /vmlinuz-2.6.18-274.3.1.el5 ro root=/dev/sda3 crashkernel=128M@16M
        initrd /initrd-2.6.18-274.3.1.el5.img
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46.2.3.2. Configuring the Target Type
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When a kernel crash is captured, the core dump can be either stored as a file in a local file system, written directly to a device, or sent over a network using the NFS (Network File System) or SSH (Secure Shell) protocol. Note that only one of these options can be set at the moment. The default option is to store the vmcore file in the /var/crash/ directory of the local file system. To change this, open the /etc/kdump.conf configuration file as root and edit the options as described below.
To change the local directory in which the core dump is to be saved, remove the hash sign (“#”) from the beginning of the #path /var/crash line, and replace the value with a desired directory path. Optionally, if you wish to write the file to a different partition, follow the same procedure with the #ext3 /dev/sda3 line as well, and change both the file system type and the device (a device name, a file system label, and UUID are all supported) accordingly. For example: 
ext3 /dev/sda4
path /usr/local/cores
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To write the dump directly to a device, remove the hash sign (“#”) from the beginning of the #raw /dev/sda5 line, and replace the value with a desired device name. For example: 
raw /dev/sdb1
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To store the dump to a remote machine using the NFS protocol, remove the hash sign (“#”) from the beginning of the #net my.server.com:/export/tmp line, and replace the value with a valid hostname and directory path. For example: 
net penguin.example.com:/export/cores
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To store the dump to a remote machine using the SSH protocol, remove the hash sign (“#”) from the beginning of the #net user@my.server.com line, and replace the value with a valid username and hostname. For example: 
net john@penguin.example.com
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Refer to Chapter 20, OpenSSH for information on how to configure an SSH server, and how to set up a key-based authentication.
46.2.3.3. Configuring the Core Collector
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To reduce the size of the vmcore dump file, kdump allows you to specify an external application (that is, a core collector) to compress the data, and optionally leave out all irrelevant information. Currently, the only fully supported core collector is makedumpfile.
To enable the core collector, open the /etc/kdump.conf configuration file as root, remove the hash sign (“#”) from the beginning of the #core_collector makedumpfile -c --message-level 1 line, and edit the command line options as described below.
To enable the dump file compression, add the -c parameter. For example: 
core_collector makedumpfile -c
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To remove certain pages from the dump, add the -d value parameter, where value is a sum of values of pages you want to omit as described in Table 46.1, “Supported filtering levels”. For example, to remove both zero and free pages, use the following: 
core_collector makedumpfile -d 17 -c
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Refer to the manual page for makedumpfile for a complete list of available options.
Expand
Table 46.1. Supported filtering levels
Option Description
1 Zero pages
2 Cache pages
4 Cache private
8 User pages
16 Free pages
46.2.3.4. Changing the Default Action
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By default, when kdump fails to create a core dump, the root file system is mounted and /sbin/init is run. To change this behavior, open the /etc/kdump.conf configuration file as root, remove the hash sign (“#”) from the beginning of the #default shell line, and replace the value with a desired action as described in Table 46.2, “Supported actions”. For example: 
default halt
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Expand
Table 46.2. Supported actions
Option Action
reboot Reboot the system, losing the core in the process.
halt After failing to capture a core, halt the system.
shell Run the msh session from within the initramfs, allowing a user to record the core manually.
46.2.3.5. Enabling the Service
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To start the kdump daemon at boot time, type the following at a shell prompt as root: 
~]# chkconfig kdump on
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This will enable the service for runlevels 2, 3, 4, and 5. Similarly, typing chkconfig kdump off will disable it for all runlevels. To start the service in the current session, use the following command as root: 
~]# service kdump start
No kdump initial ramdisk found.                            [WARNING]
Rebuilding /boot/initrd-2.6.18-194.8.1.el5kdump.img
Starting kdump:                                            [  OK  ]
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For more information on runlevels and configuring services in general, refer to Chapter 18, Controlling Access to Services.

46.2.4. Testing the Configuration
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Warning

The commands below will cause the kernel to crash. Use caution when following these steps, and by no means use them on a production machine.
To test the configuration, reboot the system with kdump enabled, and as root, make sure that the service is running: 
~]# service kdump status
Kdump is operational
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Then type the following commands at a shell prompt as root: 
~]# echo 1 > /proc/sys/kernel/sysrq
~]# echo c > /proc/sysrq-trigger
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This will force the Linux kernel to crash, and the YYYY-MM-DD-HH:MM/vmcore file will be copied to the location you have selected in the configuration (that is, to /var/crash/ by default).

46.3. Analyzing the Core Dump
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Note

To analyze the vmcore dump file, you must have the crash and kernel-debuginfo packages installed. To do so, type the following at a shell prompt as root: 
~]# yum install --enablerepo=rhel-debuginfo crash kernel-debuginfo
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Refer to Part II, “Package Management” for more information on how to install new packages in Red Hat Enterprise Linux.
To determine the cause of the system crash, you can use the crash utility. This utility allows you to interactively analyze a running Linux system as well as a core dump created by netdump, diskdump, xendump, or kdump. When started, it presents you with an interactive prompt very similar to the GNU Debugger (GDB).
To start the utility, type the command in the following form at a shell prompt: 
crash /var/crash/timestamp/vmcore /usr/lib/debug/lib/modules/kernel/vmlinux
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Note that the kernel version should be the same as the one that was captured by kdump. To find out which kernel you are currently running, use the uname -r command.

Example 46.2. Running the crash utility

~]# crash /var/crash/2010-08-04-17\:55/vmcore \
/usr/lib/debug/lib/modules/2.6.18-194.8.1.el5/vmlinux

crash 4.1.2-4.el5_5.1
Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009  Red Hat, Inc.
Copyright (C) 2004, 2005, 2006  IBM Corporation
Copyright (C) 1999-2006  Hewlett-Packard Co
Copyright (C) 2005, 2006  Fujitsu Limited
Copyright (C) 2006, 2007  VA Linux Systems Japan K.K.
Copyright (C) 2005  NEC Corporation
Copyright (C) 1999, 2002, 2007  Silicon Graphics, Inc.
Copyright (C) 1999, 2000, 2001, 2002  Mission Critical Linux, Inc.
This program is free software, covered by the GNU General Public License,
and you are welcome to change it and/or distribute copies of it under
certain conditions.  Enter "help copying" to see the conditions.
This program has absolutely no warranty.  Enter "help warranty" for details.
 
GNU gdb 6.1
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i686-pc-linux-gnu"...

      KERNEL: /usr/lib/debug/lib/modules/2.6.18-194.8.1.el5/vmlinux
    DUMPFILE: /var/crash/2010-08-04-17:55/vmcore
        CPUS: 1
        DATE: Wed Aug  4 17:50:41 2010
      UPTIME: 00:56:53
LOAD AVERAGE: 0.47, 0.47, 0.55
       TASKS: 128
    NODENAME: localhost.localdomain
     RELEASE: 2.6.18-194.el5
     VERSION: #1 SMP Tue Mar 16 21:52:43 EDT 2010
     MACHINE: i686  (2702 Mhz)
      MEMORY: 1 GB
       PANIC: "SysRq : Trigger a crashdump"
         PID: 6042
     COMMAND: "bash"
        TASK: f09c7000  [THREAD_INFO: e1ba9000]
         CPU: 0
       STATE: TASK_RUNNING (SYSRQ)

crash>
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To exit the interactive prompt and terminate crash, type exit.

46.3.1. Displaying the Message Buffer
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To display the kernel message buffer, type the log command at the interactive prompt.

Example 46.3. Displaying the kernel message buffer

crash> log
Linux version 2.6.18-194.el5 (mockbuild@x86-007.build.bos.redhat.com) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-48)) #1 SMP Tue Mar 16 21:52:43 EDT 2010
BIOS-provided physical RAM map:
 BIOS-e820: 0000000000010000 - 000000000009fc00 (usable)
 BIOS-e820: 000000000009fc00 - 00000000000a0000 (reserved)
 BIOS-e820: 00000000000f0000 - 0000000000100000 (reserved)
 BIOS-e820: 0000000000100000 - 000000003fff0000 (usable)
 BIOS-e820: 000000003fff0000 - 0000000040000000 (ACPI data)
 BIOS-e820: 00000000fffc0000 - 0000000100000000 (reserved)
127MB HIGHMEM available.
896MB LOWMEM available.
Using x86 segment limits to approximate NX protection
On node 0 totalpages: 262128
  DMA zone: 4096 pages, LIFO batch:0
  Normal zone: 225280 pages, LIFO batch:31
  HighMem zone: 32752 pages, LIFO batch:7
DMI 2.5 present.
Using APIC driver default
... several lines omitted ...
SysRq : Trigger a crashdump
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Type help log for more information on the command usage.

46.3.2. Displaying a Backtrace
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To display the kernel stack trace, type the bt command at the interactive prompt. You can use bt pid to display the backtrace of the selected process.

Example 46.4. Displaying the kernel stack trace

crash> bt
PID: 6042   TASK: f09c7000  CPU: 0   COMMAND: "bash"
 #0 [e1ba9d10] schedule at c061c738
 #1 [e1ba9d28] netlink_getsockopt at c05d50bb
 #2 [e1ba9d34] netlink_queue_skip at c05d40d5
 #3 [e1ba9d40] netlink_sock_destruct at c05d506d
 #4 [e1ba9d84] sock_recvmsg at c05b6cc8
 #5 [e1ba9dd4] enqueue_task at c041eed5
 #6 [e1ba9dec] try_to_wake_up at c041f798
 #7 [e1ba9e10] vsnprintf at c04efef2
 #8 [e1ba9ec0] machine_kexec at c0419bf0
 #9 [e1ba9f04] sys_kexec_load at c04448a1
#10 [e1ba9f4c] tty_audit_exit at c0549f06
#11 [e1ba9f50] tty_audit_add_data at c0549d5d
#12 [e1ba9f84] do_readv_writev at c0476055
#13 [e1ba9fb8] system_call at c0404f10
    EAX: ffffffda  EBX: 00000001  ECX: b7f7f000  EDX: 00000002 
    DS:  007b      ESI: 00000002  ES:  007b      EDI: b7f7f000
    SS:  007b      ESP: bf83f478  EBP: bf83f498
    CS:  0073      EIP: 009ac402  ERR: 00000004  EFLAGS: 00000246
Copy to Clipboard Toggle word wrap
Type help bt for more information on the command usage.

46.3.3. Displaying a Process Status
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To display a status of processes in the system, type the ps command at the interactive prompt. You can use ps pid to display the status of the selected process.

Example 46.5. Displaying status of processes in the system

crash> ps
   PID    PPID  CPU   TASK    ST  %MEM     VSZ    RSS  COMM
      0      0   0  c068a3c0  RU   0.0       0      0  [swapper]
      1      0   0  f7c81aa0  IN   0.1    2152    616  init
... several lines omitted ...
   6017      1   0  e39f6550  IN   1.2   40200  13000  gnome-terminal
   6019   6017   0  e39f6000  IN   0.1    2568    708  gnome-pty-helpe
   6020   6017   0  f0421550  IN   0.1    4620   1480  bash
   6021      1   0  f7f69aa0  ??   1.2   40200  13000  gnome-terminal
   6039   6020   0  e7e84aa0  IN   0.1    5004   1300  su
>  6042   6039   0  f09c7000  RU   0.1    4620   1464  bash
Copy to Clipboard Toggle word wrap
Type help ps for more information on the command usage.

46.3.4. Displaying Virtual Memory Information
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To display basic virtual memory information, type the vm command at the interactive prompt. You can use vm pid to display information on the selected process.

Example 46.6. Displaying virtual memory information of the current context

crash> vm
PID: 6042   TASK: f09c7000  CPU: 0   COMMAND: "bash"
   MM       PGD      RSS    TOTAL_VM
e275ee40  e2b08000  1464k    4620k  
  VMA       START      END    FLAGS  FILE
e315d764    1fe000    201000     75  /lib/libtermcap.so.2.0.8
e315de9c    201000    202000 100073  /lib/libtermcap.so.2.0.8
c9b040d4    318000    46a000     75  /lib/libc-2.5.so
e315da04    46a000    46c000 100071  /lib/libc-2.5.so
e315d7b8    46c000    46d000 100073  /lib/libc-2.5.so
e315de48    46d000    470000 100073  
e315dba8    9ac000    9ad000 8040075  
c9b04a04    a2f000    a4a000    875  /lib/ld-2.5.so
c9b04374    a4a000    a4b000 100871  /lib/ld-2.5.so
e315d6bc    a4b000    a4c000 100873  /lib/ld-2.5.so
e315d908    fa1000    fa4000     75  /lib/libdl-2.5.so
e315db00    fa4000    fa5000 100071  /lib/libdl-2.5.so
e315df44    fa5000    fa6000 100073  /lib/libdl-2.5.so
e315d320    ff0000    ffa000     75  /lib/libnss_files-2.5.so
e315d668    ffa000    ffb000 100071  /lib/libnss_files-2.5.so
e315def0    ffb000    ffc000 100073  /lib/libnss_files-2.5.so
e315d374   8048000   80f5000   1875  /bin/bash
c9b045c0   80f5000   80fa000 101873  /bin/bash
... several lines omitted ...
Copy to Clipboard Toggle word wrap
Type help vm for more information on the command usage.

46.3.5. Displaying Open Files
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To display information about open files, type the files command at the interactive prompt. You can use files pid to display files opened by the selected process.

Example 46.7. Displaying information about open files of the current context

crash> files
PID: 6042   TASK: f09c7000  CPU: 0   COMMAND: "bash"
ROOT: /    CWD: /root
 FD    FILE     DENTRY    INODE    TYPE  PATH
  0  e33be480  e609bf70  f0e1d880  CHR   /dev/pts/1
  1  e424d8c0  d637add8  f7809b78  REG   /proc/sysrq-trigger
  2  e33be480  e609bf70  f0e1d880  CHR   /dev/pts/1
 10  e33be480  e609bf70  f0e1d880  CHR   /dev/pts/1
255  e33be480  e609bf70  f0e1d880  CHR   /dev/pts/1
Copy to Clipboard Toggle word wrap
Type help files for more information on the command usage.

46.4. Additional Resources
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46.4.1. Installed Documentation
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man kdump.conf
The manual page for the /etc/kdump.conf configuration file containing the full documentation of available options.
man kexec
The manual page for kexec containing the full documentation on its usage.
man crash
The manual page for the crash utility containing the full documentation on its usage.
/usr/share/doc/kexec-tools-version/kexec-kdump-howto.txt
An overview of the kdump and kexec installation and usage.

46.4.2. Useful Websites
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https://access.redhat.com/kb/docs/DOC-6039
The Red Hat Knowledgebase article about the kexec and kdump configuration.
http://people.redhat.com/anderson/
The crash utility homepage.

Part VII. Security And Authentication
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Whether system administrators need to secure their mission-critical systems, services, or data, Red Hat Enterprise Linux provides a range of tools and methods to serve as part of a comprehensive security strategy.
This chapter provides a general introduction to security, and from the perspective of Red Hat Enterprise Linux in particular. It provides conceptual information in the areas of security assessment, common exploits, and intrusion and incident response. It also provides conceptual and specific configuration information on how to use SELinux to harden Workstation, Server, VPN, firewall and other implementations.
This chapter assumes a basic knowledge of IT security, and consequently provides only minimal coverage of common security practices such as controlling physical access, sound account-keeping policies and procedures, auditing, etc. Where appropriate, reference is made to external resources for this and related information.

Chapter 47. Security Overview
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Because of the increased reliance on powerful, networked computers to help run businesses and keep track of our personal information, industries have been formed around the practice of network and computer security. Enterprises have solicited the knowledge and skills of security experts to properly audit systems and tailor solutions to fit the operating requirements of the organization. Because most organizations are dynamic in nature, with workers accessing company IT resources locally and remotely, the need for secure computing environments has become more pronounced.
Unfortunately, most organizations (as well as individual users) regard security as an afterthought, a process that is overlooked in favor of increased power, productivity, and budgetary concerns. Proper security implementation is often enacted postmortem — after an unauthorized intrusion has already occurred. Security experts agree that the right measures taken prior to connecting a site to an untrusted network, such as the Internet, is an effective means of thwarting most attempts at intrusion.

47.1. Introduction to Security
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47.1.1. What is Computer Security?
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Computer security is a general term that covers a wide area of computing and information processing. Industries that depend on computer systems and networks to conduct daily business transactions and access crucial information regard their data as an important part of their overall assets. Several terms and metrics have entered our daily business vocabulary, such as total cost of ownership (TCO) and quality of service (QoS). In these metrics, industries calculate aspects such as data integrity and high-availability as part of their planning and process management costs. In some industries, such as electronic commerce, the availability and trustworthiness of data can be the difference between success and failure.
47.1.1.1. How did Computer Security Come about?
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Information security has evolved over the years due to the increasing reliance on public networks not to disclose personal, financial, and other restricted information. There are numerous instances such as the Mitnick and the Vladimir Levin cases that prompted organizations across all industries to rethink the way they handle information transmission and disclosure. The popularity of the Internet was one of the most important developments that prompted an intensified effort in data security.
An ever-growing number of people are using their personal computers to gain access to the resources that the Internet has to offer. From research and information retrieval to electronic mail and commerce transaction, the Internet has been regarded as one of the most important developments of the 20th century.
The Internet and its earlier protocols, however, were developed as a trust-based system. That is, the Internet Protocol was not designed to be secure in itself. There are no approved security standards built into the TCP/IP communications stack, leaving it open to potentially malicious users and processes across the network. Modern developments have made Internet communication more secure, but there are still several incidents that gain national attention and alert us to the fact that nothing is completely safe.
47.1.1.2. Security Today
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In February of 2000, a Distributed Denial of Service (DDoS) attack was unleashed on several of the most heavily-trafficked sites on the Internet. The attack rendered yahoo.com, cnn.com, amazon.com, fbi.gov, and several other sites completely unreachable to normal users, as it tied up routers for several hours with large-byte ICMP packet transfers, also called a ping flood. The attack was brought on by unknown assailants using specially created, widely available programs that scanned vulnerable network servers, installed client applications called Trojans on the servers, and timed an attack with every infected server flooding the victim sites and rendering them unavailable. Many blame the attack on fundamental flaws in the way routers and the protocols used are structured to accept all incoming data, no matter where or for what purpose the packets are sent.
Currently, an estimated 945 million people use or have used the Internet worldwide (Computer Industry Almanac, 2004). At the same time:
  • On any given day, there are approximately 225 major incidences of security breach reported to the CERT Coordination Center at Carnegie Mellon University.[10]
  • In 2003, the number of CERT reported incidences jumped to 137,529 from 82,094 in 2002 and from 52,658 in 2001.[11]
  • The worldwide economic impact of the three most dangerous Internet Viruses of the last three years was estimated at US$13.2 Billion.[12]
Computer security has become a quantifiable and justifiable expense for all IT budgets. Organizations that require data integrity and high availability elicit the skills of system administrators, developers, and engineers to ensure 24x7 reliability of their systems, services, and information. Falling victim to malicious users, processes, or coordinated attacks is a direct threat to the success of the organization.
Unfortunately, system and network security can be a difficult proposition, requiring an intricate knowledge of how an organization regards, uses, manipulates, and transmits its information. Understanding the way an organization (and the people that make up the organization) conducts business is paramount to implementing a proper security plan.
47.1.1.3. Standardizing Security
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Enterprises in every industry rely on regulations and rules that are set by standards making bodies such as the American Medical Association (AMA) or the Institute of Electrical and Electronics Engineers (IEEE). The same ideals hold true for information security. Many security consultants and vendors agree upon the standard security model known as CIA, or Confidentiality, Integrity, and Availability. This three-tiered model is a generally accepted component to assessing risks of sensitive information and establishing security policy. The following describes the CIA model in further detail:
  • Confidentiality — Sensitive information must be available only to a set of pre-defined individuals. Unauthorized transmission and usage of information should be restricted. For example, confidentiality of information ensures that a customer's personal or financial information is not obtained by an unauthorized individual for malicious purposes such as identity theft or credit fraud.
  • Integrity — Information should not be altered in ways that render it incomplete or incorrect. Unauthorized users should be restricted from the ability to modify or destroy sensitive information.
  • Availability — Information should be accessible to authorized users any time that it is needed. Availability is a warranty that information can be obtained with an agreed-upon frequency and timeliness. This is often measured in terms of percentages and agreed to formally in Service Level Agreements (SLAs) used by network service providers and their enterprise clients.

47.1.2. Security Controls
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Computer security is often divided into three distinct master categories, commonly referred to as controls:
  • Physical
  • Technical
  • Administrative
These three broad categories define the main objectives of proper security implementation. Within these controls are sub-categories that further detail the controls and how to implement them.
47.1.2.1. Physical Controls
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Physical control is the implementation of security measures in a defined structure used to deter or prevent unauthorized access to sensitive material. Examples of physical controls are:
  • Closed-circuit surveillance cameras
  • Motion or thermal alarm systems
  • Security guards
  • Picture IDs
  • Locked and dead-bolted steel doors
  • Biometrics (includes fingerprint, voice, face, iris, handwriting, and other automated methods used to recognize individuals)
47.1.2.2. Technical Controls
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Technical controls use technology as a basis for controlling the access and usage of sensitive data throughout a physical structure and over a network. Technical controls are far-reaching in scope and encompass such technologies as:
  • Encryption
  • Smart cards
  • Network authentication
  • Access control lists (ACLs)
  • File integrity auditing software
47.1.2.3. Administrative Controls
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Administrative controls define the human factors of security. It involves all levels of personnel within an organization and determines which users have access to what resources and information by such means as:
  • Training and awareness
  • Disaster preparedness and recovery plans
  • Personnel recruitment and separation strategies
  • Personnel registration and accounting

47.1.3. Conclusion
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Now that you have learned about the origins, reasons, and aspects of security, you can determine the appropriate course of action with regard to Red Hat Enterprise Linux. It is important to know what factors and conditions make up security in order to plan and implement a proper strategy. With this information in mind, the process can be formalized and the path becomes clearer as you delve deeper into the specifics of the security process.

47.2. Vulnerability Assessment
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Given time, resources, and motivation, a cracker can break into nearly any system. At the end of the day, all of the security procedures and technologies currently available cannot guarantee that any systems are safe from intrusion. Routers help secure gateways to the Internet. Firewalls help secure the edge of the network. Virtual Private Networks safely pass data in an encrypted stream. Intrusion detection systems warn you of malicious activity. However, the success of each of these technologies is dependent upon a number of variables, including:
  • The expertise of the staff responsible for configuring, monitoring, and maintaining the technologies.
  • The ability to patch and update services and kernels quickly and efficiently.
  • The ability of those responsible to keep constant vigilance over the network.
Given the dynamic state of data systems and technologies, securing corporate resources can be quite complex. Due to this complexity, it is often difficult to find expert resources for all of your systems. While it is possible to have personnel knowledgeable in many areas of information security at a high level, it is difficult to retain staff who are experts in more than a few subject areas. This is mainly because each subject area of information security requires constant attention and focus. Information security does not stand still.

47.2.1. Thinking Like the Enemy
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Suppose that you administer an enterprise network. Such networks are commonly comprised of operating systems, applications, servers, network monitors, firewalls, intrusion detection systems, and more. Now imagine trying to keep current with each of these. Given the complexity of today's software and networking environments, exploits and bugs are a certainty. Keeping current with patches and updates for an entire network can prove to be a daunting task in a large organization with heterogeneous systems.
Combine the expertise requirements with the task of keeping current, and it is inevitable that adverse incidents occur, systems are breached, data is corrupted, and service is interrupted.
To augment security technologies and aid in protecting systems, networks, and data, you must think like a cracker and gauge the security of your systems by checking for weaknesses. Preventative vulnerability assessments against your own systems and network resources can reveal potential issues that can be addressed before a cracker exploits it.
A vulnerability assessment is an internal audit of your network and system security; the results of which indicate the confidentiality, integrity, and availability of your network (as explained in Section 47.1.1.3, “Standardizing Security”). Typically, vulnerability assessment starts with a reconnaissance phase, during which important data regarding the target systems and resources is gathered. This phase leads to the system readiness phase, whereby the target is essentially checked for all known vulnerabilities. The readiness phase culminates in the reporting phase, where the findings are classified into categories of high, medium, and low risk; and methods for improving the security (or mitigating the risk of vulnerability) of the target are discussed.
If you were to perform a vulnerability assessment of your home, you would likely check each door to your home to see if they are closed and locked. You would also check every window, making sure that they closed completely and latch correctly. This same concept applies to systems, networks, and electronic data. Malicious users are the thieves and vandals of your data. Focus on their tools, mentality, and motivations, and you can then react swiftly to their actions.

47.2.2. Defining Assessment and Testing
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Vulnerability assessments may be broken down into one of two types: Outside looking in and inside looking around.
When performing an outside looking in vulnerability assessment, you are attempting to compromise your systems from the outside. Being external to your company provides you with the cracker's viewpoint. You see what a cracker sees — publicly-routable IP addresses, systems on your DMZ, external interfaces of your firewall, and more. DMZ stands for "demilitarized zone", which corresponds to a computer or small subnetwork that sits between a trusted internal network, such as a corporate private LAN, and an untrusted external network, such as the public Internet. Typically, the DMZ contains devices accessible to Internet traffic, such as Web (HTTP ) servers, FTP servers, SMTP (e-mail) servers and DNS servers.
When you perform an inside looking around vulnerability assessment, you are somewhat at an advantage since you are internal and your status is elevated to trusted. This is the viewpoint you and your co-workers have once logged on to your systems. You see print servers, file servers, databases, and other resources.
There are striking distinctions between these two types of vulnerability assessments. Being internal to your company gives you elevated privileges more so than any outsider. Still today in most organizations, security is configured in such a manner as to keep intruders out. Very little is done to secure the internals of the organization (such as departmental firewalls, user-level access controls, authentication procedures for internal resources, and more). Typically, there are many more resources when looking around inside as most systems are internal to a company. Once you set yourself outside of the company, you immediately are given an untrusted status. The systems and resources available to you externally are usually very limited.
Consider the difference between vulnerability assessments and penetration tests. Think of a vulnerability assessment as the first step to a penetration test. The information gleaned from the assessment is used for testing. Whereas, the assessment is checking for holes and potential vulnerabilities, the penetration testing actually attempts to exploit the findings.
Assessing network infrastructure is a dynamic process. Security, both information and physical, is dynamic. Performing an assessment shows an overview, which can turn up false positives and false negatives.
Security administrators are only as good as the tools they use and the knowledge they retain. Take any of the assessment tools currently available, run them against your system, and it is almost a guarantee that there are some false positives. Whether by program fault or user error, the result is the same. The tool may find vulnerabilities which in reality do not exist (false positive); or, even worse, the tool may not find vulnerabilities that actually do exist (false negative).
Now that the difference between a vulnerability assessment and a penetration test is defined, take the findings of the assessment and review them carefully before conducting a penetration test as part of your new best practices approach.

Warning

Attempting to exploit vulnerabilities on production resources can have adverse effects to the productivity and efficiency of your systems and network.
The following list examines some of the benefits to performing vulnerability assessments.
  • Creates proactive focus on information security
  • Finds potential exploits before crackers find them
  • Results in systems being kept up to date and patched
  • Promotes growth and aids in developing staff expertise
  • Abates Financial loss and negative publicity
47.2.2.1. Establishing a Methodology
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To aid in the selection of tools for a vulnerability assessment, it is helpful to establish a vulnerability assessment methodology. Unfortunately, there is no predefined or industry approved methodology at this time; however, common sense and best practices can act as a sufficient guide.
What is the target? Are we looking at one server, or are we looking at our entire network and everything within the network? Are we external or internal to the company? The answers to these questions are important as they help determine not only which tools to select but also the manner in which they are used.
To learn more about establishing methodologies, refer to the following websites:

47.2.3. Evaluating the Tools
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An assessment can start by using some form of an information gathering tool. When assessing the entire network, map the layout first to find the hosts that are running. Once located, examine each host individually. Focusing on these hosts requires another set of tools. Knowing which tools to use may be the most crucial step in finding vulnerabilities.
Just as in any aspect of everyday life, there are many different tools that perform the same job. This concept applies to performing vulnerability assessments as well. There are tools specific to operating systems, applications, and even networks (based on the protocols used). Some tools are free; others are not. Some tools are intuitive and easy to use, while others are cryptic and poorly documented but have features that other tools do not.
Finding the right tools may be a daunting task and in the end, experience counts. If possible, set up a test lab and try out as many tools as you can, noting the strengths and weaknesses of each. Review the README file or man page for the tool. Additionally, look to the Internet for more information, such as articles, step-by-step guides, or even mailing lists specific to a tool.
The tools discussed below are just a small sampling of the available tools.
47.2.3.1. Scanning Hosts with Nmap
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Nmap is a popular tool included in Red Hat Enterprise Linux that can be used to determine the layout of a network. Nmap has been available for many years and is probably the most often used tool when gathering information. An excellent man page is included that provides a detailed description of its options and usage. Administrators can use Nmap on a network to find host systems and open ports on those systems.
Nmap is a competent first step in vulnerability assessment. You can map out all the hosts within your network and even pass an option that allows Nmap to attempt to identify the operating system running on a particular host. Nmap is a good foundation for establishing a policy of using secure services and stopping unused services.
47.2.3.1.1. Using Nmap
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Nmap can be run from a shell prompt by typing the nmap command followed by the hostname or IP address of the machine to scan. 
nmap foo.example.com
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The results of the scan (which could take up to a few minutes, depending on where the host is located) should look similar to the following: 
Starting nmap V. 3.50 ( www.insecure.org/nmap/ )
Interesting ports on localhost.localdomain (127.0.0.1):
(The 1591 ports scanned but not shown below are in state: closed)
Port       State       Service
22/tcp     open        ssh
25/tcp     open        smtp
111/tcp    open        sunrpc
443/tcp    open        https
515/tcp    open        printer
950/tcp    open        oftep-rpc
6000/tcp   open        X11

Nmap run completed -- 1 IP address (1 host up) scanned in 71.825 seconds
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Nmap tests the most common network communication ports for listening or waiting services. This knowledge can be helpful to an administrator who wants to close down unnecessary or unused services.
For more information about using Nmap, refer to the official homepage at the following URL:
http://www.insecure.org/
47.2.3.2. Nessus
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Nessus is a full-service security scanner. The plug-in architecture of Nessus allows users to customize it for their systems and networks. As with any scanner, Nessus is only as good as the signature database it relies upon. Fortunately, Nessus is frequently updated and features full reporting, host scanning, and real-time vulnerability searches. Remember that there could be false positives and false negatives, even in a tool as powerful and as frequently updated as Nessus.

Note

Nessus is not included with Red Hat Enterprise Linux and is not supported. It has been included in this document as a reference to users who may be interested in using this popular application.
For more information about Nessus, refer to the official website at the following URL:
http://www.nessus.org/
47.2.3.3. Nikto
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Nikto is an excellent common gateway interface (CGI) script scanner. Nikto not only checks for CGI vulnerabilities but does so in an evasive manner, so as to elude intrusion detection systems. It comes with thorough documentation which should be carefully reviewed prior to running the program. If you have Web servers serving up CGI scripts, Nikto can be an excellent resource for checking the security of these servers.

Note

Nikto is not included with Red Hat Enterprise Linux and is not supported. It has been included in this document as a reference to users who may be interested in using this popular application.
More information about Nikto can be found at the following URL:
http://www.cirt.net/code/nikto.shtml
47.2.3.4. VLAD the Scanner
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VLAD is a vulnerabilities scanner developed by the RAZOR team at Bindview, Inc., which checks for the SANS Top Ten list of common security issues (SNMP issues, file sharing issues, etc.). While not as full-featured as Nessus, VLAD is worth investigating.

Note

VLAD is not included with Red Hat Enterprise Linux and is not supported. It has been included in this document as a reference to users who may be interested in using this popular application.
More information about VLAD can be found on the RAZOR team website at the following URL:
http://www.bindview.com/Support/Razor/Utilities/
47.2.3.5. Anticipating Your Future Needs
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Depending upon your target and resources, there are many tools available. There are tools for wireless networks, Novell networks, Windows systems, Linux systems, and more. Another essential part of performing assessments may include reviewing physical security, personnel screening, or voice/PBX network assessment. New concepts, such as war walking scanning the perimeter of your enterprise's physical structures for wireless network vulnerabilities are some emerging concepts that you can investigate and, if needed, incorporate into your assessments. Imagination and exposure are the only limits of planning and conducting vulnerability assessments.

47.3. Attackers and Vulnerabilities
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To plan and implement a good security strategy, first be aware of some of the issues which determined, motivated attackers exploit to compromise systems. But before detailing these issues, the terminology used when identifying an attacker must be defined.

47.3.1. A Quick History of Hackers
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The modern meaning of the term hacker has origins dating back to the 1960s and the Massachusetts Institute of Technology (MIT) Tech Model Railroad Club, which designed train sets of large scale and intricate detail. Hacker was a name used for club members who discovered a clever trick or workaround for a problem.
The term hacker has since come to describe everything from computer buffs to gifted programmers. A common trait among most hackers is a willingness to explore in detail how computer systems and networks function with little or no outside motivation. Open source software developers often consider themselves and their colleagues to be hackers, and use the word as a term of respect.
Typically, hackers follow a form of the hacker ethic which dictates that the quest for information and expertise is essential, and that sharing this knowledge is the hackers duty to the community. During this quest for knowledge, some hackers enjoy the academic challenges of circumventing security controls on computer systems. For this reason, the press often uses the term hacker to describe those who illicitly access systems and networks with unscrupulous, malicious, or criminal intent. The more accurate term for this type of computer hacker is cracker — a term created by hackers in the mid-1980s to differentiate the two communities.
47.3.1.1. Shades of Gray
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Within the community of individuals who find and exploit vulnerabilities in systems and networks are several distinct groups. These groups are often described by the shade of hat that they "wear" when performing their security investigations and this shade is indicative of their intent.
The white hat hacker is one who tests networks and systems to examine their performance and determine how vulnerable they are to intrusion. Usually, white hat hackers crack their own systems or the systems of a client who has specifically employed them for the purposes of security auditing. Academic researchers and professional security consultants are two examples of white hat hackers.
A black hat hacker is synonymous with a cracker. In general, crackers are less focused on programming and the academic side of breaking into systems. They often rely on available cracking programs and exploit well known vulnerabilities in systems to uncover sensitive information for personal gain or to inflict damage on the target system or network.
The gray hat hacker, on the other hand, has the skills and intent of a white hat hacker in most situations but uses his knowledge for less than noble purposes on occasion. A gray hat hacker can be thought of as a white hat hacker who wears a black hat at times to accomplish his own agenda.
Gray hat hackers typically subscribe to another form of the hacker ethic, which says it is acceptable to break into systems as long as the hacker does not commit theft or breach confidentiality. Some would argue, however, that the act of breaking into a system is in itself unethical.
Regardless of the intent of the intruder, it is important to know the weaknesses a cracker may likely attempt to exploit. The remainder of the chapter focuses on these issues.

47.3.2. Threats to Network Security
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Bad practices when configuring the following aspects of a network can increase the risk of attack.
47.3.2.1. Insecure Architectures
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A misconfigured network is a primary entry point for unauthorized users. Leaving a trust-based, open local network vulnerable to the highly-insecure Internet is much like leaving a door ajar in a crime-ridden neighborhood — nothing may happen for an arbitrary amount of time, but eventually someone exploits the opportunity.
47.3.2.1.1. Broadcast Networks
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System administrators often fail to realize the importance of networking hardware in their security schemes. Simple hardware such as hubs and routers rely on the broadcast or non-switched principle; that is, whenever a node transmits data across the network to a recipient node, the hub or router sends a broadcast of the data packets until the recipient node receives and processes the data. This method is the most vulnerable to address resolution protocol (arp) or media access control (MAC) address spoofing by both outside intruders and unauthorized users on local hosts.
47.3.2.1.2. Centralized Servers
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Another potential networking pitfall is the use of centralized computing. A common cost-cutting measure for many businesses is to consolidate all services to a single powerful machine. This can be convenient as it is easier to manage and costs considerably less than multiple-server configurations. However, a centralized server introduces a single point of failure on the network. If the central server is compromised, it may render the network completely useless or worse, prone to data manipulation or theft. In these situations, a central server becomes an open door which allows access to the entire network.

47.3.3. Threats to Server Security
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Server security is as important as network security because servers often hold a great deal of an organization's vital information. If a server is compromised, all of its contents may become available for the cracker to steal or manipulate at will. The following sections detail some of the main issues.
47.3.3.1. Unused Services and Open Ports
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A full installation of Red Hat Enterprise Linux contains 1000+ application and library packages. However, most server administrators do not opt to install every single package in the distribution, preferring instead to install a base installation of packages, including several server applications.
A common occurrence among system administrators is to install the operating system without paying attention to what programs are actually being installed. This can be problematic because unneeded services may be installed, configured with the default settings, and possibly turned on. This can cause unwanted services, such as Telnet, DHCP, or DNS, to run on a server or workstation without the administrator realizing it, which in turn can cause unwanted traffic to the server, or even, a potential pathway into the system for crackers. Refer To Section 48.2, “Server Security” for information on closing ports and disabling unused services.
47.3.3.2. Unpatched Services
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Most server applications that are included in a default installation are solid, thoroughly tested pieces of software. Having been in use in production environments for many years, their code has been thoroughly refined and many of the bugs have been found and fixed.
However, there is no such thing as perfect software and there is always room for further refinement. Moreover, newer software is often not as rigorously tested as one might expect, because of its recent arrival to production environments or because it may not be as popular as other server software.
Developers and system administrators often find exploitable bugs in server applications and publish the information on bug tracking and security-related websites such as the Bugtraq mailing list (http://www.securityfocus.com) or the Computer Emergency Response Team (CERT) website (http://www.cert.org). Although these mechanisms are an effective way of alerting the community to security vulnerabilities, it is up to system administrators to patch their systems promptly. This is particularly true because crackers have access to these same vulnerability tracking services and will use the information to crack unpatched systems whenever they can. Good system administration requires vigilance, constant bug tracking, and proper system maintenance to ensure a more secure computing environment.
Refer to Section 47.5, “Security Updates” for more information about keeping a system up-to-date.
47.3.3.3. Inattentive Administration
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Administrators who fail to patch their systems are one of the greatest threats to server security. According to the System Administration Network and Security Institute (SANS), the primary cause of computer security vulnerability is to "assign untrained people to maintain security and provide neither the training nor the time to make it possible to do the job."[13] This applies as much to inexperienced administrators as it does to overconfident or amotivated administrators.
Some administrators fail to patch their servers and workstations, while others fail to watch log messages from the system kernel or network traffic. Another common error is when default passwords or keys to services are left unchanged. For example, some databases have default administration passwords because the database developers assume that the system administrator changes these passwords immediately after installation. If a database administrator fails to change this password, even an inexperienced cracker can use a widely-known default password to gain administrative privileges to the database. These are only a few examples of how inattentive administration can lead to compromised servers.
47.3.3.4. Inherently Insecure Services
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Even the most vigilant organization can fall victim to vulnerabilities if the network services they choose are inherently insecure. For instance, there are many services developed under the assumption that they are used over trusted networks; however, this assumption fails as soon as the service becomes available over the Internet — which is itself inherently untrusted.
One category of insecure network services are those that require unencrypted usernames and passwords for authentication. Telnet and FTP are two such services. If packet sniffing software is monitoring traffic between the remote user and such a service usernames and passwords can be easily intercepted.
Inherently, such services can also more easily fall prey to what the security industry terms the man-in-the-middle attack. In this type of attack, a cracker redirects network traffic by tricking a cracked name server on the network to point to his machine instead of the intended server. Once someone opens a remote session to the server, the attacker's machine acts as an invisible conduit, sitting quietly between the remote service and the unsuspecting user capturing information. In this way a cracker can gather administrative passwords and raw data without the server or the user realizing it.
Another category of insecure services include network file systems and information services such as NFS or NIS, which are developed explicitly for LAN usage but are, unfortunately, extended to include WANs (for remote users). NFS does not, by default, have any authentication or security mechanisms configured to prevent a cracker from mounting the NFS share and accessing anything contained therein. NIS, as well, has vital information that must be known by every computer on a network, including passwords and file permissions, within a plain text ASCII or DBM (ASCII-derived) database. A cracker who gains access to this database can then access every user account on a network, including the administrator's account.
By default, Red Hat Enterprise Linux is released with all such services turned off. However, since administrators often find themselves forced to use these services, careful configuration is critical. Refer to Section 48.2, “Server Security” for more information about setting up services in a safe manner.
Workstations and home PCs may not be as prone to attack as networks or servers, but since they often contain sensitive data, such as credit card information, they are targeted by system crackers. Workstations can also be co-opted without the user's knowledge and used by attackers as "slave" machines in coordinated attacks. For these reasons, knowing the vulnerabilities of a workstation can save users the headache of reinstalling the operating system, or worse, recovering from data theft.
47.3.4.1. Bad Passwords
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Bad passwords are one of the easiest ways for an attacker to gain access to a system. For more on how to avoid common pitfalls when creating a password, refer to Section 48.1.3, “Password Security”.
47.3.4.2. Vulnerable Client Applications
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Although an administrator may have a fully secure and patched server, that does not mean remote users are secure when accessing it. For instance, if the server offers Telnet or FTP services over a public network, an attacker can capture the plain text usernames and passwords as they pass over the network, and then use the account information to access the remote user's workstation.
Even when using secure protocols, such as SSH, a remote user may be vulnerable to certain attacks if they do not keep their client applications updated. For instance, v.1 SSH clients are vulnerable to an X-forwarding attack from malicious SSH servers. Once connected to the server, the attacker can quietly capture any keystrokes and mouse clicks made by the client over the network. This problem was fixed in the v.2 SSH protocol, but it is up to the user to keep track of what applications have such vulnerabilities and update them as necessary.
Section 48.1, “Workstation Security” discusses in more detail what steps administrators and home users should take to limit the vulnerability of computer workstations.

47.4. Common Exploits and Attacks
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Table 47.1, “Common Exploits” details some of the most common exploits and entry points used by intruders to access organizational network resources. Key to these common exploits are the explanations of how they are performed and how administrators can properly safeguard their network against such attacks.
Expand
Table 47.1. Common Exploits
Exploit Description Notes
Null or Default Passwords Leaving administrative passwords blank or using a default password set by the product vendor. This is most common in hardware such as routers and firewalls, though some services that run on Linux can contain default administrator passwords (though Red Hat Enterprise Linux 5 does not ship with them).
Commonly associated with networking hardware such as routers, firewalls, VPNs, and network attached storage (NAS) appliances.
Common in many legacy operating systems, especially OSes that bundle services (such as UNIX and Windows.)
Administrators sometimes create privileged user accounts in a rush and leave the password null, a perfect entry point for malicious users who discover the account.
Default Shared Keys Secure services sometimes package default security keys for development or evaluation testing purposes. If these keys are left unchanged and are placed in a production environment on the Internet, all users with the same default keys have access to that shared-key resource, and any sensitive information that it contains.
Most common in wireless access points and preconfigured secure server appliances.
IP Spoofing A remote machine acts as a node on your local network, finds vulnerabilities with your servers, and installs a backdoor program or Trojan horse to gain control over your network resources.
Spoofing is quite difficult as it involves the attacker predicting TCP/IP SYN-ACK numbers to coordinate a connection to target systems, but several tools are available to assist crackers in performing such a vulnerability.
Depends on target system running services (such as rsh, telnet, FTP and others) that use source-based authentication techniques, which are not recommended when compared to PKI or other forms of encrypted authentication used in ssh or SSL/TLS.
Eavesdropping Collecting data that passes between two active nodes on a network by eavesdropping on the connection between the two nodes.
This type of attack works mostly with plain text transmission protocols such as Telnet, FTP, and HTTP transfers.
Remote attacker must have access to a compromised system on a LAN in order to perform such an attack; usually the cracker has used an active attack (such as IP spoofing or man-in-the-middle) to compromise a system on the LAN.
Preventative measures include services with cryptographic key exchange, one-time passwords, or encrypted authentication to prevent password snooping; strong encryption during transmission is also advised.
Service Vulnerabilities An attacker finds a flaw or loophole in a service run over the Internet; through this vulnerability, the attacker compromises the entire system and any data that it may hold, and could possibly compromise other systems on the network.
HTTP-based services such as CGI are vulnerable to remote command execution and even interactive shell access. Even if the HTTP service runs as a non-privileged user such as "nobody", information such as configuration files and network maps can be read, or the attacker can start a denial of service attack which drains system resources or renders it unavailable to other users.
Services sometimes can have vulnerabilities that go unnoticed during development and testing; these vulnerabilities (such as buffer overflows, where attackers crash a service using arbitrary values that fill the memory buffer of an application, giving the attacker an interactive command prompt from which they may execute arbitrary commands) can give complete administrative control to an attacker.
Administrators should make sure that services do not run as the root user, and should stay vigilant of patches and errata updates for applications from vendors or security organizations such as CERT and CVE.
Application Vulnerabilities Attackers find faults in desktop and workstation applications (such as e-mail clients) and execute arbitrary code, implant Trojan horses for future compromise, or crash systems. Further exploitation can occur if the compromised workstation has administrative privileges on the rest of the network.
Workstations and desktops are more prone to exploitation as workers do not have the expertise or experience to prevent or detect a compromise; it is imperative to inform individuals of the risks they are taking when they install unauthorized software or open unsolicited email attachments.
Safeguards can be implemented such that email client software does not automatically open or execute attachments. Additionally, the automatic update of workstation software via Red Hat Network or other system management services can alleviate the burdens of multi-seat security deployments.
Denial of Service (DoS) Attacks Attacker or group of attackers coordinate against an organization's network or server resources by sending unauthorized packets to the target host (either server, router, or workstation). This forces the resource to become unavailable to legitimate users.
The most reported DoS case in the US occurred in 2000. Several highly-trafficked commercial and government sites were rendered unavailable by a coordinated ping flood attack using several compromised systems with high bandwidth connections acting as zombies, or redirected broadcast nodes.
Source packets are usually forged (as well as rebroadcasted), making investigation as to the true source of the attack difficult.
Advances in ingress filtering (IETF rfc2267) using iptables and Network IDSes such as snort assist administrators in tracking down and preventing distributed DoS attacks.

47.5. Security Updates
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As security vulnerabilities are discovered, the affected software must be updated in order to limit any potential security risks. If the software is part of a package within a Red Hat Enterprise Linux distribution that is currently supported, Red Hat, Inc. is committed to releasing updated packages that fix the vulnerability as soon as possible. Often, announcements about a given security exploit are accompanied with a patch (or source code that fixes the problem). This patch is then applied to the Red Hat Enterprise Linux package, tested by the Red Hat quality assurance team, and released as an errata update. However, if an announcement does not include a patch, a Red Hat developer works with the maintainer of the software to fix the problem. Once the problem is fixed, the package is tested and released as an errata update.
If an errata update is released for software used on your system, it is highly recommended that you update the effected packages as soon as possible to minimize the amount of time the system is potentially vulnerable.

47.5.1. Updating Packages
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When updating software on a system, it is important to download the update from a trusted source. An attacker can easily rebuild a package with the same version number as the one that is supposed to fix the problem but with a different security exploit and release it on the Internet. If this happens, using security measures such as verifying files against the original RPM does not detect the exploit. Thus, it is very important to only download RPMs from trusted sources, such as from Red Hat, Inc. and check the signature of the package to verify its integrity.
Red Hat offers two ways to find information on errata updates:
  1. Listed and available for download on Red Hat Network
  2. Listed and unlinked on the Red Hat Errata website

Note

Beginning with the Red Hat Enterprise Linux product line, updated packages can be downloaded only from Red Hat Network. Although the Red Hat Errata website contains updated information, it does not contain the actual packages for download.
47.5.1.1. Using Automatic Updates with RHN Classic
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Warning

Automatic system updates are only available using RHN Classic, which basis subscription consumption on access to content repository channels. RHN Classic is available as a convenience for customer environments with legacy systems which have not updated to Certificate-Based Red Hat Network.
The update and content stream is different for Certificate-Based Red Hat Network, so automatic updates are not used.
The new Certificate-Based Red Hat Network and the differences between Certificate-Based Red Hat Network and RHN Classic are described in Chapter 15, Registering a System and Managing Subscriptions.
RHN Classic allows the majority of the update process to be automated. It determines which RPM packages are necessary for the system, downloads them from a secure repository, verifies the RPM signature to make sure they have not been tampered with, and updates them. The package install can occur immediately or can be scheduled during a certain time period.
RHN Classic requires a system profile for each machine, which contains hardware and software information about the system. This information is kept confidential and is not given to anyone else. It is only used to determine which errata updates are applicable to each system, and, without it, RHN Classic can not determine whether a given system needs updates. When a security errata (or any type of errata) is released, RHN Classic sends an email with a description of the errata as well as a list of systems which are affected. To apply the update, use the Red Hat Update Agent or schedule the package to be updated through the RHN Classic Subscription Management area of the Customer Portal.

Important

Before installing any security errata, be sure to read any special instructions contained in the errata report and execute them accordingly. Refer to Section 47.5.1.5, “Applying the Changes” for general instructions about applying the changes made by an errata update.
47.5.1.2. Using the Red Hat Errata Website
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When security errata reports are released, they are published on the Red Hat Errata website available at http://www.redhat.com/security/. From this page, select the product and version for your system, and then select security at the top of the page to display only Red Hat Enterprise Linux Security Advisories. If the synopsis of one of the advisories describes a package used on your system, click on the synopsis for more details.
The details page describes the security exploit and any special instructions that must be performed in addition to updating the package to fix the security hole.
To download the updated package(s), click on the link to login to Red Hat Network, click the package name(s) and save to the hard drive. It is highly recommended that you create a new directory, such as /tmp/updates, and save all the downloaded packages to it.
47.5.1.3. Verifying Signed Packages
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All Red Hat Enterprise Linux packages are signed with the Red Hat, Inc. GPG key. GPG stands for GNU Privacy Guard, or GnuPG, a free software package used for ensuring the authenticity of distributed files. For example, a private key (secret key) held by Red Hat locks the package while the public key unlocks and verifies the package. If the public key distributed by Red Hat does not match the private key during RPM verification, the package may have been altered and therefore cannot be trusted.
The RPM utility within Red Hat Enterprise Linux automatically tries to verify the GPG signature of an RPM package before installing it. If the Red Hat GPG key is not installed, install it from a secure, static location, such as an Red Hat Enterprise Linux installation CD-ROM.
Assuming the CD-ROM is mounted in /mnt/cdrom, use the following command to import it into the keyring (a database of trusted keys on the system): 
rpm --import /mnt/cdrom/RPM-GPG-KEY-redhat-release
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To display a list of all keys installed for RPM verification, execute the following command: 
rpm -qa gpg-pubkey*
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For the Red Hat key, the output includes the following: 
gpg-pubkey-37017186-45761324
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To display details about a specific key, use the rpm -qi command followed by the output from the previous command, as in this example: 
rpm -qi gpg-pubkey-37017186-45761324
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It is extremely important to verify the signature of the RPM files before installing them to ensure that they have not been altered from the Red Hat, Inc. release of the packages. To verify all the downloaded packages at once, issue the following command: 
rpm -K /tmp/updates/*.rpm
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For each package, if the GPG key verifies successfully, the command returns gpg OK. If it doesn't, make sure you are using the correct Red Hat public key, as well as verifying the source of the content. Packages that do not pass GPG verifications should not be installed, as they may have been altered by a third party.
After verifying the GPG key and downloading all the packages associated with the errata report, install the packages as root at a shell prompt.
47.5.1.4. Installing Signed Packages
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Installation for most packages can be done safely (except kernel packages) by issuing the following command: 
rpm -Uvh /tmp/updates/*.rpm
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For kernel packages use the following command: 
rpm -ivh /tmp/updates/<kernel-package>
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Replace <kernel-package> in the previous example with the name of the kernel RPM.
Once the machine has been safely rebooted using the new kernel, the old kernel may be removed using the following command: 
rpm -e <old-kernel-package>
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Replace <old-kernel-package> in the previous example with the name of the older kernel RPM.

Note

It is not a requirement that the old kernel be removed. The default boot loader, GRUB, allows for multiple kernels to be installed, then chosen from a menu at boot time.

Important

Before installing any security errata, be sure to read any special instructions contained in the errata report and execute them accordingly. Refer to Section 47.5.1.5, “Applying the Changes” for general instructions about applying the changes made by an errata update.
47.5.1.5. Applying the Changes
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After downloading and installing security errata via Red Hat Network or the Red Hat errata website, it is important to halt usage of the older software and begin using the new software. How this is done depends on the type of software that has been updated. The following list itemizes the general categories of software and provides instructions for using the updated versions after a package upgrade.

Note

In general, rebooting the system is the surest way to ensure that the latest version of a software package is used; however, this option is not always available to the system administrator.
Applications
User-space applications are any programs that can be initiated by a system user. Typically, such applications are used only when a user, script, or automated task utility launches them and they do not persist for long periods of time.
Once such a user-space application is updated, halt any instances of the application on the system and launch the program again to use the updated version.
Kernel
The kernel is the core software component for the Red Hat Enterprise Linux operating system. It manages access to memory, the processor, and peripherals as well as schedules all tasks.
Because of its central role, the kernel cannot be restarted without also stopping the computer. Therefore, an updated version of the kernel cannot be used until the system is rebooted.
Shared Libraries
Shared libraries are units of code, such as glibc, which are used by a number of applications and services. Applications utilizing a shared library typically load the shared code when the application is initialized, so any applications using the updated library must be halted and relaunched.
To determine which running applications link against a particular library, use the lsof command as in the following example: 
lsof /usr/lib/libwrap.so*
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This command returns a list of all the running programs which use TCP wrappers for host access control. Therefore, any program listed must be halted and relaunched if the tcp_wrappers package is updated.
SysV Services
SysV services are persistent server programs launched during the boot process. Examples of SysV services include sshd, vsftpd, and xinetd.
Because these programs usually persist in memory as long as the machine is booted, each updated SysV service must be halted and relaunched after the package is upgraded. This can be done using the Services Configuration Tool or by logging into a root shell prompt and issuing the /sbin/service command as in the following example: 
service <service-name> restart
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In the previous example, replace <service-name> with the name of the service, such as sshd.
Refer to Chapter 17, Network Configuration for more information on the Services Configuration Tool.
xinetd Services
Services controlled by the xinetd super service only run when a there is an active connection. Examples of services controlled by xinetd include Telnet, IMAP, and POP3.
Because new instances of these services are launched by xinetd each time a new request is received, connections that occur after an upgrade are handled by the updated software. However, if there are active connections at the time the xinetd controlled service is upgraded, they are serviced by the older version of the software.
To kill off older instances of a particular xinetd controlled service, upgrade the package for the service then halt all processes currently running. To determine if the process is running, use the ps command and then use the kill or killall command to halt current instances of the service.
For example, if security errata imap packages are released, upgrade the packages, then type the following command as root into a shell prompt: 
ps -aux | grep imap
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This command returns all active IMAP sessions. Individual sessions can then be terminated by issuing the following command: 
kill <PID>
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If this fails to terminate the session, use the following command instead: 
kill -9 <PID>
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In the previous examples, replace <PID> with the process identification number (found in the second column of the ps command) for an IMAP session.
To kill all active IMAP sessions, issue the following command: 
killall imapd
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Chapter 48. Securing Your Network
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48.1. Workstation Security
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Securing a Linux environment begins with the workstation. Whether locking down a personal machine or securing an enterprise system, sound security policy begins with the individual computer. A computer network is only as secure as its weakest node.

48.1.1. Evaluating Workstation Security
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When evaluating the security of a Red Hat Enterprise Linux workstation, consider the following:
  • BIOS and Boot Loader Security — Can an unauthorized user physically access the machine and boot into single user or rescue mode without a password?
  • Password Security — How secure are the user account passwords on the machine?
  • Administrative Controls — Who has an account on the system and how much administrative control do they have?
  • Available Network Services — What services are listening for requests from the network and should they be running at all?
  • Personal Firewalls — What type of firewall, if any, is necessary?
  • Security Enhanced Communication Tools — Which tools should be used to communicate between workstations and which should be avoided?

48.1.2. BIOS and Boot Loader Security
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Password protection for the BIOS (or BIOS equivalent) and the boot loader can prevent unauthorized users who have physical access to systems from booting using removable media or obtaining root privileges through single user mode. The security measures you should take to protect against such attacks depends both on the sensitivity of the information on the workstation and the location of the machine.
For example, if a machine is used in a trade show and contains no sensitive information, then it may not be critical to prevent such attacks. However, if an employee's laptop with private, unencrypted SSH keys for the corporate network is left unattended at that same trade show, it could lead to a major security breach with ramifications for the entire company.
If the workstation is located in a place where only authorized or trusted people have access, however, then securing the BIOS or the boot loader may not be necessary.
48.1.2.1. BIOS Passwords
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The two primary reasons for password protecting the BIOS of a computer are[14]:
  1. Preventing Changes to BIOS Settings — If an intruder has access to the BIOS, they can set it to boot from a diskette or CD-ROM. This makes it possible for them to enter rescue mode or single user mode, which in turn allows them to start arbitrary processes on the system or copy sensitive data.
  2. Preventing System Booting — Some BIOSes allow password protection of the boot process. When activated, an attacker is forced to enter a password before the BIOS launches the boot loader.
Because the methods for setting a BIOS password vary between computer manufacturers, consult the computer's manual for specific instructions.
If you forget the BIOS password, it can either be reset with jumpers on the motherboard or by disconnecting the CMOS battery. For this reason, it is good practice to lock the computer case if possible. However, consult the manual for the computer or motherboard before attempting to disconnect the CMOS battery.
48.1.2.1.1. Securing Non-x86 Platforms
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Other architectures use different programs to perform low-level tasks roughly equivalent to those of the BIOS on x86 systems. For instance, Intel® Itanium™ computers use the Extensible Firmware Interface (EFI) shell.
For instructions on password protecting BIOS-like programs on other architectures, refer to the manufacturer's instructions.
48.1.2.2. Boot Loader Passwords
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The primary reasons for password protecting a Linux boot loader are as follows:
  1. Preventing Access to Single User Mode — If attackers can boot the system into single user mode, they are logged in automatically as root without being prompted for the root password.
  2. Preventing Access to the GRUB Console — If the machine uses GRUB as its boot loader, an attacker can use the GRUB editor interface to change its configuration or to gather information using the cat command.
  3. Preventing Access to Insecure Operating Systems — If it is a dual-boot system, an attacker can select an operating system at boot time (for example, DOS), which ignores access controls and file permissions.
Red Hat Enterprise Linux ships with the GRUB boot loader on the x86 platform. For a detailed look at GRUB, refer to the Red Hat Installation Guide.
48.1.2.2.1. Password Protecting GRUB
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You can configure GRUB to address the first two issues listed in Section 48.1.2.2, “Boot Loader Passwords” by adding a password directive to its configuration file. To do this, first choose a strong password, open a shell, log in as root, and then type the following command: 
grub-md5-crypt
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When prompted, type the GRUB password and press Enter. This returns an MD5 hash of the password.
Next, edit the GRUB configuration file /boot/grub/grub.conf. Open the file and below the timeout line in the main section of the document, add the following line: 
password --md5 <password-hash>
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Replace <password-hash> with the value returned by /sbin/grub-md5-crypt[15].
The next time the system boots, the GRUB menu prevents access to the editor or command interface without first pressing p followed by the GRUB password.
Unfortunately, this solution does not prevent an attacker from booting into an insecure operating system in a dual-boot environment. For this, a different part of the /boot/grub/grub.conf file must be edited.
Look for the title line of the operating system that you want to secure, and add a line with the lock directive immediately beneath it.
For a DOS system, the stanza should begin similar to the following: 
title DOS lock
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Warning

A password line must be present in the main section of the /boot/grub/grub.conf file for this method to work properly. Otherwise, an attacker can access the GRUB editor interface and remove the lock line.
To create a different password for a particular kernel or operating system, add a lock line to the stanza, followed by a password line.
Each stanza protected with a unique password should begin with lines similar to the following example: 
title DOS lock password --md5 <password-hash>
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48.1.3. Password Security
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Passwords are the primary method that Red Hat Enterprise Linux uses to verify a user's identity. This is why password security is so important for protection of the user, the workstation, and the network.
For security purposes, the installation program configures the system to use Message-Digest Algorithm (MD5) and shadow passwords. It is highly recommended that you do not alter these settings.
If MD5 passwords are deselected during installation, the older Data Encryption Standard (DES) format is used. This format limits passwords to eight alphanumeric characters (disallowing punctuation and other special characters), and provides a modest 56-bit level of encryption.
If shadow passwords are deselected during installation, all passwords are stored as a one-way hash in the world-readable /etc/passwd file, which makes the system vulnerable to offline password cracking attacks. If an intruder can gain access to the machine as a regular user, they can copy the /etc/passwd file to their own machine and run any number of password cracking programs against it. If there is an insecure password in the file, it is only a matter of time before the password cracker discovers it.
Shadow passwords eliminate this type of attack by storing the password hashes in the file /etc/shadow, which is readable only by the root user.
This forces a potential attacker to attempt password cracking remotely by logging into a network service on the machine, such as SSH or FTP. This sort of brute-force attack is much slower and leaves an obvious trail as hundreds of failed login attempts are written to system files. Of course, if the cracker starts an attack in the middle of the night on a system with weak passwords, the cracker may have gained access before dawn and edited the log files to cover their tracks.
In addition to format and storage considerations is the issue of content. The single most important thing a user can do to protect their account against a password cracking attack is create a strong password.
48.1.3.1. Creating Strong Passwords
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When creating a secure password, it is a good idea to follow these guidelines:
  • Do Not Use Only Words or Numbers — Never use only numbers or words in a password.
    Some insecure examples include the following:
    • 8675309
    • juan
    • hackme
  • Do Not Use Recognizable Words — Words such as proper names, dictionary words, or even terms from television shows or novels should be avoided, even if they are bookended with numbers.
    Some insecure examples include the following:
    • john1
    • DS-9
    • mentat123
  • Do Not Use Words in Foreign Languages — Password cracking programs often check against word lists that encompass dictionaries of many languages. Relying on foreign languages for secure passwords is not secure.
    Some insecure examples include the following:
    • cheguevara
    • bienvenido1
    • 1dumbKopf
  • Do Not Use Hacker Terminology — If you think you are elite because you use hacker terminology — also called l337 (LEET) speak — in your password, think again. Many word lists include LEET speak.
    Some insecure examples include the following:
    • H4X0R
    • 1337
  • Do Not Use Personal Information — Avoid using any personal information in your passwords. If the attacker knows your identity, the task of deducing your password becomes easier. The following is a list of the types of information to avoid when creating a password:
    Some insecure examples include the following:
    • Your name
    • The names of pets
    • The names of family members
    • Any birth dates
    • Your phone number or zip code
  • Do Not Invert Recognizable Words — Good password checkers always reverse common words, so inverting a bad password does not make it any more secure.
    Some insecure examples include the following:
    • R0X4H
    • nauj
    • 9-DS
  • Do Not Write Down Your Password — Never store a password on paper. It is much safer to memorize it.
  • Do Not Use the Same Password For All Machines — It is important to make separate passwords for each machine. This way if one system is compromised, all of your machines are not immediately at risk.
The following guidelines will help you to create a strong password:
  • Make the Password at Least Eight Characters Long — The longer the password, the better. If using MD5 passwords, it should be 15 characters or longer. With DES passwords, use the maximum length (eight characters).
  • Mix Upper and Lower Case Letters — Red Hat Enterprise Linux is case sensitive, so mix cases to enhance the strength of the password.
  • Mix Letters and Numbers — Adding numbers to passwords, especially when added to the middle (not just at the beginning or the end), can enhance password strength.
  • Include Non-Alphanumeric Characters — Special characters such as &, $, and > can greatly improve the strength of a password (this is not possible if using DES passwords).
  • Pick a Password You Can Remember — The best password in the world does little good if you cannot remember it; use acronyms or other mnemonic devices to aid in memorizing passwords.
With all these rules, it may seem difficult to create a password that meets all of the criteria for good passwords while avoiding the traits of a bad one. Fortunately, there are some steps you can take to generate an easily-remembered, secure password.
48.1.3.1.1. Secure Password Creation Methodology
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There are many methods that people use to create secure passwords. One of the more popular methods involves acronyms. For example:
  • Think of an easily-remembered phrase, such as:
    "over the river and through the woods, to grandmother's house we go."
  • Next, turn it into an acronym (including the punctuation).
    otrattw,tghwg.
  • Add complexity by substituting numbers and symbols for letters in the acronym. For example, substitute 7 for t and the at symbol (@) for a:
    o7r@77w,7ghwg.
  • Add more complexity by capitalizing at least one letter, such as H.
    o7r@77w,7gHwg.
  • Finally, do not use the example password above for any systems, ever.
While creating secure passwords is imperative, managing them properly is also important, especially for system administrators within larger organizations. The following section details good practices for creating and managing user passwords within an organization.
If an organization has a large number of users, the system administrators have two basic options available to force the use of good passwords. They can create passwords for the user, or they can let users create their own passwords, while verifying the passwords are of acceptable quality.
Creating the passwords for the users ensures that the passwords are good, but it becomes a daunting task as the organization grows. It also increases the risk of users writing their passwords down.
For these reasons, most system administrators prefer to have the users create their own passwords, but actively verify that the passwords are good and, in some cases, force users to change their passwords periodically through password aging.
48.1.3.2.1. Forcing Strong Passwords
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To protect the network from intrusion it is a good idea for system administrators to verify that the passwords used within an organization are strong ones. When users are asked to create or change passwords, they can use the command line application passwd, which is Pluggable Authentication Manager (PAM) aware and therefore checks to see if the password is too short or otherwise easy to crack. This check is performed using the pam_cracklib.so PAM module. Since PAM is customizable, it is possible to add more password integrity checkers, such as pam_passwdqc (available from http://www.openwall.com/passwdqc/) or to write a new module. For a list of available PAM modules, refer to http://www.kernel.org/pub/linux/libs/pam/modules.html. For more information about PAM, refer to Section 48.4, “Pluggable Authentication Modules (PAM)”.
The password check that is performed at the time of their creation does not discover bad passwords as effectively as running a password cracking program against the passwords.
Many password cracking programs are available that run under Red Hat Enterprise Linux, although none ship with the operating system. Below is a brief list of some of the more popular password cracking programs:

Note

None of these tools are supplied with Red Hat Enterprise Linux and are therefore not supported by Red Hat, Inc. in any way.
  • John The Ripper — A fast and flexible password cracking program. It allows the use of multiple word lists and is capable of brute-force password cracking. It is available online at http://www.openwall.com/john/.
  • Crack — Perhaps the most well known password cracking software, Crack is also very fast, though not as easy to use as John The Ripper. It can be found online at http://www.openwall.com/john/.
  • Slurpie — Slurpie is similar to John The Ripper and Crack, but it is designed to run on multiple computers simultaneously, creating a distributed password cracking attack. It can be found along with a number of other distributed attack security evaluation tools online at http://www.ussrback.com/distributed.htm.

Warning

Always get authorization in writing before attempting to crack passwords within an organization.
48.1.3.2.2. Password Aging
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Password aging is another technique used by system administrators to defend against bad passwords within an organization. Password aging means that after a specified period (usually 90 days), the user is prompted to create a new password. The theory behind this is that if a user is forced to change their password periodically, a cracked password is only useful to an intruder for a limited amount of time. The downside to password aging, however, is that users are more likely to write their passwords down.
There are two primary programs used to specify password aging under Red Hat Enterprise Linux: the chage command or the graphical User Manager (system-config-users) application.
The -M option of the chage command specifies the maximum number of days the password is valid. For example, to set a user's password to expire in 90 days, use the following command: 
chage -M 90 <username>
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In the above command, replace <username> with the name of the user. To disable password expiration, it is traditional to use a value of 99999 after the -M option (this equates to a little over 273 years).
You can also use the chage command in interactive mode to modify multiple password aging and account details. Use the following command to enter interactive mode: 
chage <username>
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The following is a sample interactive session using this command: 
~]# chage davido
Changing the aging information for davido
Enter the new value, or press ENTER for the default

        Minimum Password Age [0]: 10
        Maximum Password Age [99999]: 90
        Last Password Change (YYYY-MM-DD) [2006-08-18]:
        Password Expiration Warning [7]:
        Password Inactive [-1]:
        Account Expiration Date (YYYY-MM-DD) [1969-12-31]:
~]#
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Refer to the man page for chage for more information on the available options.
You can also use the graphical User Manager application to create password aging policies, as follows. Note: you need Administrator privileges to perform this procedure.
  1. Click the System menu on the Panel, point to Administration and then click Users and Groups to display the User Manager. Alternatively, type the command system-config-users at a shell prompt.
  2. Click the Users tab, and select the required user in the list of users.
  3. Click Properties on the toolbar to display the User Properties dialog box (or choose Properties on the File menu).
  4. Click the Password Info tab, and select the check box for Enable password expiration.
  5. Enter the required value in the Days before change required field, and click OK.
Specifying password aging options
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Specifying password aging options

Figure 48.1. Specifying password aging options

For more information about user and group configuration (including instructions on forcing first time passwords), refer to Chapter 37, Users and Groups.

48.1.4. Administrative Controls
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When administering a home machine, the user must perform some tasks as the root user or by acquiring effective root privileges via a setuid program, such as sudo or su. A setuid program is one that operates with the user ID (UID) of the program's owner rather than the user operating the program. Such programs are denoted by an s in the owner section of a long format listing, as in the following example: 
-rwsr-xr-x    1 root     root        47324 May  1 08:09 /bin/su
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Note

The s may be upper case or lower case. If it appears as upper case, it means that the underlying permission bit has not been set.
For the system administrators of an organization, however, choices must be made as to how much administrative access users within the organization should have to their machine. Through a PAM module called pam_console.so, some activities normally reserved only for the root user, such as rebooting and mounting removable media are allowed for the first user that logs in at the physical console (refer to Section 48.4, “Pluggable Authentication Modules (PAM)” for more information about the pam_console.so module.) However, other important system administration tasks, such as altering network settings, configuring a new mouse, or mounting network devices, are not possible without administrative privileges. As a result, system administrators must decide how much access the users on their network should receive.
48.1.4.1. Allowing Root Access
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If the users within an organization are trusted and computer-literate, then allowing them root access may not be an issue. Allowing root access by users means that minor activities, like adding devices or configuring network interfaces, can be handled by the individual users, leaving system administrators free to deal with network security and other important issues.
On the other hand, giving root access to individual users can lead to the following issues:
  • Machine Misconfiguration — Users with root access can misconfigure their machines and require assistance to resolve issues. Even worse, they might open up security holes without knowing it.
  • Running Insecure Services — Users with root access might run insecure servers on their machine, such as FTP or Telnet, potentially putting usernames and passwords at risk. These services transmit this information over the network in plain text.
  • Running Email Attachments As Root — Although rare, email viruses that affect Linux do exist. The only time they are a threat, however, is when they are run by the root user.
48.1.4.2. Disallowing Root Access
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If an administrator is uncomfortable allowing users to log in as root for these or other reasons, the root password should be kept secret, and access to runlevel one or single user mode should be disallowed through boot loader password protection (refer to Section 48.1.2.2, “Boot Loader Passwords” for more information on this topic.)
The following are four different ways that an administrator can further ensure that root logins are disallowed:
Changing the root shell
To prevent users from logging in directly as root, the system administrator can set the root account's shell to /sbin/nologin in the /etc/passwd file.
Expand
Table 48.1. Disabling the Root Shell
Effects Does Not Affect
Prevents access to the root shell and logs any such attempts. The following programs are prevented from accessing the root account:
  • login
  • gdm
  • kdm
  • xdm
  • su
  • ssh
  • scp
  • sftp
Programs that do not require a shell, such as FTP clients, mail clients, and many setuid programs. The following programs are not prevented from accessing the root account:
  • sudo
  • FTP clients
  • Email clients
Disabling root access via any console device (tty)
To further limit access to the root account, administrators can disable root logins at the console by editing the /etc/securetty file. This file lists all devices the root user is allowed to log into. If the file does not exist at all, the root user can log in through any communication device on the system, whether via the console or a raw network interface. This is dangerous, because a user can log in to their machine as root via Telnet, which transmits the password in plain text over the network.
By default, Red Hat Enterprise Linux's /etc/securetty file only allows the root user to log in at the console physically attached to the machine. To prevent the root user from logging in, remove the contents of this file by typing the following command at a shell prompt as root: 
echo > /etc/securetty
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To enable securetty support in the KDM, GDM, and XDM login managers, add the following line: 
auth [user_unknown=ignore success=ok ignore=ignore default=bad] pam_securetty.so
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to the files listed below:
  • /etc/pam.d/gdm
  • /etc/pam.d/gdm-autologin
  • /etc/pam.d/gdm-fingerprint
  • /etc/pam.d/gdm-password
  • /etc/pam.d/gdm-smartcard
  • /etc/pam.d/kdm
  • /etc/pam.d/kdm-np
  • /etc/pam.d/xdm

Warning

A blank /etc/securetty file does not prevent the root user from logging in remotely using the OpenSSH suite of tools because the console is not opened until after authentication.
Expand
Table 48.2. Disabling Root Logins
Effects Does Not Affect
Prevents access to the root account via the console or the network. The following programs are prevented from accessing the root account:
  • login
  • gdm
  • kdm
  • xdm
  • Other network services that open a tty
Programs that do not log in as root, but perform administrative tasks through setuid or other mechanisms. The following programs are not prevented from accessing the root account:
  • su
  • sudo
  • ssh
  • scp
  • sftp
Disabling root SSH logins
To prevent root logins via the SSH protocol, edit the SSH daemon's configuration file, /etc/ssh/sshd_config, and change the line that reads: 
#PermitRootLogin yes
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to read as follows: 
PermitRootLogin no
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Expand
Table 48.3. Disabling Root SSH Logins
Effects Does Not Affect
Prevents root access via the OpenSSH suite of tools. The following programs are prevented from accessing the root account:
  • ssh
  • scp
  • sftp
Programs that are not part of the OpenSSH suite of tools.
Using PAM to limit root access to services
PAM, through the /lib/security/pam_listfile.so module, allows great flexibility in denying specific accounts. The administrator can use this module to reference a list of users who are not allowed to log in. To limit root access to a system service, edit the file for the target service in the /etc/pam.d/ directory and make sure the pam_listfile.so module is required for authentication.
The following is an example of how the module is used for the vsftpd FTP server in the /etc/pam.d/vsftpd PAM configuration file (the \ character at the end of the first line is not necessary if the directive is on a single line): 
auth   required   /lib/security/pam_listfile.so   item=user \
 sense=deny file=/etc/vsftpd.ftpusers onerr=succeed
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This instructs PAM to consult the /etc/vsftpd.ftpusers file and deny access to the service for any listed user. The administrator can change the name of this file, and can keep separate lists for each service or use one central list to deny access to multiple services.
If the administrator wants to deny access to multiple services, a similar line can be added to the PAM configuration files, such as /etc/pam.d/pop and /etc/pam.d/imap for mail clients, or /etc/pam.d/ssh for SSH clients.
For more information about PAM, refer to Section 48.4, “Pluggable Authentication Modules (PAM)”.
Expand
Table 48.4. Disabling Root Using PAM
Effects Does Not Affect
Prevents root access to network services that are PAM aware. The following services are prevented from accessing the root account:
  • login
  • gdm
  • kdm
  • xdm
  • ssh
  • scp
  • sftp
  • FTP clients
  • Email clients
  • Any PAM aware services
Programs and services that are not PAM aware.
48.1.4.3. Limiting Root Access
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Rather than completely denying access to the root user, the administrator may want to allow access only via setuid programs, such as su or sudo.
48.1.4.3.1. The su Command
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When a user executes the su command, they are prompted for the root password and, after authentication, is given a root shell prompt.
Once logged in via the su command, the user is the root user and has absolute administrative access to the system[16]. In addition, once a user has become root, it is possible for them to use the su command to change to any other user on the system without being prompted for a password.
Because this program is so powerful, administrators within an organization may wish to limit who has access to the command.
One of the simplest ways to do this is to add users to the special administrative group called wheel. To do this, type the following command as root: 
usermod -G wheel <username>
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In the previous command, replace <username> with the username you want to add to the wheel group.
You can also use the User Manager to modify group memberships, as follows. Note: you need Administrator privileges to perform this procedure.
  1. Click the System menu on the Panel, point to Administration and then click Users and Groups to display the User Manager. Alternatively, type the command system-config-users at a shell prompt.
  2. Click the Users tab, and select the required user in the list of users.
  3. Click Properties on the toolbar to display the User Properties dialog box (or choose Properties on the File menu).
  4. Click the Groups tab, select the check box for the wheel group, and then click OK. Refer to Figure 48.2, “Adding users to the "wheel" group.”.
  5. Open the PAM configuration file for su (/etc/pam.d/su) in a text editor and remove the comment # from the following line: 
    auth           required        pam_wheel.so use_uid
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    This change means that only members of the administrative group wheel can switch to another user using the su command.
Adding users to the
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Adding users to the

Figure 48.2. Adding users to the "wheel" group.

Note

The root user is part of the wheel group by default.
48.1.4.3.2. The sudo Command
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The sudo command offers another approach to giving users administrative access. When trusted users precede an administrative command with sudo, they are prompted for their own password. Then, when they have been authenticated and assuming that the command is permitted, the administrative command is executed as if they were the root user.
The basic format of the sudo command is as follows: 
sudo <command>
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In the above example, <command> would be replaced by a command normally reserved for the root user, such as mount.

Important

Users of the sudo command should take extra care to log out before walking away from their machines since sudoers can use the command again without being asked for a password within a five minute period. This setting can be altered via the configuration file, /etc/sudoers.
The sudo command allows for a high degree of flexibility. For instance, only users listed in the /etc/sudoers configuration file are allowed to use the sudo command and the command is executed in the user's shell, not a root shell. This means the root shell can be completely disabled, as shown in Section 48.1.4.2, “Disallowing Root Access”.
The sudo command also provides a comprehensive audit trail. Each successful authentication is logged to the file /var/log/messages and the command issued along with the issuer's user name is logged to the file /var/log/secure.
Another advantage of the sudo command is that an administrator can allow different users access to specific commands based on their needs.
Administrators wanting to edit the sudo configuration file, /etc/sudoers, should use the visudo command.
To give someone full administrative privileges, type visudo and add a line similar to the following in the user privilege specification section: 
juan ALL=(ALL) ALL
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This example states that the user, juan, can use sudo from any host and execute any command.
The example below illustrates the granularity possible when configuring sudo: 
%users  localhost=/sbin/shutdown -h now
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This example states that any user can issue the command /sbin/shutdown -h now as long as it is issued from the console.
The man page for sudoers has a detailed listing of options for this file.

48.1.5. Available Network Services
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While user access to administrative controls is an important issue for system administrators within an organization, monitoring which network services are active is of paramount importance to anyone who administers and operates a Linux system.
Many services under Red Hat Enterprise Linux behave as network servers. If a network service is running on a machine, then a server application (called a daemon), is listening for connections on one or more network ports. Each of these servers should be treated as a potential avenue of attack.
48.1.5.1. Risks To Services
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Network services can pose many risks for Linux systems. Below is a list of some of the primary issues:
  • Denial of Service Attacks (DoS) — By flooding a service with requests, a denial of service attack can render a system unusable as it tries to log and answer each request.
  • Script Vulnerability Attacks — If a server is using scripts to execute server-side actions, as Web servers commonly do, a cracker can attack improperly written scripts. These script vulnerability attacks can lead to a buffer overflow condition or allow the attacker to alter files on the system.
  • Buffer Overflow Attacks — Services that connect to ports numbered 0 through 1023 must run as an administrative user. If the application has an exploitable buffer overflow, an attacker could gain access to the system as the user running the daemon. Because exploitable buffer overflows exist, crackers use automated tools to identify systems with vulnerabilities, and once they have gained access, they use automated rootkits to maintain their access to the system.

Note

The threat of buffer overflow vulnerabilities is mitigated in Red Hat Enterprise Linux by ExecShield, an executable memory segmentation and protection technology supported by x86-compatible uni- and multi-processor kernels. ExecShield reduces the risk of buffer overflow by separating virtual memory into executable and non-executable segments. Any program code that tries to execute outside of the executable segment (such as malicious code injected from a buffer overflow exploit) triggers a segmentation fault and terminates.
Execshield also includes support for No eXecute (NX) technology on AMD64 platforms and eXecute Disable (XD) technology on Itanium and Intel® 64 systems. These technologies work in conjunction with ExecShield to prevent malicious code from running in the executable portion of virtual memory with a granularity of 4KB of executable code, lowering the risk of attack from stealthy buffer overflow exploits.

Note

To limit exposure to attacks over the network, all services that are unused should be turned off.
48.1.5.2. Identifying and Configuring Services
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To enhance security, most network services installed with Red Hat Enterprise Linux are turned off by default. There are, however, some notable exceptions:
  • cupsd — The default print server for Red Hat Enterprise Linux.
  • lpd — An alternative print server.
  • xinetd — A super server that controls connections to a range of subordinate servers, such as gssftp and telnet.
  • sendmail — The Sendmail Mail Transport Agent (MTA) is enabled by default, but only listens for connections from the localhost.
  • sshd — The OpenSSH server, which is a secure replacement for Telnet.
When determining whether to leave these services running, it is best to use common sense and err on the side of caution. For example, if a printer is not available, do not leave cupsd running. The same is true for portmap. If you do not mount NFSv3 volumes or use NIS (the ypbind service), then portmap should be disabled.
Red Hat Enterprise Linux ships with three programs designed to switch services on or off. They are the Services Configuration Tool (system-config-services), ntsysv, and chkconfig. For information on using these tools, refer to Chapter 18, Controlling Access to Services.
Services Configuration Tool
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Services Configuration Tool

Figure 48.3. Services Configuration Tool

If unsure of the purpose for a particular service, the Services Configuration Tool has a description field, illustrated in Figure 48.3, “Services Configuration Tool”, that provides additional information.
Checking which network services are available to start at boot time is only part of the story. You should also check which ports are open and listening. Refer to Section 48.2.8, “Verifying Which Ports Are Listening” for more information.
48.1.5.3. Insecure Services
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Potentially, any network service is insecure. This is why turning off unused services is so important. Exploits for services are routinely revealed and patched, making it very important to regularly update packages associated with any network service. Refer to Section 47.5, “Security Updates” for more information.
Some network protocols are inherently more insecure than others. These include any services that:
  • Transmit Usernames and Passwords Over a Network Unencrypted — Many older protocols, such as Telnet and FTP, do not encrypt the authentication session and should be avoided whenever possible.
  • Transmit Sensitive Data Over a Network Unencrypted — Many protocols transmit data over the network unencrypted. These protocols include Telnet, FTP, HTTP, and SMTP. Many network file systems, such as NFS and SMB, also transmit information over the network unencrypted. It is the user's responsibility when using these protocols to limit what type of data is transmitted.
    Remote memory dump services, like netdump, transmit the contents of memory over the network unencrypted. Memory dumps can contain passwords or, even worse, database entries and other sensitive information.
    Other services like finger and rwhod reveal information about users of the system.
Examples of inherently insecure services include rlogin, rsh, telnet, and vsftpd.
All remote login and shell programs (rlogin, rsh, and telnet) should be avoided in favor of SSH. Refer to Section 48.1.7, “Security Enhanced Communication Tools” for more information about sshd.
FTP is not as inherently dangerous to the security of the system as remote shells, but FTP servers must be carefully configured and monitored to avoid problems. Refer to Section 48.2.6, “Securing FTP” for more information about securing FTP servers.
Services that should be carefully implemented and behind a firewall include:
  • finger
  • authd (this was called identd in previous Red Hat Enterprise Linux releases.)
  • netdump
  • netdump-server
  • nfs
  • rwhod
  • sendmail
  • smb (Samba)
  • yppasswdd
  • ypserv
  • ypxfrd
More information on securing network services is available in Section 48.2, “Server Security”.
The next section discusses tools available to set up a simple firewall.

48.1.6. Personal Firewalls
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After the necessary network services are configured, it is important to implement a firewall.

Important

You should configure the necessary services and implement a firewall before connecting to the Internet or any other network that you do not trust.
Firewalls prevent network packets from accessing the system's network interface. If a request is made to a port that is blocked by a firewall, the request is ignored. If a service is listening on one of these blocked ports, it does not receive the packets and is effectively disabled. For this reason, care should be taken when configuring a firewall to block access to ports not in use, while not blocking access to ports used by configured services.
For most users, the best tool for configuring a simple firewall is the graphical firewall configuration tool which ships with Red Hat Enterprise Linux: the Security Level Configuration Tool (system-config-securitylevel). This tool creates broad iptables rules for a general-purpose firewall using a control panel interface.
Refer to Section 48.8.2, “Basic Firewall Configuration” for more information about using this application and its available options.
For advanced users and server administrators, manually configuring a firewall with iptables is probably a better option. Refer to Section 48.8, “Firewalls” for more information. Refer to Section 48.9, “IPTables” for a comprehensive guide to the iptables command.

48.1.7. Security Enhanced Communication Tools
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As the size and popularity of the Internet has grown, so has the threat of communication interception. Over the years, tools have been developed to encrypt communications as they are transferred over the network.
Red Hat Enterprise Linux ships with two basic tools that use high-level, public-key-cryptography-based encryption algorithms to protect information as it travels over the network.
  • OpenSSH — A free implementation of the SSH protocol for encrypting network communication.
  • Gnu Privacy Guard (GPG) — A free implementation of the PGP (Pretty Good Privacy) encryption application for encrypting data.
OpenSSH is a safer way to access a remote machine and replaces older, unencrypted services like telnet and rsh. OpenSSH includes a network service called sshd and three command line client applications:
  • ssh — A secure remote console access client.
  • scp — A secure remote copy command.
  • sftp — A secure pseudo-ftp client that allows interactive file transfer sessions.

Important

Although the sshd service is inherently secure, the service must be kept up-to-date to prevent security threats. Refer to Section 47.5, “Security Updates” for more information.
GPG is one way to ensure private email communication. It can be used both to email sensitive data over public networks and to protect sensitive data on hard drives.

48.2. Server Security
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When a system is used as a server on a public network, it becomes a target for attacks. Hardening the system and locking down services is therefore of paramount importance for the system administrator.
Before delving into specific issues, review the following general tips for enhancing server security:
  • Keep all services current, to protect against the latest threats.
  • Use secure protocols whenever possible.
  • Serve only one type of network service per machine whenever possible.
  • Monitor all servers carefully for suspicious activity.
TCP Wrappers provide access control to a variety of services. Most modern network services, such as SSH, Telnet, and FTP, make use of TCP Wrappers, which stand guard between an incoming request and the requested service.
The benefits offered by TCP Wrappers are enhanced when used in conjunction with xinetd, a super server that provides additional access, logging, binding, redirection, and resource utilization control.

Note

It is a good idea to use iptables firewall rules in conjunction with TCP Wrappers and xinetd to create redundancy within service access controls. Refer to Section 48.8, “Firewalls” for more information about implementing firewalls with iptables commands.
Refer to Section 18.2, “TCP Wrappers” for more information on configuring TCP Wrappers and xinetd.
The following subsections assume a basic knowledge of each topic and focus on specific security options.
48.2.1.1. Enhancing Security With TCP Wrappers
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TCP Wrappers are capable of much more than denying access to services. This section illustrates how they can be used to send connection banners, warn of attacks from particular hosts, and enhance logging functionality. Refer to the hosts_options man page for information about the TCP Wrapper functionality and control language.
48.2.1.1.1. TCP Wrappers and Connection Banners
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Displaying a suitable banner when users connect to a service is a good way to let potential attackers know that the system administrator is being vigilant. You can also control what information about the system is presented to users. To implement a TCP Wrappers banner for a service, use the banner option.
This example implements a banner for vsftpd. To begin, create a banner file. It can be anywhere on the system, but it must have same name as the daemon. For this example, the file is called /etc/banners/vsftpd and contains the following line: 
220-Hello, %c
220-All activity on ftp.example.com is logged.
220-Inappropriate use will result in your access privileges being removed.
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The %c token supplies a variety of client information, such as the username and hostname, or the username and IP address to make the connection even more intimidating.
For this banner to be displayed to incoming connections, add the following line to the /etc/hosts.allow file: 
vsftpd : ALL : banners /etc/banners/
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48.2.1.1.2. TCP Wrappers and Attack Warnings
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If a particular host or network has been detected attacking the server, TCP Wrappers can be used to warn the administrator of subsequent attacks from that host or network using the spawn directive.
In this example, assume that a cracker from the 206.182.68.0/24 network has been detected attempting to attack the server. Place the following line in the /etc/hosts.deny file to deny any connection attempts from that network, and to log the attempts to a special file: 
ALL : 206.182.68.0 : spawn /bin/ 'date' %c %d >> /var/log/intruder_alert
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The %d token supplies the name of the service that the attacker was trying to access.
To allow the connection and log it, place the spawn directive in the /etc/hosts.allow file.

Note

Because the spawn directive executes any shell command, create a special script to notify the administrator or execute a chain of commands in the event that a particular client attempts to connect to the server.
48.2.1.1.3. TCP Wrappers and Enhanced Logging
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If certain types of connections are of more concern than others, the log level can be elevated for that service using the severity option.
For this example, assume that anyone attempting to connect to port 23 (the Telnet port) on an FTP server is a cracker. To denote this, place an emerg flag in the log files instead of the default flag, info, and deny the connection.
To do this, place the following line in /etc/hosts.deny: 
in.telnetd : ALL : severity emerg
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This uses the default authpriv logging facility, but elevates the priority from the default value of info to emerg, which posts log messages directly to the console.
48.2.1.2. Enhancing Security With xinetd
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This section focuses on using xinetd to set a trap service and using it to control resource levels available to any given xinetd service. Setting resource limits for services can help thwart Denial of Service (DoS) attacks. Refer to the man pages for xinetd and xinetd.conf for a list of available options.
48.2.1.2.1. Setting a Trap
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One important feature of xinetd is its ability to add hosts to a global no_access list. Hosts on this list are denied subsequent connections to services managed by xinetd for a specified period or until xinetd is restarted. You can do this using the SENSOR attribute. This is an easy way to block hosts attempting to scan the ports on the server.
The first step in setting up a SENSOR is to choose a service you do not plan on using. For this example, Telnet is used.
Edit the file /etc/xinetd.d/telnet and change the flags line to read: 
flags           = SENSOR
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Add the following line: 
deny_time       = 30
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This denies any further connection attempts to that port by that host for 30 minutes. Other acceptable values for the deny_time attribute are FOREVER, which keeps the ban in effect until xinetd is restarted, and NEVER, which allows the connection and logs it.
Finally, the last line should read: 
disable         = no
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This enables the trap itself.
While using SENSOR is a good way to detect and stop connections from undesirable hosts, it has two drawbacks:
  • It does not work against stealth scans.
  • An attacker who knows that a SENSOR is running can mount a Denial of Service attack against particular hosts by forging their IP addresses and connecting to the forbidden port.
48.2.1.2.2. Controlling Server Resources
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Another important feature of xinetd is its ability to set resource limits for services under its control.
It does this using the following directives:
  • cps = <number_of_connections> <wait_period> — Limits the rate of incoming connections. This directive takes two arguments:
    • <number_of_connections> — The number of connections per second to handle. If the rate of incoming connections is higher than this, the service is temporarily disabled. The default value is fifty (50).
    • <wait_period> — The number of seconds to wait before re-enabling the service after it has been disabled. The default interval is ten (10) seconds.
  • instances = <number_of_connections> — Specifies the total number of connections allowed to a service. This directive accepts either an integer value or UNLIMITED.
  • per_source = <number_of_connections> — Specifies the number of connections allowed to a service by each host. This directive accepts either an integer value or UNLIMITED.
  • rlimit_as = <number[K|M]> — Specifies the amount of memory address space the service can occupy in kilobytes or megabytes. This directive accepts either an integer value or UNLIMITED.
  • rlimit_cpu = <number_of_seconds> — Specifies the amount of time in seconds that a service may occupy the CPU. This directive accepts either an integer value or UNLIMITED.
Using these directives can help prevent any single xinetd service from overwhelming the system, resulting in a denial of service.

48.2.2. Securing Portmap
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The portmap service is a dynamic port assignment daemon for RPC services such as NIS and NFS. It has weak authentication mechanisms and has the ability to assign a wide range of ports for the services it controls. For these reasons, it is difficult to secure.

Note

Securing portmap only affects NFSv2 and NFSv3 implementations, since NFSv4 no longer requires it. If you plan to implement an NFSv2 or NFSv3 server, then portmap is required, and the following section applies.
If running RPC services, follow these basic rules.
48.2.2.1. Protect portmap With TCP Wrappers
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It is important to use TCP Wrappers to limit which networks or hosts have access to the portmap service since it has no built-in form of authentication.
Further, use only IP addresses when limiting access to the service. Avoid using hostnames, as they can be forged by DNS poisoning and other methods.
48.2.2.2. Protect portmap With iptables
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To further restrict access to the portmap service, it is a good idea to add iptables rules to the server and restrict access to specific networks.
Below are two example iptables commands. The first allows TCP connections to the port 111 (used by the portmap service) from the 192.168.0.0/24 network. The second allows TCP connections to the same port from the localhost. This is necessary for the sgi_fam service used by Nautilus. All other packets are dropped. 
iptables -A INPUT -p tcp -s! 192.168.0.0/24 --dport 111 -j DROP
iptables -A INPUT -p tcp -s 127.0.0.1 --dport 111 -j ACCEPT
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To similarly limit UDP traffic, use the following command. 
iptables -A INPUT -p udp -s! 192.168.0.0/24 --dport 111 -j DROP
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Note

Refer to Section 48.8, “Firewalls” for more information about implementing firewalls with iptables commands.

48.2.3. Securing NIS
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The Network Information Service (NIS) is an RPC service, called ypserv,--> which is used in conjunction with portmap and other related services to distribute maps of usernames, passwords, and other sensitive information to any computer claiming to be within its domain.
An NIS server is comprised of several applications. They include the following:
  • /usr/sbin/rpc.yppasswdd — Also called the yppasswdd service, this daemon allows users to change their NIS passwords.
  • /usr/sbin/rpc.ypxfrd — Also called the ypxfrd service, this daemon is responsible for NIS map transfers over the network.
  • /usr/sbin/yppush — This application propagates changed NIS databases to multiple NIS servers.
  • /usr/sbin/ypserv — This is the NIS server daemon.
NIS is somewhat insecure by today's standards. It has no host authentication mechanisms and transmits all of its information over the network unencrypted, including password hashes. As a result, extreme care must be taken when setting up a network that uses NIS. This is further complicated by the fact that the default configuration of NIS is inherently insecure.
It is recommended that anyone planning to implement an NIS server first secure the portmap service as outlined in Section 48.2.2, “Securing Portmap”, then address the following issues, such as network planning.
48.2.3.1. Carefully Plan the Network
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Because NIS transmits sensitive information unencrypted over the network, it is important the service be run behind a firewall and on a segmented and secure network. Whenever NIS information is transmitted over an insecure network, it risks being intercepted. Careful network design can help prevent severe security breaches.
Any machine within an NIS domain can use commands to extract information from the server without authentication, as long as the user knows the NIS server's DNS hostname and NIS domain name.
For instance, if someone either connects a laptop computer into the network or breaks into the network from outside (and manages to spoof an internal IP address), the following command reveals the /etc/passwd map: 
ypcat -d <NIS_domain> -h <DNS_hostname> passwd
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If this attacker is a root user, they can obtain the /etc/shadow file by typing the following command: 
ypcat -d <NIS_domain> -h <DNS_hostname> shadow
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Note

If Kerberos is used, the /etc/shadow file is not stored within an NIS map.
To make access to NIS maps harder for an attacker, create a random string for the DNS hostname, such as o7hfawtgmhwg.domain.com. Similarly, create a different randomized NIS domain name. This makes it much more difficult for an attacker to access the NIS server.
48.2.3.3. Edit the /var/yp/securenets File
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If the /var/yp/securenets file is blank or does not exist (as is the case after a default installation), NIS listens to all networks. One of the first things to do is to put netmask/network pairs in the file so that ypserv only responds to requests from the appropriate network.
Below is a sample entry from a /var/yp/securenets file: 
255.255.255.0     192.168.0.0
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Warning

Never start an NIS server for the first time without creating the /var/yp/securenets file.
This technique does not provide protection from an IP spoofing attack, but it does at least place limits on what networks the NIS server services.
All of the servers related to NIS can be assigned specific ports except for rpc.yppasswdd — the daemon that allows users to change their login passwords. Assigning ports to the other two NIS server daemons, rpc.ypxfrd and ypserv, allows for the creation of firewall rules to further protect the NIS server daemons from intruders.
To do this, add the following lines to /etc/sysconfig/network: 
YPSERV_ARGS="-p 834" YPXFRD_ARGS="-p 835"
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The following iptables rules can then be used to enforce which network the server listens to for these ports: 
iptables -A INPUT -p tcp -s! 192.168.0.0/24 --dport 834 -j DROP
iptables -A INPUT -p tcp -s! 192.168.0.0/24 --dport 835 -j DROP
iptables -A INPUT -p udp -s! 192.168.0.0/24 --dport 834 -j DROP
iptables -A INPUT -p udp -s! 192.168.0.0/24 --dport 835 -j DROP
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This means that the server only allows connections to ports 834 and 835 if the requests come from the 192.168.0.0/24 network.

Note

Refer to Section 48.8, “Firewalls” for more information about implementing firewalls with iptables commands.
48.2.3.5. Use Kerberos Authentication
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One of the issues to consider when NIS is used for authentication is that whenever a user logs into a machine, a password hash from the /etc/shadow map is sent over the network. If an intruder gains access to an NIS domain and sniffs network traffic, they can collect usernames and password hashes. With enough time, a password cracking program can guess weak passwords, and an attacker can gain access to a valid account on the network.
Since Kerberos uses secret-key cryptography, no password hashes are ever sent over the network, making the system far more secure. Refer to Section 48.6, “Kerberos” for more information about Kerberos.

48.2.4. Securing NFS
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The Network File System (NFS) is a service that provides network accessible file systems for client machines. Refer to Chapter 21, Network File System (NFS) for more information about NFS. The following subsections assume a basic knowledge of NFS.

Important

The version of NFS included in Red Hat Enterprise Linux, NFSv4, no longer requires the portmap service as outlined in Section 48.2.2, “Securing Portmap”. NFS traffic now utilizes TCP in all versions, rather than UDP, and requires it when using NFSv4. NFSv4 now includes Kerberos user and group authentication, as part of the RPCSEC_GSS kernel module. Information on portmap is still included, since Red Hat Enterprise Linux supports NFSv2 and NFSv3, both of which utilize portmap.
48.2.4.1. Carefully Plan the Network
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Now that NFSv4 has the ability to pass all information encrypted using Kerberos over a network, it is important that the service be configured correctly if it is behind a firewall or on a segmented network. NFSv2 and NFSv3 still pass data insecurely, and this should be taken into consideration. Careful network design in all of these regards can help prevent security breaches.
48.2.4.2. Beware of Syntax Errors
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The NFS server determines which file systems to export and which hosts to export these directories to by consulting the /etc/exports file. Be careful not to add extraneous spaces when editing this file.
For instance, the following line in the /etc/exports file shares the directory /tmp/nfs/ to the host bob.example.com with read/write permissions. 
/tmp/nfs/     bob.example.com(rw)
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The following line in the /etc/exports file, on the other hand, shares the same directory to the host bob.example.com with read-only permissions and shares it to the world with read/write permissions due to a single space character after the hostname. 
/tmp/nfs/     bob.example.com (rw)
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It is good practice to check any configured NFS shares by using the showmount command to verify what is being shared: 
showmount -e <hostname>
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48.2.4.3. Do Not Use the no_root_squash Option
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By default, NFS shares change the root user to the nfsnobody user, an unprivileged user account. This changes the owner of all root-created files to nfsnobody, which prevents uploading of programs with the setuid bit set.
If no_root_squash is used, remote root users are able to change any file on the shared file system and leave applications infected by Trojans for other users to inadvertently execute.

48.2.5. Securing the Apache HTTP Server
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The Apache HTTP Server is one of the most stable and secure services that ships with Red Hat Enterprise Linux. A large number of options and techniques are available to secure the Apache HTTP Server — too numerous to delve into deeply here.
When configuring the Apache HTTP Server, it is important to read the documentation available for the application. This includes Chapter 25, Apache HTTP Server, and the Stronghold manuals, available at http://www.redhat.com/docs/manuals/stronghold/.
System Administrators should be careful when using the following configuration options:
48.2.5.1.  FollowSymLinks
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This directive is enabled by default, so be sure to use caution when creating symbolic links to the document root of the Web server. For instance, it is a bad idea to provide a symbolic link to /.
48.2.5.2. The Indexes Directive
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This directive is enabled by default, but may not be desirable. To prevent visitors from browsing files on the server, remove this directive.
48.2.5.3. The UserDir Directive
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The UserDir directive is disabled by default because it can confirm the presence of a user account on the system. To enable user directory browsing on the server, use the following directives: 
UserDir enabled
UserDir disabled root
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These directives activate user directory browsing for all user directories other than /root/. To add users to the list of disabled accounts, add a space-delimited list of users on the UserDir disabled line.
By default, the Server-Side Includes (SSI) module cannot execute commands. It is recommended that you do not change this setting unless absolutely necessary, as it could potentially enable an attacker to execute commands on the system.
Ensure that only the root user has write permissions to any directory containing scripts or CGIs. To do this, type the following commands: 
chown root <directory_name>
chmod 755 <directory_name>
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Important

Always verify that any scripts running on the system work as intended before putting them into production.

48.2.6. Securing FTP
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The File Transfer Protocol (FTP) is an older TCP protocol designed to transfer files over a network. Because all transactions with the server, including user authentication, are unencrypted, it is considered an insecure protocol and should be carefully configured.
Red Hat Enterprise Linux provides three FTP servers.
  • gssftpd — A Kerberos-aware xinetd-based FTP daemon that does not transmit authentication information over the network.
  • Red Hat Content Accelerator (tux) — A kernel-space Web server with FTP capabilities.
  • vsftpd — A standalone, security oriented implementation of the FTP service.
The following security guidelines are for setting up the vsftpd FTP service.
48.2.6.1. FTP Greeting Banner
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Before submitting a username and password, all users are presented with a greeting banner. By default, this banner includes version information useful to crackers trying to identify weaknesses in a system.
To change the greeting banner for vsftpd, add the following directive to the /etc/vsftpd/vsftpd.conf file: 
ftpd_banner=<insert_greeting_here>
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Replace <insert_greeting_here> in the above directive with the text of the greeting message.
For mutli-line banners, it is best to use a banner file. To simplify management of multiple banners, place all banners in a new directory called /etc/banners/. The banner file for FTP connections in this example is /etc/banners/ftp.msg. Below is an example of what such a file may look like: 
######### # Hello, all activity on ftp.example.com is logged. #########
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Note

It is not necessary to begin each line of the file with 220 as specified in Section 48.2.1.1.1, “TCP Wrappers and Connection Banners”.
To reference this greeting banner file for vsftpd, add the following directive to the /etc/vsftpd/vsftpd.conf file: 
banner_file=/etc/banners/ftp.msg
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Important

Make sure that you specify the path to the banner file correctly in /etc/vsftpd/vsftpd.conf, or else every attempt to connect to vsftpd will result in the connection being closed immediately and a 500 OOPS: cannot open banner <path_to_banner_file> error message.
Note that the banner_file directive in /etc/vsftpd/vfsftpd.conf takes precedence over any ftpd_banner directives in the configuration file: if banner_file is specified, then ftpd_banner is ignored.
It also is possible to send additional banners to incoming connections using TCP Wrappers as described in Section 48.2.1.1.1, “TCP Wrappers and Connection Banners”.
48.2.6.2. Anonymous Access
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The presence of the /var/ftp/ directory activates the anonymous account.
The easiest way to create this directory is to install the vsftpd package. This package establishes a directory tree for anonymous users and configures the permissions on directories to read-only for anonymous users.
By default the anonymous user cannot write to any directories.

Warning

If enabling anonymous access to an FTP server, be aware of where sensitive data is stored.
48.2.6.2.1. Anonymous Upload
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To allow anonymous users to upload files, it is recommended that a write-only directory be created within /var/ftp/pub/.
To do this, type the following command: 
mkdir /var/ftp/pub/upload
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Next, change the permissions so that anonymous users cannot view the contents of the directory: 
chmod 730 /var/ftp/pub/upload
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A long format listing of the directory should look like this: 
drwx-wx---    2 root     ftp          4096 Feb 13 20:05 upload
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Warning

Administrators who allow anonymous users to read and write in directories often find that their servers become a repository of stolen software.
Additionally, under vsftpd, add the following line to the /etc/vsftpd/vsftpd.conf file: 
anon_upload_enable=YES
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48.2.6.3. User Accounts
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Because FTP transmits unencrypted usernames and passwords over insecure networks for authentication, it is a good idea to deny system users access to the server from their user accounts.
To disable all user accounts in vsftpd, add the following directive to /etc/vsftpd/vsftpd.conf: 
local_enable=NO
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48.2.6.3.1. Restricting User Accounts
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To disable FTP access for specific accounts or specific groups of accounts, such as the root user and those with sudo privileges, the easiest way is to use a PAM list file as described in Section 48.1.4.2, “Disallowing Root Access”. The PAM configuration file for vsftpd is /etc/pam.d/vsftpd.
It is also possible to disable user accounts within each service directly.
To disable specific user accounts in vsftpd, add the username to /etc/vsftpd.ftpusers
48.2.6.4. Use TCP Wrappers To Control Access
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Use TCP Wrappers to control access to either FTP daemon as outlined in Section 48.2.1.1, “Enhancing Security With TCP Wrappers”.

48.2.7. Securing Sendmail
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Sendmail is a Mail Transport Agent (MTA) that uses the Simple Mail Transport Protocol (SMTP) to deliver electronic messages between other MTAs and to email clients or delivery agents. Although many MTAs are capable of encrypting traffic between one another, most do not, so sending email over any public networks is considered an inherently insecure form of communication.
Refer to Chapter 27, Email for more information about how email works and an overview of common configuration settings. This section assumes a basic knowledge of how to generate a valid /etc/mail/sendmail.cf by editing the /etc/mail/sendmail.mc and using the m4 command.
It is recommended that anyone planning to implement a Sendmail server address the following issues.
48.2.7.1. Limiting a Denial of Service Attack
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Because of the nature of email, a determined attacker can flood the server with mail fairly easily and cause a denial of service. By setting limits to the following directives in /etc/mail/sendmail.mc, the effectiveness of such attacks is limited.
  • confCONNECTION_RATE_THROTTLE — The number of connections the server can receive per second. By default, Sendmail does not limit the number of connections. If a limit is set and reached, further connections are delayed.
  • confMAX_DAEMON_CHILDREN — The maximum number of child processes that can be spawned by the server. By default, Sendmail does not assign a limit to the number of child processes. If a limit is set and reached, further connections are delayed.
  • confMIN_FREE_BLOCKS — The minimum number of free blocks which must be available for the server to accept mail. The default is 100 blocks.
  • confMAX_HEADERS_LENGTH — The maximum acceptable size (in bytes) for a message header.
  • confMAX_MESSAGE_SIZE — The maximum acceptable size (in bytes) for a single message.
48.2.7.2. NFS and Sendmail
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Never put the mail spool directory, /var/spool/mail/, on an NFS shared volume.
Because NFSv2 and NFSv3 do not maintain control over user and group IDs, two or more users can have the same UID, and receive and read each other's mail.

Note

With NFSv4 using Kerberos, this is not the case, since the SECRPC_GSS kernel module does not utilize UID-based authentication. However, it is considered good practice not to put the mail spool directory on NFS shared volumes.
48.2.7.3. Mail-only Users
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To help prevent local user exploits on the Sendmail server, it is best for mail users to only access the Sendmail server using an email program. Shell accounts on the mail server should not be allowed and all user shells in the /etc/passwd file should be set to /sbin/nologin (with the possible exception of the root user).

48.2.8. Verifying Which Ports Are Listening
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After configuring network services, it is important to pay attention to which ports are actually listening on the system's network interfaces. Any open ports can be evidence of an intrusion.
There are two basic approaches for listing the ports that are listening on the network. The less reliable approach is to query the network stack using commands such as netstat -an or lsof -i. This method is less reliable since these programs do not connect to the machine from the network, but rather check to see what is running on the system. For this reason, these applications are frequent targets for replacement by attackers. Crackers attempt to cover their tracks if they open unauthorized network ports by replacing netstat and lsof with their own, modified versions.
A more reliable way to check which ports are listening on the network is to use a port scanner such as nmap.
The following command issued from the console determines which ports are listening for TCP connections from the network: 
nmap -sT -O localhost
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The output of this command appears as follows: 
Starting nmap 3.55 ( http://www.insecure.org/nmap/ ) at 2004-09-24 13:49 EDT
Interesting ports on localhost.localdomain (127.0.0.1):
(The 1653 ports scanned but not shown below are in state: closed)
PORT      STATE SERVICE
22/tcp    open  ssh
25/tcp    open  smtp
111/tcp   open  rpcbind
113/tcp   open  auth
631/tcp   open  ipp
834/tcp   open  unknown
2601/tcp  open  zebra
32774/tcp open  sometimes-rpc11
Device type: general purpose
Running: Linux 2.4.X|2.5.X|2.6.X OS details: Linux 2.5.25 - 2.6.3 or Gentoo 1.2 Linux 2.4.19 rc1-rc7)
Uptime 12.857 days (since Sat Sep 11 17:16:20 2004)
Nmap run completed -- 1 IP address (1 host up) scanned in 5.190 seconds
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This output shows the system is running portmap due to the presence of the sunrpc service. However, there is also a mystery service on port 834. To check if the port is associated with the official list of known services, type: 
cat /etc/services | grep 834
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This command returns no output. This indicates that while the port is in the reserved range (meaning 0 through 1023) and requires root access to open, it is not associated with a known service.
Next, check for information about the port using netstat or lsof. To check for port 834 using netstat, use the following command: 
netstat -anp | grep 834
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The command returns the following output: 
tcp   0    0 0.0.0.0:834    0.0.0.0:*   LISTEN   653/ypbind
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The presence of the open port in netstat is reassuring because a cracker opening a port surreptitiously on a hacked system is not likely to allow it to be revealed through this command. Also, the [p] option reveals the process ID (PID) of the service that opened the port. In this case, the open port belongs to ypbind (NIS), which is an RPC service handled in conjunction with the portmap service.
The lsof command reveals similar information to netstat since it is also capable of linking open ports to services: 
lsof -i | grep 834
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The relevant portion of the output from this command follows: 
ypbind      653        0    7u  IPv4       1319                 TCP *:834 (LISTEN)
ypbind      655        0    7u  IPv4       1319                 TCP *:834 (LISTEN)
ypbind      656        0    7u  IPv4       1319                 TCP *:834 (LISTEN)
ypbind      657        0    7u  IPv4       1319                 TCP *:834 (LISTEN)
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These tools reveal a great deal about the status of the services running on a machine. These tools are flexible and can provide a wealth of information about network services and configuration. Refer to the man pages for lsof, netstat, nmap, and services for more information.

48.3. Single Sign-on (SSO)
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48.3.1. Introduction
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The Red Hat Enterprise Linux SSO functionality reduces the number of times Red Hat Enterprise Linux desktop users have to enter their passwords. Several major applications leverage the same underlying authentication and authorization mechanisms so that users can log in to Red Hat Enterprise Linux from the log-in screen, and then not need to re-enter their passwords. These applications are detailed below.
In addition, users can log in to their machines even when there is no network (offline mode) or where network connectivity is unreliable, for example, wireless access. In the latter case, services will degrade gracefully.
48.3.1.1. Supported Applications
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The following applications are currently supported by the unified log-in scheme in Red Hat Enterprise Linux:
  • Login
  • Screensaver
  • Firefox and Thunderbird
48.3.1.2. Supported Authentication Mechanisms
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Red Hat Enterprise Linux currently supports the following authentication mechanisms:
  • Kerberos name/password login
  • Smart card/PIN login
48.3.1.3. Supported Smart Cards
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Red Hat Enterprise Linux has been tested with the Cyberflex e-gate card and reader, but any card that complies with both Java card 2.1.1 and Global Platform 2.0.1 specifications should operate correctly, as should any reader that is supported by PCSC-lite.
Red Hat Enterprise Linux has also been tested with Common Access Cards (CAC). The supported reader for CAC is the SCM SCR 331 USB Reader.
As of Red Hat Enterprise Linux 5.2, Gemalto smart cards (Cyberflex Access 64k v2, standard with DER SHA1 value configured as in PKCSI v2.1) are now supported. These smart cards now use readers compliant with Chip/Smart Card Interface Devices (CCID).
Numerous security mechanisms currently exist that utilize a large number of protocols and credential stores. Examples include SSL, SSH, IPsec, and Kerberos. Red Hat Enterprise Linux SSO aims to unify these schemes to support the requirements listed above. This does not mean replacing Kerberos with X.509v3 certificates, but rather uniting them to reduce the burden on both system users and the administrators who manage them.
To achieve this goal, Red Hat Enterprise Linux:
  • Provides a single, shared instance of the NSS crypto libraries on each operating system.
  • Ships the Certificate System's Enterprise Security Client (ESC) with the base operating system. The ESC application monitors smart card insertion events. If it detects that the user has inserted a smart card that was designed to be used with the Red Hat Enterprise Linux Certificate System server product, it displays a user interface instructing the user how to enroll that smart card.
  • Unifies Kerberos and NSS so that users who log in to the operating system using a smart card also obtain a Kerberos credential (which allows them to log in to file servers, etc.)

48.3.2. Getting Started with your new Smart Card
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Before you can use your smart card to log in to your system and take advantage of the increased security options this technology provides, you need to perform some basic installation and configuration steps. These are described below.

Note

This section provides a high-level view of getting started with your smart card. More detailed information is available in the Red Hat Certificate System Enterprise Security Client Guide.
  1. Log in with your Kerberos name and password
  2. Make sure you have the nss-tools package loaded.
  3. Download and install your corporate-specific root certificates. Use the following command to install the root CA certificate: 
    certutil -A -d /etc/pki/nssdb -n "root ca cert" -t "CT,C,C" \
	-i ./ca_cert_in_base64_format.crt
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  4. Verify that you have the following RPMs installed on your system: esc, pam_pkcs11, coolkey, ifd-egate, ccid, gdm, authconfig, and authconfig-gtk.
  5. Enable Smart Card Login Support
    1. On the Gnome Title Bar, select System->Administration->Authentication.
    2. Type your machine's root password if necessary.
    3. In the Authentication Configuration dialog, click the Authentication tab.
    4. Select the Enable Smart Card Support check box.
    5. Click the Configure Smart Card... button to display the Smartcard Settings dialog, and specify the required settings:
      • Require smart card for login — Clear this check box. After you have successfully logged in with the smart card you can select this option to prevent users from logging in without a smart card.
      • Card Removal Action — This controls what happens when you remove the smart card after you have logged in. The available options are:
        • Lock — Removing the smart card locks the X screen.
        • Ignore — Removing the smart card has no effect.
  6. If you need to enable the Online Certificate Status Protocol (OCSP), open the /etc/pam_pkcs11/pam_pkcs11.conf file, and locate the following line:
    enable_ocsp = false;
    Change this value to true, as follows:
    enable_ocsp = true;
  7. Enroll your smart card
  8. If you are using a CAC card, you also need to perform the following steps:
    1. Change to the root account and create a file called /etc/pam_pkcs11/cn_map.
    2. Add the following entry to the cn_map file:
      MY.CAC_CN.123454 -> myloginid
      where MY.CAC_CN.123454 is the Common Name on your CAC and myloginid is your UNIX login ID.
  9. Logout
48.3.2.1. Troubleshooting
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If you have trouble getting your smart card to work, try using the following command to locate the source of the problem: 
pklogin_finder debug
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If you run the pklogin_finder tool in debug mode while an enrolled smart card is plugged in, it attempts to output information about the validity of certificates, and if it is successful in attempting to map a login ID from the certificates that are on the card.

48.3.3. How Smart Card Enrollment Works
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Smart cards are said to be enrolled when they have received an appropriate certificate signed by a valid Certificate Authority (CA). This involves several steps, described below:
  1. The user inserts their smart card into the smart card reader on their workstation. This event is recognized by the Enterprise Security Client (ESC).
  2. The enrollment page is displayed on the user's desktop. The user completes the required details and the user's system then connects to the Token Processing System (TPS) and the CA.
  3. The TPS enrolls the smart card using a certificate signed by the CA.
How Smart Card Enrollment Works
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How Smart Card Enrollment Works

Figure 48.4. How Smart Card Enrollment Works

48.3.4. How Smart Card Login Works
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This section provides a brief overview of the process of logging in using a smart card.
  1. When the user inserts their smart card into the smart card reader, this event is recognized by the PAM facility, which prompts for the user's PIN.
  2. The system then looks up the user's current certificates and verifies their validity. The certificate is then mapped to the user's UID.
  3. This is validated against the KDC and login granted.
How Smart Card Login Works
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How Smart Card Login Works

Figure 48.5. How Smart Card Login Works

Note

You cannot log in with a card that has not been enrolled, even if it has been formatted. You need to log in with a formatted, enrolled card, or not using a smart card, before you can enroll a new card.
Refer to Section 48.6, “Kerberos” and Section 48.4, “Pluggable Authentication Modules (PAM)” for more information on Kerberos and PAM.
You can configure Firefox to use Kerberos for Single Sign-on. In order for this functionality to work correctly, you need to configure your web browser to send your Kerberos credentials to the appropriate KDC.The following section describes the configuration changes and other requirements to achieve this.
  1. In the address bar of Firefox, type about:config to display the list of current configuration options.
  2. In the Filter field, type negotiate to restrict the list of options.
  3. Double-click the network.negotiate-auth.trusted-uris entry to display the Enter string value dialog box.
  4. Enter the name of the domain against which you want to authenticate, for example, .example.com.
  5. Repeat the above procedure for the network.negotiate-auth.delegation-uris entry, using the same domain.

    Note

    You can leave this value blank, as it allows Kerberos ticket passing, which is not required.
    If you do not see these two configuration options listed, your version of Firefox may be too old to support Negotiate authentication, and you should consider upgrading.
Configuring Firefox for SSO with Kerberos
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Configuring Firefox for SSO with Kerberos

Figure 48.6. Configuring Firefox for SSO with Kerberos

You now need to ensure that you have Kerberos tickets. In a command shell, type kinit to retrieve Kerberos tickets. To display the list of available tickets, type klist. The following shows an example output from these commands: 
~]$ kinit
Password for user@EXAMPLE.COM:

~]$ klist
Ticket cache: FILE:/tmp/krb5cc_10920
Default principal: user@EXAMPLE.COM

Valid starting     Expires            Service principal
10/26/06 23:47:54  10/27/06 09:47:54  krbtgt/USER.COM@USER.COM
        renew until 10/26/06 23:47:54

Kerberos 4 ticket cache: /tmp/tkt10920
klist: You have no tickets cached
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48.3.5.1. Troubleshooting
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If you have followed the configuration steps above and Negotiate authentication is not working, you can turn on verbose logging of the authentication process. This could help you find the cause of the problem. To enable verbose logging, use the following procedure:
  1. Close all instances of Firefox.
  2. Open a command shell, and enter the following commands: 
    export NSPR_LOG_MODULES=negotiateauth:5
export NSPR_LOG_FILE=/tmp/moz.log
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  3. Restart Firefox from that shell, and visit the website you were unable to authenticate to earlier. Information will be logged to /tmp/moz.log, and may give a clue to the problem. For example: 
    -1208550944[90039d0]: entering nsNegotiateAuth::GetNextToken()
-1208550944[90039d0]: gss_init_sec_context() failed: Miscellaneous failure
No credentials cache found
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    This indicates that you do not have Kerberos tickets, and need to run kinit.
If you are able to run kinit successfully from your machine but you are unable to authenticate, you might see something like this in the log file: 
-1208994096[8d683d8]: entering nsAuthGSSAPI::GetNextToken()
-1208994096[8d683d8]: gss_init_sec_context() failed: Miscellaneous failure
Server not found in Kerberos database
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This generally indicates a Kerberos configuration problem. Make sure that you have the correct entries in the [domain_realm] section of the /etc/krb5.conf file. For example: 
.example.com = EXAMPLE.COM
example.com = EXAMPLE.COM
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If nothing appears in the log it is possible that you are behind a proxy, and that proxy is stripping off the HTTP headers required for Negotiate authentication. As a workaround, you can try to connect to the server using HTTPS instead, which allows the request to pass through unmodified. Then proceed to debug using the log file, as described above.

48.4. Pluggable Authentication Modules (PAM)
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Programs that grant users access to a system use authentication to verify each other's identity (that is, to establish that a user is who they say they are).
Historically, each program had its own way of authenticating users. In Red Hat Enterprise Linux, many programs are configured to use a centralized authentication mechanism called Pluggable Authentication Modules (PAM).
PAM uses a pluggable, modular architecture, which affords the system administrator a great deal of flexibility in setting authentication policies for the system.
In most situations, the default PAM configuration file for a PAM-aware application is sufficient. Sometimes, however, it is necessary to edit a PAM configuration file. Because misconfiguration of PAM can compromise system security, it is important to understand the structure of these files before making any modifications. Refer to Section 48.4.3, “PAM Configuration File Format” for more information.

48.4.1. Advantages of PAM
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PAM offers the following advantages:
  • a common authentication scheme that can be used with a wide variety of applications.
  • significant flexibility and control over authentication for both system administrators and application developers.
  • a single, fully-documented library which allows developers to write programs without having to create their own authentication schemes.

48.4.2. PAM Configuration Files
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The /etc/pam.d/ directory contains the PAM configuration files for each PAM-aware application. In earlier versions of PAM, the /etc/pam.conf file was used, but this file is now deprecated and is only used if the /etc/pam.d/ directory does not exist.
48.4.2.1. PAM Service Files
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Each PAM-aware application or service has a file in the /etc/pam.d/ directory. Each file in this directory has the same name as the service to which it controls access.
The PAM-aware program is responsible for defining its service name and installing its own PAM configuration file in the /etc/pam.d/ directory. For example, the login program defines its service name as login and installs the /etc/pam.d/login PAM configuration file.

48.4.3. PAM Configuration File Format
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Each PAM configuration file contains a group of directives formatted as follows: 
<module interface>  <control flag>   <module name>   <module arguments>
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Each of these elements is explained in the following sections.
48.4.3.1. Module Interface
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Four types of PAM module interface are currently available. Each of these corresponds to a different aspect of the authorization process:
  • auth — This module interface authenticates use. For example, it requests and verifies the validity of a password. Modules with this interface can also set credentials, such as group memberships or Kerberos tickets.
  • account — This module interface verifies that access is allowed. For example, it may check if a user account has expired or if a user is allowed to log in at a particular time of day.
  • password — This module interface is used for changing user passwords.
  • session — This module interface configures and manages user sessions. Modules with this interface can also perform additional tasks that are needed to allow access, like mounting a user's home directory and making the user's mailbox available.

Note

An individual module can provide any or all module interfaces. For instance, pam_unix.so provides all four module interfaces.
In a PAM configuration file, the module interface is the first field defined. For example, a typical line in a configuration may look like this: 
auth	required	pam_unix.so
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This instructs PAM to use the pam_unix.so module's auth interface.
48.4.3.1.1. Stacking Module Interfaces
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Module interface directives can be stacked, or placed upon one another, so that multiple modules are used together for one purpose. If a module's control flag uses the "sufficient" or "requisite" value (refer to Section 48.4.3.2, “Control Flag” for more information on these flags), then the order in which the modules are listed is important to the authentication process.
Stacking makes it easy for an administrator to require specific conditions to exist before allowing the user to authenticate. For example, the reboot command normally uses several stacked modules, as seen in its PAM configuration file: 
~]# cat /etc/pam.d/reboot
#%PAM-1.0
auth	sufficient	pam_rootok.so
auth	required	pam_console.so
#auth	include	system-auth
account	required	pam_permit.so
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  • The first line is a comment and is not processed.
  • auth sufficient pam_rootok.so — This line uses the pam_rootok.so module to check whether the current user is root, by verifying that their UID is 0. If this test succeeds, no other modules are consulted and the command is executed. If this test fails, the next module is consulted.
  • auth required pam_console.so — This line uses the pam_console.so module to attempt to authenticate the user. If this user is already logged in at the console, pam_console.so checks whether there is a file in the /etc/security/console.apps/ directory with the same name as the service name (reboot). If such a file exists, authentication succeeds and control is passed to the next module.
  • #auth include system-auth — This line is commented and is not processed.
  • account required pam_permit.so — This line uses the pam_permit.so module to allow the root user or anyone logged in at the console to reboot the system.
48.4.3.2. Control Flag
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All PAM modules generate a success or failure result when called. Control flags tell PAM what do with the result. Modules can be stacked in a particular order, and the control flags determine how important the success or failure of a particular module is to the overall goal of authenticating the user to the service.
There are four predefined control flags:
  • required — The module result must be successful for authentication to continue. If the test fails at this point, the user is not notified until the results of all module tests that reference that interface are complete.
  • requisite — The module result must be successful for authentication to continue. However, if a test fails at this point, the user is notified immediately with a message reflecting the first failed required or requisite module test.
  • sufficient — The module result is ignored if it fails. However, if the result of a module flagged sufficient is successful and no previous modules flagged required have failed, then no other results are required and the user is authenticated to the service.
  • optional — The module result is ignored. A module flagged as optional only becomes necessary for successful authentication when no other modules reference the interface.

Important

The order in which required modules are called is not critical. Only the sufficient and requisite control flags cause order to become important.
A newer control flag syntax that allows for more precise control is now available for PAM.
The pam.d man page, and the PAM documentation, located in the /usr/share/doc/pam-<version-number>/ directory, where <version-number> is the version number for PAM on your system, describe this newer syntax in detail.
48.4.3.3. Module Name
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The module name provides PAM with the name of the pluggable module containing the specified module interface. In older versions of Red Hat Enterprise Linux, the full path to the module was provided in the PAM configuration file. However, since the advent of multilib systems, which store 64-bit PAM modules in the /lib64/security/ directory, the directory name is omitted because the application is linked to the appropriate version of libpam, which can locate the correct version of the module.
48.4.3.4. Module Arguments
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PAM uses arguments to pass information to a pluggable module during authentication for some modules.
For example, the pam_userdb.so module uses information stored in a Berkeley DB file to authenticate the user. Berkeley DB is an open source database system embedded in many applications. The module takes a db argument so that Berkeley DB knows which database to use for the requested service.
The following is a typical pam_userdb.so line in a PAM configuration. The <path-to-file> is the full path to the Berkeley DB database file: 
auth	required	pam_userdb.so db=<path-to-file>
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Invalid arguments are generally ignored and do not otherwise affect the success or failure of the PAM module. Some modules, however, may fail on invalid arguments. Most modules report errors to the /var/log/secure file.

48.4.4. Sample PAM Configuration Files
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The following is a sample PAM application configuration file: 
#%PAM-1.0
auth	required  pam_securetty.so
auth	required  pam_unix.so nullok
auth	required  pam_nologin.so
account	required  pam_unix.so
password	required  pam_cracklib.so retry=3
password	required  pam_unix.so shadow nullok use_authtok
session	required  pam_unix.so
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  • The first line is a comment, indicated by the hash mark (#) at the beginning of the line.
  • Lines two through four stack three modules for login authentication.
    auth required pam_securetty.so — This module ensures that if the user is trying to log in as root, the tty on which the user is logging in is listed in the /etc/securetty file, if that file exists.
    If the tty is not listed in the file, any attempt to log in as root fails with a Login incorrect message.
    auth required pam_unix.so nullok — This module prompts the user for a password and then checks the password using the information stored in /etc/passwd and, if it exists, /etc/shadow.
    In the authentication phase, the pam_unix.so module automatically detects whether the user's password is in the passwd file or the shadow file. Refer to Section 37.6, “Shadow Passwords” for more information.
    • The argument nullok instructs the pam_unix.so module to allow a blank password.
  • auth required pam_nologin.so — This is the final authentication step. It checks whether the /etc/nologin file exists. If it exists and the user is not root, authentication fails.

    Note

    In this example, all three auth modules are checked, even if the first auth module fails. This prevents the user from knowing at what stage their authentication failed. Such knowledge in the hands of an attacker could allow them to more easily deduce how to crack the system.
  • account required pam_unix.so — This module performs any necessary account verification. For example, if shadow passwords have been enabled, the account interface of the pam_unix.so module checks to see if the account has expired or if the user has not changed the password within the allowed grace period.
  • password required pam_cracklib.so retry=3 — If a password has expired, the password component of the pam_cracklib.so module prompts for a new password. It then tests the newly created password to see whether it can easily be determined by a dictionary-based password cracking program.
    • The argument retry=3 specifies that if the test fails the first time, the user has two more chances to create a strong password.
  • password required pam_unix.so shadow nullok use_authtok — This line specifies that if the program changes the user's password, it should use the password interface of the pam_unix.so module to do so.
    • The argument shadow instructs the module to create shadow passwords when updating a user's password.
    • The argument nullok instructs the module to allow the user to change their password from a blank password, otherwise a null password is treated as an account lock.
    • The final argument on this line, use_authtok, provides a good example of the importance of order when stacking PAM modules. This argument instructs the module not to prompt the user for a new password. Instead, it accepts any password that was recorded by a previous password module. In this way, all new passwords must pass the pam_cracklib.so test for secure passwords before being accepted.
  • session required pam_unix.so — The final line instructs the session interface of the pam_unix.so module to manage the session. This module logs the user name and the service type to /var/log/secure at the beginning and end of each session. This module can be supplemented by stacking it with other session modules for additional functionality.

48.4.5. Creating PAM Modules
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You can create or add new PAM modules at any time for use by PAM-aware applications.
For example, a developer might create a one-time-password creation method and write a PAM module to support it. PAM-aware programs can immediately use the new module and password method without being recompiled or otherwise modified.
This allows developers and system administrators to mix-and-match, as well as test, authentication methods for different programs without recompiling them.
Documentation on writing modules is included in the /usr/share/doc/pam-<version-number>/ directory, where <version-number> is the version number for PAM on your system.

48.4.6. PAM and Administrative Credential Caching
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A number of graphical administrative tools in Red Hat Enterprise Linux provide users with elevated privileges for up to five minutes using the pam_timestamp.so module. It is important to understand how this mechanism works, because a user who walks away from a terminal while pam_timestamp.so is in effect leaves the machine open to manipulation by anyone with physical access to the console.
In the PAM timestamp scheme, the graphical administrative application prompts the user for the root password when it is launched. When the user has been authenticated, the pam_timestamp.so module creates a timestamp file. By default, this is created in the /var/run/sudo/ directory. If the timestamp file already exists, graphical administrative programs do not prompt for a password. Instead, the pam_timestamp.so module freshens the timestamp file, reserving an extra five minutes of unchallenged administrative access for the user.
You can verify the actual state of the timestamp file by inspecting the /var/run/sudo/<user> file. For the desktop, the relevant file is unknown:root. If it is present and its timestamp is less than five minutes old, the credentials are valid.
The existence of the timestamp file is indicated by an authentication icon, which appears in the notification area of the panel.
The Authentication Icon
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The Authentication Icon

Figure 48.7. The Authentication Icon

48.4.6.1. Removing the Timestamp File
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Before abandoning a console where a PAM timestamp is active, it is recommended that the timestamp file be destroyed. To do this from a graphical environment, click the authentication icon on the panel. This causes a dialog box to appear. Click the Forget Authorization button to destroy the active timestamp file.
Dismiss Authentication Dialog
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Dismiss Authentication Dialog

Figure 48.8. Dismiss Authentication Dialog

You should be aware of the following with respect to the PAM timestamp file:
  • If logged in to the system remotely using ssh, use the /sbin/pam_timestamp_check -k root command to destroy the timestamp file.
  • You need to run the /sbin/pam_timestamp_check -k root command from the same terminal window from which you launched the privileged application.
  • You must be logged in as the user who originally invoked the pam_timestamp.so module in order to use the /sbin/pam_timestamp_check -k command. Do not log in as root to use this command.
  • If you want to kill the credentials on the desktop (without using the Forget Authorization action on the icon), use the following command: 
    pam_timestamp_check -k root </dev/null >/dev/null 2>/dev/null
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    Failure to use this command will only remove the credentials (if any) from the pty where you run the command.
Refer to the pam_timestamp_check man page for more information about destroying the timestamp file using pam_timestamp_check.
48.4.6.2. Common pam_timestamp Directives
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The pam_timestamp.so module accepts several directives. The following are the two most commonly used options:
  • timestamp_timeout — Specifies the period (in seconds) for which the timestamp file is valid. The default value is 300 (five minutes).
  • timestampdir — Specifies the directory in which the timestamp file is stored. The default value is /var/run/sudo/.
Refer to Section 48.4.8.1, “Installed Documentation” for more information about controlling the pam_timestamp.so module.

48.4.7. PAM and Device Ownership
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In Red Hat Enterprise Linux, the first user who logs in at the physical console of the machine can manipulate certain devices and perform certain tasks normally reserved for the root user. This is controlled by a PAM module called pam_console.so.
48.4.7.1. Device Ownership
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When a user logs in to a Red Hat Enterprise Linux system, the pam_console.so module is called by login or the graphical login programs, gdm, kdm, and xdm. If this user is the first user to log in at the physical console — referred to as the console user — the module grants the user ownership of a variety of devices normally owned by root. The console user owns these devices until the last local session for that user ends. After this user has logged out, ownership of the devices reverts back to the root user.
The devices affected include, but are not limited to, sound cards, diskette drives, and CD-ROM drives.
This facility allows a local user to manipulate these devices without obtaining root access, thus simplifying common tasks for the console user.
You can modify the list of devices controlled by pam_console.so by editing the following files:
  • /etc/security/console.perms
  • /etc/security/console.perms.d/50-default.perms
You can change the permissions of different devices than those listed in the above files, or override the specified defaults. Rather than modify the 50-default.perms file, you should create a new file (for example, xx-name.perms) and enter the required modifications. The name of the new default file must begin with a number higher than 50 (for example, 51-default.perms). This will override the defaults in the 50-default.perms file.

Warning

If the gdm, kdm, or xdm display manager configuration file has been altered to allow remote users to log in and the host is configured to run at runlevel 5, it is advisable to change the <console> and <xconsole> directives in the /etc/security/console.perms to the following values: 
<console>=tty[0-9][0-9]* vc/[0-9][0-9]* :0\.[0-9] :0
<xconsole>=:0\.[0-9] :0
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This prevents remote users from gaining access to devices and restricted applications on the machine.
If the gdm, kdm, or xdm display manager configuration file has been altered to allow remote users to log in and the host is configured to run at any multiple user runlevel other than 5, it is advisable to remove the <xconsole> directive entirely and change the <console> directive to the following value: 
<console>=tty[0-9][0-9]* vc/[0-9][0-9]*
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48.4.7.2. Application Access
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The console user also has access to certain programs configured for use in the /etc/security/console.apps/ directory.
This directory contains configuration files which enable the console user to run certain applications in /sbin and /usr/sbin.
These configuration files have the same name as the applications that they set up.
One notable group of applications that the console user has access to are three programs that shut down or reboot the system:
  • /sbin/halt
  • /sbin/reboot
  • /sbin/poweroff
Because these are PAM-aware applications, they call the pam_console.so module as a requirement for use.
Refer to Section 48.4.8.1, “Installed Documentation” for more information.

48.4.8. Additional Resources
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The following resources further explain methods to use and configure PAM. In addition to these resources, read the PAM configuration files on the system to better understand how they are structured.
48.4.8.1. Installed Documentation
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  • PAM-related man pages — Several man pages exist for the various applications and configuration files involved with PAM. The following is a list of some of the more important man pages.
    Configuration Files
    • pam — Good introductory information on PAM, including the structure and purpose of the PAM configuration files.
      Note that this man page discusses both /etc/pam.conf and individual configuration files in the /etc/pam.d/ directory. By default, Red Hat Enterprise Linux uses the individual configuration files in the /etc/pam.d/ directory, ignoring /etc/pam.conf even if it exists.
    • pam_console — Describes the purpose of the pam_console.so module. It also describes the appropriate syntax for an entry within a PAM configuration file.
    • console.apps — Describes the format and options available in the /etc/security/console.apps configuration file, which defines which applications are accessible by the console user assigned by PAM.
    • console.perms — Describes the format and options available in the /etc/security/console.perms configuration file, which specifies the console user permissions assigned by PAM.
    • pam_timestamp — Describes the pam_timestamp.so module.
  • /usr/share/doc/pam-<version-number> — Contains a System Administrators' Guide, a Module Writers' Manual, and the Application Developers' Manual, as well as a copy of the PAM standard, DCE-RFC 86.0, where <version-number> is the version number of PAM.
  • /usr/share/doc/pam-<version-number>/txts/README.pam_timestamp — Contains information about the pam_timestamp.so PAM module, where <version-number> is the version number of PAM.
48.4.8.2. Useful Websites
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  • http://www.kernel.org/pub/linux/libs/pam/ — The primary distribution website for the Linux-PAM project, containing information on various PAM modules, a FAQ, and additional PAM documentation.

    Note

    The documentation in the above website is for the last released upstream version of PAM and might not be 100% accurate for the PAM version included in Red Hat Enterprise Linux.

48.5. TCP Wrappers and xinetd
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Controlling access to network services is one of the most important security tasks facing a server administrator. Red Hat Enterprise Linux provides several tools for this purpose. For example, an iptables-based firewall filters out unwelcome network packets within the kernel's network stack. For network services that utilize it, TCP Wrappers add an additional layer of protection by defining which hosts are or are not allowed to connect to "wrapped" network services. One such wrapped network service is the xinetd super server. This service is called a super server because it controls connections to a subset of network services and further refines access control.
Figure 48.9, “Access Control to Network Services” is a basic illustration of how these tools work together to protect network services.
Access Control to Network Services
View larger image
Access Control to Network Services

Figure 48.9. Access Control to Network Services

This chapter focuses on the role of TCP Wrappers and xinetd in controlling access to network services and reviews how these tools can be used to enhance both logging and utilization management. Refer to Section 48.9, “IPTables” for information about using firewalls with iptables.

48.5.1. TCP Wrappers
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The TCP Wrappers package (tcp_wrappers) is installed by default and provides host-based access control to network services. The most important component within the package is the /usr/lib/libwrap.a library. In general terms, a TCP-wrapped service is one that has been compiled against the libwrap.a library.
When a connection attempt is made to a TCP-wrapped service, the service first references the host's access files (/etc/hosts.allow and /etc/hosts.deny) to determine whether or not the client is allowed to connect. In most cases, it then uses the syslog daemon (syslogd) to write the name of the requesting client and the requested service to /var/log/secure or /var/log/messages.
If a client is allowed to connect, TCP Wrappers release control of the connection to the requested service and take no further part in the communication between the client and the server.
In addition to access control and logging, TCP Wrappers can execute commands to interact with the client before denying or releasing control of the connection to the requested network service.
Because TCP Wrappers are a valuable addition to any server administrator's arsenal of security tools, most network services within Red Hat Enterprise Linux are linked to the libwrap.a library. Some such applications include /usr/sbin/sshd, /usr/sbin/sendmail, and /usr/sbin/xinetd.

Note

To determine if a network service binary is linked to libwrap.a, type the following command as the root user: 
ldd <binary-name> | grep libwrap
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Replace <binary-name> with the name of the network service binary.
If the command returns straight to the prompt with no output, then the network service is not linked to libwrap.a.
The following example indicates that /usr/sbin/sshd is linked to libwrap.a: 
~]# ldd /usr/sbin/sshd | grep libwrap
        libwrap.so.0 => /usr/lib/libwrap.so.0 (0x00655000)
~]#
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48.5.1.1. Advantages of TCP Wrappers
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TCP Wrappers provide the following advantages over other network service control techniques:
  • Transparency to both the client and the wrapped network service — Both the connecting client and the wrapped network service are unaware that TCP Wrappers are in use. Legitimate users are logged and connected to the requested service while connections from banned clients fail.
  • Centralized management of multiple protocols — TCP Wrappers operate separately from the network services they protect, allowing many server applications to share a common set of access control configuration files, making for simpler management.

48.5.2. TCP Wrappers Configuration Files
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To determine if a client is allowed to connect to a service, TCP Wrappers reference the following two files, which are commonly referred to as hosts access files:
  • /etc/hosts.allow
  • /etc/hosts.deny
When a TCP-wrapped service receives a client request, it performs the following steps:
  1. It references /etc/hosts.allow. — The TCP-wrapped service sequentially parses the /etc/hosts.allow file and applies the first rule specified for that service. If it finds a matching rule, it allows the connection. If not, it moves on to the next step.
  2. It references /etc/hosts.deny. — The TCP-wrapped service sequentially parses the /etc/hosts.deny file. If it finds a matching rule, it denies the connection. If not, it grants access to the service.
The following are important points to consider when using TCP Wrappers to protect network services:
  • Because access rules in hosts.allow are applied first, they take precedence over rules specified in hosts.deny. Therefore, if access to a service is allowed in hosts.allow, a rule denying access to that same service in hosts.deny is ignored.
  • The rules in each file are read from the top down and the first matching rule for a given service is the only one applied. The order of the rules is extremely important.
  • If no rules for the service are found in either file, or if neither file exists, access to the service is granted.
  • TCP-wrapped services do not cache the rules from the hosts access files, so any changes to hosts.allow or hosts.deny take effect immediately, without restarting network services.

Warning

If the last line of a hosts access file is not a newline character (created by pressing the Enter key), the last rule in the file fails and an error is logged to either /var/log/messages or /var/log/secure. This is also the case for a rule that spans multiple lines without using the backslash character. The following example illustrates the relevant portion of a log message for a rule failure due to either of these circumstances: 
warning: /etc/hosts.allow, line 20: missing newline or line too long
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48.5.2.1. Formatting Access Rules
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The format for both /etc/hosts.allow and /etc/hosts.deny is identical. Each rule must be on its own line. Blank lines or lines that start with a hash (#) are ignored.
Each rule uses the following basic format to control access to network services: 
<daemon list>: <client list> [: <option>: <option>: ...]
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  • <daemon list> — A comma-separated list of process names (not service names) or the ALL wildcard. The daemon list also accepts operators (refer to Section 48.5.2.1.4, “Operators”) to allow greater flexibility.
  • <client list> — A comma-separated list of hostnames, host IP addresses, special patterns, or wildcards which identify the hosts affected by the rule. The client list also accepts operators listed in Section 48.5.2.1.4, “Operators” to allow greater flexibility.
  • <option> — An optional action or colon-separated list of actions performed when the rule is triggered. Option fields support expansions, launch shell commands, allow or deny access, and alter logging behavior.

Note

More information on the specialist terms above can be found elsewhere in this Guide:
The following is a basic sample hosts access rule: 
vsftpd : .example.com
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This rule instructs TCP Wrappers to watch for connections to the FTP daemon (vsftpd) from any host in the example.com domain. If this rule appears in hosts.allow, the connection is accepted. If this rule appears in hosts.deny, the connection is rejected.
The next sample hosts access rule is more complex and uses two option fields: 
sshd : .example.com  \ : spawn /bin/echo `/bin/date` access denied>>/var/log/sshd.log \ : deny
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Note that each option field is preceded by the backslash (\). Use of the backslash prevents failure of the rule due to length.
This sample rule states that if a connection to the SSH daemon (sshd) is attempted from a host in the example.com domain, execute the echo command to append the attempt to a special log file, and deny the connection. Because the optional deny directive is used, this line denies access even if it appears in the hosts.allow file. Refer to Section 48.5.2.2, “Option Fields” for a more detailed look at available options.
48.5.2.1.1. Wildcards
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Wildcards allow TCP Wrappers to more easily match groups of daemons or hosts. They are used most frequently in the client list field of access rules.
The following wildcards are available:
  • ALL — Matches everything. It can be used for both the daemon list and the client list.
  • LOCAL — Matches any host that does not contain a period (.), such as localhost.
  • KNOWN — Matches any host where the hostname and host address are known or where the user is known.
  • UNKNOWN — Matches any host where the hostname or host address are unknown or where the user is unknown.
  • PARANOID — Matches any host where the hostname does not match the host address.

Warning

The KNOWN, UNKNOWN, and PARANOID wildcards should be used with care, because they rely on functioning DNS server for correct operation. Any disruption to name resolution may prevent legitimate users from gaining access to a service.
48.5.2.1.2. Patterns
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Patterns can be used in the client field of access rules to more precisely specify groups of client hosts.
The following is a list of common patterns for entries in the client field:
  • Hostname beginning with a period (.) — Placing a period at the beginning of a hostname matches all hosts sharing the listed components of the name. The following example applies to any host within the example.com domain: 
    ALL : .example.com
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  • IP address ending with a period (.) — Placing a period at the end of an IP address matches all hosts sharing the initial numeric groups of an IP address. The following example applies to any host within the 192.168.x.x network: 
    ALL : 192.168.
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  • IP address/netmask pair — Netmask expressions can also be used as a pattern to control access to a particular group of IP addresses. The following example applies to any host with an address range of 192.168.0.0 through 192.168.1.255: 
    ALL : 192.168.0.0/255.255.254.0
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    Important

    When working in the IPv4 address space, the address/prefix length (prefixlen) pair declarations (CIDR notation) are not supported. Only IPv6 rules can use this format.
  • [IPv6 address]/prefixlen pair — [net]/prefixlen pairs can also be used as a pattern to control access to a particular group of IPv6 addresses. The following example would apply to any host with an address range of 3ffe:505:2:1:: through 3ffe:505:2:1:ffff:ffff:ffff:ffff: 
    ALL : [3ffe:505:2:1::]/64
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  • The asterisk (*) — Asterisks can be used to match entire groups of hostnames or IP addresses, as long as they are not mixed in a client list containing other types of patterns. The following example would apply to any host within the example.com domain: 
    ALL : *.example.com
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  • The slash (/) — If a client list begins with a slash, it is treated as a file name. This is useful if rules specifying large numbers of hosts are necessary. The following example refers TCP Wrappers to the /etc/telnet.hosts file for all Telnet connections: 
    in.telnetd : /etc/telnet.hosts
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Other, lesser used, patterns are also accepted by TCP Wrappers. Refer to the hosts_access man 5 page for more information.

Warning

Be very careful when using hostnames and domain names. Attackers can use a variety of tricks to circumvent accurate name resolution. In addition, disruption to DNS service prevents even authorized users from using network services. It is, therefore, best to use IP addresses whenever possible.
48.5.2.1.3. Portmap and TCP Wrappers
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Portmap's implementation of TCP Wrappers does not support host look-ups, which means portmap can not use hostnames to identify hosts. Consequently, access control rules for portmap in hosts.allow or hosts.deny must use IP addresses, or the keyword ALL, for specifying hosts.
Changes to portmap access control rules may not take effect immediately. You may need to restart the portmap service.
Widely used services, such as NIS and NFS, depend on portmap to operate, so be aware of these limitations.
48.5.2.1.4. Operators
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At present, access control rules accept one operator, EXCEPT. It can be used in both the daemon list and the client list of a rule.
The EXCEPT operator allows specific exceptions to broader matches within the same rule.
In the following example from a hosts.allow file, all example.com hosts are allowed to connect to all services except cracker.example.com: 
ALL: .example.com EXCEPT cracker.example.com
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In another example from a hosts.allow file, clients from the 192.168.0.x network can use all services except for FTP: 
ALL EXCEPT vsftpd: 192.168.0.
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Note

Organizationally, it is often easier to avoid using EXCEPT operators. This allows other administrators to quickly scan the appropriate files to see what hosts are allowed or denied access to services, without having to sort through EXCEPT operators.
48.5.2.2. Option Fields
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In addition to basic rules that allow and deny access, the Red Hat Enterprise Linux implementation of TCP Wrappers supports extensions to the access control language through option fields. By using option fields in hosts access rules, administrators can accomplish a variety of tasks such as altering log behavior, consolidating access control, and launching shell commands.
48.5.2.2.1. Logging
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Option fields let administrators easily change the log facility and priority level for a rule by using the severity directive.
In the following example, connections to the SSH daemon from any host in the example.com domain are logged to the default authpriv syslog facility (because no facility value is specified) with a priority of emerg: 
sshd : .example.com : severity emerg
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It is also possible to specify a facility using the severity option. The following example logs any SSH connection attempts by hosts from the example.com domain to the local0 facility with a priority of alert: 
sshd : .example.com : severity local0.alert
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Note

In practice, this example does not work until the syslog daemon (syslogd) is configured to log to the local0 facility. Refer to the syslog.conf man page for information about configuring custom log facilities.
48.5.2.2.2. Access Control
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Option fields also allow administrators to explicitly allow or deny hosts in a single rule by adding the allow or deny directive as the final option.
For example, the following two rules allow SSH connections from client-1.example.com, but deny connections from client-2.example.com: 
sshd : client-1.example.com : allow
sshd : client-2.example.com : deny
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By allowing access control on a per-rule basis, the option field allows administrators to consolidate all access rules into a single file: either hosts.allow or hosts.deny. Some administrators consider this an easier way of organizing access rules.
48.5.2.2.3. Shell Commands
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Option fields allow access rules to launch shell commands through the following two directives:
  • spawn — Launches a shell command as a child process. This directive can perform tasks like using /usr/sbin/safe_finger to get more information about the requesting client or create special log files using the echo command.
    In the following example, clients attempting to access Telnet services from the example.com domain are quietly logged to a special file: 
    in.telnetd : .example.com \
	: spawn /bin/echo `/bin/date` from %h>>/var/log/telnet.log \
	: allow
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  • twist — Replaces the requested service with the specified command. This directive is often used to set up traps for intruders (also called "honey pots"). It can also be used to send messages to connecting clients. The twist directive must occur at the end of the rule line.
    In the following example, clients attempting to access FTP services from the example.com domain are sent a message using the echo command: 
    vsftpd : .example.com \
	: twist /bin/echo "421 This domain has been black-listed. Access denied!"
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For more information about shell command options, refer to the hosts_options man page.
48.5.2.2.4. Expansions
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Expansions, when used in conjunction with the spawn and twist directives, provide information about the client, server, and processes involved.
The following is a list of supported expansions:
  • %a — Returns the client's IP address.
  • %A — Returns the server's IP address.
  • %c — Returns a variety of client information, such as the username and hostname, or the username and IP address.
  • %d — Returns the daemon process name.
  • %h — Returns the client's hostname (or IP address, if the hostname is unavailable).
  • %H — Returns the server's hostname (or IP address, if the hostname is unavailable).
  • %n — Returns the client's hostname. If unavailable, unknown is printed. If the client's hostname and host address do not match, paranoid is printed.
  • %N — Returns the server's hostname. If unavailable, unknown is printed. If the server's hostname and host address do not match, paranoid is printed.
  • %p — Returns the daemon's process ID.
  • %s —Returns various types of server information, such as the daemon process and the host or IP address of the server.
  • %u — Returns the client's username. If unavailable, unknown is printed.
The following sample rule uses an expansion in conjunction with the spawn command to identify the client host in a customized log file.
When connections to the SSH daemon (sshd) are attempted from a host in the example.com domain, execute the echo command to log the attempt, including the client hostname (by using the %h expansion), to a special file: 
sshd : .example.com  \
	: spawn /bin/echo `/bin/date` access denied to %h>>/var/log/sshd.log \
	: deny
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Similarly, expansions can be used to personalize messages back to the client. In the following example, clients attempting to access FTP services from the example.com domain are informed that they have been banned from the server: 
vsftpd : .example.com \
: twist /bin/echo "421 %h has been banned from this server!"
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For a full explanation of available expansions, as well as additional access control options, refer to section 5 of the man pages for hosts_access (man 5 hosts_access) and the man page for hosts_options.
Refer to Section 48.5.5, “Additional Resources” for more information about TCP Wrappers.

48.5.3. xinetd
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The xinetd daemon is a TCP-wrapped super service which controls access to a subset of popular network services, including FTP, IMAP, and Telnet. It also provides service-specific configuration options for access control, enhanced logging, binding, redirection, and resource utilization control.
When a client attempts to connect to a network service controlled by xinetd, the super service receives the request and checks for any TCP Wrappers access control rules.
If access is allowed, xinetd verifies that the connection is allowed under its own access rules for that service. It also checks that the service can have more resources allotted to it and that it is not in breach of any defined rules.
If all these conditions are met (that is, access is allowed to the service; the service has not reached its resource limit; and the service is not in breach of any defined rule), xinetd then starts an instance of the requested service and passes control of the connection to it. After the connection has been established, xinetd takes no further part in the communication between the client and the server.

48.5.4. xinetd Configuration Files
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The configuration files for xinetd are as follows:
  • /etc/xinetd.conf — The global xinetd configuration file.
  • /etc/xinetd.d/ — The directory containing all service-specific files.
48.5.4.1. The /etc/xinetd.conf File
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The /etc/xinetd.conf file contains general configuration settings which affect every service under xinetd's control. It is read when the xinetd service is first started, so for configuration changes to take effect, you need to restart the xinetd service. The following is a sample /etc/xinetd.conf file: 
defaults
{
         instances               = 60
	 log_type                = SYSLOG	authpriv
	 log_on_success          = HOST PID
	 log_on_failure          = HOST
	 cps                     = 25 30
}
includedir /etc/xinetd.d
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These lines control the following aspects of xinetd:
  • instances — Specifies the maximum number of simultaneous requests that xinetd can process.
  • log_type — Configures xinetd to use the authpriv log facility, which writes log entries to the /var/log/secure file. Adding a directive such as FILE /var/log/xinetdlog would create a custom log file called xinetdlog in the /var/log/ directory.
  • log_on_success — Configures xinetd to log successful connection attempts. By default, the remote host's IP address and the process ID of the server processing the request are recorded.
  • log_on_failure — Configures xinetd to log failed connection attempts or if the connection was denied.
  • cps — Configures xinetd to allow no more than 25 connections per second to any given service. If this limit is exceeded, the service is retired for 30 seconds.
  • includedir /etc/xinetd.d/ — Includes options declared in the service-specific configuration files located in the /etc/xinetd.d/ directory. Refer to Section 48.5.4.2, “The /etc/xinetd.d/ Directory” for more information.

Note

Often, both the log_on_success and log_on_failure settings in /etc/xinetd.conf are further modified in the service-specific configuration files. More information may therefore appear in a given service's log file than the /etc/xinetd.conf file may indicate. Refer to Section 48.5.4.3.1, “Logging Options” for further information.
48.5.4.2. The /etc/xinetd.d/ Directory
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The /etc/xinetd.d/ directory contains the configuration files for each service managed by xinetd and the names of the files correlate to the service. As with xinetd.conf, this directory is read only when the xinetd service is started. For any changes to take effect, the administrator must restart the xinetd service.
The format of files in the /etc/xinetd.d/ directory use the same conventions as /etc/xinetd.conf. The primary reason the configuration for each service is stored in a separate file is to make customization easier and less likely to affect other services.
To gain an understanding of how these files are structured, consider the /etc/xinetd.d/krb5-telnet file: 
service telnet
{
         flags           = REUSE
	 socket_type     = stream
	 wait            = no
	 user            = root
	 server          = /usr/kerberos/sbin/telnetd
	 log_on_failure  += USERID
	 disable         = yes
}
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These lines control various aspects of the telnet service:
  • service — Specifies the service name, usually one of those listed in the /etc/services file.
  • flags — Sets any of a number of attributes for the connection. REUSE instructs xinetd to reuse the socket for a Telnet connection.

    Note

    The REUSE flag is deprecated. All services now implicitly use the REUSE flag.
  • socket_type — Sets the network socket type to stream.
  • wait — Specifies whether the service is single-threaded (yes) or multi-threaded (no).
  • user — Specifies which user ID the process runs under.
  • server — Specifies which binary executable to launch.
  • log_on_failure — Specifies logging parameters for log_on_failure in addition to those already defined in xinetd.conf.
  • disable — Specifies whether the service is disabled (yes) or enabled (no).
Refer to the xinetd.conf man page for more information about these options and their usage.
48.5.4.3. Altering xinetd Configuration Files
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A range of directives is available for services protected by xinetd. This section highlights some of the more commonly used options.
48.5.4.3.1. Logging Options
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The following logging options are available for both /etc/xinetd.conf and the service-specific configuration files within the /etc/xinetd.d/ directory.
The following is a list of some of the more commonly used logging options:
  • ATTEMPT — Logs the fact that a failed attempt was made (log_on_failure).
  • DURATION — Logs the length of time the service is used by a remote system (log_on_success).
  • EXIT — Logs the exit status or termination signal of the service (log_on_success).
  • HOST — Logs the remote host's IP address (log_on_failure and log_on_success).
  • PID — Logs the process ID of the server receiving the request (log_on_success).
  • USERID — Logs the remote user using the method defined in RFC 1413 for all multi-threaded stream services (log_on_failure andlog_on_success).
For a complete list of logging options, refer to the xinetd.conf man page.
48.5.4.3.2. Access Control Options
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Users of xinetd services can choose to use the TCP Wrappers hosts access rules, provide access control via the xinetd configuration files, or a mixture of both. Refer to Section 48.5.2, “TCP Wrappers Configuration Files” for more information about TCP Wrappers hosts access control files.
This section discusses using xinetd to control access to services.

Note

Unlike TCP Wrappers, changes to access control only take effect if the xinetd administrator restarts the xinetd service.
Also, unlike TCP Wrappers, access control through xinetd only affects services controlled by xinetd.
The xinetd hosts access control differs from the method used by TCP Wrappers. While TCP Wrappers places all of the access configuration within two files, /etc/hosts.allow and /etc/hosts.deny, xinetd's access control is found in each service's configuration file in the /etc/xinetd.d/ directory.
The following hosts access options are supported by xinetd:
  • only_from — Allows only the specified hosts to use the service.
  • no_access — Blocks listed hosts from using the service.
  • access_times — Specifies the time range when a particular service may be used. The time range must be stated in 24-hour format notation, HH:MM-HH:MM.
The only_from and no_access options can use a list of IP addresses or host names, or can specify an entire network. Like TCP Wrappers, combining xinetd access control with the enhanced logging configuration can increase security by blocking requests from banned hosts while verbosely recording each connection attempt.
For example, the following /etc/xinetd.d/telnet file can be used to block Telnet access from a particular network group and restrict the overall time range that even allowed users can log in: 
service telnet
{
         disable         = no
	 flags           = REUSE
	 socket_type     = stream
	 wait            = no
	 user            = root
	 server          = /usr/kerberos/sbin/telnetd
	 log_on_failure  += USERID
	 no_access       = 172.16.45.0/24
	 log_on_success  += PID HOST EXIT
	 access_times    = 09:45-16:15
}
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In this example, when a client system from the 10.0.1.0/24 network, such as 10.0.1.2, tries to access the Telnet service, it receives the following message: 
Connection closed by foreign host.
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In addition, their login attempts are logged in /var/log/messages as follows: 
Sep  7 14:58:33 localhost xinetd[5285]: FAIL: telnet address from=172.16.45.107
Sep  7 14:58:33 localhost xinetd[5283]: START: telnet pid=5285 from=172.16.45.107
Sep  7 14:58:33 localhost xinetd[5283]: EXIT: telnet status=0 pid=5285 duration=0(sec)
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When using TCP Wrappers in conjunction with xinetd access controls, it is important to understand the relationship between the two access control mechanisms.
The following is the sequence of events followed by xinetd when a client requests a connection:
  1. The xinetd daemon accesses the TCP Wrappers hosts access rules using a libwrap.a library call. If a deny rule matches the client, the connection is dropped. If an allow rule matches the client, the connection is passed to xinetd.
  2. The xinetd daemon checks its own access control rules both for the xinetd service and the requested service. If a deny rule matches the client, the connection is dropped. Otherwise, xinetd starts an instance of the requested service and passes control of the connection to that service.

Important

Care should be taken when using TCP Wrappers access controls in conjunction with xinetd access controls. Misconfiguration can cause undesirable effects.
48.5.4.3.3. Binding and Redirection Options
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The service configuration files for xinetd support binding the service to an IP address and redirecting incoming requests for that service to another IP address, hostname, or port.
Binding is controlled with the bind option in the service-specific configuration files and links the service to one IP address on the system. When this is configured, the bind option only allows requests to the correct IP address to access the service. You can use this method to bind different services to different network interfaces based on requirements.
This is particularly useful for systems with multiple network adapters or with multiple IP addresses. On such a system, insecure services (for example, Telnet), can be configured to listen only on the interface connected to a private network and not to the interface connected to the Internet.
The redirect option accepts an IP address or hostname followed by a port number. It configures the service to redirect any requests for this service to the specified host and port number. This feature can be used to point to another port number on the same system, redirect the request to a different IP address on the same machine, shift the request to a totally different system and port number, or any combination of these options. A user connecting to a certain service on a system may therefore be rerouted to another system without disruption.
The xinetd daemon is able to accomplish this redirection by spawning a process that stays alive for the duration of the connection between the requesting client machine and the host actually providing the service, transferring data between the two systems.
The advantages of the bind and redirect options are most clearly evident when they are used together. By binding a service to a particular IP address on a system and then redirecting requests for this service to a second machine that only the first machine can see, an internal system can be used to provide services for a totally different network. Alternatively, these options can be used to limit the exposure of a particular service on a multi-homed machine to a known IP address, as well as redirect any requests for that service to another machine especially configured for that purpose.
For example, consider a system that is used as a firewall with this setting for its Telnet service: 
service telnet
{
         socket_type		= stream
	 wait			= no
	 server			= /usr/kerberos/sbin/telnetd
	 log_on_success		+= DURATION USERID
	 log_on_failure		+= USERID
	 bind                    = 123.123.123.123
	 redirect                = 10.0.1.13 23
}
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The bind and redirect options in this file ensure that the Telnet service on the machine is bound to the external IP address (123.123.123.123), the one facing the Internet. In addition, any requests for Telnet service sent to 123.123.123.123 are redirected via a second network adapter to an internal IP address (10.0.1.13) that only the firewall and internal systems can access. The firewall then sends the communication between the two systems, and the connecting system thinks it is connected to 123.123.123.123 when it is actually connected to a different machine.
This feature is particularly useful for users with broadband connections and only one fixed IP address. When using Network Address Translation (NAT), the systems behind the gateway machine, which are using internal-only IP addresses, are not available from outside the gateway system. However, when certain services controlled by xinetd are configured with the bind and redirect options, the gateway machine can act as a proxy between outside systems and a particular internal machine configured to provide the service. In addition, the various xinetd access control and logging options are also available for additional protection.
48.5.4.3.4. Resource Management Options
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The xinetd daemon can add a basic level of protection from Denial of Service (DoS) attacks. The following is a list of directives which can aid in limiting the effectiveness of such attacks:
  • per_source — Defines the maximum number of instances for a service per source IP address. It accepts only integers as an argument and can be used in both xinetd.conf and in the service-specific configuration files in the xinetd.d/ directory.
  • cps — Defines the maximum number of connections per second. This directive takes two integer arguments separated by white space. The first argument is the maximum number of connections allowed to the service per second. The second argument is the number of seconds that xinetd must wait before re-enabling the service. It accepts only integers as arguments and can be used in either the xinetd.conf file or the service-specific configuration files in the xinetd.d/ directory.
  • max_load — Defines the CPU usage or load average threshold for a service. It accepts a floating point number argument.
    The load average is a rough measure of how many processes are active at a given time. See the uptime, who, and procinfo commands for more information about load average.
There are more resource management options available for xinetd. Refer to the xinetd.conf man page for more information.

48.5.5. Additional Resources
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More information about TCP Wrappers and xinetd is available from system documentation and on the Internet.
48.5.5.1. Installed Documentation
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The documentation on your system is a good place to start looking for additional configuration options for TCP Wrappers, xinetd, and access control.
  • /usr/share/doc/tcp_wrappers-<version>/ — This directory contains a README file that discusses how TCP Wrappers work and the various hostname and host address spoofing risks that exist.
  • /usr/share/doc/xinetd-<version>/ — This directory contains a README file that discusses aspects of access control and a sample.conf file with various ideas for modifying service-specific configuration files in the /etc/xinetd.d/ directory.
  • TCP Wrappers and xinetd-related man pages — A number of man pages exist for the various applications and configuration files involved with TCP Wrappers and xinetd. The following are some of the more important man pages:
    Server Applications
    • man xinetd — The man page for xinetd.
    Configuration Files
    • man 5 hosts_access — The man page for the TCP Wrappers hosts access control files.
    • man hosts_options — The man page for the TCP Wrappers options fields.
    • man xinetd.conf — The man page listing xinetd configuration options.
48.5.5.2. Useful Websites
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48.6. Kerberos
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System security and integrity within a network can be unwieldy. It can occupy the time of several administrators just to keep track of what services are being run on a network and the manner in which these services are used.
Further, authenticating users to network services can prove dangerous when the method used by the protocol is inherently insecure, as evidenced by the transfer of unencrypted passwords over a network using the traditional FTP and Telnet protocols.
Kerberos is a way to eliminate the need for protocols that allow unsafe methods of authentication, thereby enhancing overall network security.

48.6.1. What is Kerberos?
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Kerberos is a network authentication protocol created by MIT, and uses symmetric-key cryptography[17] to authenticate users to network services, which means passwords are never actually sent over the network.
Consequently, when users authenticate to network services using Kerberos, unauthorized users attempting to gather passwords by monitoring network traffic are effectively thwarted.
48.6.1.1. Advantages of Kerberos
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Most conventional network services use password-based authentication schemes. Such schemes require a user to authenticate to a given network server by supplying their username and password. Unfortunately, the transmission of authentication information for many services is unencrypted. For such a scheme to be secure, the network has to be inaccessible to outsiders, and all computers and users on the network must be trusted and trustworthy.
Even if this is the case, a network that is connected to the Internet can no longer be assumed to be secure. Any attacker who gains access to the network can use a simple packet analyzer, also known as a packet sniffer, to intercept usernames and passwords, compromising user accounts and the integrity of the entire security infrastructure.
The primary design goal of Kerberos is to eliminate the transmission of unencrypted passwords across the network. If used properly, Kerberos effectively eliminates the threat that packet sniffers would otherwise pose on a network.
48.6.1.2. Disadvantages of Kerberos
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Although Kerberos removes a common and severe security threat, it may be difficult to implement for a variety of reasons:
  • Migrating user passwords from a standard UNIX password database, such as /etc/passwd or /etc/shadow, to a Kerberos password database can be tedious, as there is no automated mechanism to perform this task. Refer to Question 2.23 in the online Kerberos FAQ:
    http://www.nrl.navy.mil/CCS/people/kenh/kerberos-faq.html
  • Kerberos has only partial compatibility with the Pluggable Authentication Modules (PAM) system used by most Red Hat Enterprise Linux servers. Refer to Section 48.6.4, “Kerberos and PAM” for more information about this issue.
  • Kerberos assumes that each user is trusted but is using an untrusted host on an untrusted network. Its primary goal is to prevent unencrypted passwords from being transmitted across that network. However, if anyone other than the proper user has access to the one host that issues tickets used for authentication — called the key distribution center (KDC) — the entire Kerberos authentication system is at risk.
  • For an application to use Kerberos, its source must be modified to make the appropriate calls into the Kerberos libraries. Applications modified in this way are considered to be Kerberos-aware, or kerberized. For some applications, this can be quite problematic due to the size of the application or its design. For other incompatible applications, changes must be made to the way in which the server and client communicate. Again, this may require extensive programming. Closed-source applications that do not have Kerberos support by default are often the most problematic.
  • Kerberos is an all-or-nothing solution. If Kerberos is used on the network, any unencrypted passwords transferred to a non-Kerberos aware service is at risk. Thus, the network gains no benefit from the use of Kerberos. To secure a network with Kerberos, one must either use Kerberos-aware versions of all client/server applications that transmit passwords unencrypted, or not use any such client/server applications at all.

48.6.2. Kerberos Terminology
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Kerberos has its own terminology to define various aspects of the service. Before learning how Kerberos works, it is important to learn the following terms.
authentication server (AS)
A server that issues tickets for a desired service which are in turn given to users for access to the service. The AS responds to requests from clients who do not have or do not send credentials with a request. It is usually used to gain access to the ticket-granting server (TGS) service by issuing a ticket-granting ticket (TGT). The AS usually runs on the same host as the key distribution center (KDC).
ciphertext
Encrypted data.
client
An entity on the network (a user, a host, or an application) that can receive a ticket from Kerberos.
credentials
A temporary set of electronic credentials that verify the identity of a client for a particular service. Also called a ticket.
credential cache or ticket file
A file which contains the keys for encrypting communications between a user and various network services. Kerberos 5 supports a framework for using other cache types, such as shared memory, but files are more thoroughly supported.
crypt hash
A one-way hash used to authenticate users. These are more secure than using unencrypted data, but they are still relatively easy to decrypt for an experienced cracker.
GSS-API
The Generic Security Service Application Program Interface (defined in RFC-2743 published by The Internet Engineering Task Force) is a set of functions which provide security services. This API is used by clients and services to authenticate to each other without either program having specific knowledge of the underlying mechanism. If a network service (such as cyrus-IMAP) uses GSS-API, it can authenticate using Kerberos.
hash
Also known as a hash value. A value generated by passing a string through a hash function. These values are typically used to ensure that transmitted data has not been tampered with.
hash function
A way of generating a digital "fingerprint" from input data. These functions rearrange, transpose or otherwise alter data to produce a hash value.
key
Data used when encrypting or decrypting other data. Encrypted data cannot be decrypted without the proper key or extremely good fortune on the part of the cracker.
key distribution center (KDC)
A service that issues Kerberos tickets, and which usually run on the same host as the ticket-granting server (TGS).
keytab (or key table)
A file that includes an unencrypted list of principals and their keys. Servers retrieve the keys they need from keytab files instead of using kinit. The default keytab file is /etc/krb5.keytab. The KDC administration server, /usr/kerberos/sbin/kadmind, is the only service that uses any other file (it uses /var/kerberos/krb5kdc/kadm5.keytab).
kinit
The kinit command allows a principal who has already logged in to obtain and cache the initial ticket-granting ticket (TGT). Refer to the kinit man page for more information.
principal (or principal name)
The principal is the unique name of a user or service allowed to authenticate using Kerberos. A principal follows the form root[/instance]@REALM. For a typical user, the root is the same as their login ID. The instance is optional. If the principal has an instance, it is separated from the root with a forward slash ("/"). An empty string ("") is considered a valid instance (which differs from the default NULL instance), but using it can be confusing. All principals in a realm have their own key, which for users is derived from a password or is randomly set for services.
realm
A network that uses Kerberos, composed of one or more servers called KDCs and a potentially large number of clients.
service
A program accessed over the network.
ticket
A temporary set of electronic credentials that verify the identity of a client for a particular service. Also called credentials.
ticket-granting server (TGS)
A server that issues tickets for a desired service which are in turn given to users for access to the service. The TGS usually runs on the same host as the KDC.
ticket-granting ticket (TGT)
A special ticket that allows the client to obtain additional tickets without applying for them from the KDC.
unencrypted password
A plain text, human-readable password.

48.6.3. How Kerberos Works
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Kerberos differs from username/password authentication methods. Instead of authenticating each user to each network service, Kerberos uses symmetric encryption and a trusted third party (a KDC), to authenticate users to a suite of network services. When a user authenticates to the KDC, the KDC sends a ticket specific to that session back to the user's machine, and any Kerberos-aware services look for the ticket on the user's machine rather than requiring the user to authenticate using a password.
When a user on a Kerberos-aware network logs in to their workstation, their principal is sent to the KDC as part of a request for a TGT from the Authentication Server. This request can be sent by the log-in program so that it is transparent to the user, or can be sent by the kinit program after the user logs in.
The KDC then checks for the principal in its database. If the principal is found, the KDC creates a TGT, which is encrypted using the user's key and returned to that user.
The login or kinit program on the client then decrypts the TGT using the user's key, which it computes from the user's password. The user's key is used only on the client machine and is not transmitted over the network.
The TGT is set to expire after a certain period of time (usually ten to twenty-four hours) and is stored in the client machine's credentials cache. An expiration time is set so that a compromised TGT is of use to an attacker for only a short period of time. After the TGT has been issued, the user does not have to re-enter their password until the TGT expires or until they log out and log in again.
Whenever the user needs access to a network service, the client software uses the TGT to request a new ticket for that specific service from the TGS. The service ticket is then used to authenticate the user to that service transparently.

Warning

The Kerberos system can be compromised if a user on the network authenticates against a non-Kerberos aware service by transmitting a password in plain text. The use of non-Kerberos aware services is highly discouraged. Such services include Telnet and FTP. The use of other encrypted protocols, such as SSH or SSL-secured services, however, is preferred, although not ideal.
This is only a broad overview of how Kerberos authentication works. Refer to Section 48.6.10, “Additional Resources” for links to more in-depth information.

Note

Kerberos depends on the following network services to function correctly.
  • Approximate clock synchronization between the machines on the network.
    A clock synchronization program should be set up for the network, such as ntpd. Refer to /usr/share/doc/ntp-<version-number>/index.html for details on setting up Network Time Protocol servers (where <version-number> is the version number of the ntp package installed on your system).
  • Domain Name Service (DNS).
    You should ensure that the DNS entries and hosts on the network are all properly configured. Refer to the Kerberos V5 System Administrator's Guide in /usr/share/doc/krb5-server-<version-number> for more information (where <version-number> is the version number of the krb5-server package installed on your system).

48.6.4. Kerberos and PAM
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Kerberos-aware services do not currently make use of Pluggable Authentication Modules (PAM) — these services bypass PAM completely. However, applications that use PAM can make use of Kerberos for authentication if the pam_krb5 module (provided in the pam_krb5 package) is installed. The pam_krb5 package contains sample configuration files that allow services such as login and gdm to authenticate users as well as obtain initial credentials using their passwords. If access to network servers is always performed using Kerberos-aware services or services that use GSS-API, such as IMAP, the network can be considered reasonably safe.

Note

Administrators should be careful not to allow users to authenticate to most network services using Kerberos passwords. Many protocols used by these services do not encrypt the password before sending it over the network, destroying the benefits of the Kerberos system. For example, users should not be allowed to authenticate to Telnet services with the same password they use for Kerberos authentication.

48.6.5. Configuring a Kerberos 5 Server
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When setting up Kerberos, install the KDC first. If it is necessary to set up slave servers, install the master first.
To configure the first Kerberos KDC, follow these steps:
  1. Ensure that time synchronization and DNS are functioning correctly on all client and server machines before configuring Kerberos. Pay particular attention to time synchronization between the Kerberos server and its clients. If the time difference between the server and client is greater than five minutes (this is configurable in Kerberos 5), Kerberos clients can not authenticate to the server. This time synchronization is necessary to prevent an attacker from using an old Kerberos ticket to masquerade as a valid user.
    It is advisable to set up a Network Time Protocol (NTP) compatible client/server network even if Kerberos is not being used. Red Hat Enterprise Linux includes the ntp package for this purpose. Refer to /usr/share/doc/ntp-<version-number>/index.html (where <version-number> is the version number of the ntp package installed on your system) for details about how to set up Network Time Protocol servers, and http://www.ntp.org for more information about NTP.
  2. Install the krb5-libs, krb5-server, and krb5-workstation packages on the dedicated machine which runs the KDC. This machine needs to be very secure — if possible, it should not run any services other than the KDC.
  3. Edit the /etc/krb5.conf and /var/kerberos/krb5kdc/kdc.conf configuration files to reflect the realm name and domain-to-realm mappings. A simple realm can be constructed by replacing instances of EXAMPLE.COM and example.com with the correct domain name — being certain to keep uppercase and lowercase names in the correct format — and by changing the KDC from kerberos.example.com to the name of the Kerberos server. By convention, all realm names are uppercase and all DNS hostnames and domain names are lowercase. For full details about the formats of these configuration files, refer to their respective man pages.
  4. Create the database using the kdb5_util utility from a shell prompt: 
    /usr/kerberos/sbin/kdb5_util create -s
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    The create command creates the database that stores keys for the Kerberos realm. The -s switch forces creation of a stash file in which the master server key is stored. If no stash file is present from which to read the key, the Kerberos server (krb5kdc) prompts the user for the master server password (which can be used to regenerate the key) every time it starts.
  5. Edit the /var/kerberos/krb5kdc/kadm5.acl file. This file is used by kadmind to determine which principals have administrative access to the Kerberos database and their level of access. Most organizations can get by with a single line: 
    */admin@EXAMPLE.COM  *
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    Most users are represented in the database by a single principal (with a NULL, or empty, instance, such as joe@EXAMPLE.COM). In this configuration, users with a second principal with an instance of admin (for example, joe/admin@EXAMPLE.COM) are able to wield full power over the realm's Kerberos database.
    After kadmind has been started on the server, any user can access its services by running kadmin on any of the clients or servers in the realm. However, only users listed in the kadm5.acl file can modify the database in any way, except for changing their own passwords.

    Note

    The kadmin utility communicates with the kadmind server over the network, and uses Kerberos to handle authentication. Consequently, the first principal must already exist before connecting to the server over the network to administer it. Create the first principal with the kadmin.local command, which is specifically designed to be used on the same host as the KDC and does not use Kerberos for authentication.
    Type the following kadmin.local command at the KDC terminal to create the first principal: 
    /usr/kerberos/sbin/kadmin.local -q "addprinc username/admin"
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  6. Start Kerberos using the following commands: 
    service krb5kdc start
service kadmin start
service krb524 start
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  7. Add principals for the users using the addprinc command within kadmin. kadmin and kadmin.local are command line interfaces to the KDC. As such, many commands — such as addprinc — are available after launching the kadmin program. Refer to the kadmin man page for more information.
  8. Verify that the KDC is issuing tickets. First, run kinit to obtain a ticket and store it in a credential cache file. Next, use klist to view the list of credentials in the cache and use kdestroy to destroy the cache and the credentials it contains.

    Note

    By default, kinit attempts to authenticate using the same system login username (not the Kerberos server). If that username does not correspond to a principal in the Kerberos database, kinit issues an error message. If that happens, supply kinit with the name of the correct principal as an argument on the command line (kinit <principal>).
Once these steps are completed, the Kerberos server should be up and running.

48.6.6. Configuring a Kerberos 5 Client
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Setting up a Kerberos 5 client is less involved than setting up a server. At a minimum, install the client packages and provide each client with a valid krb5.conf configuration file. While ssh and slogin are the preferred method of remotely logging in to client systems, Kerberized versions of rsh and rlogin are still available, though deploying them requires that a few more configuration changes be made.
  1. Be sure that time synchronization is in place between the Kerberos client and the KDC. Refer to Section 48.6.5, “Configuring a Kerberos 5 Server” for more information. In addition, verify that DNS is working properly on the Kerberos client before configuring the Kerberos client programs.
  2. Install the krb5-libs and krb5-workstation packages on all of the client machines. Supply a valid /etc/krb5.conf file for each client (usually this can be the same krb5.conf file used by the KDC).
  3. Before a workstation in the realm can use Kerberos to authenticate users who connect using ssh or Kerberized rsh or rlogin, it must have its own host principal in the Kerberos database. The sshd, kshd, and klogind server programs all need access to the keys for the host service's principal. Additionally, in order to use the kerberized rsh and rlogin services, that workstation must have the xinetd package installed.
    Using kadmin, add a host principal for the workstation on the KDC. The instance in this case is the hostname of the workstation. Use the -randkey option for the kadmin's addprinc command to create the principal and assign it a random key: 
    addprinc -randkey host/blah.example.com
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    Now that the principal has been created, keys can be extracted for the workstation by running kadmin on the workstation itself, and using the ktadd command within kadmin: 
    ktadd -k /etc/krb5.keytab host/blah.example.com
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  4. To use other kerberized network services, they must first be started. Below is a list of some common kerberized services and instructions about enabling them:
    • ssh — OpenSSH uses GSS-API to authenticate users to servers if the client's and server's configuration both have GSSAPIAuthentication enabled. If the client also has GSSAPIDelegateCredentials enabled, the user's credentials are made available on the remote system.
    • rsh and rlogin — To use the kerberized versions of rsh and rlogin, enable klogin, eklogin, and kshell.
    • Telnet — To use kerberized Telnet, krb5-telnet must be enabled.
    • FTP — To provide FTP access, create and extract a key for the principal with a root of ftp. Be certain to set the instance to the fully qualified hostname of the FTP server, then enable gssftp.
    • IMAP — To use a kerberized IMAP server, the cyrus-imap package uses Kerberos 5 if it also has the cyrus-sasl-gssapi package installed. The cyrus-sasl-gssapi package contains the Cyrus SASL plugins which support GSS-API authentication. Cyrus IMAP should function properly with Kerberos as long as the cyrus user is able to find the proper key in /etc/krb5.keytab, and the root for the principal is set to imap (created with kadmin).
      An alternative to cyrus-imap can be found in the dovecot package, which is also included in Red Hat Enterprise Linux. This package contains an IMAP server but does not, to date, support GSS-API and Kerberos.
    • CVS — To use a kerberized CVS server, gserver uses a principal with a root of cvs and is otherwise identical to the CVS pserver.
    Refer to Chapter 18, Controlling Access to Services for details about how to enable services.

48.6.7. Domain-to-Realm Mapping
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When a client attempts to access a service running on a particular server, it knows the name of the service (host) and the name of the server (foo.example.com), but because more than one realm may be deployed on your network, it must guess at the name of the realm in which the service resides.
By default, the name of the realm is taken to be the DNS domain name of the server, upper-cased.

foo.example.org → EXAMPLE.ORG
foo.example.com → EXAMPLE.COM
foo.hq.example.com → HQ.EXAMPLE.COM

In some configurations, this will be sufficient, but in others, the realm name which is derived will be the name of a non-existent realm. In these cases, the mapping from the server's DNS domain name to the name of its realm must be specified in the domain_realm section of the client system's krb5.conf. For example: 
[domain_realm]
.example.com = EXAMPLE.COM
example.com = EXAMPLE.COM
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The above configuration specifies two mappings. The first mapping specifies that any system in the "example.com" DNS domain belongs to the EXAMPLE.COM realm. The second specifies that a system with the exact name "example.com" is also in the realm. (The distinction between a domain and a specific host is marked by the presence or lack of an initial ".".) The mapping can also be stored directly in DNS.

48.6.8. Setting Up Secondary KDCs
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For a number of reasons, you may choose to run multiple KDCs for a given realm. In this scenario, one KDC (the master KDC) keeps a writable copy of the realm database and runs kadmind (it is also your realm's admin server), and one or more KDCs (slave KDCs) keep read-only copies of the database and run kpropd.
The master-slave propagation procedure entails the master KDC dumping its database to a temporary dump file and then transmitting that file to each of its slaves, which then overwrite their previously-received read-only copies of the database with the contents of the dump file.
To set up a slave KDC, first ensure that the master KDC's krb5.conf and kdc.conf files are copied to the slave KDC.
Start kadmin.local from a root shell on the master KDC and use its add_principal command to create a new entry for the master KDC's host service, and then use its ktadd command to simultaneously set a random key for the service and store the random key in the master's default keytab file. This key will be used by the kprop command to authenticate to the slave servers. You will only need to do this once, regardless of how many slave servers you install. 
~]# kadmin.local -r EXAMPLE.COM
Authenticating as principal root/admin@EXAMPLE.COM with password.
kadmin: add_principal -randkey host/masterkdc.example.com
Principal "host/host/masterkdc.example.com@EXAMPLE.COM" created.
kadmin: ktadd host/masterkdc.example.com
Entry for principal host/masterkdc.example.com with kvno 3, encryption type Triple DES cbc mode with \
	HMAC/sha1 added to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/masterkdc.example.com with kvno 3, encryption type ArcFour with HMAC/md5 \
	added to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/masterkdc.example.com with kvno 3, encryption type DES with HMAC/sha1 added \
	to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/masterkdc.example.com with kvno 3, encryption type DES cbc mode with RSA-MD5 \
	added to keytab WRFILE:/etc/krb5.keytab.
kadmin: quit
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Start kadmin from a root shell on the slave KDC and use its add_principal command to create a new entry for the slave KDC's host service, and then use kadmin's ktadd command to simultaneously set a random key for the service and store the random key in the slave's default keytab file. This key is used by the kpropd service when authenticating clients. 
~]# kadmin -p jimbo/admin@EXAMPLE.COM -r EXAMPLE.COM
Authenticating as principal jimbo/admin@EXAMPLE.COM with password.
Password for jimbo/admin@EXAMPLE.COM:
kadmin: add_principal -randkey host/slavekdc.example.com
Principal "host/slavekdc.example.com@EXAMPLE.COM" created.
kadmin: ktadd host/slavekdc.example.com@EXAMPLE.COM
Entry for principal host/slavekdc.example.com with kvno 3, encryption type Triple DES cbc mode with \
	HMAC/sha1 added to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/slavekdc.example.com with kvno 3, encryption type ArcFour with HMAC/md5 added \
	to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/slavekdc.example.com with kvno 3, encryption type DES with HMAC/sha1 added \
	to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/slavekdc.example.com with kvno 3, encryption type DES cbc mode with RSA-MD5 added \
	to keytab WRFILE:/etc/krb5.keytab.
kadmin: quit
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With its service key, the slave KDC could authenticate any client which would connect to it. Obviously, not all of them should be allowed to provide the slave's kprop service with a new realm database. To restrict access, the kprop service on the slave KDC will only accept updates from clients whose principal names are listed in /var/kerberos/krb5kdc/kpropd.acl. Add the master KDC's host service's name to that file. 
~]# echo host/masterkdc.example.com@EXAMPLE.COM > /var/kerberos/krb5kdc/kpropd.acl
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Once the slave KDC has obtained a copy of the database, it will also need the master key which was used to encrypt it. If your KDC database's master key is stored in a stash file on the master KDC (typically named /var/kerberos/krb5kdc/.k5.REALM, either copy it to the slave KDC using any available secure method, or create a dummy database and identical stash file on the slave KDC by running kdb5_util create -s (the dummy database will be overwritten by the first successful database propagation) and supplying the same password.
Ensure that the slave KDC's firewall allows the master KDC to contact it using TCP on port 754 (krb5_prop), and start the kprop service. Then, double-check that the kadmin service is disabled.
Now perform a manual database propagation test by dumping the realm database, on the master KDC, to the default data file which the kprop command will read (/var/kerberos/krb5kdc/slave_datatrans), and then use the kprop command to transmit its contents to the slave KDC. 
~]# /usr/kerberos/sbin/kdb5_util dump /var/kerberos/krb5kdc/slave_datatrans
~]# kprop slavekdc.example.com
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Using kinit, verify that a client system whose krb5.conf lists only the slave KDC in its list of KDCs for your realm is now correctly able to obtain initial credentials from the slave KDC.
That done, simply create a script which dumps the realm database and runs the kprop command to transmit the database to each slave KDC in turn, and configure the cron service to run the script periodically.

48.6.9. Setting Up Cross Realm Authentication
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Cross-realm authentication is the term which is used to describe situations in which clients (typically users) of one realm use Kerberos to authenticate to services (typically server processes running on a particular server system) which belong to a realm other than their own.
For the simplest case, in order for a client of a realm named A.EXAMPLE.COM to access a service in the B.EXAMPLE.COM realm, both realms must share a key for a principal named krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM, and both keys must have the same key version number associated with them.
To accomplish this, select a very strong password or passphrase, and create an entry for the principal in both realms using kadmin. 
~]# kadmin -r A.EXAMPLE.COM
kadmin: add_principal krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM
Enter password for principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM":
Re-enter password for principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM":
Principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM" created.
kadmin:	quit
~]# kadmin -r B.EXAMPLE.COM
kadmin: add_principal krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM
Enter password for principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM":
Re-enter password for principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM":
Principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM" created.
kadmin: quit
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Use the get_principal command to verify that both entries have matching key version numbers (kvno values) and encryption types.

Warning

Security-conscious administrators may attempt to use the add_principal command's -randkey option to assign a random key instead of a password, dump the new entry from the database of the first realm, and import it into the second. This will not work unless the master keys for the realm databases are identical, as the keys contained in a database dump are themselves encrypted using the master key.
Clients in the A.EXAMPLE.COM realm are now able to authenticate to services in the B.EXAMPLE.COM realm. Put another way, the B.EXAMPLE.COM realm now trusts the A.EXAMPLE.COM realm, or phrased even more simply, B.EXAMPLE.COM now trusts A.EXAMPLE.COM.
This brings us to an important point: cross-realm trust is unidirectional by default. The KDC for the B.EXAMPLE.COM realm may trust clients from the A.EXAMPLE.COM to authenticate to services in the B.EXAMPLE.COM realm, but the fact that it does has no effect on whether or not clients in the B.EXAMPLE.COM realm are trusted to authenticate to services in the A.EXAMPLE.COM realm. To establish trust in the other direction, both realms would need to share keys for the krbtgt/A.EXAMPLE.COM@B.EXAMPLE.COM service (take note of the reversed in order of the two realms compared to the example above).
If direct trust relationships were the only method for providing trust between realms, networks which contain multiple realms would be very difficult to set up. Luckily, cross-realm trust is transitive. If clients from A.EXAMPLE.COM can authenticate to services in B.EXAMPLE.COM, and clients from B.EXAMPLE.COM can authenticate to services in C.EXAMPLE.COM, then clients in A.EXAMPLE.COM can also authenticate to services in C.EXAMPLE.COM, even if C.EXAMPLE.COM doesn't directly trust A.EXAMPLE.COM. This means that, on a network with multiple realms which all need to trust each other, making good choices about which trust relationships to set up can greatly reduce the amount of effort required.
Now you face the more conventional problems: the client's system must be configured so that it can properly deduce the realm to which a particular service belongs, and it must be able to determine how to obtain credentials for services in that realm.
First things first: the principal name for a service provided from a specific server system in a given realm typically looks like this: 
service/server.example.com@EXAMPLE.COM
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In this example, service is typically either the name of the protocol in use (other common values include ldap, imap, cvs, and HTTP) or host, server.example.com is the fully-qualified domain name of the system which runs the service, and EXAMPLE.COM is the name of the realm.
To deduce the realm to which the service belongs, clients will most often consult DNS or the domain_realm section of /etc/krb5.conf to map either a hostname (server.example.com) or a DNS domain name (.example.com) to the name of a realm (EXAMPLE.COM).
Having determined which to which realm a service belongs, a client then has to determine the set of realms which it needs to contact, and in which order it must contact them, to obtain credentials for use in authenticating to the service.
This can be done in one of two ways.
The default method, which requires no explicit configuration, is to give the realms names within a shared hierarchy. For an example, assume realms named A.EXAMPLE.COM, B.EXAMPLE.COM, and EXAMPLE.COM. When a client in the A.EXAMPLE.COM realm attempts to authenticate to a service in B.EXAMPLE.COM, it will, by default, first attempt to get credentials for the EXAMPLE.COM realm, and then to use those credentials to obtain credentials for use in the B.EXAMPLE.COM realm.
The client in this scenario treats the realm name as one might treat a DNS name. It repeatedly strips off the components of its own realm's name to generate the names of realms which are "above" it in the hierarchy until it reaches a point which is also "above" the service's realm. At that point it begins prepending components of the service's realm name until it reaches the service's realm. Each realm which is involved in the process is another "hop".
For example, using credentials in A.EXAMPLE.COM, authenticating to a service in B.EXAMPLE.COM:


A.EXAMPLE.COM → EXAMPLE.COM → B.EXAMPLE.COM

  • A.EXAMPLE.COM and EXAMPLE.COM share a key for krbtgt/EXAMPLE.COM@A.EXAMPLE.COM
  • EXAMPLE.COM and B.EXAMPLE.COM share a key for krbtgt/B.EXAMPLE.COM@EXAMPLE.COM
Another example, using credentials in SITE1.SALES.EXAMPLE.COM, authenticating to a service in EVERYWHERE.EXAMPLE.COM:


SITE1.SALES.EXAMPLE.COM → SALES.EXAMPLE.COM → EXAMPLE.COM → EVERYWHERE.EXAMPLE.COM

  • SITE1.SALES.EXAMPLE.COM and SALES.EXAMPLE.COM share a key for krbtgt/SALES.EXAMPLE.COM@SITE1.SALES.EXAMPLE.COM
  • SALES.EXAMPLE.COM and EXAMPLE.COM share a key for krbtgt/EXAMPLE.COM@SALES.EXAMPLE.COM
  • EXAMPLE.COM and EVERYWHERE.EXAMPLE.COM share a key for krbtgt/EVERYWHERE.EXAMPLE.COM@EXAMPLE.COM
Another example, this time using realm names whose names share no common suffix (DEVEL.EXAMPLE.COM and PROD.EXAMPLE.ORG):


DEVEL.EXAMPLE.COM → EXAMPLE.COM → COM → ORG → EXAMPLE.ORG → PROD.EXAMPLE.ORG

  • DEVEL.EXAMPLE.COM and EXAMPLE.COM share a key for krbtgt/EXAMPLE.COM@DEVEL.EXAMPLE.COM
  • EXAMPLE.COM and COM share a key for krbtgt/COM@EXAMPLE.COM
  • COM and ORG share a key for krbtgt/ORG@COM
  • ORG and EXAMPLE.ORG share a key for krbtgt/EXAMPLE.ORG@ORG
  • EXAMPLE.ORG and PROD.EXAMPLE.ORG share a key for krbtgt/PROD.EXAMPLE.ORG@EXAMPLE.ORG
The more complicated, but also more flexible, method involves configuring the capaths section of /etc/krb5.conf, so that clients which have credentials for one realm will be able to look up which realm is next in the chain which will eventually lead to the being able to authenticate to servers.
The format of the capaths section is relatively straightforward: each entry in the section is named after a realm in which a client might exist. Inside of that subsection, the set of intermediate realms from which the client must obtain credentials is listed as values of the key which corresponds to the realm in which a service might reside. If there are no intermediate realms, the value "." is used.
Here's an example: 
[capaths]
A.EXAMPLE.COM = {
	B.EXAMPLE.COM = .
	C.EXAMPLE.COM = B.EXAMPLE.COM
	D.EXAMPLE.COM = B.EXAMPLE.COM
	D.EXAMPLE.COM = C.EXAMPLE.COM
}
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In this example, clients in the A.EXAMPLE.COM realm can obtain cross-realm credentials for B.EXAMPLE.COM directly from the A.EXAMPLE.COM KDC.
If those clients wish to contact a service in theC.EXAMPLE.COM realm, they will first need to obtain necessary credentials from the B.EXAMPLE.COM realm (this requires that krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM exist), and then use those credentials to obtain credentials for use in the C.EXAMPLE.COM realm (using krbtgt/C.EXAMPLE.COM@B.EXAMPLE.COM).
If those clients wish to contact a service in the D.EXAMPLE.COM realm, they will first need to obtain necessary credentials from the B.EXAMPLE.COM realm, and then credentials from the C.EXAMPLE.COM realm, before finally obtaining credentials for use with the D.EXAMPLE.COM realm.

Note

Without a capath entry indicating otherwise, Kerberos assumes that cross-realm trust relationships form a hierarchy.
Clients in the A.EXAMPLE.COM realm can obtain cross-realm credentials from B.EXAMPLE.COM realm directly. Without the "." indicating this, the client would instead attempt to use a hierarchical path, in this case:


A.EXAMPLE.COM → EXAMPLE.COM → B.EXAMPLE.COM

48.6.10. Additional Resources
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For more information about Kerberos, refer to the following resources.
48.6.10.1. Installed Documentation
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  • The Kerberos V5 Installation Guide and the Kerberos V5 System Administrator's Guide in PostScript and HTML formats. These can be found in the /usr/share/doc/krb5-server-<version-number>/ directory (where <version-number> is the version number of the krb5-server package installed on your system).
  • The Kerberos V5 UNIX User's Guide in PostScript and HTML formats. These can be found in the /usr/share/doc/krb5-workstation-<version-number>/ directory (where <version-number> is the version number of the krb5-workstation package installed on your system).
  • Kerberos man pages — There are a number of man pages for the various applications and configuration files involved with a Kerberos implementation. The following is a list of some of the more important man pages.
    Client Applications
    • man kerberos — An introduction to the Kerberos system which describes how credentials work and provides recommendations for obtaining and destroying Kerberos tickets. The bottom of the man page references a number of related man pages.
    • man kinit — Describes how to use this command to obtain and cache a ticket-granting ticket.
    • man kdestroy — Describes how to use this command to destroy Kerberos credentials.
    • man klist — Describes how to use this command to list cached Kerberos credentials.
    Administrative Applications
    • man kadmin — Describes how to use this command to administer the Kerberos V5 database.
    • man kdb5_util — Describes how to use this command to create and perform low-level administrative functions on the Kerberos V5 database.
    Server Applications
    • man krb5kdc — Describes available command line options for the Kerberos V5 KDC.
    • man kadmind — Describes available command line options for the Kerberos V5 administration server.
    Configuration Files
    • man krb5.conf — Describes the format and options available within the configuration file for the Kerberos V5 library.
    • man kdc.conf — Describes the format and options available within the configuration file for the Kerberos V5 AS and KDC.
48.6.10.2. Useful Websites
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48.7. Virtual Private Networks (VPNs)
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Organizations with several satellite offices often connect to each other with dedicated lines for efficiency and protection of sensitive data in transit. For example, many businesses use frame relay or Asynchronous Transfer Mode (ATM) lines as an end-to-end networking solution to link one office with others. This can be an expensive proposition, especially for small to medium sized businesses (SMBs) that want to expand without paying the high costs associated with enterprise-level, dedicated digital circuits.
To address this need, Virtual Private Networks (VPNs) were developed. Following the same functional principles as dedicated circuits, VPNs allow for secured digital communication between two parties (or networks), creating a Wide Area Network (WAN) from existing Local Area Networks (LANs). Where it differs from frame relay or ATM is in its transport medium. VPNs transmit over IP using datagrams as the transport layer, making it a secure conduit through the Internet to an intended destination. Most free software VPN implementations incorporate open standard encryption methods to further mask data in transit.
Some organizations employ hardware VPN solutions to augment security, while others use software or protocol-based implementations. Several vendors provide hardware VPN solutions, such as Cisco, Nortel, IBM, and Checkpoint. There is a free software-based VPN solution for Linux called FreeS/Wan that utilizes a standardized Internet Protocol Security (IPsec) implementation. These VPN solutions, irrespective of whether they are hardware or software based, act as specialized routers that exist between the IP connection from one office to another.

48.7.1. How Does a VPN Work?
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When a packet is transmitted from a client, it sends it through the VPN router or gateway, which adds an Authentication Header (AH) for routing and authentication. The data is then encrypted and, finally, enclosed with an Encapsulating Security Payload (ESP). This latter constitutes the decryption and handling instructions.
The receiving VPN router strips the header information, decrypts the data, and routes it to its intended destination (either a workstation or other node on a network). Using a network-to-network connection, the receiving node on the local network receives the packets already decrypted and ready for processing. The encryption/decryption process in a network-to-network VPN connection is transparent to a local node.
With such a heightened level of security, an attacker must not only intercept a packet, but decrypt the packet as well. Intruders who employ a man-in-the-middle attack between a server and client must also have access to at least one of the private keys for authenticating sessions. Because they employ several layers of authentication and encryption, VPNs are a secure and effective means of connecting multiple remote nodes to act as a unified intranet.

48.7.2. VPNs and Red Hat Enterprise Linux
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Red Hat Enterprise Linux provides various options in terms of implementing a software solution to securely connect to a WAN. Internet Protocol Security (IPsec) is the supported VPN implementation for Red Hat Enterprise Linux, and sufficiently addresses the usability needs of organizations with branch offices or remote users.

48.7.3. IPsec
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Red Hat Enterprise Linux supports IPsec for connecting remote hosts and networks to each other using a secure tunnel on a common carrier network such as the Internet. IPsec can be implemented using a host-to-host (one computer workstation to another) or network-to-network (one LAN/WAN to another) configuration.
The IPsec implementation in Red Hat Enterprise Linux uses Internet Key Exchange (IKE), a protocol implemented by the Internet Engineering Task Force (IETF), used for mutual authentication and secure associations between connecting systems.

48.7.4. Creating an IPsec Connection
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An IPsec connection is split into two logical phases. In phase 1, an IPsec node initializes the connection with the remote node or network. The remote node or network checks the requesting node's credentials and both parties negotiate the authentication method for the connection.
On Red Hat Enterprise Linux systems, an IPsec connection uses the pre-shared key method of IPsec node authentication. In a pre-shared key IPsec connection, both hosts must use the same key in order to move to Phase 2 of the IPsec connection.
Phase 2 of the IPsec connection is where the Security Association (SA) is created between IPsec nodes. This phase establishes an SA database with configuration information, such as the encryption method, secret session key exchange parameters, and more. This phase manages the actual IPsec connection between remote nodes and networks.
The Red Hat Enterprise Linux implementation of IPsec uses IKE for sharing keys between hosts across the Internet. The racoon keying daemon handles the IKE key distribution and exchange. Refer to the racoon man page for more information about this daemon.

48.7.5. IPsec Installation
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Implementing IPsec requires that the ipsec-tools RPM package be installed on all IPsec hosts (if using a host-to-host configuration) or routers (if using a network-to-network configuration). The RPM package contains essential libraries, daemons, and configuration files for setting up the IPsec connection, including:
  • /sbin/setkey — manipulates the key management and security attributes of IPsec in the kernel. This executable is controlled by the racoon key management daemon. Refer to the setkey(8) man page for more information.
  • /usr/sbin/racoon — the IKE key management daemon, used to manage and control security associations and key sharing between IPsec-connected systems.
  • /etc/racoon/racoon.conf — the racoon daemon configuration file used to configure various aspects of the IPsec connection, including authentication methods and encryption algorithms used in the connection. Refer to the racoon.conf(5) man page for a complete listing of available directives.
To configure IPsec on Red Hat Enterprise Linux, you can use the Network Administration Tool, or manually edit the networking and IPsec configuration files.

48.7.6. IPsec Host-to-Host Configuration
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IPsec can be configured to connect one desktop or workstation (host) to another using a host-to-host connection. This type of connection uses the network to which each host is connected to create a secure tunnel between each host. The requirements of a host-to-host connection are minimal, as is the configuration of IPsec on each host. The hosts need only a dedicated connection to a carrier network (such as the Internet) and Red Hat Enterprise Linux to create the IPsec connection.
48.7.6.1. Host-to-Host Connection
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A host-to-host IPsec connection is an encrypted connection between two systems, both running IPsec with the same authentication key. With the IPsec connection active, any network traffic between the two hosts is encrypted.
To configure a host-to-host IPsec connection, use the following steps for each host:

Note

You should perform the following procedures on the actual machine that you are configuring. Avoid attempting to configure and establish IPsec connections remotely.
  1. In a command shell, type system-config-network to start the Network Administration Tool.
  2. On the IPsec tab, click New to start the IPsec configuration wizard.
  3. Click Forward to start configuring a host-to-host IPsec connection.
  4. Enter a unique name for the connection, for example, ipsec0. If required, select the check box to automatically activate the connection when the computer starts. Click Forward to continue.
  5. Select Host to Host encryption as the connection type, and then click Forward.
  6. Select the type of encryption to use: manual or automatic.
    If you select manual encryption, an encryption key must be provided later in the process. If you select automatic encryption, the racoon daemon manages the encryption key. The ipsec-tools package must be installed if you want to use automatic encryption.
    Click Forward to continue.
  7. Enter the IP address of the remote host.
    To determine the IP address of the remote host, use the following command on the remote host: 
    ifconfig <device>
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    where <device> is the Ethernet device that you want to use for the VPN connection.
    If only one Ethernet card exists in the system, the device name is typically eth0. The following example shows the relevant information from this command (note that this is an example output only): 
    eth0      Link encap:Ethernet  HWaddr 00:0C:6E:E8:98:1D
          inet addr:172.16.44.192  Bcast:172.16.45.255  Mask:255.255.254.0
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    The IP address is the number following the inet addr: label.

    Note

    For host-to-host connections, both hosts should have a public, routable address. Alternatively, both hosts can have a private, non-routable address (for example, from the 10.x.x.x or 192.168.x.x ranges) as long as they are on the sam LAN.
    If the hosts are on different LANs, or one has a public address while the other has a private address, refer to Section 48.7.7, “IPsec Network-to-Network Configuration”.
    Click Forward to continue.
  8. If manual encryption was selected in step 6, specify the encryption key to use, or click Generate to create one.
    1. Specify an authentication key or click Generate to generate one. It can be any combination of numbers and letters.
    2. Click Forward to continue.
  9. Verify the information on the IPsec — Summary page, and then click Apply.
  10. Click File > Save to save the configuration.
    You may need to restart the network for the changes to take effect. To restart the network, use the following command: 
    service network restart
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  11. Select the IPsec connection from the list and click the Activate button.
  12. Repeat the entire procedure for the other host. It is essential that the same keys from step 8 be used on the other hosts. Otherwise, IPsec will not work.
After configuring the IPsec connection, it appears in the IPsec list as shown in Figure 48.10, “IPsec Connection”.
IPsec Connection
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IPsec Connection

Figure 48.10. IPsec Connection

The following files are created when the IPsec connection is configured:
  • /etc/sysconfig/network-scripts/ifcfg-<nickname>
  • /etc/sysconfig/network-scripts/keys-<nickname>
  • /etc/racoon/<remote-ip>.conf
  • /etc/racoon/psk.txt
If automatic encryption is selected, /etc/racoon/racoon.conf is also created.
When the interface is up, /etc/racoon/racoon.conf is modified to include <remote-ip>.conf.
48.7.6.2. Manual IPsec Host-to-Host Configuration
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The first step in creating a connection is to gather system and network information from each workstation. For a host-to-host connection, you need the following:
  • The IP address of each host
  • A unique name, for example, ipsec1. This is used to identify the IPsec connection and to distinguish it from other devices or connections.
  • A fixed encryption key or one automatically generated by racoon.
  • A pre-shared authentication key that is used during the initial stage of the connection and to exchange encryption keys during the session.
For example, suppose Workstation A and Workstation B want to connect to each other through an IPsec tunnel. They want to connect using a pre-shared key with the value of Key_Value01, and the users agree to let racoon automatically generate and share an authentication key between each host. Both host users decide to name their connections ipsec1.

Note

You should choose a PSK that uses a mixture of upper- and lower-case characters, numbers and punctuation. An easily-guessable PSK constitutes a security risk.
It is not necessary to use the same connection name for each host. You should choose a name that is convenient and meaningful for your installation.
The following is the IPsec configuration file for Workstation A for a host-to-host IPsec connection with Workstation B. The unique name to identify the connection in this example is ipsec1, so the resulting file is called /etc/sysconfig/network-scripts/ifcfg-ipsec1. 
DST=X.X.X.X
TYPE=IPSEC
ONBOOT=no
IKE_METHOD=PSK
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For Workstation A, X.X.X.X is the IP address of Workstation B. For Workstation B, X.X.X.X is the IP address of Workstation A. This connection is not set to initiate on boot-up (ONBOOT=no) and it uses the pre-shared key method of authentication (IKE_METHOD=PSK).
The following is the content of the pre-shared key file (called /etc/sysconfig/network-scripts/keys-ipsec1) that both workstations need to authenticate each other. The contents of this file should be identical on both workstations, and only the root user should be able to read or write this file. 
IKE_PSK=Key_Value01
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Important

To change the keys-ipsec1 file so that only the root user can read or edit the file, use the following command after creating the file: 
chmod 600 /etc/sysconfig/network-scripts/keys-ipsec1
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To change the authentication key at any time, edit the keys-ipsec1 file on both workstations. Both authentication keys must be identical for proper connectivity.
The next example shows the specific configuration for the phase 1 connection to the remote host. The file is called X.X.X.X.conf, where X.X.X.X is the IP address of the remote IPsec host. Note that this file is automatically generated when the IPsec tunnel is activated and should not be edited directly. 
remote X.X.X.X
{
         exchange_mode aggressive, main;
	 my_identifier address;
	 proposal {
	 	encryption_algorithm 3des;
		hash_algorithm sha1;
		authentication_method pre_shared_key;
		dh_group 2 ;
	}
}
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The default phase 1 configuration file that is created when an IPsec connection is initialized contains the following statements used by the Red Hat Enterprise Linux implementation of IPsec:
remote X.X.X.X
Specifies that the subsequent stanzas of this configuration file apply only to the remote node identified by the X.X.X.X IP address.
exchange_mode aggressive
The default configuration for IPsec on Red Hat Enterprise Linux uses an aggressive authentication mode, which lowers the connection overhead while allowing configuration of several IPsec connections with multiple hosts.
my_identifier address
Specifies the identification method to use when authenticating nodes. Red Hat Enterprise Linux uses IP addresses to identify nodes.
encryption_algorithm 3des
Specifies the encryption cipher used during authentication. By default, Triple Data Encryption Standard (3DES) is used.
hash_algorithm sha1;
Specifies the hash algorithm used during phase 1 negotiation between nodes. By default, Secure Hash Algorithm version 1 is used.
authentication_method pre_shared_key
Specifies the authentication method used during node negotiation. By default, Red Hat Enterprise Linux uses pre-shared keys for authentication.
dh_group 2
Specifies the Diffie-Hellman group number for establishing dynamically-generated session keys. By default, modp1024 (group 2) is used.
48.7.6.2.1. The Racoon Configuration File
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The /etc/racoon/racoon.conf files should be identical on all IPsec nodes except for the include "/etc/racoon/X.X.X.X.conf" statement. This statement (and the file it references) is generated when the IPsec tunnel is activated. For Workstation A, the X.X.X.X in the include statement is Workstation B's IP address. The opposite is true of Workstation B. The following shows a typical racoon.conf file when the IPsec connection is activated. 
# Racoon IKE daemon configuration file.
# See 'man racoon.conf' for a description of the format and entries.

path include "/etc/racoon";
path pre_shared_key "/etc/racoon/psk.txt";
path certificate "/etc/racoon/certs";

sainfo anonymous
{
        pfs_group 2;
        lifetime time 1 hour ;
        encryption_algorithm 3des, blowfish 448, rijndael ;
        authentication_algorithm hmac_sha1, hmac_md5 ;
        compression_algorithm deflate ;
}
include "/etc/racoon/X.X.X.X.conf";
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This default racoon.conf file includes defined paths for IPsec configuration, pre-shared key files, and certificates. The fields in sainfo anonymous describe the phase 2 SA between the IPsec nodes — the nature of the IPsec connection (including the supported encryption algorithms used) and the method of exchanging keys. The following list defines the fields of phase 2:
sainfo anonymous
Denotes that SA can anonymously initialize with any peer provided that the IPsec credentials match.
pfs_group 2
Defines the Diffie-Hellman key exchange protocol, which determines the method by which the IPsec nodes establish a mutual temporary session key for the second phase of IPsec connectivity. By default, the Red Hat Enterprise Linux implementation of IPsec uses group 2 (or modp1024) of the Diffie-Hellman cryptographic key exchange groups. Group 2 uses a 1024-bit modular exponentiation that prevents attackers from decrypting previous IPsec transmissions even if a private key is compromised.
lifetime time 1 hour
This parameter specifies the lifetime of an SA and can be quantified either by time or by bytes of data. The default Red Hat Enterprise Linux implementation of IPsec specifies a one hour lifetime.
encryption_algorithm 3des, blowfish 448, rijndael
Specifies the supported encryption ciphers for phase 2. Red Hat Enterprise Linux supports 3DES, 448-bit Blowfish, and Rijndael (the cipher used in the Advanced Encryption Standard, or AES).
authentication_algorithm hmac_sha1, hmac_md5
Lists the supported hash algorithms for authentication. Supported modes are sha1 and md5 hashed message authentication codes (HMAC).
compression_algorithm deflate
Defines the Deflate compression algorithm for IP Payload Compression (IPCOMP) support, which allows for potentially faster transmission of IP datagrams over slow connections.
To start the connection, use the following command on each host: 
ifup <nickname>
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where <nickname> is the name you specified for the IPsec connection.
To test the IPsec connection, run the tcpdump utility to view the network packets being transferred between the hosts and verify that they are encrypted via IPsec. The packet should include an AH header and should be shown as ESP packets. ESP means it is encrypted. For example: 
~]# tcpdump -n -i eth0 host <targetSystem>

IP 172.16.45.107 > 172.16.44.192: AH(spi=0x0954ccb6,seq=0xbb): ESP(spi=0x0c9f2164,seq=0xbb)
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48.7.7. IPsec Network-to-Network Configuration
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IPsec can also be configured to connect an entire network (such as a LAN or WAN) to a remote network using a network-to-network connection. A network-to-network connection requires the setup of IPsec routers on each side of the connecting networks to transparently process and route information from one node on a LAN to a node on a remote LAN. Figure 48.11, “A network-to-network IPsec tunneled connection” shows a network-to-network IPsec tunneled connection.
A network-to-network IPsec tunneled connection
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A network-to-network IPsec tunneled connection

Figure 48.11. A network-to-network IPsec tunneled connection

This diagram shows two separate LANs separated by the Internet. These LANs use IPsec routers to authenticate and initiate a connection using a secure tunnel through the Internet. Packets that are intercepted in transit would require brute-force decryption in order to crack the cipher protecting the packets between these LANs. The process of communicating from one node in the 192.168.1.0/24 IP range to another in the 192.168.2.0/24 range is completely transparent to the nodes as the processing, encryption/decryption, and routing of the IPsec packets are completely handled by the IPsec router.
The information needed for a network-to-network connection include:
  • The externally-accessible IP addresses of the dedicated IPsec routers
  • The network address ranges of the LAN/WAN served by the IPsec routers (such as 192.168.1.0/24 or 10.0.1.0/24)
  • The IP addresses of the gateway devices that route the data from the network nodes to the Internet
  • A unique name, for example, ipsec1. This is used to identify the IPsec connection and to distinguish it from other devices or connections.
  • A fixed encryption key or one automatically generated by racoon
  • A pre-shared authentication key that is used during the initial stage of the connection and to exchange encryption keys during the session.
48.7.7.1. Network-to-Network (VPN) Connection
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A network-to-network IPsec connection uses two IPsec routers, one for each network, through which the network traffic for the private subnets is routed.
For example, as shown in Figure 48.12, “Network-to-Network IPsec”, if the 192.168.1.0/24 private network sends network traffic to the 192.168.2.0/24 private network, the packets go through gateway0, to ipsec0, through the Internet, to ipsec1, to gateway1, and to the 192.168.2.0/24 subnet.
IPsec routers require publicly addressable IP addresses and a second Ethernet device connected to their respective private networks. Traffic only travels through an IPsec router if it is intended for another IPsec router with which it has an encrypted connection.
Network-to-Network IPsec
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Network-to-Network IPsec

Figure 48.12. Network-to-Network IPsec

Alternate network configuration options include a firewall between each IP router and the Internet, and an intranet firewall between each IPsec router and subnet gateway. The IPsec router and the gateway for the subnet can be one system with two Ethernet devices: one with a public IP address that acts as the IPsec router; and one with a private IP address that acts as the gateway for the private subnet. Each IPsec router can use the gateway for its private network or a public gateway to send the packets to the other IPsec router.
Use the following procedure to configure a network-to-network IPsec connection:
  1. In a command shell, type system-config-network to start the Network Administration Tool.
  2. On the IPsec tab, click New to start the IPsec configuration wizard.
  3. Click Forward to start configuring a network-to-network IPsec connection.
  4. Enter a unique nickname for the connection, for example, ipsec0. If required, select the check box to automatically activate the connection when the computer starts. Click Forward to continue.
  5. Select Network to Network encryption (VPN) as the connection type, and then click Forward.
  6. Select the type of encryption to use: manual or automatic.
    If you select manual encryption, an encryption key must be provided later in the process. If you select automatic encryption, the racoon daemon manages the encryption key. The ipsec-tools package must be installed if you want to use automatic encryption.
    Click Forward to continue.
  7. On the Local Network page, enter the following information:
    • Local Network Address — The IP address of the device on the IPsec router connected to the private network.
    • Local Subnet Mask — The subnet mask of the local network IP address.
    • Local Network Gateway — The gateway for the private subnet.
    Click Forward to continue.
    Local Network Information
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    Local Network Information

    Figure 48.13. Local Network Information

  8. On the Remote Network page, enter the following information:
    • Remote IP Address — The publicly addressable IP address of the IPsec router for the other private network. In our example, for ipsec0, enter the publicly addressable IP address of ipsec1, and vice versa.
    • Remote Network Address — The network address of the private subnet behind the other IPsec router. In our example, enter 192.168.1.0 if configuring ipsec1, and enter 192.168.2.0 if configuring ipsec0.
    • Remote Subnet Mask — The subnet mask of the remote IP address.
    • Remote Network Gateway — The IP address of the gateway for the remote network address.
    • If manual encryption was selected in step 6, specify the encryption key to use or click Generate to create one.
      Specify an authentication key or click Generate to generate one. This key can be any combination of numbers and letters.
    Click Forward to continue.
    Remote Network Information
    View larger image
    Remote Network Information

    Figure 48.14. Remote Network Information

  9. Verify the information on the IPsec — Summary page, and then click Apply.
  10. Select File > Save to save the configuration.
  11. Select the IPsec connection from the list, and then click Activate to activate the connection.
  12. Enable IP forwarding:
    1. Edit /etc/sysctl.conf and set net.ipv4.ip_forward to 1.
    2. Use the following command to enable the change: 
      sysctl -p /etc/sysctl.conf
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The network script to activate the IPsec connection automatically creates network routes to send packets through the IPsec router if necessary.
Suppose LAN A (lana.example.com) and LAN B (lanb.example.com) want to connect to each other through an IPsec tunnel. The network address for LAN A is in the 192.168.1.0/24 range, while LAN B uses the 192.168.2.0/24 range. The gateway IP address is 192.168.1.254 for LAN A and 192.168.2.254 for LAN B. The IPsec routers are separate from each LAN gateway and use two network devices: eth0 is assigned to an externally-accessible static IP address which accesses the Internet, while eth1 acts as a routing point to process and transmit LAN packets from one network node to the remote network nodes.
The IPsec connection between each network uses a pre-shared key with the value of r3dh4tl1nux, and the administrators of A and B agree to let racoon automatically generate and share an authentication key between each IPsec router. The administrator of LAN A decides to name the IPsec connection ipsec0, while the administrator of LAN B names the IPsec connection ipsec1.
The following example shows the contents of the ifcfg file for a network-to-network IPsec connection for LAN A. The unique name to identify the connection in this example is ipsec0, so the resulting file is called /etc/sysconfig/network-scripts/ifcfg-ipsec0. 
TYPE=IPSEC
ONBOOT=yes
IKE_METHOD=PSK
SRCGW=192.168.1.254
DSTGW=192.168.2.254
SRCNET=192.168.1.0/24
DSTNET=192.168.2.0/24
DST=X.X.X.X
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The following list describes the contents of this file:
TYPE=IPSEC
Specifies the type of connection.
ONBOOT=yes
Specifies that the connection should initiate on boot-up.
IKE_METHOD=PSK
Specifies that the connection uses the pre-shared key method of authentication.
SRCGW=192.168.1.254
The IP address of the source gateway. For LAN A, this is the LAN A gateway, and for LAN B, the LAN B gateway.
DSTGW=192.168.2.254
The IP address of the destination gateway. For LAN A, this is the LAN B gateway, and for LAN B, the LAN A gateway.
SRCNET=192.168.1.0/24
Specifies the source network for the IPsec connection, which in this example is the network range for LAN A.
DSTNET=192.168.2.0/24
Specifies the destination network for the IPsec connection, which in this example is the network range for LAN B.
DST=X.X.X.X
The externally-accessible IP address of LAN B.
The following example is the content of the pre-shared key file called /etc/sysconfig/network-scripts/keys-ipsecX (where X is 0 for LAN A and 1 for LAN B) that both networks use to authenticate each other. The contents of this file should be identical and only the root user should be able to read or write this file. 
IKE_PSK=r3dh4tl1nux
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Important

To change the keys-ipsecX file so that only the root user can read or edit the file, use the following command after creating the file: 
chmod 600 /etc/sysconfig/network-scripts/keys-ipsec1
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To change the authentication key at any time, edit the keys-ipsecX file on both IPsec routers. Both keys must be identical for proper connectivity.
The following example is the contents of the /etc/racoon/racoon.conf configuration file for the IPsec connection. Note that the include line at the bottom of the file is automatically generated and only appears if the IPsec tunnel is running. 
# Racoon IKE daemon configuration file.
# See 'man racoon.conf' for a description of the format and entries.
path include "/etc/racoon";
path pre_shared_key "/etc/racoon/psk.txt";
path certificate "/etc/racoon/certs";

sainfo anonymous
{
	pfs_group 2;
	lifetime time 1 hour ;
	encryption_algorithm 3des, blowfish 448, rijndael ;
	authentication_algorithm hmac_sha1, hmac_md5 ;
	compression_algorithm deflate ;
}
include "/etc/racoon/X.X.X.X.conf"
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The following is the specific configuration for the connection to the remote network. The file is called X.X.X.X.conf (where X.X.X.X is the IP address of the remote IPsec router). Note that this file is automatically generated when the IPsec tunnel is activated and should not be edited directly. 
remote X.X.X.X
{
        exchange_mode aggressive, main;
	my_identifier address;
	proposal {
		encryption_algorithm 3des;
		hash_algorithm sha1;
		authentication_method pre_shared_key;
		dh_group 2 ;
	}
}
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Prior to starting the IPsec connection, IP forwarding should be enabled in the kernel. To enable IP forwarding:
  1. Edit /etc/sysctl.conf and set net.ipv4.ip_forward to 1.
  2. Use the following command to enable the change: 
    sysctl -p /etc/sysctl.conf
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To start the IPsec connection, use the following command on each router: 
ifup ipsec0
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The connections are activated, and both LAN A and LAN B are able to communicate with each other. The routes are created automatically via the initialization script called by running ifup on the IPsec connection. To show a list of routes for the network, use the following command: 
ip route list
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To test the IPsec connection, run the tcpdump utility on the externally-routable device (eth0 in this example) to view the network packets being transferred between the hosts (or networks), and verify that they are encrypted via IPsec. For example, to check the IPsec connectivity of LAN A, use the following command: 
tcpdump -n -i eth0 host lana.example.com
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The packet should include an AH header and should be shown as ESP packets. ESP means it is encrypted. For example (back slashes denote a continuation of one line): 
12:24:26.155529 lanb.example.com > lana.example.com: AH(spi=0x021c9834,seq=0x358): \
	lanb.example.com > lana.example.com: ESP(spi=0x00c887ad,seq=0x358) (DF) \
	(ipip-proto-4)
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48.7.8. Starting and Stopping an IPsec Connection
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If the IPsec connection was not configured to activate on boot, you can control it from the command line.
To start the connection, use the following command on each host for host-to-host IPsec, or each IPsec router for network-to-network IPsec: 
ifup <nickname>
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where <nickname> is the nickname configured earlier, such as ipsec0.
To stop the connection, use the following command: 
ifdown <nickname>
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48.8. Firewalls
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Information security is commonly thought of as a process and not a product. However, standard security implementations usually employ some form of dedicated mechanism to control access privileges and restrict network resources to users who are authorized, identifiable, and traceable. Red Hat Enterprise Linux includes several tools to assist administrators and security engineers with network-level access control issues.
Firewalls are one of the core components of a network security implementation. Several vendors market firewall solutions catering to all levels of the marketplace: from home users protecting one PC to data center solutions safeguarding vital enterprise information. Firewalls can be stand-alone hardware solutions, such as firewall appliances by Cisco, Nokia, and Sonicwall. Vendors such as Checkpoint, McAfee, and Symantec have also developed proprietary software firewall solutions for home and business markets.
Apart from the differences between hardware and software firewalls, there are also differences in the way firewalls function that separate one solution from another. Table 48.5, “Firewall Types” details three common types of firewalls and how they function:
Expand
Table 48.5. Firewall Types
Method Description Advantages Disadvantages
NAT Network Address Translation (NAT) places private IP subnetworks behind one or a small pool of public IP addresses, masquerading all requests to one source rather than several. The Linux kernel has built-in NAT functionality through the Netfilter kernel subsystem.
· Can be configured transparently to machines on a LAN
· Protection of many machines and services behind one or more external IP addresses simplifies administration duties
· Restriction of user access to and from the LAN can be configured by opening and closing ports on the NAT firewall/gateway
· Cannot prevent malicious activity once users connect to a service outside of the firewall
Packet Filter A packet filtering firewall reads each data packet that passes through a LAN. It can read and process packets by header information and filters the packet based on sets of programmable rules implemented by the firewall administrator. The Linux kernel has built-in packet filtering functionality through the Netfilter kernel subsystem.
· Customizable through the iptables front-end utility
· Does not require any customization on the client side, as all network activity is filtered at the router level rather than the application level
· Since packets are not transmitted through a proxy, network performance is faster due to direct connection from client to remote host
· Cannot filter packets for content like proxy firewalls
· Processes packets at the protocol layer, but cannot filter packets at an application layer
· Complex network architectures can make establishing packet filtering rules difficult, especially if coupled with IP masquerading or local subnets and DMZ networks
Proxy Proxy firewalls filter all requests of a certain protocol or type from LAN clients to a proxy machine, which then makes those requests to the Internet on behalf of the local client. A proxy machine acts as a buffer between malicious remote users and the internal network client machines.
· Gives administrators control over what applications and protocols function outside of the LAN
· Some proxy servers can cache frequently-accessed data locally rather than having to use the Internet connection to request it. This helps to reduce bandwidth consumption
· Proxy services can be logged and monitored closely, allowing tighter control over resource utilization on the network
· Proxies are often application-specific (HTTP, Telnet, etc.), or protocol-restricted (most proxies work with TCP-connected services only)
· Application services cannot run behind a proxy, so your application servers must use a separate form of network security
· Proxies can become a network bottleneck, as all requests and transmissions are passed through one source rather than directly from a client to a remote service

48.8.1. Netfilter and IPTables
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The Linux kernel features a powerful networking subsystem called Netfilter. The Netfilter subsystem provides stateful or stateless packet filtering as well as NAT and IP masquerading services. Netfilter also has the ability to mangle IP header information for advanced routing and connection state management. Netfilter is controlled using the iptables tool.
48.8.1.1. IPTables Overview
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The power and flexibility of Netfilter is implemented using the iptables administration tool, a command line tool similar in syntax to its predecessor, ipchains.
A similar syntax does not mean similar implementation, however. ipchains requires intricate rule sets for: filtering source paths; filtering destination paths; and filtering both source and destination connection ports.
By contrast, iptables uses the Netfilter subsystem to enhance network connection, inspection, and processing. iptables features advanced logging, pre- and post-routing actions, network address translation, and port forwarding, all in one command line interface.
This section provides an overview of iptables. For more detailed information, refer to Section 48.9, “IPTables”.

48.8.2. Basic Firewall Configuration
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Just as a firewall in a building attempts to prevent a fire from spreading, a computer firewall attempts to prevent malicious software from spreading to your computer. It also helps to prevent unauthorized users from accessing your computer.
In a default Red Hat Enterprise Linux installation, a firewall exists between your computer or network and any untrusted networks, for example the Internet. It determines which services on your computer remote users can access. A properly configured firewall can greatly increase the security of your system. It is recommended that you configure a firewall for any Red Hat Enterprise Linux system with an Internet connection.
48.8.2.1. Security Level Configuration Tool
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During the Firewall Configuration screen of the Red Hat Enterprise Linux installation, you were given the option to enable a basic firewall as well as to allow specific devices, incoming services, and ports.
After installation, you can change this preference by using the Security Level Configuration Tool.
To start this application, use the following command: 
system-config-securitylevel
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Security Level Configuration Tool
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Security Level Configuration Tool

Figure 48.15. Security Level Configuration Tool

Note

The Security Level Configuration Tool only configures a basic firewall. If the system needs more complex rules, refer to Section 48.9, “IPTables” for details on configuring specific iptables rules.
48.8.2.2. Enabling and Disabling the Firewall
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Select one of the following options for the firewall:
  • Disabled — Disabling the firewall provides complete access to your system and does no security checking. This should only be selected if you are running on a trusted network (not the Internet) or need to configure a custom firewall using the iptables command line tool.

    Warning

    Firewall configurations and any customized firewall rules are stored in the /etc/sysconfig/iptables file. If you choose Disabled and click OK, these configurations and firewall rules will be lost.
  • Enabled — This option configures the system to reject incoming connections that are not in response to outbound requests, such as DNS replies or DHCP requests. If access to services running on this machine is needed, you can choose to allow specific services through the firewall.
    If you are connecting your system to the Internet, but do not plan to run a server, this is the safest choice.
48.8.2.3. Trusted Services
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Enabling options in the Trusted services list allows the specified service to pass through the firewall.
WWW (HTTP)
The HTTP protocol is used by Apache (and by other Web servers) to serve web pages. If you plan on making your Web server publicly available, select this check box. This option is not required for viewing pages locally or for developing web pages. This service requires that the httpd package be installed.
Enabling WWW (HTTP) will not open a port for HTTPS, the SSL version of HTTP. If this service is required, select the Secure WWW (HTTPS) check box.
FTP
The FTP protocol is used to transfer files between machines on a network. If you plan on making your FTP server publicly available, select this check box. This service requires that the vsftpd package be installed.
SSH
Secure Shell (SSH) is a suite of tools for logging into and executing commands on a remote machine. To allow remote access to the machine via ssh, select this check box. This service requires that the openssh-server package be installed.
Telnet
Telnet is a protocol for logging into remote machines. Telnet communications are unencrypted and provide no security from network snooping. Allowing incoming Telnet access is not recommended. To allow remote access to the machine via telnet, select this check box. This service requires that the telnet-server package be installed.
Mail (SMTP)
SMTP is a protocol that allows remote hosts to connect directly to your machine to deliver mail. You do not need to enable this service if you collect your mail from your ISP's server using POP3 or IMAP, or if you use a tool such as fetchmail. To allow delivery of mail to your machine, select this check box. Note that an improperly configured SMTP server can allow remote machines to use your server to send spam.
NFS4
The Network File System (NFS) is a file sharing protocol commonly used on *NIX systems. Version 4 of this protocol is more secure than its predecessors. If you want to share files or directories on your system with other network users, select this check box.
Samba
Samba is an implementation of Microsoft's proprietary SMB networking protocol. If you need to share files, directories, or locally-connected printers with Microsoft Windows machines, select this check box.
48.8.2.4. Other Ports
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The Security Level Configuration Tool includes an Other ports section for specifying custom IP ports as being trusted by iptables. For example, to allow IRC and Internet printing protocol (IPP) to pass through the firewall, add the following to the Other ports section:
194:tcp,631:tcp
48.8.2.5. Saving the Settings
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Click OK to save the changes and enable or disable the firewall. If Enable firewall was selected, the options selected are translated to iptables commands and written to the /etc/sysconfig/iptables file. The iptables service is also started so that the firewall is activated immediately after saving the selected options. If Disable firewall was selected, the /etc/sysconfig/iptables file is removed and the iptables service is stopped immediately.
The selected options are also written to the /etc/sysconfig/system-config-securitylevel file so that the settings can be restored the next time the application is started. Do not edit this file by hand.
Even though the firewall is activated immediately, the iptables service is not configured to start automatically at boot time. Refer to Section 48.8.2.6, “Activating the IPTables Service” for more information.
48.8.2.6. Activating the IPTables Service
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The firewall rules are only active if the iptables service is running. To manually start the service, use the following command: 
service iptables restart
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To ensure that iptables starts when the system is booted, use the following command: 
chkconfig --level 345 iptables on
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The ipchains service is not included in Red Hat Enterprise Linux. However, if ipchains is installed (for example, an upgrade was performed and the system had ipchains previously installed), the ipchains and iptables services should not be activated simultaneously. To make sure the ipchains service is disabled and configured not to start at boot time, use the following two commands: 
service ipchains stop
chkconfig --level 345 ipchains off
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48.8.3. Using IPTables
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The first step in using iptables is to start the iptables service. Use the following command to start the iptables service: 
service iptables start
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Note

The ip6tables service can be turned off if you intend to use the iptables service only. If you deactivate the ip6tables service, remember to deactivate the IPv6 network also. Never leave a network device active without the matching firewall.
To force iptables to start by default when the system is booted, use the following command: 
chkconfig --level 345 iptables on
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This forces iptables to start whenever the system is booted into runlevel 3, 4, or 5.
48.8.3.1. IPTables Command Syntax
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The following sample iptables command illustrates the basic command syntax: 
iptables -A <chain> -j <target>
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The -A option specifies that the rule be appended to <chain>. Each chain is comprised of one or more rules, and is therefore also known as a ruleset.
The three built-in chains are INPUT, OUTPUT, and FORWARD. These chains are permanent and cannot be deleted. The chain specifies the point at which a packet is manipulated.
The -j <target> option specifies the target of the rule; i.e., what to do if the packet matches the rule. Examples of built-in targets are ACCEPT, DROP, and REJECT.
Refer to the iptables man page for more information on the available chains, options, and targets.
48.8.3.2. Basic Firewall Policies
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Establishing basic firewall policies creates a foundation for building more detailed, user-defined rules.
Each iptables chain is comprised of a default policy, and zero or more rules which work in concert with the default policy to define the overall ruleset for the firewall.
The default policy for a chain can be either DROP or ACCEPT. Security-minded administrators typically implement a default policy of DROP, and only allow specific packets on a case-by-case basis. For example, the following policies block all incoming and outgoing packets on a network gateway: 
iptables -P INPUT DROP
iptables -P OUTPUT DROP
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It is also recommended that any forwarded packets — network traffic that is to be routed from the firewall to its destination node — be denied as well, to restrict internal clients from inadvertent exposure to the Internet. To do this, use the following rule: 
iptables -P FORWARD DROP
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When you have established the default policies for each chain, you can create and save further rules for your particular network and security requirements.
The following sections describe how to save iptables rules and outline some of the rules you might implement in the course of building your iptables firewall.
48.8.3.3. Saving and Restoring IPTables Rules
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Changes to iptables are transitory; if the system is rebooted or if the iptables service is restarted, the rules are automatically flushed and reset. To save the rules so that they are loaded when the iptables service is started, use the following command: 
service iptables save
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The rules are stored in the file /etc/sysconfig/iptables and are applied whenever the service is started or the machine is rebooted.

48.8.4. Common IPTables Filtering
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Preventing remote attackers from accessing a LAN is one of the most important aspects of network security. The integrity of a LAN should be protected from malicious remote users through the use of stringent firewall rules.
However, with a default policy set to block all incoming, outgoing, and forwarded packets, it is impossible for the firewall/gateway and internal LAN users to communicate with each other or with external resources.
To allow users to perform network-related functions and to use networking applications, administrators must open certain ports for communication.
For example, to allow access to port 80 on the firewall, append the following rule: 
iptables -A INPUT -p tcp -m tcp --dport 80 -j ACCEPT
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This allows users to browse websites that communicate using the standard port 80. To allow access to secure websites (for example, https://www.example.com/), you also need to provide access to port 443, as follows: 
iptables -A INPUT -p tcp -m tcp --dport 443 -j ACCEPT
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Important

When creating an iptables ruleset, order is important.
If a rule specifies that any packets from the 192.168.100.0/24 subnet be dropped, and this is followed by a rule that allows packets from 192.168.100.13 (which is within the dropped subnet), then the second rule is ignored.
The rule to allow packets from 192.168.100.13 must precede the rule that drops the remainder of the subnet.
To insert a rule in a specific location in an existing chain, use the -I option. For example: 
iptables -I INPUT 1 -i lo -p all -j ACCEPT
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This rule is inserted as the first rule in the INPUT chain to allow local loopback device traffic.
There may be times when you require remote access to the LAN. Secure services, for example SSH, can be used for encrypted remote connection to LAN services.
Administrators with PPP-based resources (such as modem banks or bulk ISP accounts), dial-up access can be used to securely circumvent firewall barriers. Because they are direct connections, modem connections are typically behind a firewall/gateway.
For remote users with broadband connections, however, special cases can be made. You can configure iptables to accept connections from remote SSH clients. For example, the following rules allow remote SSH access: 
iptables -A INPUT -p tcp --dport 22 -j ACCEPT
iptables -A OUTPUT -p tcp --sport 22 -j ACCEPT
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These rules allow incoming and outbound access for an individual system, such as a single PC directly connected to the Internet or a firewall/gateway. However, they do not allow nodes behind the firewall/gateway to access these services. To allow LAN access to these services, you can use Network Address Translation (NAT) with iptables filtering rules.

48.8.5. FORWARD and NAT Rules
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Most ISPs provide only a limited number of publicly routable IP addresses to the organizations they serve.
Administrators must, therefore, find alternative ways to share access to Internet services without giving public IP addresses to every node on the LAN. Using private IP addresses is the most common way of allowing all nodes on a LAN to properly access internal and external network services.
Edge routers (such as firewalls) can receive incoming transmissions from the Internet and route the packets to the intended LAN node. At the same time, firewalls/gateways can also route outgoing requests from a LAN node to the remote Internet service.
This forwarding of network traffic can become dangerous at times, especially with the availability of modern cracking tools that can spoof internal IP addresses and make the remote attacker's machine act as a node on your LAN.
To prevent this, iptables provides routing and forwarding policies that can be implemented to prevent abnormal usage of network resources.
The FORWARD chain allows an administrator to control where packets can be routed within a LAN. For example, to allow forwarding for the entire LAN (assuming the firewall/gateway is assigned an internal IP address on eth1), use the following rules: 
iptables -A FORWARD -i eth1 -j ACCEPT
iptables -A FORWARD -o eth1 -j ACCEPT
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This rule gives systems behind the firewall/gateway access to the internal network. The gateway routes packets from one LAN node to its intended destination node, passing all packets through its eth1 device.

Note

By default, the IPv4 policy in Red Hat Enterprise Linux kernels disables support for IP forwarding. This prevents machines that run Red Hat Enterprise Linux from functioning as dedicated edge routers. To enable IP forwarding, use the following command: 
sysctl -w net.ipv4.ip_forward=1
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This configuration change is only valid for the current session; it does not persist beyond a reboot or network service restart. To permanently set IP forwarding, edit the /etc/sysctl.conf file as follows:
Locate the following line: 
net.ipv4.ip_forward = 0
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Edit it to read as follows: 
net.ipv4.ip_forward = 1
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Use the following command to enable the change to the sysctl.conf file: 
sysctl -p /etc/sysctl.conf
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48.8.5.1. Postrouting and IP Masquerading
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Accepting forwarded packets via the firewall's internal IP device allows LAN nodes to communicate with each other; however they still cannot communicate externally to the Internet.
To allow LAN nodes with private IP addresses to communicate with external public networks, configure the firewall for IP masquerading, which masks requests from LAN nodes with the IP address of the firewall's external device (in this case, eth0): 
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
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This rule uses the NAT packet matching table (-t nat) and specifies the built-in POSTROUTING chain for NAT (-A POSTROUTING) on the firewall's external networking device (-o eth0).
POSTROUTING allows packets to be altered as they are leaving the firewall's external device.
The -j MASQUERADE target is specified to mask the private IP address of a node with the external IP address of the firewall/gateway.
48.8.5.2. Prerouting
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If you have a server on your internal network that you want make available externally, you can use the -j DNAT target of the PREROUTING chain in NAT to specify a destination IP address and port where incoming packets requesting a connection to your internal service can be forwarded.
For example, if you want to forward incoming HTTP requests to your dedicated Apache HTTP Server at 172.31.0.23, use the following command: 
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT --to 172.31.0.23:80
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This rule specifies that the nat table use the built-in PREROUTING chain to forward incoming HTTP requests exclusively to the listed destination IP address of 172.31.0.23.

Note

If you have a default policy of DROP in your FORWARD chain, you must append a rule to forward all incoming HTTP requests so that destination NAT routing is possible. To do this, use the following command: 
iptables -A FORWARD -i eth0 -p tcp --dport 80 -d 172.31.0.23 -j ACCEPT
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This rule forwards all incoming HTTP requests from the firewall to the intended destination; the Apache HTTP Server behind the firewall.
48.8.5.3. DMZs and IPTables
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You can create iptables rules to route traffic to certain machines, such as a dedicated HTTP or FTP server, in a demilitarized zone (DMZ). A DMZ is a special local subnetwork dedicated to providing services on a public carrier, such as the Internet.
For example, to set a rule for routing incoming HTTP requests to a dedicated HTTP server at 10.0.4.2 (outside of the 192.168.1.0/24 range of the LAN), NAT uses the PREROUTING table to forward the packets to the appropriate destination: 
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT --to-destination 10.0.4.2:80
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With this command, all HTTP connections to port 80 from outside of the LAN are routed to the HTTP server on a network separate from the rest of the internal network. This form of network segmentation can prove safer than allowing HTTP connections to a machine on the network.
If the HTTP server is configured to accept secure connections, then port 443 must be forwarded as well.
More elaborate rules can be created that control access to specific subnets, or even specific nodes, within a LAN. You can also restrict certain dubious applications or programs such as Trojans, worms, and other client/server viruses from contacting their server.
For example, some Trojans scan networks for services on ports from 31337 to 31340 (called the elite ports in cracking terminology).
Since there are no legitimate services that communicate via these non-standard ports, blocking them can effectively diminish the chances that potentially infected nodes on your network independently communicate with their remote master servers.
The following rules drop all TCP traffic that attempts to use port 31337: 
iptables -A OUTPUT -o eth0 -p tcp --dport 31337 --sport 31337 -j DROP
iptables -A FORWARD -o eth0 -p tcp --dport 31337 --sport 31337 -j DROP
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You can also block outside connections that attempt to spoof private IP address ranges to infiltrate your LAN.
For example, if your LAN uses the 192.168.1.0/24 range, you can design a rule that instructs the Internet-facing network device (for example, eth0) to drop any packets to that device with an address in your LAN IP range.
Because it is recommended to reject forwarded packets as a default policy, any other spoofed IP address to the external-facing device (eth0) is rejected automatically. 
iptables -A FORWARD -s 192.168.1.0/24 -i eth0 -j DROP
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Note

There is a distinction between the DROP and REJECT targets when dealing with appended rules.
The REJECT target denies access and returns a connection refused error to users who attempt to connect to the service. The DROP target, as the name implies, drops the packet without any warning.
Administrators can use their own discretion when using these targets. However, to avoid user confusion and attempts to continue connecting, the REJECT target is recommended.

48.8.7. IPTables and Connection Tracking
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You can inspect and restrict connections to services based on their connection state. A module within iptables uses a method called connection tracking to store information about incoming connections. You can allow or deny access based on the following connection states:
  • NEW — A packet requesting a new connection, such as an HTTP request.
  • ESTABLISHED — A packet that is part of an existing connection.
  • RELATED — A packet that is requesting a new connection but is part of an existing connection. For example, FTP uses port 21 to establish a connection, but data is transferred on a different port (typically port 20).
  • INVALID — A packet that is not part of any connections in the connection tracking table.
You can use the stateful functionality of iptables connection tracking with any network protocol, even if the protocol itself is stateless (such as UDP). The following example shows a rule that uses connection tracking to forward only the packets that are associated with an established connection: 
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
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48.8.8. IPv6
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The introduction of the next-generation Internet Protocol, called IPv6, expands beyond the 32-bit address limit of IPv4 (or IP). IPv6 supports 128-bit addresses, and carrier networks that are IPv6 aware are therefore able to address a larger number of routable addresses than IPv4.
Red Hat Enterprise Linux supports IPv6 firewall rules using the Netfilter 6 subsystem and the ip6tables command. In Red Hat Enterprise Linux 5, both IPv4 and IPv6 services are enabled by default.
The ip6tables command syntax is identical to iptables in every aspect except that it supports 128-bit addresses. For example, use the following command to enable SSH connections on an IPv6-aware network server: 
ip6tables -A INPUT -i eth0 -p tcp -s 3ffe:ffff:100::1/128 --dport 22 -j ACCEPT
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For more information about IPv6 networking, refer to the IPv6 Information Page at http://www.ipv6.org/.

48.8.9. Additional Resources
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There are several aspects to firewalls and the Linux Netfilter subsystem that could not be covered in this chapter. For more information, refer to the following resources.
48.8.9.1. Installed Documentation
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  • Refer to Section 48.9, “IPTables” for more detailed information on the iptables command, including definitions for many command options.
  • The iptables man page contains a brief summary of the various options.
48.8.9.2. Useful Websites
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48.8.9.3. Related Documentation
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  • Red Hat Linux Firewalls, by Bill McCarty; Red Hat Press — a comprehensive reference to building network and server firewalls using open source packet filtering technology such as Netfilter and iptables. It includes topics that cover analyzing firewall logs, developing firewall rules, and customizing your firewall using various graphical tools.
  • Linux Firewalls, by Robert Ziegler; New Riders Press — contains a wealth of information on building firewalls using both 2.2 kernel ipchains as well as Netfilter and iptables. Additional security topics such as remote access issues and intrusion detection systems are also covered.

48.9. IPTables
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Included with Red Hat Enterprise Linux are advanced tools for network packet filtering — the process of controlling network packets as they enter, move through, and exit the network stack within the kernel. Kernel versions prior to 2.4 relied on ipchains for packet filtering and used lists of rules applied to packets at each step of the filtering process. The 2.4 kernel introduced iptables (also called netfilter), which is similar to ipchains but greatly expands the scope and control available for filtering network packets.
This chapter focuses on packet filtering basics, defines the differences between ipchains and iptables, explains various options available with iptables commands, and explains how filtering rules can be preserved between system reboots.
Refer to Section 48.9.7, “Additional Resources” for instructions on how to construct iptables rules and setting up a firewall based on these rules.

Warning

The default firewall mechanism in the 2.4 and later kernels is iptables, but iptables cannot be used if ipchains is already running. If ipchains is present at boot time, the kernel issues an error and fails to start iptables.
The functionality of ipchains is not affected by these errors.

48.9.1. Packet Filtering
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The Linux kernel uses the Netfilter facility to filter packets, allowing some of them to be received by or pass through the system while stopping others. This facility is built in to the Linux kernel, and has three built-in tables or rules lists, as follows:
  • filter — The default table for handling network packets.
  • nat — Used to alter packets that create a new connection and used for Network Address Translation (NAT).
  • mangle — Used for specific types of packet alteration.
Each table has a group of built-in chains, which correspond to the actions performed on the packet by netfilter.
The built-in chains for the filter table are as follows:
  • INPUT — Applies to network packets that are targeted for the host.
  • OUTPUT — Applies to locally-generated network packets.
  • FORWARD — Applies to network packets routed through the host.
The built-in chains for the nat table are as follows:
  • PREROUTING — Alters network packets when they arrive.
  • OUTPUT — Alters locally-generated network packets before they are sent out.
  • POSTROUTING — Alters network packets before they are sent out.
The built-in chains for the mangle table are as follows:
  • INPUT — Alters network packets targeted for the host.
  • OUTPUT — Alters locally-generated network packets before they are sent out.
  • FORWARD — Alters network packets routed through the host.
  • PREROUTING — Alters incoming network packets before they are routed.
  • POSTROUTING — Alters network packets before they are sent out.
Every network packet received by or sent from a Linux system is subject to at least one table. However, a packet may be subjected to multiple rules within each table before emerging at the end of the chain. The structure and purpose of these rules may vary, but they usually seek to identify a packet coming from or going to a particular IP address, or set of addresses, when using a particular protocol and network service.

Note

By default, firewall rules are saved in the /etc/sysconfig/iptables or /etc/sysconfig/ip6tables files.
The iptables service starts before any DNS-related services when a Linux system is booted. This means that firewall rules can only reference numeric IP addresses (for example, 192.168.0.1). Domain names (for example, host.example.com) in such rules produce errors.
Regardless of their destination, when packets match a particular rule in one of the tables, a target or action is applied to them. If the rule specifies an ACCEPT target for a matching packet, the packet skips the rest of the rule checks and is allowed to continue to its destination. If a rule specifies a DROP target, that packet is refused access to the system and nothing is sent back to the host that sent the packet. If a rule specifies a QUEUE target, the packet is passed to user-space. If a rule specifies the optional REJECT target, the packet is dropped, but an error packet is sent to the packet's originator.
Every chain has a default policy to ACCEPT, DROP, REJECT, or QUEUE. If none of the rules in the chain apply to the packet, then the packet is dealt with in accordance with the default policy.
The iptables command configures these tables, as well as sets up new tables if necessary.

48.9.2. Differences Between IPTables and IPChains
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Both ipchains and iptables use chains of rules that operate within the Linux kernel to filter packets based on matches with specified rules or rule sets. However, iptables offers a more extensible way of filtering packets, giving the administrator greater control without building undue complexity into the system.
You should be aware of the following significant differences between ipchains and iptables:
Using iptables, each filtered packet is processed using rules from only one chain rather than multiple chains.
For example, a FORWARD packet coming into a system using ipchains would have to go through the INPUT, FORWARD, and OUTPUT chains to continue to its destination. However, iptables only sends packets to the INPUT chain if they are destined for the local system, and only sends them to the OUTPUT chain if the local system generated the packets. It is therefore important to place the rule designed to catch a particular packet within the chain that actually handles the packet.
The DENY target has been changed to DROP.
In ipchains, packets that matched a rule in a chain could be directed to the DENY target. This target must be changed to DROP in iptables.
Order matters when placing options in a rule.
In ipchains, the order of the rule options does not matter.
The iptables command has a stricter syntax. The iptables command requires that the protocol (ICMP, TCP, or UDP) be specified before the source or destination ports.
Network interfaces must be associated with the correct chains in firewall rules.
For example, incoming interfaces (-i option) can only be used in INPUT or FORWARD chains. Similarly, outgoing interfaces (-o option) can only be used in FORWARD or OUTPUT chains.
In other words, INPUT chains and incoming interfaces work together; OUTPUT chains and outgoing interfaces work together. FORWARD chains work with both incoming and outgoing interfaces.
OUTPUT chains are no longer used by incoming interfaces, and INPUT chains are not seen by packets moving through outgoing interfaces.
This is not a comprehensive list of the changes. Refer to Section 48.9.7, “Additional Resources” for more specific information.

48.9.3. Command Options for IPTables
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Rules for filtering packets are created using the iptables command. The following aspects of the packet are most often used as criteria:
  • Packet Type — Specifies the type of packets the command filters.
  • Packet Source/Destination — Specifies which packets the command filters based on the source or destination of the packet.
  • Target — Specifies what action is taken on packets matching the above criteria.
Refer to Section 48.9.3.4, “IPTables Match Options” and Section 48.9.3.5, “Target Options” for more information about specific options that address these aspects of a packet.
The options used with specific iptables rules must be grouped logically, based on the purpose and conditions of the overall rule, for the rule to be valid. The remainder of this section explains commonly-used options for the iptables command.
48.9.3.1. Structure of IPTables Command Options
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Many iptables commands have the following structure: 
iptables [-t <table-name>] <command> <chain-name> \
			<parameter-1> <option-1> \
			<parameter-n> <option-n>
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<table-name> — Specifies which table the rule applies to. If omitted, the filter table is used.
<command> — Specifies the action to perform, such as appending or deleting a rule.
<chain-name> — Specifies the chain to edit, create, or delete.
<parameter>-<option> pairs — Parameters and associated options that specify how to process a packet that matches the rule.
The length and complexity of an iptables command can change significantly, based on its purpose.
For example, a command to remove a rule from a chain can be very short:
iptables -D <chain-name> <line-number>
In contrast, a command that adds a rule which filters packets from a particular subnet using a variety of specific parameters and options can be rather long. When constructing iptables commands, it is important to remember that some parameters and options require further parameters and options to construct a valid rule. This can produce a cascading effect, with the further parameters requiring yet more parameters. Until every parameter and option that requires another set of options is satisfied, the rule is not valid.
Type iptables -h to view a comprehensive list of iptables command structures.
48.9.3.2. Command Options
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Command options instruct iptables to perform a specific action. Only one command option is allowed per iptables command. With the exception of the help command, all commands are written in upper-case characters.
The iptables commands are as follows:
  • -A — Appends the rule to the end of the specified chain. Unlike the -I option described below, it does not take an integer argument. It always appends the rule to the end of the specified chain.
  • -C — Checks a particular rule before adding it to the user-specified chain. This command can help you construct complex iptables rules by prompting you for additional parameters and options.
  • -D <integer> | <rule> — Deletes a rule in a particular chain by number (such as 5 for the fifth rule in a chain), or by rule specification. The rule specification must exactly match an existing rule.
  • -E — Renames a user-defined chain. A user-defined chain is any chain other than the default, pre-existing chains. (Refer to the -N option, below, for information on creating user-defined chains.) This is a cosmetic change and does not affect the structure of the table.

    Note

    If you attempt to rename one of the default chains, the system reports a Match not found error. You cannot rename the default chains.
  • -F — Flushes the selected chain, which effectively deletes every rule in the chain. If no chain is specified, this command flushes every rule from every chain.
  • -h — Provides a list of command structures, as well as a quick summary of command parameters and options.
  • -I [<integer>] — Inserts the rule in the specified chain at a point specified by a user-defined integer argument. If no argument is specified, the rule is inserted at the top of the chain.

    Warning

    As noted above, the order of rules in a chain determines which rules apply to which packets. This is important to remember when adding rules using either the -A or -I option.
    This is especially important when adding rules using the -I with an integer argument. If you specify an existing number when adding a rule to a chain, iptables adds the new rule before (or above) the existing rule.
  • -L — Lists all of the rules in the chain specified after the command. To list all rules in all chains in the default filter table, do not specify a chain or table. Otherwise, the following syntax should be used to list the rules in a specific chain in a particular table: 
    iptables -L <chain-name> -t <table-name>
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    Additional options for the -L command option, which provide rule numbers and allow more verbose rule descriptions, are described in Section 48.9.3.6, “Listing Options”.
  • -N — Creates a new chain with a user-specified name. The chain name must be unique, otherwise an error message is displayed.
  • -P — Sets the default policy for the specified chain, so that when packets traverse an entire chain without matching a rule, they are sent to the specified target, such as ACCEPT or DROP.
  • -R — Replaces a rule in the specified chain. The rule's number must be specified after the chain's name. The first rule in a chain corresponds to rule number one.
  • -X — Deletes a user-specified chain. You cannot delete a built-in chain.
  • -Z — Sets the byte and packet counters in all chains for a table to zero.
48.9.3.3. IPTables Parameter Options
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Certain iptables commands, including those used to add, append, delete, insert, or replace rules within a particular chain, require various parameters to construct a packet filtering rule.
  • -c — Resets the counters for a particular rule. This parameter accepts the PKTS and BYTES options to specify which counter to reset.
  • -d — Sets the destination hostname, IP address, or network of a packet that matches the rule. When matching a network, the following IP address/netmask formats are supported:
    • N.N.N.N/M.M.M.M — Where N.N.N.N is the IP address range and M.M.M.M is the netmask.
    • N.N.N.N/M — Where N.N.N.N is the IP address range and M is the bitmask.
  • -f — Applies this rule only to fragmented packets.
    You can use the exclamation point character (!) option after this parameter to specify that only unfragmented packets are matched.

    Note

    Distinguishing between fragmented and unfragmented packets is desirable, despite fragmented packets being a standard part of the IP protocol.
    Originally designed to allow IP packets to travel over networks with differing frame sizes, these days fragmentation is more commonly used to generate DoS attacks using mal-formed packets. It's also worth noting that IPv6 disallows fragmentation entirely.
  • -i — Sets the incoming network interface, such as eth0 or ppp0. With iptables, this optional parameter may only be used with the INPUT and FORWARD chains when used with the filter table and the PREROUTING chain with the nat and mangle tables.
    This parameter also supports the following special options:
    • Exclamation point character (!) — Reverses the directive, meaning any specified interfaces are excluded from this rule.
    • Plus character (+) — A wildcard character used to match all interfaces that match the specified string. For example, the parameter -i eth+ would apply this rule to any Ethernet interfaces but exclude any other interfaces, such as ppp0.
    If the -i parameter is used but no interface is specified, then every interface is affected by the rule.
  • -j — Jumps to the specified target when a packet matches a particular rule.
    The standard targets are ACCEPT, DROP, QUEUE, and RETURN.
    Extended options are also available through modules loaded by default with the Red Hat Enterprise Linux iptables RPM package. Valid targets in these modules include LOG, MARK, and REJECT, among others. Refer to the iptables man page for more information about these and other targets.
    This option can also be used to direct a packet matching a particular rule to a user-defined chain outside of the current chain so that other rules can be applied to the packet.
    If no target is specified, the packet moves past the rule with no action taken. The counter for this rule, however, increases by one.
  • -o — Sets the outgoing network interface for a rule. This option is only valid for the OUTPUT and FORWARD chains in the filter table, and the POSTROUTING chain in the nat and mangle tables. This parameter accepts the same options as the incoming network interface parameter (-i).
  • -p <protocol> — Sets the IP protocol affected by the rule. This can be either icmp, tcp, udp, or all, or it can be a numeric value, representing one of these or a different protocol. You can also use any protocols listed in the /etc/protocols file.
    The "all" protocol means the rule applies to every supported protocol. If no protocol is listed with this rule, it defaults to "all".
  • -s — Sets the source for a particular packet using the same syntax as the destination (-d) parameter.
48.9.3.4. IPTables Match Options
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Different network protocols provide specialized matching options which can be configured to match a particular packet using that protocol. However, the protocol must first be specified in the iptables command. For example, -p <protocol-name> enables options for the specified protocol. Note that you can also use the protocol ID, instead of the protocol name. Refer to the following examples, each of which have the same effect: 
iptables -A INPUT -p icmp --icmp-type any -j ACCEPT
iptables -A INPUT -p 5813 --icmp-type any -j ACCEPT
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Service definitions are provided in the /etc/services file. For readability, it is recommended that you use the service names rather than the port numbers.

Important

Secure the /etc/services file to prevent unauthorized editing. If this file is editable, crackers can use it to enable ports on your machine you have otherwise closed. To secure this file, type the following commands as root: 
chown root.root /etc/services
chmod 0644 /etc/services
chattr +i /etc/services
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This prevents the file from being renamed, deleted or having links made to it.
48.9.3.4.1. TCP Protocol
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These match options are available for the TCP protocol (-p tcp):
  • --dport — Sets the destination port for the packet.
    To configure this option, use a network service name (such as www or smtp); a port number; or a range of port numbers.
    To specify a range of port numbers, separate the two numbers with a colon (:). For example: -p tcp --dport 3000:3200. The largest acceptable valid range is 0:65535.
    Use an exclamation point character (!) after the --dport option to match all packets that do not use that network service or port.
    To browse the names and aliases of network services and the port numbers they use, view the /etc/services file.
    The --destination-port match option is synonymous with --dport.
  • --sport — Sets the source port of the packet using the same options as --dport. The --source-port match option is synonymous with --sport.
  • --syn — Applies to all TCP packets designed to initiate communication, commonly called SYN packets. Any packets that carry a data payload are not touched.
    Use an exclamation point character (!) after the --syn option to match all non-SYN packets.
  • --tcp-flags <tested flag list> <set flag list> — Allows TCP packets that have specific bits (flags) set, to match a rule.
    The --tcp-flags match option accepts two parameters. The first parameter is the mask; a comma-separated list of flags to be examined in the packet. The second parameter is a comma-separated list of flags that must be set for the rule to match.
    The possible flags are:
    • ACK
    • FIN
    • PSH
    • RST
    • SYN
    • URG
    • ALL
    • NONE
    For example, an iptables rule that contains the following specification only matches TCP packets that have the SYN flag set and the ACK and FIN flags not set:
    --tcp-flags ACK,FIN,SYN SYN
    Use the exclamation point character (!) after the --tcp-flags to reverse the effect of the match option.
  • --tcp-option — Attempts to match with TCP-specific options that can be set within a particular packet. This match option can also be reversed with the exclamation point character (!).
48.9.3.4.2. UDP Protocol
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These match options are available for the UDP protocol (-p udp):
  • --dport — Specifies the destination port of the UDP packet, using the service name, port number, or range of port numbers. The --destination-port match option is synonymous with --dport.
  • --sport — Specifies the source port of the UDP packet, using the service name, port number, or range of port numbers. The --source-port match option is synonymous with --sport.
For the --dport and --sport options, to specify a range of port numbers, separate the two numbers with a colon (:). For example: -p tcp --dport 3000:3200. The largest acceptable valid range is 0:65535.
48.9.3.4.3. ICMP Protocol
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The following match options are available for the Internet Control Message Protocol (ICMP) (-p icmp):
  • --icmp-type — Sets the name or number of the ICMP type to match with the rule. A list of valid ICMP names can be retrieved by typing the iptables -p icmp -h command.
48.9.3.4.4. Additional Match Option Modules
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Additional match options are available through modules loaded by the iptables command.
To use a match option module, load the module by name using the -m <module-name>, where <module-name> is the name of the module.
Many modules are available by default. You can also create modules to provide additional functionality.
The following is a partial list of the most commonly used modules:
  • limit module — Places limits on how many packets are matched to a particular rule.
    When used in conjunction with the LOG target, the limit module can prevent a flood of matching packets from filling up the system log with repetitive messages or using up system resources.
    Refer to Section 48.9.3.5, “Target Options” for more information about the LOG target.
    The limit module enables the following options:
    • --limit — Sets the maximum number of matches for a particular time period, specified as a <value>/<period> pair. For example, using --limit 5/hour allows five rule matches per hour.
      Periods can be specified in seconds, minutes, hours, or days.
      If a number and time modifier are not used, the default value of 3/hour is assumed.
    • --limit-burst — Sets a limit on the number of packets able to match a rule at one time.
      This option is specified as an integer and should be used in conjunction with the --limit option.
      If no value is specified, the default value of five (5) is assumed.
  • state module — Enables state matching.
    The state module enables the following options:
    • --state — match a packet with the following connection states:
      • ESTABLISHED — The matching packet is associated with other packets in an established connection. You need to accept this state if you want to maintain a connection between a client and a server.
      • INVALID — The matching packet cannot be tied to a known connection.
      • NEW — The matching packet is either creating a new connection or is part of a two-way connection not previously seen. You need to accept this state if you want to allow new connections to a service.
      • RELATED — The matching packet is starting a new connection related in some way to an existing connection. An example of this is FTP, which uses one connection for control traffic (port 21), and a separate connection for data transfer (port 20).
      These connection states can be used in combination with one another by separating them with commas, such as -m state --state INVALID,NEW.
  • mac module — Enables hardware MAC address matching.
    The mac module enables the following option:
    • --mac-source — Matches a MAC address of the network interface card that sent the packet. To exclude a MAC address from a rule, place an exclamation point character (!) after the --mac-source match option.
Refer to the iptables man page for more match options available through modules.
48.9.3.5. Target Options
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When a packet has matched a particular rule, the rule can direct the packet to a number of different targets which determine the appropriate action. Each chain has a default target, which is used if none of the rules on that chain match a packet or if none of the rules which match the packet specify a target.
The following are the standard targets:
  • <user-defined-chain> — A user-defined chain within the table. User-defined chain names must be unique. This target passes the packet to the specified chain.
  • ACCEPT — Allows the packet through to its destination or to another chain.
  • DROP — Drops the packet without responding to the requester. The system that sent the packet is not notified of the failure.
  • QUEUE — The packet is queued for handling by a user-space application.
  • RETURN — Stops checking the packet against rules in the current chain. If the packet with a RETURN target matches a rule in a chain called from another chain, the packet is returned to the first chain to resume rule checking where it left off. If the RETURN rule is used on a built-in chain and the packet cannot move up to its previous chain, the default target for the current chain is used.
In addition, extensions are available which allow other targets to be specified. These extensions are called target modules or match option modules and most only apply to specific tables and situations. Refer to Section 48.9.3.4.4, “Additional Match Option Modules” for more information about match option modules.
Many extended target modules exist, most of which only apply to specific tables or situations. Some of the most popular target modules included by default in Red Hat Enterprise Linux are:
  • LOG — Logs all packets that match this rule. Because the packets are logged by the kernel, the /etc/syslog.conf file determines where these log entries are written. By default, they are placed in the /var/log/messages file.
    Additional options can be used after the LOG target to specify the way in which logging occurs:
    • --log-level — Sets the priority level of a logging event. Refer to the syslog.conf man page for a list of priority levels.
    • --log-ip-options — Logs any options set in the header of an IP packet.
    • --log-prefix — Places a string of up to 29 characters before the log line when it is written. This is useful for writing syslog filters for use in conjunction with packet logging.

      Note

      Due to an issue with this option, you should add a trailing space to the log-prefix value.
    • --log-tcp-options — Logs any options set in the header of a TCP packet.
    • --log-tcp-sequence — Writes the TCP sequence number for the packet in the log.
  • REJECT — Sends an error packet back to the remote system and drops the packet.
    The REJECT target accepts --reject-with <type> (where <type> is the rejection type) allowing more detailed information to be returned with the error packet. The message port-unreachable is the default error type given if no other option is used. Refer to the iptables man page for a full list of <type> options.
Other target extensions, including several that are useful for IP masquerading using the nat table, or with packet alteration using the mangle table, can be found in the iptables man page.
48.9.3.6. Listing Options
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The default list command, iptables -L [<chain-name>], provides a very basic overview of the default filter table's current chains. Additional options provide more information:
  • -v — Displays verbose output, such as the number of packets and bytes each chain has processed, the number of packets and bytes each rule has matched, and which interfaces apply to a particular rule.
  • -x — Expands numbers into their exact values. On a busy system, the number of packets and bytes processed by a particular chain or rule may be abbreviated to Kilobytes, Megabytes (Megabytes) or Gigabytes. This option forces the full number to be displayed.
  • -n — Displays IP addresses and port numbers in numeric format, rather than the default hostname and network service format.
  • --line-numbers — Lists rules in each chain next to their numeric order in the chain. This option is useful when attempting to delete the specific rule in a chain or to locate where to insert a rule within a chain.
  • -t <table-name> — Specifies a table name. If omitted, defaults to the filter table.
The following examples illustrate the use of several of these options. Note the difference in the byte display by including the -x option. 
~]# iptables -L OUTPUT -v -n -x
Chain OUTPUT (policy ACCEPT 64005 packets, 6445791 bytes)
    pkts      bytes target     prot opt in     out     source               destination
    1593   133812 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0

~]# iptables -L OUTPUT -v -n
Chain OUTPUT (policy ACCEPT 64783 packets, 6492K bytes)
    pkts bytes target     prot opt in     out     source               destination
    1819  153K ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0
~]#
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48.9.4. Saving IPTables Rules
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Rules created with the iptables command are stored in memory. If the system is restarted before saving the iptables rule set, all rules are lost. For netfilter rules to persist through a system reboot, they need to be saved. To save netfilter rules, type the following command as root: 
service iptables save
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This executes the iptables init script, which runs the /sbin/iptables-save program and writes the current iptables configuration to /etc/sysconfig/iptables. The existing /etc/sysconfig/iptables file is saved as /etc/sysconfig/iptables.save.
The next time the system boots, the iptables init script reapplies the rules saved in /etc/sysconfig/iptables by using the /sbin/iptables-restore command.
While it is always a good idea to test a new iptables rule before committing it to the /etc/sysconfig/iptables file, it is possible to copy iptables rules into this file from another system's version of this file. This provides a quick way to distribute sets of iptables rules to multiple machines.
You can also save the iptables rules to a separate file for distribution, backup or other purposes. To save your iptables rules, type the following command as root: 
iptables-save > <filename>
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where <filename> is a user-defined name for your ruleset.

Important

If distributing the /etc/sysconfig/iptables file to other machines, type /sbin/service iptables restart for the new rules to take effect.

Note

Note the difference between the iptables command (/sbin/iptables), which is used to manipulate the tables and chains that constitute the iptables functionality, and the iptables service (/sbin/iptables service), which is used to enable and disable the iptables service itself.

48.9.5. IPTables Control Scripts
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There are two basic methods for controlling iptables in Red Hat Enterprise Linux:
  • Security Level Configuration Tool (system-config-securitylevel) — A graphical interface for creating, activating, and saving basic firewall rules. Refer to Section 48.8.2, “Basic Firewall Configuration” for more information.
  • /sbin/service iptables <option> — Used to manipulate various functions of iptables using its initscript. The following options are available:
    • start — If a firewall is configured (that is, /etc/sysconfig/iptables exists), all running iptables are stopped completely and then started using the /sbin/iptables-restore command. This option only works if the ipchains kernel module is not loaded. To check if this module is loaded, type the following command as root: 
      lsmod | grep ipchains
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      If this command returns no output, it means the module is not loaded. If necessary, use the /sbin/rmmod command to remove the module.
    • stop — If a firewall is running, the firewall rules in memory are flushed, and all iptables modules and helpers are unloaded.
      If the IPTABLES_SAVE_ON_STOP directive in the /etc/sysconfig/iptables-config configuration file is changed from its default value to yes, current rules are saved to /etc/sysconfig/iptables and any existing rules are moved to the file /etc/sysconfig/iptables.save.
      Refer to Section 48.9.5.1, “IPTables Control Scripts Configuration File” for more information about the iptables-config file.
    • restart — If a firewall is running, the firewall rules in memory are flushed, and the firewall is started again if it is configured in /etc/sysconfig/iptables. This option only works if the ipchains kernel module is not loaded.
      If the IPTABLES_SAVE_ON_RESTART directive in the /etc/sysconfig/iptables-config configuration file is changed from its default value to yes, current rules are saved to /etc/sysconfig/iptables and any existing rules are moved to the file /etc/sysconfig/iptables.save.
      Refer to Section 48.9.5.1, “IPTables Control Scripts Configuration File” for more information about the iptables-config file.
    • status — Displays the status of the firewall and lists all active rules.
      The default configuration for this option displays IP addresses in each rule. To display domain and hostname information, edit the /etc/sysconfig/iptables-config file and change the value of IPTABLES_STATUS_NUMERIC to no. Refer to Section 48.9.5.1, “IPTables Control Scripts Configuration File” for more information about the iptables-config file.
    • panic — Flushes all firewall rules. The policy of all configured tables is set to DROP.
      This option could be useful if a server is known to be compromised. Rather than physically disconnecting from the network or shutting down the system, you can use this option to stop all further network traffic but leave the machine in a state ready for analysis or other forensics.
    • save — Saves firewall rules to /etc/sysconfig/iptables using iptables-save. Refer to Section 48.9.4, “Saving IPTables Rules” for more information.

Note

To use the same initscript commands to control netfilter for IPv6, substitute ip6tables for iptables in the /sbin/service commands listed in this section. For more information about IPv6 and netfilter, refer to Section 48.9.6, “IPTables and IPv6”.
The behavior of the iptables initscripts is controlled by the /etc/sysconfig/iptables-config configuration file. The following is a list of directives contained in this file:
  • IPTABLES_MODULES — Specifies a space-separated list of additional iptables modules to load when a firewall is activated. These can include connection tracking and NAT helpers.
  • IPTABLES_MODULES_UNLOAD — Unloads modules on restart and stop. This directive accepts the following values:
    • yes — The default value. This option must be set to achieve a correct state for a firewall restart or stop.
    • no — This option should only be set if there are problems unloading the netfilter modules.
  • IPTABLES_SAVE_ON_STOP — Saves current firewall rules to /etc/sysconfig/iptables when the firewall is stopped. This directive accepts the following values:
    • yes — Saves existing rules to /etc/sysconfig/iptables when the firewall is stopped, moving the previous version to the /etc/sysconfig/iptables.save file.
    • no — The default value. Does not save existing rules when the firewall is stopped.
  • IPTABLES_SAVE_ON_RESTART — Saves current firewall rules when the firewall is restarted. This directive accepts the following values:
    • yes — Saves existing rules to /etc/sysconfig/iptables when the firewall is restarted, moving the previous version to the /etc/sysconfig/iptables.save file.
    • no — The default value. Does not save existing rules when the firewall is restarted.
  • IPTABLES_SAVE_COUNTER — Saves and restores all packet and byte counters in all chains and rules. This directive accepts the following values:
    • yes — Saves the counter values.
    • no — The default value. Does not save the counter values.
  • IPTABLES_STATUS_NUMERIC — Outputs IP addresses in numeric form instead of domain or hostnames. This directive accepts the following values:
    • yes — The default value. Returns only IP addresses within a status output.
    • no — Returns domain or hostnames within a status output.

48.9.6. IPTables and IPv6
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If the iptables-ipv6 package is installed, netfilter in Red Hat Enterprise Linux can filter the next-generation IPv6 Internet protocol. The command used to manipulate the IPv6 netfilter is ip6tables.
Most directives for this command are identical to those used for iptables, except the nat table is not yet supported. This means that it is not yet possible to perform IPv6 network address translation tasks, such as masquerading and port forwarding.
Rules for ip6tables are saved in the /etc/sysconfig/ip6tables file. Previous rules saved by the ip6tables initscripts are saved in the /etc/sysconfig/ip6tables.save file.
Configuration options for the ip6tables init script are stored in /etc/sysconfig/ip6tables-config, and the names for each directive vary slightly from their iptables counterparts.
For example, the iptables-config directive IPTABLES_MODULES:the equivalent in the ip6tables-config file is IP6TABLES_MODULES.

48.9.7. Additional Resources
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Refer to the following sources for additional information on packet filtering with iptables.
  • Section 48.8, “Firewalls” — Contains a chapter about the role of firewalls within an overall security strategy as well as strategies for constructing firewall rules.
48.9.7.1. Installed Documentation
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  • man iptables — Contains a description of iptables as well as a comprehensive list of targets, options, and match extensions.
48.9.7.2. Useful Websites
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  • http://www.netfilter.org/ — The home of the netfilter/iptables project. Contains assorted information about iptables, including a FAQ addressing specific problems and various helpful guides by Rusty Russell, the Linux IP firewall maintainer. The HOWTO documents on the site cover subjects such as basic networking concepts, kernel packet filtering, and NAT configurations.
  • http://www.linuxnewbie.org/nhf/Security/IPtables_Basics.html — An introduction to the way packets move through the Linux kernel, plus an introduction to constructing basic iptables commands.

[14] Since system BIOSes differ between manufacturers, some may not support password protection of either type, while others may support one type but not the other.
[15] GRUB also accepts unencrypted passwords, but it is recommended that an MD5 hash be used for added security.
[16] This access is still subject to the restrictions imposed by SELinux, if it is enabled.
[17] A system where both the client and the server share a common key that is used to encrypt and decrypt network communication.

Chapter 49. Security and SELinux
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49.1. Access Control Mechanisms (ACMs)
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This section provides a basic introduction to Access Control Mechanisms (ACMs). ACMs provide a means for system administrators to control which users and processes can access different files, devices, interfaces, etc., in a computer system. This is a primary consideration when securing a computer system or network of any size.

49.1.1. Discretionary Access Control (DAC)
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Discretionary Access Control (DAC) defines the basic access controls for objects in a filesystem. This is the typical access control provided by file permissions, sharing, etc. Such access is generally at the discretion of the owner of the object (file, directory, device, etc.).
DAC provides a means of restricting access to objects based on the identity of the users or groups (subjects) that try to access those objects. Depending on a subject's access permissions, they may also be able to pass permissions to other subjects.

49.1.2. Access Control Lists (ACLs)
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Access Control Lists (ACLs) provide further control over which objects a subject can access. For more information, refer to Chapter 10, Access Control Lists.

49.1.3. Mandatory Access Control (MAC)
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Mandatory Access Control (MAC) is a security mechanism that restricts the level of control that users (subjects) have over the objects that they create. Unlike in a DAC implementation, where users have full control over their own files, directories, etc., MAC adds additional labels, or categories, to all file system objects. Users and processes must have the appropriate access to these categories before they can interact with these objects.
In Red Hat Enterprise Linux, MAC is enforced by SELinux. For more information, refer to Section 49.2, “Introduction to SELinux”.

49.1.4. Role-based Access Control (RBAC)
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Role-based Access Control (RBAC) is an alternative method of controlling user access to file system objects. Instead of access being controlled by user permissions, the system administrator establishes Roles based on business functional requirements or similar criteria. These Roles have different types and levels of access to objects.
In contrast to DAC or MAC systems, where users have access to objects based on their own and the object's permissions, users in an RBAC system must be members of the appropriate group, or Role, before they can interact with files, directories, devices, etc.
From an administrative point of view, this makes it easier to control who has access to various parts of the file system, just by controlling their group memberships.

49.1.5. Multi-Level Security (MLS)
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Multi-Level Security (MLS) is a specific Mandatory Access Control (MAC) security scheme. Under this scheme, processes are called Subjects. Files, sockets and other passive operating system entities are called Objects. For more information, refer to Section 49.6, “Multi-Level Security (MLS)”.

49.1.6. Multi-Category Security (MCS)
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Multi-Category Security (MCS) is an enhancement to SELinux, and allows users to label files with categories. MCS is an adaptation of MLS and re-uses much of the MLS framework in SELinux. For more information, refer to Section 49.4.1, “Introduction”

49.2. Introduction to SELinux
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Security-Enhanced Linux (SELinux) is a security architecture integrated into the 2.6.x kernel using the Linux Security Modules (LSM). It is a project of the United States National Security Agency (NSA) and the SELinux community. SELinux integration into Red Hat Enterprise Linux was a joint effort between the NSA and Red Hat.

49.2.1. SELinux Overview
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SELinux provides a flexible Mandatory Access Control (MAC) system built into the Linux kernel. Under standard Linux Discretionary Access Control (DAC), an application or process running as a user (UID or SUID) has the user's permissions to objects such as files, sockets, and other processes. Running a MAC kernel protects the system from malicious or flawed applications that can damage or destroy the system.
SELinux defines the access and transition rights of every user, application, process, and file on the system. SELinux then governs the interactions of these entities using a security policy that specifies how strict or lenient a given Red Hat Enterprise Linux installation should be.
On a day-to-day basis, system users will be largely unaware of SELinux. Only system administrators need to consider how strict a policy to implement for their server environment. The policy can be as strict or as lenient as needed, and is very finely detailed. This detail gives the SELinux kernel complete, granular control over the entire system.
The SELinux Decision Making Process

When a subject, (for example, an application), attempts to access an object (for example, a file), the policy enforcement server in the kernel checks an access vector cache (AVC), where subject and object permissions are cached. If a decision cannot be made based on data in the AVC, the request continues to the security server, which looks up the security context of the application and the file in a matrix. Permission is then granted or denied, with an avc: denied message detailed in /var/log/messages if permission is denied. The security context of subjects and objects is applied from the installed policy, which also provides the information to populate the security server's matrix.

Refer to the following diagram:
SELinux Decision Process
View larger image
SELinux Decision Process

Figure 49.1. SELinux Decision Process

SELinux Operating Modes

Instead of running in enforcing mode, SELinux can run in permissive mode, where the AVC is checked and denials are logged, but SELinux does not enforce the policy. This can be useful for troubleshooting and for developing or fine-tuning SELinux policy.

For more information about how SELinux works, refer to Section 49.2.3, “Additional Resources”.

49.2.2. Files Related to SELinux
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The following sections describe SELinux configuration files and related file systems.
49.2.2.1. The SELinux Pseudo-File System
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The /selinux/ pseudo-file system contains commands that are most commonly used by the kernel subsystem. This type of file system is similar to the /proc/ pseudo-file system.
Administrators and users do not normally need to manipulate this component.
The following example shows sample contents of the /selinux/ directory: 
-rw-rw-rw-  1 root root 0 Sep 22 13:14 access
dr-xr-xr-x  1 root root 0 Sep 22 13:14 booleans
--w-------  1 root root 0 Sep 22 13:14 commit_pending_bools
-rw-rw-rw-  1 root root 0 Sep 22 13:14 context
-rw-rw-rw-  1 root root 0 Sep 22 13:14 create
--w-------  1 root root 0 Sep 22 13:14 disable
-rw-r--r--  1 root root 0 Sep 22 13:14 enforce
-rw-------  1 root root 0 Sep 22 13:14 load
-r--r--r--  1 root root 0 Sep 22 13:14 mls
-r--r--r--  1 root root 0 Sep 22 13:14 policyvers
-rw-rw-rw-  1 root root 0 Sep 22 13:14 relabel
-rw-rw-rw-  1 root root 0 Sep 22 13:14 user
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For example, running the cat command on the enforce file reveals either a 1 for enforcing mode or 0 for permissive mode.
49.2.2.2. SELinux Configuration Files
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The following sections describe SELinux configuration and policy files, and related file systems located in the /etc/ directory.
There are two ways to configure SELinux under Red Hat Enterprise Linux: using the SELinux Administration Tool (system-config-selinux), or manually editing the configuration file (/etc/sysconfig/selinux).
The /etc/sysconfig/selinux file is the primary configuration file for enabling or disabling SELinux, as well as for setting which policy to enforce on the system and how to enforce it.

Note

The /etc/sysconfig/selinux contains a symbolic link to the actual configuration file, /etc/selinux/config.
The following explains the full subset of options available for configuration:
  • SELINUX=enforcing|permissive|disabled — Defines the top-level state of SELinux on a system.
    • enforcing — The SELinux security policy is enforced.
    • permissive — The SELinux system prints warnings but does not enforce policy.
      This is useful for debugging and troubleshooting purposes. In permissive mode, more denials are logged because subjects can continue with actions that would otherwise be denied in enforcing mode. For example, traversing a directory tree in permissive mode produces avc: denied messages for every directory level read. In enforcing mode, SELinux would have stopped the initial traversal and kept further denial messages from occurring.
    • disabled — SELinux is fully disabled. SELinux hooks are disengaged from the kernel and the pseudo-file system is unregistered.

      Note

      Actions made while SELinux is disabled may result in the file system no longer having the correct security context. That is, the security context defined by the policy. The best way to relabel the file system is to create the flag file /.autorelabel and reboot the machine. This causes the relabel to occur very early in the boot process, before any processes are running on the system. Using this procedure means that processes can not accidentally create files in the wrong context or start up in the wrong context.
      It is possible to use the fixfiles relabel command prior to enabling SELinux to relabel the file system. This method is not recommended, however, because after it is complete, it is still possible to have processes potentially running on the system in the wrong context. These processes could create files that would also be in the wrong context.

    Note

    Additional white space at the end of a configuration line or as extra lines at the end of the file may cause unexpected behavior. To be safe, remove unnecessary white space.
  • SELINUXTYPE=targeted|strict — Specifies which policy SELinux should enforce.
    • targeted — Only targeted network daemons are protected.

      Important

      The following daemons are protected in the default targeted policy: dhcpd, httpd (apache.te), named, nscd, ntpd, portmap, snmpd, squid, and syslogd. The rest of the system runs in the unconfined_t domain. This domain allows subjects and objects with that security context to operate using standard Linux security.
      The policy files for these daemons are located in /etc/selinux/targeted/src/policy/domains/program. These files are subject to change as newer versions of Red Hat Enterprise Linux are released.
      Policy enforcement for these daemons can be turned on or off, using Boolean values controlled by the SELinux Administration Tool (system-config-selinux).
      Setting a Boolean value for a targeted daemon to 1 disables SELinux protection for the daemon. For example, you can set dhcpd_disable_trans to 1 to prevent init, which executes apps labeled dhcpd_exec_t, from transitioning to the dhcpd_t domain.
      Use the getsebool -a command to list all SELinux booleans. The following is an example of using the setsebool command to set an SELinux boolean. The -P option makes the change permanent. Without this option, the boolean would be reset to 1 at reboot. 
      setsebool -P dhcpd_disable_trans=0
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    • strict — Full SELinux protection, for all daemons. Security contexts are defined for all subjects and objects, and every action is processed by the policy enforcement server.
  • SETLOCALDEFS=0|1 — Controls how local definitions (users and booleans) are set. Set this value to 1 to have these definitions controlled by load_policy from files in /etc/selinux/<policyname>. or set it to 0 to have them controlled by semanage.

    Warning

    You should not change this value from the default (0) unless you are fully aware of the impact of such a change.
49.2.2.2.2. The /etc/selinux/ Directory
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The /etc/selinux/ directory is the primary location for all policy files as well as the main configuration file.
The following example shows sample contents of the /etc/selinux/ directory: 
-rw-r--r--  1 root root  448 Sep 22 17:34 config
drwxr-xr-x  5 root root 4096 Sep 22 17:27 strict
drwxr-xr-x  5 root root 4096 Sep 22 17:28 targeted
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The two subdirectories, strict/ and targeted/, are the specific directories where the policy files of the same name (that is, strict and targeted) are contained.
49.2.2.3. SELinux Utilities
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The following are some of the commonly used SELinux utilities:
  • /usr/sbin/setenforce — Modifies in real-time the mode in which SELinux runs.
    For example:
    setenforce 1 — SELinux runs in enforcing mode.
    setenforce 0 — SELinux runs in permissive mode.
    To actually disable SELinux, you need to either specify the appropriate setenforce parameter in /etc/sysconfig/selinux or pass the parameter selinux=0 to the kernel, either in /etc/grub.conf or at boot time.
  • /usr/sbin/sestatus -v — Displays the detailed status of a system running SELinux. The following example shows an excerpt of sestatus -v output: 
    SELinux status:                 enabled
SELinuxfs mount:                /selinux
Current mode:                   enforcing
Mode from config file:          enforcing
Policy version:                 21
Policy from config file:        targeted

Process contexts:
Current context:                user_u:system_r:unconfined_t:s0
Init context:                   system_u:system_r:init_t:s0
/sbin/mingetty                  system_u:system_r:getty_t:s0
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  • /usr/bin/newrole — Runs a new shell in a new context, or role. Policy must allow the transition to the new role.

    Note

    This command is only available if you have the policycoreutils-newrole package installed, which is required for the strict and MLS policies.
  • /sbin/restorecon — Sets the security context of one or more files by marking the extended attributes with the appropriate file or security context.
  • /sbin/fixfiles — Checks or corrects the security context database on the file system.
Refer to the man page associated with these utilities for more information.
Refer to the setools or policycoreutils package contents for more information on all available binary utilities. To view the contents of a package, use the following command:
rpm -ql <package-name>

49.2.3. Additional Resources
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Refer to the following resources for more detailed information on SELinux.
49.2.3.1. Installed Documentation
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  • /usr/share/doc/setools-<version-number>/ All documentation for utilities contained in the setools package. This includes all helper scripts, sample configuration files, and documentation.
49.2.3.2. Useful Websites
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49.3. Brief Background and History of SELinux
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SELinux was originally a development project from the National Security Agency (NSA )[18] and others. It is an implementation of the Flask operating system security architecture.[19]The NSA integrated SELinux into the Linux kernel using the Linux Security Modules (LSM ) framework. SELinux motivated the creation of LSM, at the suggestion of Linus Torvalds, who wanted a modular approach to security instead of just accepting SELinux into the kernel.
Originally, the SELinux implementation used persistent security IDs (PSIDs) stored in an unused field of the ext2 inode. These numerical representations (i.e., non-human-readable) were mapped by SELinux to a security context label. Unfortunately, this required modifying each file system type to support PSIDs, so was not a scalable solution or one that would be supported upstream in the Linux kernel.
The next evolution of SELinux was as a loadable kernel module for the 2.4.<x> series of Linux kernels. This module stored PSIDs in a normal file, and SELinux was able to support more file systems. This solution was not optimal for performance, and was inconsistent across platforms. Finally, the SELinux code was integrated upstream to the 2.6.x kernel, which has full support for LSM and has extended attributes (xattrs ) in the ext3 file system. SELinux was moved to using xattrs to store security context information. The xattr namespace provides useful separation for multiple security modules existing on the same system.
Much of the work to get the kernel ready for upstream, as well as subsequent SELinux development, has been a joint effort between the NSA, Red Hat, and the community of SELinux developers.
For more information about the history of SELinux, the definitive website is http://www.nsa.gov/research/selinux/index.shtml.

49.4. Multi-Category Security (MCS)
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49.4.1. Introduction
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Multi-Category Security (MCS) is an enhancement to SELinux, and allows users to label files with categories. These categories are used to further constrain Discretionary Access Control (DAC) and Type Enforcement (TE) logic. They may also be used when displaying or printing files. An example of a category is "Company_Confidential". Only users with access to this category can access files labeled with the category, assuming the existing DAC and TE rules also permit access.
The term categories refers to the same non-hierarchical categories used by Multi-Level Security (MLS). Under MLS, objects and subjects are labeled with Security Levels. These Security Levels consist of a hierarchical sensitivity value (such as "Top Secret") and zero or more non-hierarchical categories (such as "Crypto"). Categories provide compartments within sensitivity levels and enforce the need-to-know security principle. Refer to Section 49.6, “Multi-Level Security (MLS)” for more information about Multi-Level Security.
49.4.1.1. What is Multi-Category Security?
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MCS is an adaptation of MLS. From a technical point of view, MCS is a policy change, combined with a few userland modifications to hide some of the unneeded MLS technology. Some kernel changes were also made, but only relating to making it easy to upgrade to MCS (or MLS) without invoking a full file system relabel.
The hope is to improve the quality of the system as a whole, reduce costs, leverage the open source process, increase transparency, and make the technology base useful to more than a small group of extremely special-case users.

49.4.2. Applications for Multi-Category Security
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Beyond access control, MCS could be used to display the MCS categories at the top and bottom of printed pages. This may also include a cover sheet to indicate document handling procedures. It should also be possible to integrate MCS with future developments in SELinux, such as Security Enhanced X. Integration with a directory server will also make MCS support for email easier. This could involve users manually labeling outgoing emails or by attaching suitably labeled files. The email client can then determine whether the recipients are known to be cleared to access the categories associated with the emails.

49.4.3. SELinux Security Contexts
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SELinux stores security contexts as an extended attribute of a file. The "security." namespace is used for security modules, and the security.selinux name is used to persistently store SELinux security labels on files. The contents of this attribute will vary depending on the file or directory you inspect and the policy the machine is enforcing.

Note

This is expected to change in the 2.6.15 kernel (and already has in the latest -mm kernels), so that getxattr(2) always returns the kernel's canonicalized version of the label.
You can use the ls -Z command to view the category label of a file: 
~]# ls -Z gravityControl.txt
-rw-r--r--  user     user     user_u:object_r:tmp_t:Moonbase_Plans gravityControl.txt
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You can use the gefattr(1) command to view the internal category value (c10): 
~]# getfattr -n security.selinux gravityControl.txt
# file: gravityControl.txt
security.selinux="user_u:object_r:tmp_t:s0:c10\000"
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Refer to Section 49.5, “Getting Started with Multi-Category Security (MCS)” for details on creating categories and assigning them to files.
This section provides an introduction to using MCS labels to extend the Mandatory Access Control (MAC) capabilities of SELinux. It discusses MCS categories, SELinux user identities, and how they apply to Linux user accounts and files. It builds on the conceptual information provided in Section 49.4, “Multi-Category Security (MCS)”, and introduces some basic examples of usage.

49.5.1. Introduction
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MCS labeling from a user and system administrator standpoint is straightforward. It consists of configuring a set of categories, which are simply text labels, such as "Company_Confidential" or "Medical_Records", and then assigning users to those categories. The system administrator first configures the categories, then assigns users to them as required. The users can then use the labels as they see fit.
The names of the categories and their meanings are set by the system administrator, and can be set to whatever is required for the specific deployment. A system in a home environment may have only one category of "Private", and be configured so that only trusted local users are assigned to this category.
In a corporate environment, categories could be used to identify documents confidential to specific departments. Categories could be established for "Finance", "Payroll", "Marketing", and "Personnel". Only users assigned to those categories can access resources labeled with the same category.
After users have been assigned to categories, they can label any of their own files with any of the categories to which they have been assigned. For example, a home user in the system described above could label all of their personal files as "Private", and no service such as Apache or vsftp would ever be able to access those files, because they don't have access to the "Private" category.
MCS works on a simple principle: to access a file, a user needs to be assigned to all of the categories with which the file is labeled. The MCS check is applied after normal Linux Discretionary Access Control (DAC) and Type Enforcement (TE) rules, so it can only further restrict security.
SELinux maintains its own user identity for processes, separately from Linux user identities. In the targeted policy (the default for Red Hat Enterprise Linux), only a minimal number of SELinux user identities exist:
  • system_u — System processes
  • root — System administrator
  • user_u — All login users
Use the semanage user -l command to list SELinux users: 
~]# semanage user -l

                Labeling   MLS/       MLS/
SELinux User    Prefix     MCS Level  MCS Range            SELinux Roles

root            user       s0         s0-s0:c0.c1023       system_r sysadm_r user_r
system_u        user       s0         s0-s0:c0.c1023       system_r
user_u          user       s0         s0-s0:c0.c1023       system_r sysadm_r user_r
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Refer to Section 49.8.3, “Understanding the Users and Roles in the Targeted Policy” for more information about SELinux users and roles.
SELinux Logins

One of the properties of targeted policy is that login users all run in the same security context. From a TE point of view, in targeted policy, they are security-equivalent. To effectively use MCS, however, we need to be able to assign different sets of categories to different Linux users, even though they are all the same SELinux user (user_u). This is solved by introducing the concept of an SELinux login. This is used during the login process to assign MCS categories to Linux users when their shell is launched.

Use the semanage login -a command to assign Linux users to SELinux user identities: 
~]# semanage login -a james
~]# semanage login -a daniel
~]# semanage login -a olga
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Now when you list the SELinux users, you can see the Linux users assigned to a specific SELinux user identity: 
~]# semanage login -l

Login Name                SELinux User              MLS/MCS Range

__default__               user_u                    s0
james                     user_u                    s0
daniel                    user_u                    s0
root                      root                      s0-s0:c0.c1023
olga                      user_u                    s0
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Notice that at this stage only the root account is assigned to any categories. By default, the root account is configured with access to all categories.
Red Hat Enterprise Linux and SELinux are preconfigured with several default categories, but to make effective use of MCS, the system administrator typically modifies these or creates further categories to suit local requirements.

49.5.3. Configuring Categories
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SELinux maintains a mapping between internal sensitivity and category levels and their human-readable representations in the setrans.conf file. The system administrator edits this file to manage and maintain the required categories.
Use the chcat -L command to list the current categories: 
~]# chcat -L
s0
s0-s0:c0.c1023                 SystemLow-SystemHigh
s0:c0.c1023                    SystemHigh
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To modify the categories or to start creating your own, modify the /etc/selinux/<selinuxtype>/setrans.conf file. For the example introduced above, add the Marketing, Finance, Payroll, and Personnel categories as follows (this example uses the targeted policy, and irrelevant sections of the file have been omitted): 
~]# vi /etc/selinux/targeted/setrans.conf
s0:c0=Marketing
s0:c1=Finance
s0:c2=Payroll
s0:c3=Personnel
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Use the chcat -L command to check the newly-added categories: 
~]# chcat -L
s0:c0                          Marketing
s0:c1                          Finance
s0:c2                          Payroll
s0:c3                          Personnel
s0
s0-s0:c0.c1023                 SystemLow-SystemHigh
s0:c0.c1023                    SystemHigh
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Note

After you make any changes to the setrans.conf file, you need to restart the MCS translation service before those changes take effect. Use the following command to restart the service: 
~]# service mcstrans restart
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49.5.4. Assigning Categories to Users
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Now that the required categories have been added to the system, you can start assigning them to SELinux users and files. To further develop the example above, assume that James is in the Marketing department, Daniel is in the Finance and Payroll departments, and Olga is in the Personnel department. Each of these users has already been assigned an SELinux login.
Use the chcat command to assign MCS categories to SELinux logins: 
~]# chcat -l -- +Marketing james
~]# chcat -l -- +Finance,+Payroll daniel
~]# chcat -l -- +Personnel olga
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You can also use the chcat command with additional command-line arguments to list the categories that are assigned to users: 
~]# chcat -L -l daniel james olga
daniel: Finance,Payroll
james: Marketing
olga: Personnel
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You can add further Linux users, assign them to SELinux user identities and then assign categories to them as required. For example, if the company director also requires a user account with access to all categories, follow the same procedure as above: 
# Create a user account for the company director (Karl)
~]# useradd karl
~]# passwd karl
Changing password for user karl.
New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully.

# Assign the user account to an SELinux login
~]# semanage login -a karl

# Assign all the MCS categories to the new login
~]# chcat -l -- +Marketing,+Finance,+Payroll,+Personnel karl
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Use the chcat command to verify the addition of the new user: 
~]# chcat -L -l daniel james olga karl
daniel: Finance,Payroll
james: Marketing
olga: Personnel
karl: Marketing,Finance,Payroll,Personnel
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Note

MCS category access is assigned during login. Consequently, a user does not have access to newly-assigned categories until they log in again. Similarly, if access to a category is revoked, this is only apparent to the user after the next login.

49.5.5. Assigning Categories to Files
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At this point we have a system that has several user accounts, each of which is mapped to an SELinux user identity. We have also established a number of categories that are suitable for the particular deployment, and assigned those categories to different users.
All of the files on the system, however, still fall under the same category, and are therefore accessible by everyone (but still according to the standard Linux DAC and TE constraints). We now need to assign categories to the various files on the system so that only the appropriate users can access them.
For this example, we create a file in Daniel's home directory: 
[daniel@dhcp-133 ~]$ echo "Financial Records 2006" > financeRecords.txt
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Use the ls -Z command to check the initial security context of the file: 
[daniel@dhcp-133 ~]$ ls -Z financeRecords.txt
-rw-r--r--  daniel daniel user_u:object_r:user_home_t      financeRecords.txt
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Notice that at this stage the file has the default context for a file created in the user's home directory (user_home_t) and has no categories assigned to it. We can add the required category using the chcat command. Now when you check the security context of the file, you can see the category has been applied. 
[daniel@dhcp-133 ~]$ chcat -- +Finance financeRecords.txt
[daniel@dhcp-133 ~]$ ls -Z financeRecords.txt
-rw-r--r--  daniel daniel root:object_r:user_home_t:Finance financeRecords.txt
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In many cases, you need to assign more than one category to a file. For example, some files may need to be accessible to users from both the Finance and Payroll departments. 
[daniel@dhcp-133 ~]$ chcat -- +Payroll financeRecords.txt
[daniel@dhcp-133 ~]$ ls -Z financeRecords.txt
-rw-r--r--  daniel daniel root:object_r:user_home_t:Finance,Payroll financeRecords.txt
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Each of the categories that have been assigned to the file are displayed in the security context. You can add and delete categories to files as required. Only users assigned to those categories can access that file, assuming that Linux DAC and TE permissions would already allow the access.
If a user who is assigned to a different category tries to access the file, they receive an error message: 
[olga@dhcp-133 ~]$ cat financeRecords.txt
cat: financeRecords.txt: Permission Denied
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Note

Refer to the man pages for semanage and chcat for more information on the available options for these commands.

49.6. Multi-Level Security (MLS)
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Protecting sensitive or confidential data is paramount in many businesses. In the event such information is made public, businesses may face legal or financial ramifications. At the very least, they will suffer a loss of customer trust. In most cases, however, they can recover from these financial and other losses with appropriate investment or compensation.
The same cannot be said of the defense and related communities, which includes military services, intelligence organizations and some areas of police service. These organizations cannot easily recover should sensitive information be leaked, and may not recover at all. These communities require higher levels of security than those employed by businesses and other organizations.
Having information of different security levels on the same computer systems poses a real threat. It is not a straight-forward matter to isolate different information security levels, even though different users log in using different accounts, with different permissions and different access controls.
Some organizations go as far as to purchase dedicated systems for each security level. This is often prohibitively expensive, however. A mechanism is required to enable users at different security levels to access systems simultaneously, without fear of information contamination.

49.6.1. Why Multi-Level?
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The term multi-level arises from the defense community's security classifications: Confidential, Secret, and Top Secret.
Individuals must be granted appropriate clearances before they can see classified information. Those with Confidential clearance are only authorized to view Confidential documents; they are not trusted to look at Secret or Top Secret information. The rules that apply to data flow operate from lower levels to higher levels, and never the reverse. This is illustrated below.
Information Security Levels
View larger image
Information Security Levels

Figure 49.2. Information Security Levels

49.6.1.1. The Bell-La Padula Model (BLP)
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SELinux, like most other systems that protect multi-level data, uses the BLP model. This model specifies how information can flow within the system based on labels attached to each subject and object. Refer to the following diagram:
Available data flows using an MLS system
View larger image
Available data flows using an MLS system

Figure 49.3. Available data flows using an MLS system

Under such a system, users, computers, and networks use labels to indicate security levels. Data can flow between like levels, for example between "Secret" and "Secret", or from a lower level to a higher level. This means that users at level "Secret" can share data with one another, and can also retrieve information from Confidential-level (i.e., lower-level), users. However, data cannot flow from a higher level to a lower level. This prevents processes at the "Secret" level from viewing information classified as "Top Secret". It also prevents processes at a higher level from accidentally writing information to a lower level. This is referred to as the "no read up, no write down" model.
49.6.1.2. MLS and System Privileges
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MLS access rules are always combined with conventional access permissions (file permissions). For example, if a user with a security level of "Secret" uses Discretionary Access Control (DAC) to block access to a file by other users, this also blocks access by users with a security level of "Top Secret". A higher security clearance does not automatically give permission to arbitrarily browse a file system.
Users with top-level clearances do not automatically acquire administrative rights on multi-level systems. While they may have access to all information on the computer, this is different from having administrative rights.

49.6.2. Security Levels, Objects and Subjects
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As discussed above, subjects and objects are labeled with Security Levels (SLs), which are composed of two types of entities:
  1. Sensitivity: — A hierarchical attribute such as "Secret" or "Top Secret".
  2. Categories: — A set of non-hierarchical attributes such as "US Only" or "UFO".
An SL must have one sensitivity, and may have zero or more categories.
Examples of SLs are: { Secret / UFO, Crypto }, { Top Secret / UFO, Crypto, Stargate } and { Unclassified }
Note the hierarchical sensitivity followed by zero or more categories. The reason for having categories as well as sensitivities is so that sensitivities can be further compartmentalized on a need-to-know basis. For example, while a process may be cleared to the "Secret" sensitivity level, it may not need any type of access to the project "Warp Drive" (which could be the name of a category).

Note

  1. Security Levels on objects are called Classifications.
  2. Security Levels on subjects are called Clearances.
Thus, objects are labeled with a Classification, while subjects operate with a specific Clearance. Security Levels can have also Ranges, but these are beyond the scope of this introduction.

49.6.3. MLS Policy
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SELinux uses the Bell-La Padula BLP model, with Type Enforcement (TE) for integrity. In simple terms, MLS policy ensures that a Subject has an appropriate clearance to access an Object of a particular classification.
For example, under MLS, the system needs to know how to process a request such as: Can a process running with a clearance of { Top Secret / UFO, Rail gun } write to a file classified as { Top Secret / UFO } ?
The MLS model and the policy implemented for it will determine the answer. (Consider, for example, the problem of information leaking out of the Rail gun category into the file).
MLS meets a very narrow (yet critical) set of security requirements based around the way information and personnel are managed in rigidly controlled environments such as the military. MLS is typically difficult to work with and does not map well to general-case scenarios.
Type Enforcement (TE) under SELinux is a more flexible and expressive security scheme, which is in many cases more suitable than MLS.
There are, however, several scenarios where traditional MLS is still required. For example, a file server where the stored data may be of mixed classification and where clients connect at different clearances. This results in a large number of Security Levels and a need for strong isolation all on a single system.
This type of scenario is the reason that SELinux includes MLS as a security model, as an adjunct to TE.

49.6.4. Enabling MLS in SELinux
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Note

It is not recommended to use the MLS policy on a system that is running the X Window System.
Follow these steps to enable the SELinux MLS policy on your system.
  1. Install the selinux-policy-mls package: 
    ~]# yum install selinux-policy-mls
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  2. Before the MLS policy is enabled, each file on the file system must be relabeled with an MLS label. When the file system is relabeled, confined domains may be denied access, which may prevent your system from booting correctly. To prevent this from happening, configure SELINUX=permissive in the /etc/selinux/config file. Also, enable the MLS policy by configuring SELINUXTYPE=mls. Your configuration file should look like this: 
    # This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#     enforcing - SELinux security policy is enforced.
#     permissive - SELinux prints warnings instead of enforcing.
#     disabled - No SELinux policy is loaded.
SELINUX=permissive
# SELINUXTYPE= can take one of these two values:
#     targeted - Targeted processes are protected,
#     minimum - Modification of targeted policy. Only selected processes are protected. 
#     mls - Multi Level Security protection.
SELINUXTYPE=mls
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  3. Make sure SELinux is running in the permissive mode: 
    ~]# setenforce 0
~]# getenforce
Permissive
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  4. Create the .autorelabel file in root's home directory to ensure that files are relabeled upon next reboot: 
    ~]# touch /.autorelabel
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  5. Reboot your system. During the next boot, all file systems will be relabeled according to the MLS policy. The label process labels all files with an appropriate SELinux context: 
    *** Warning -- SELinux mls policy relabel is required.
*** Relabeling could take a very long time, depending on file
*** system size and speed of hard drives.
***********
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    Each * (asterisk) character on the bottom line represents 1000 files that have been labeled. In the above example, eleven * characters represent 11000 files which have been labeled. The time it takes to label all files depends upon the number of files on the system, and the speed of the hard disk drives. On modern systems, this process can take as little as 10 minutes. Once the labeling process finishes, the system will automatically reboot.
  6. Once the file system is relabeled, execute the following commands to assure that the /root directory and all other home directories are properly labeled: 
    ~]# genhomedircon
~]# restorecon -R -v /root /home <other_home_directories>
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  7. In permissive mode, SELinux policy is not enforced, but denials are still logged for actions that would have been denied if running in enforcing mode. Before changing to enforcing mode, as the Linux root user, run the grep "SELinux is preventing" /var/log/messages command to confirm that SELinux did not deny actions during the last boot. If SELinux did not deny actions during the last boot, this command does not return any output.
  8. If there were no denial messages in /var/log/messages, or you have resolved all existing denials, configure SELINUX=enforcing in the /etc/selinux/config file: 
    # This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#     enforcing - SELinux security policy is enforced.
#     permissive - SELinux prints warnings instead of enforcing.
#     disabled - No SELinux policy is loaded.
SELINUX=enforcing
# SELINUXTYPE= can take one of these two values:
#     targeted - Targeted processes are protected,
#     minimum - Modification of targeted policy. Only selected processes are protected. 
#     mls - Multi Level Security protection.
SELINUXTYPE=mls
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  9. Reboot your system and make sure SELinux is running in permissive mode: 
    ~]$ getenforce
Enforcing
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    and the MLS policy is enabled: 
    ~]# sestatus |grep mls
Policy from config file:        mls
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49.6.5. LSPP Certification
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Efforts are being made to have Linux certified as an MLS operating system. The certification is equivalent to the old B1 rating, which has been reworked into the Labeled Security Protection Profile under the Common Criteria scheme.

49.7. SELinux Policy Overview
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This chapter is an overview of SELinux policy, some of its internals, and how it works. It discusses the policy in general terms, while Section 49.8, “Targeted Policy Overview” focuses on the details of the targeted policy as it ships in Red Hat Enterprise Linux. This chapter starts with a brief overview of what policy is and where it resides.
Following on from this, the role of SELinux during the boot process is discussed. This is followed by discussions on file security contexts, object classes and permissions, attributes, types, access vectors, macros, users and roles, constraints, and a brief discussion summarizing special kernel interfaces.

49.7.1. What is the SELinux Policy?
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The SELinux Policy is the set of rules that guide the SELinux security engine. It defines types for file objects and domains for processes. It uses roles to limit the domains that can be entered, and has user identities to specify the roles that can be attained. In essence, types and domains are equivalent, the difference being that types apply to objects while domains apply to processes.
49.7.1.1. SELinux Types
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A type is a way of grouping items based on their similarity from a security perspective. This is not necessarily related to the unique purpose of an application or the content of a document. For example, a file can have any type of content and be for any purpose, but if it belongs to a user and exists in that user's home directory, it is considered to be of a specific security type, user_home_t.
These object types are considered alike because they are accessible in the same way by the same set of subjects. Similarly, processes tend to be of the same type if they have the same permissions as other subjects. In the targeted policy, programs that run in the unconfined_t domain have an executable file with a type such as sbin_t. From an SELinux perspective, this means they are all equivalent in terms of what they can and cannot do on the system.
For example, the binary executable file object at /usr/bin/postgres has the type postgresql_exec_t. All of the targeted daemons have their own *_exec_t type for their executable applications. In fact, the entire set of PostgreSQL executables such as createlang, pg_dump, and pg_restore have the same type, postgresql_exec_t, and they transition to the same domain, postgresql_t, upon execution.
The SELinux policy defines various rules which determine how each domain may access each type. Only what is specifically allowed by the rules is permitted. By default, every operation is denied and audited, meaning it is logged in the $AUDIT_LOG file. In Red Hat Enterprise Linux, this is set to /var/log/messages. The policy is compiled into binary format for loading into the kernel security server, and each time the security server makes a decision, it is cached in the AVC to optimize performance.
The policy can be defined either by modifying the existing files or by adding local Type Enforcement (TE) and File Context (FC) files to the policy tree. These new policies can be loaded into the kernel in real time. Otherwise, the policy is loaded during the boot process by init, as explained in Section 49.7.3, “The Role of Policy in the Boot Process”. Ultimately, every system operation is determined by the policy and the type-labeling of the files.

Important

After loading a new policy, it is recommended that you restart any services that may have new or changed labeling. Generally speaking, this is only the targeted daemons, as listed in Section 49.8.1, “What is the Targeted Policy?”.
49.7.1.2. SELinux and Mandatory Access Control
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SELinux is an implementation of Mandatory Access Control (MAC). Depending on the security policy type, SELinux implements either Type Enforcement (TE), Roles Based Access Control (RBAC) or Bell-La Padula Model Multi-Level Security (MLS).
The policy specifies the rules in the implemented environment. It is written in a language created specifically for writing security policy. Policy writers use m4 macros to capture common sets of low-level rules. A number of m4 macros are defined in the existing policy, which facilitate the writing of new policy. These rules are preprocessed into many additional rules as part of building the policy.conf file, which is compiled into the binary policy.
Access rights are divided differently among domains, and no domain is required to act as a master for all other domains. Moving between domains is controlled by the policy, through login programs, userspace programs such as newrole, or by requiring a new process execution in the new domain. This movement between domains is referred to as a transition .

49.7.2. Where is the Policy?
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There are two components to the policy: the binary tree and the source tree. The binary tree is provided by the selinux-policy-<policyname> package and supplies the binary policy file.
Alternatively, the binary policy can be built from source when the selinux-policy-devel package is installed.

Note

Information on how to edit, write and compile policy is currently outside the scope of this document.
49.7.2.1. Binary Tree Files
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  • /etc/selinux/targeted/ — this is the root directory for the targeted policy, and contains the binary tree.
  • /etc/selinux/targeted/policy/ — this is the location of the binary policy file policy.<xx>. In this guide, the variable SELINUX_POLICY is used for this directory.
  • /etc/selinux/targeted/contexts/ — this is the location of the security context information and configuration files, which are used during runtime by various applications.
  • /etc/selinux/targeted/contexts/files/ — contains the default contexts for the entire file system. This is referenced by restorecon when performing relabeling operations.
  • /etc/selinux/targeted/contexts/users/ — in the targeted policy, only the root file is in this directory. These files are used for determining context when a user logs in. For example, for the root user, the context is user_u:system_r:unconfined_t.
  • /etc/selinux/targeted/modules/active/booleans* — this is where the runtime Booleans are configured.

    Note

    These files should never be manually changed. You should use the getsebool, setsebool and semanage tools to manipulate runtime Booleans.
49.7.2.2. Source Tree Files
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For developing policy modules, the selinux-policy-devel package includes all of the interface files used to build policy. It is recommended that people who build policy use these files to build the policy modules.
This package installs the policy interface files under /usr/share/selinux/devel/include and has make files installed in /usr/share/selinux/devel/Makefile.
To help applications that need the various SELinux paths, libselinux provides a number of functions that return the paths to the different configuration files and directories. This negates the need for applications to hard-code the paths, especially since the active policy location is dependent on the SELINUXTYPE setting in /etc/selinux/config.
For example, if SELINUXTYPE is set to strict, the active policy location is under /etc/selinux/strict.
To view the list of available functions, use the following command: 
man 3 selinux_binary_policy_path
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Note

This man page is available only if you have the libselinux-devel RPM installed.
The use of libselinux and related functions is outside the scope of this document.

49.7.3. The Role of Policy in the Boot Process
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SELinux plays an important role during the early stages of system start-up. Because all processes must be labeled with their correct domain, init performs some essential operations early in the boot process to maintain synchronization between labeling and policy enforcement.
  1. After the kernel has been loaded during the boot process, the initial process is assigned the predefined initial SELinux ID (initial SID) kernel. Initial SIDs are used for bootstrapping before the policy is loaded.
  2. /sbin/init mounts /proc/, and then searches for the selinuxfs file system type. If it is present, that means SELinux is enabled in the kernel.
  3. If init does not find SELinux in the kernel, or if it is disabled via the selinux=0 boot parameter, or if /etc/selinux/config specifies that SELINUX=disabled, the boot process proceeds with a non-SELinux system.
    At the same time, init sets the enforcing status if it is different from the setting in /etc/selinux/config. This happens when a parameter is passed during the boot process, such as enforcing=0 or enforcing=1. The kernel does not enforce any policy until the initial policy is loaded.
  4. If SELinux is present, /selinux/ is mounted.
  5. init checks /selinux/policyvers for the supported policy version. The version number in /selinux/policyvers is the latest policy version your kernel supports. init inspects /etc/selinux/config to determine which policy is active, such as the targeted policy, and loads the associated file at $SELINUX_POLICY/policy.<version>.
    If the binary policy is not the version supported by the kernel, init attempts to load the policy file if it is a previous version. This provides backward compatibility with older policy versions.
    If the local settings in /etc/selinux/targeted/booleans are different from those compiled in the policy, init modifies the policy in memory based on the local settings prior to loading the policy into the kernel.
  6. By this stage of the process, the policy is fully loaded into the kernel. The initial SIDs are then mapped to security contexts in the policy. In the case of the targeted policy, the new domain is user_u:system_r:unconfined_t. The kernel can now begin to retrieve security contexts dynamically from the in-kernel security server.
  7. init then re-executes itself so that it can transition to a different domain, if the policy defines it. For the targeted policy, there is no transition defined and init remains in the unconfined_t domain.
  8. At this point, init continues with its normal boot process.
The reason that init re-executes itself is to accommodate stricter SELinux policy controls. The objective of re-execution is to transition to a new domain with its own granular rules. The only way that a process can enter a domain is during execution, which means that such processes are the only entry points into the domains.
For example, if the policy has a specific domain for init, such as init_t, a method is required to change from the initial SID, such as kernel, to the correct runtime domain for init. Because this transition may need to occur, init is coded to re-execute itself after loading the policy.
This init transition occurs if the domain_auto_trans(kernel_t, init_exec_t, <target_domain_t>) rule is present in the policy. This rule states that an automatic transition occurs on anything executing in the kernel_t domain that executes a file of type init_exec_t. When this execution occurs, the new process is assigned the domain <target_domain_t>, using an actual target domain such as init_t.

49.7.4. Object Classes and Permissions
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SELinux defines a number of classes for objects, making it easier to group certain permissions by specific classes. For example:
  • File-related classes include filesystem for file systems, file for files, and dir for directories. Each class has its own associated set of permissions.
    The filesystem class can mount, unmount, get attributes, set quotas, relabel, and so forth. The file class has common file permissions such as read, write, get and set attributes, lock, relabel, link, rename, append, etc.
  • Network related classes include tcp_socket for TCP sockets, netif for network interfaces, and node for network nodes.
    The netif class, for example, can send and receive on TCP, UDP and raw sockets (tcp_recv, tcp_send, udp_send, udp_recv, rawip_recv, and rawip_send.)
The object classes have matching declarations in the kernel, meaning that it is not trivial to add or change object class details. The same is true for permissions. Development work is ongoing to make it possible to dynamically register and unregister classes and permissions.
Permissions are the actions that a subject can perform on an object, if the policy allows it. These permissions are the access requests that SELinux actively allows or denies.

49.8. Targeted Policy Overview
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This chapter is an overview and examination of the SELinux targeted policy, the current supported policy for Red Hat Enterprise Linux.
Much of the content in this chapter is applicable to all types of SELinux policy, in terms of file locations and the type of content in those files. The difference lies in which files exist in the key locations and their contents.

49.8.1. What is the Targeted Policy?
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The SELinux policy is highly configurable. For Red Hat Enterprise Linux 5, Red Hat supports a single policy, the targeted policy . Under the targeted policy, every subject and object runs in the unconfined_t domain except for the specific targeted daemons. Objects that are in the unconfined_t domain have no restrictions and fall back to using standard Linux security, that is, DAC. The daemons that are part of the targeted policy run in their own domains and are restricted in every operation they perform on the system. This way daemons that are exploited or compromised in any way are contained and can only cause limited damage.
For example, the http and ntp daemons are both protected in the default targeted policy, and run in the httpd_t and ntpd_t domains, respectively. The ssh daemon, however, is not protected in this policy, and consequently runs in the unconfined_t domain.
Refer to the following sample output, which illustrates the various domains for the daemons mentioned above: 
user_u:system_r:httpd_t         25129 ?        00:00:00 httpd
user_u:system_r:ntpd_t          25176 ?        00:00:00 ntpd
system_u:system_r:unconfined_t         25245 ? 00:00:00 sshd
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The Strict Policy

The opposite of the targeted policy is the strict policy . In the strict policy, every subject and object exists in a specific security domain, and all interactions and transitions are individually considered within the policy rules.

The strict policy is a much more complex environment, and does not ship with Red Hat Enterprise Linux. This guide focuses on the targeted policy that ships with Red Hat Enterprise Linux, and the components of SELinux used by the targeted daemons.
The targeted daemons are as follows: dhcpd; httpd; mysqld; named; nscd; ntpd; portmap; postgres; snmpd; squid; syslogd; and winbind.

Note

Depending on your installation, only some of these daemons may be present.
Refer to Section 49.7.2, “Where is the Policy?” for a list of the common files and directories used by SELinux.
This section covers the specific roles enabled for the targeted policy. The unconfined_t type exists in every role, which significantly reduces the usefulness of roles in the targeted policy. More extensive use of roles requires a change to the strict policy paradigm, where every process runs in an individually considered domain.
Effectively, there are only two roles in the targeted policy: system_r and object_r. The initial role is system_r, and everything else inherits that role. The remaining roles are defined for compatibility purposes between the targeted policy and the strict policy.[20]
Three of the four roles are defined by the policy. The fourth role, object_r, is an implied role and is not found in policy source. Because roles are created and populated by types using one or more declarations in the policy, there is no single file that declares all roles. (Remember that the policy itself is generated from a number of separate files.)
system_r
This role is for all system processes except user processes: 
system_r (28 types)
    dhcpd_t
    httpd_helper_t
    httpd_php_t
    httpd_suexec_t
    httpd_sys_script_t
    httpd_t
    httpd_unconfined_script_t
    initrc_t
    ldconfig_t
    mailman_cgi_t
    mailman_mail_t
    mailman_queue_t
    mysqld_t
    named_t
    ndc_t
    nscd_t
    ntpd_t
    pegasus_t
    portmap_t
    postgresql_t
    snmpd_t
    squid_t
    syslogd_t
    system_mail_t
    unconfined_t
    winbind_helper_t
    winbind_t
    ypbind_t
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user_r
This is the default user role for regular Linux users. In a strict policy, individual users might be used, allowing for the users to have special roles to perform privileged operations. In the targeted policy, all users run in the unconfined_t domain.
object_r
In SELinux, roles are not utilized for objects when RBAC is being used. Roles are strictly for subjects. This is because roles are task-oriented and they group together entities which perform actions (for example, processes). All such entities are collectively referred to as subjects. For this reason, all objects have the role object_r, and the role is only used as a placeholder in the label.
sysadm_r
This is the system administrator role in a strict policy. If you log in directly as the root user, the default role may actually be staff_r. If this is true, use the newrole -r sysadm_r command to change to the SELinux system administrator role to perform system administration tasks. In the targeted policy, the following retain sysadm_r for compatibility: 
sysadm_r (6 types)
    httpd_helper_t
    httpd_sys_script_t
    initrc_t
    ldconfig_t
    ndc_t
    unconfined_t
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There is effectively only one user identity in the targeted policy. The user_u identity was chosen because libselinux falls back to user_u as the default SELinux user identity. This occurs when there is no matching SELinux user for the Linux user who is logging in. Using user_u as the single user in the targeted policy makes it easier to change to the strict policy. The remaining users exist for compatibility with the strict policy.[21]
The one exception is the SELinux user root. You may notice root as the user identity in a process's context. This occurs when the SELinux user root starts daemons from the command line, or restarts a daemon originally started by init.

[18] The NSA is the cryptologic agency of the United States of America's Federal government, charged with information assurance and signals intelligence. You can read more about the NSA at their website, http://www.nsa.gov/about/.
[19] Flask grew out of a project that integrated the Distributed Trusted Operating System (DTOS ) into the Fluke research operating system. Flask was the name of the architecture and the implementation in the Fluke operating system.
[20] Any role could have been chosen for the targeted policy, but system_r already had existing authorization for the daemon domains, simplifying the process. This was done because no mechanism currently exists to alias roles.
[21] A user aliasing mechanism would also work here, to alias all identities from the strict policy to a single user identity in the targeted policy.

Chapter 50. Working With SELinux
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SELinux presents both a new security paradigm and a new set of practices and tools for administrators and some end-users. The tools and techniques discussed in this chapter focus on standard operations performed by end-users, administrators, and analysts.

50.1. End User Control of SELinux
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In general, end users have little interaction with SELinux when Red Hat Enterprise Linux is running the targeted policy. This is because users are running in the domain of unconfined_t along with the rest of the system except the targeted daemons.
In most situations, standard DAC controls prevent you from performing tasks for which you do not have the required access or permissions before SELinux is consulted. Consequently, it is likely that you will never generate an avc: denied message.
The following sections cover the general tasks and practices that an end user might need to perform on a Red Hat Enterprise Linux system. These tasks apply to users of all privilege levels, not only to end users.

50.1.1. Moving and Copying Files
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In file system operations, security context must now be considered in terms of the label of the file, the process accessing it, and the directories where the operation is happening. Because of this, moving and copying files with mv and cp may have unexpected results.
Copying Files: SELinux Options for cp

Unless you specify otherwise, cp follows the default behavior of creating a new file based on the domain of the creating process and the type of the target directory. Unless there is a specific rule to set the label, the file inherits the type from the target directory.

Use the -Z user:role:type option to specify the required label for the new file.
The -p (or --preserve=mode,ownership,timestamps) option preserves the specified attributes and, if possible, additional attributes such as links. 
touch bar foo
ls -Z bar foo
-rw-rw-r--  auser   auser   user_u:object_r:user_home_t   bar
-rw-rw-r--  auser   auser   user_u:object_r:user_home_t   foo
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If you use the cp command without any additional command-line arguments, a copy of the file is created in the new location using the default type of the creating process and the target directory. In this case, because there is no specific rule that applies to cp and /tmp, the new file has the type of the parent directory: 
cp bar /tmp
ls -Z /tmp/bar
-rw-rw-r--  auser   auser   user_u:object_r:tmp_t   /tmp/bar
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The type tmp_t is the default type for temporary files.
Use the -Z option to specify the label for the new file: 
cp -Z user_u:object_r:user_home_t foo /tmp
ls -Z /tmp/foo
-rw-rw-r--  auser   auser   user_u:object_r:user_home_t   /tmp/foo
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Moving Files: SELinux Options for mv

Moving files with mv retains the original type associated with the file. Care should be taken using this command as it can cause problems. For example, if you move files with the type user_home_t into ~/public_html, then the httpd daemon is not able to serve those files until you relabel them. Refer to Section 50.1.3, “Relabeling a File or Directory” for more information about file labeling.

Expand
Table 50.1. Behavior of mv and cp Commands
Command Behavior
mv The file retains its original label. This may cause problems, confusion, or minor insecurity. For example, the tmpwatch program running in the sbin_t domain might not be allowed to delete an aged file in the /tmp directory because of the file's type.
cp Makes a copy of the file using the default behavior based on the domain of the creating process (cp) and the type of the target directory.
cp -p Makes a copy of the file, preserving the specified attributes and security contexts, if possible. The default attributes are mode, ownership, and timestamps. Additional attributes are links and all.
cp -Z <user:role:type> Makes a copy of the file with the specified labels. The -Z option is synonymous with --context.
Checking a Process ID

In Red Hat Enterprise Linux, the -Z option is equivalent to --context, and can be used with the ps, id, ls, and cp commands. The behavior of the cp command with respect to SELinux is explained in Table 50.1, “Behavior of mv and cp Commands”.

The following example shows a small sample of the output of the ps command. Most of the processes are running in the unconfined_t domain, with a few exceptions. 
[user@localhost ~]$ ps auxZ
LABEL                           USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
system_u:system_r:init_t        root         1  0.0  0.1   2032   620 ?        Ss   15:09   0:00 init [5]
system_u:system_r:kernel_t      root         2  0.0  0.0      0     0 ?        S    15:09   0:00 [migration/0]
system_u:system_r:kernel_t      root         3  0.0  0.0      0     0 ?        SN   15:09   0:00 [ksoftirqd/0]

user_u:system_r:unconfined_t    user     3122  0.0  0.6   6908  3232 ?        S    16:47   0:01 /usr/libexec/gconfd-2 5
user_u:system_r:unconfined_t    user     3125  0.0  0.1   2540   588 ?        S    16:47   0:00 /usr/bin/gnome-keyring-daemon
user_u:system_r:unconfined_t    user     3127  0.0  1.4  33612  6988 ?        Sl   16:47   0:00 /usr/libexec/gnome-settings-daemon
user_u:system_r:unconfined_t    user     3144  0.1  1.4  16528  7360 ?        Ss   16:47   0:01 metacity --sm-client-id=default1
user_u:system_r:unconfined_t    user     3148  0.2  2.9  79544 14808 ?        Ss   16:47   0:03 gnome-panel --sm-client-id default2
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Checking a User ID

You can use the -Z option with the id command to determine a user's security context. Note that with this command you cannot combine -Z with other options. 

[root@localhost ~]# id -Z
user_u:system_r:unconfined_t
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Note that you cannot use the -Z option with the id command to inspect the security context of a different user. That is, you can only display the security context of the currently logged-in user: 
[user@localhost ~]$ id
uid=501(user) gid=501(user) groups=501(user) context=user_u:system_r:unconfined_t
[user@localhost ~]$ id root
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel)
[user@localhost ~]$ id -Z root
id: cannot display context when selinux not enabled or when displaying the id
of a different user
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Check a File ID

You can use the -Z option with the ls command to group common long-format information. You can display mode, user, group, security context, and filename information. 

cd /etc
ls -Z h* -d
drwxr-xr-x  root root  system_u:object_r:etc_t        hal
-rw-r--r--  root root  system_u:object_r:etc_t        host.conf
-rw-r--r--  root root  user_u:object_r:etc_t          hosts
-rw-r--r--  root root  system_u:object_r:etc_t        hosts.allow
-rw-r--r--  root root  system_u:object_r:etc_t        hosts.canna
-rw-r--r--  root root  system_u:object_r:etc_t        hosts.deny
drwxr-xr-x  root root  system_u:object_r:hotplug_etc_t  hotplug
drwxr-xr-x  root root  system_u:object_r:etc_t        hotplug.d
drwxr-xr-x  root root  system_u:object_r:httpd_sys_content_t htdig
drwxr-xr-x  root root  system_u:object_r:httpd_config_t httpd
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50.1.3. Relabeling a File or Directory
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You may need to relabel a file when moving or copying into special directories related to the targeted daemons, such as ~/public_html directories, or when writing scripts that work in directories outside of /home.
There are two general types of relabeling operations:
  • Deliberately changing the type of a file
  • Restoring files to the default state according to policy
There are also relabeling operations that an administrator performs. These are covered in Section 50.2.2, “Relabeling a File System”.

Note

The majority of SELinux permission control in the targeted policy is Type Enforcement (TE). Consequently, you can generally ignore the user and role information in a security label and focus on just changing the type. You do not normally need to consider the role and user settings on files.

Note

If relabeling affects the label on a daemon's executable, you should restart the daemon to be sure it is running in the correct domain. For example, if /usr/sbin/mysqld has the wrong security label, and you address this by using a relabeling operation such as restorecon, you must restart mysqld after the relabeling operation. Setting the executable file to have the correct type (mysqld_exec_t) ensures that it transitions to the proper domain when started.
Use the chcon command to change a file to the correct type. You need to know the correct type that you want to apply to use this command. The directories and files in the following example are labeled with the default type defined for file system objects created in /home: 
cd ~
ls -Zd public_html/
drwxrwxr-x  auser  auser  user_u:object_r:user_home_t public_html/

ls -Z web_files/
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   1.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   2.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   3.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   4.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   5.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   index.html
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If you move these files into the public_html directory, they retain the original type: 
mv web_files/* public_html/
ls -Z public_html/
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   1.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   2.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   3.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   4.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   5.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   index.html
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To make these files viewable from a special user public HTML folder, they need to have a type that httpd has permissions to read, presuming the Apache HTTP Server is configured for UserDir and the Boolean value httpd_enable_homedirs is enabled. 
chcon -R -t httpd_user_content_t public_html/
ls -Z public_html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t   1.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t   2.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t   3.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t   4.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t   5.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t   index.html

ls -Z public_html/ -d
drwxrwxr-x  auser  auser  user_u:object_r:httpd_user_content_t  public_html/
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Note

If the file has no label, such as a file created while SELinux was disabled in the kernel, you need to give it a full label with chcon system_u:object_r:shlib_t foo.so. Otherwise, you will receive an error about applying a partial context to an unlabeled file.
Use the restorecon command to restore files to the default values according to the policy. There are two other methods for performing this operation that work on the entire file system: fixfiles or a policy relabeling operation. Each of these methods requires superuser privileges. Cautions against both of these methods appear in Section 50.2.2, “Relabeling a File System”.
The following example demonstrates restoring the default user home directory context to a set of files that have different types. The first two sets of files have different types, and are being moved into a directory for archiving. Their contexts are different from each other, and are incorrect for a standard user's home directory: 
ls -Z /tmp/
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t            /tmp/file1
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t            /tmp/file2
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t            /tmp/file3

mv /tmp/{1,2,3} archives/
mv public_html/* archives/
ls -Z archives/
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t            file1
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t    file1.html
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t            file2
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t    file2.html
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t            file3
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t    file3.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t    file4.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t    file5.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t  index.html
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The archives/ directory already has the default type because it was created in the user's home directory: 
ls -Zd archives/
drwxrwxr-x  auser  auser  user_u:object_r:user_home_t  archives/
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Using the restorecon command to relabel the files uses the default file contexts set by the policy, so these files are labeled with the default label for their current directory. 
/sbin/restorecon -R archives/
ls -Z archives/
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file1
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file1.html
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file2
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file2.html
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file3
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file3.html
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file4.html
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file5.html
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    index.html
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You can use either the tar or star utilities to create archives that retain SELinux security contexts. The following example uses star to demonstrate how to create such an archive. You need to use the appropriate -xattr and -H=exustar options to ensure that the extra attributes are captured and that the header for the *.star file is of a type that fully supports xattrs. Refer to the man page for more information about these and other options.
The following example illustrates the creation and extraction of a set of html files and directories. Note that the two directories have different labels. Unimportant parts of the file context have been omitted for printing purposes (indicated by ellipses '...'): 
ls -Z public_html/ web_files/

public_html/:
-rw-rw-r--  auser  auser  ...httpd_user_content_t 1.html
-rw-rw-r--  auser  auser  ...httpd_user_content_t 2.html
-rw-rw-r--  auser  auser  ...httpd_user_content_t 3.html
-rw-rw-r--  auser  auser  ...httpd_user_content_t 4.html
-rw-rw-r--  auser  auser  ...httpd_user_content_t 5.html
-rw-rw-r--  auser  auser  ...httpd_user_content_t index.html
web_files/:
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  1.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  2.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  3.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  4.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  5.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  index.html
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The following command creates the archive, retaining all of the SELinux security contexts: 
star -xattr -H=exustar -c -f all_web.star public_html/ web_files/
star: 11 blocks + 0 bytes (total of 112640 bytes = 110.00k).
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Use the ls command with the -Z option to validate the security context: 
ls -Z all_web.star
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t \  all_web.star
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You can now copy the archive to a different directory. In this example, the archive is copied to /tmp. If there is no specific policy to make a derivative temporary type, the default behavior is to acquire the tmp_t type. 
cp all_web.star /tmp/ cd /tmp/

ls -Z all_web.star
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t  all_web.star
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Now you can expand the archives using star and it restores the extended attributes: 
star -xattr -x -f all_web.star
star: 11 blocks + 0 bytes (total of 112640 bytes = 110.00k).

ls -Z /tmp/public_html/ /tmp/web_files/
/tmp/public_html/:
-rw-rw-r--  auser  auser  ...httpd_sys_content_t 1.html
-rw-rw-r--  auser  auser  ...httpd_sys_content_t 2.html
-rw-rw-r--  auser  auser  ...httpd_sys_content_t 3.html
-rw-rw-r--  auser  auser  ...httpd_sys_content_t 4.html
-rw-rw-r--  auser  auser  ...httpd_sys_content_t 5.html
-rw-rw-r--  auser  auser  ...httpd_sys_content_t index.html
/tmp/web_files/:
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  1.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  2.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  3.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  4.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  5.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  \ index.html
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Warning

If you use an absolute path when you create an archive using star, the archive expands on that same path. For example, an archive made with this command restores the files to /var/log/httpd/: 
star -xattr -H=exustar -c -f httpd_logs.star /var/log/httpd/
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If you attempt to expand this archive, star issues a warning if the files in the path are newer than the ones in the archive.

50.2. Administrator Control of SELinux
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In addition to the tasks often performed by users in Section 50.1, “End User Control of SELinux”, SELinux administrators could be expected to perform a number of additional tasks. These tasks typically require root access to the system. Such tasks are significantly easier under the targeted policy. For example, there is no need to consider adding, editing, or deleting Linux users from the SELinux users, nor do you need to consider roles.
This section covers the types of tasks required of an administrator who maintains Red Hat Enterprise Linux running SELinux.

50.2.1. Viewing the Status of SELinux
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The sestatus command provides a configurable view into the status of SELinux. The simplest form of this command shows the following information: 
~]# sestatus
SELinux status:                 enabled
SELinuxfs mount:                /selinux
Current mode:                   enforcing
Mode from config file:          enforcing
Policy version:                 21
Policy from config file:        targeted
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The -v option includes information about the security contexts of a series of files that are specified in /etc/sestatus.conf: 
~]# sestatus -v
SELinux status:                 enabled
SELinuxfs mount:                /selinux
Current mode:                   enforcing
Mode from config file:          enforcing
Policy version:                 21
Policy from config file:        targeted

Process contexts:
Current context:                user_u:system_r:unconfined_t
Init context:                   system_u:system_r:init_t
/sbin/mingetty                  system_u:system_r:getty_t
/usr/sbin/sshd                  system_u:system_r:unconfined_t:s0-s0:c0.c1023

File contexts:
Controlling term:               user_u:object_r:devpts_t
/etc/passwd                     system_u:object_r:etc_t
/etc/shadow                     system_u:object_r:shadow_t
/bin/bash                       system_u:object_r:shell_exec_t
/bin/login                      system_u:object_r:login_exec_t
/bin/sh                         system_u:object_r:bin_t -> system_u:object_r:shell_exec_t
/sbin/agetty                    system_u:object_r:getty_exec_t
/sbin/init                      system_u:object_r:init_exec_t
/sbin/mingetty                  system_u:object_r:getty_exec_t
/usr/sbin/sshd                  system_u:object_r:sshd_exec_t
/lib/libc.so.6                  system_u:object_r:lib_t -> system_u:object_r:lib_t
/lib/ld-linux.so.2              system_u:object_r:lib_t -> system_u:object_r:ld_so_t
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The -b displays the current state of booleans. You can use this in combination with grep or other tools to determine the status of particular booleans: 
~]# sestatus -b | grep httpd | grep on$
httpd_builtin_scripting           on
httpd_disable_trans               on
httpd_enable_cgi                  on
httpd_enable_homedirs             on
httpd_unified                     on
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50.2.2. Relabeling a File System
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You may never need to relabel an entire file system. This usually occurs only when labeling a file system for SELinux for the first time, or when switching between different types of policy, such as changing from the targeted to the strict policy.
Relabeling a File System Using init

The recommended method for relabeling a file system is to reboot the machine. This allows the init process to perform the relabeling, ensuring that applications have the correct labels when they are started and that they are started in the right order. If you relabel a file system without rebooting, some processes may continue running with an incorrect context. Manually ensuring that all the daemons are restarted and running in the correct context can be difficult.

Use the following procedure to relabel a file system using this method. 
touch /.autorelabel
reboot
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At boot time, init.rc checks for the existence of /.autorelabel. If this file exists, SELinux performs a complete file system relabel (using the /sbin/fixfiles -f -F relabel command), and then deletes /.autorelabel.
Relabeling a File System Using fixfiles

It is possible to relabel a file system using the fixfiles command, or to relabel based on the RPM database:

Use the following command to relabel a file system only using the fixfiles command: 
fixfiles relabel
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Use the following command to relabel a file system based on the RPM database: 
fixfiles -R <packagename> restore
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Using fixfiles to restore contexts from packages is safer and quicker.

Warning

Running fixfiles on the entire file system without rebooting may make the system unstable.
If the relabeling operation applies a new policy that is different from the policy that was in place when the system booted, existing processes may be running in incorrect and insecure domains. For example, a process could be in a domain that is not an allowed transition for that process in the new policy, granting unexpected permissions to that process alone.
In addition, one of the options to fixfiles relabel prompts for approval to empty /tmp/ because it is not possible to reliably relabel /tmp/. Since fixfiles is run as root, temporary files that applications are relying upon are erased. This could make the system unstable or behave unexpectedly.

50.2.3. Managing NFS Home Directories
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In Red Hat Enterprise Linux 5, most targeted daemons do not interact with user data and are not affected by NFS-mounted home directories. One exception is the Apache HTTP Server. For example, CGI scripts that are on the mounted file system have the nfs_t type, which is not a type that httpd_t is allowed to execute.
If you are having problems with the default type of nfs_t, try mounting the home directories with a different context: 
mount -t nfs -o context=user_u:object_r:user_home_dir_t \
	fileserver.example.com:/shared/homes/ /home
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Warning

Section 50.2.9, “Specifying the Security Context of Entire File Systems” explains how to mount a directory so that httpd can execute scripts. If you do this for user home directories, it gives the Apache HTTP Server increased access to those directories. Remember that a mountpoint label applies to the entire mounted file system.
Future versions of the SELinux policy address the functionality of NFS.

50.2.4. Granting Access to a Directory or a Tree
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Similar to standard Linux DAC permissions, a targeted daemon must have SELinux permissions to be able to descend the directory tree. This does not mean that a directory and its contents need to have the same type. There are many types, such as root_t, tmp_t, and usr_t that grant read access for a directory. These types are suitable for directories that do not contain any confidential information, and that you want to be widely readable. They could also be used for a parent directory of more secured directories with different contexts.
If you are working with an avc: denied message, there are some common problems that arise with directory traversal. For example, many programs run a command equivalent to ls -l / that is not necessary to their operation but generates a denial message in the logs. For this you need to create a dontaudit rule in your local.te file.
When trying to interpret AVC denial messages, do not be misled by the path=/ component. This path is not related to the label for the root file system, /. It is actually relative to the root of the file system on the device node. For example, if your /var/ directory is located on an LVM (Logical Volume Management [22]) device, /dev/dm-0, the device node is identified in the message as dev=dm-0. When you see path=/ in this example, that is the top level of the LVM device dm-0, not necessarily the same as the root file system designation /.

50.2.6. Enabling or Disabling Enforcement
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You can enable and disable SELinux enforcement at runtime or configure it to start in the correct mode at boot time, using the command line or GUI. SELinux can operate in one of three modes: disabled , meaning not enabled in the kernel; permissive , meaning SELinux is running and logging but not controlling permissions; or enforcing , meaning SELinux is running and enforcing policy.
Use the setenforce command to change between permissive and enforcing modes at runtime. Use setenforce 0 to enter permissive mode; use setenforce 1 to enter enforcing mode.
The sestatus command displays the current mode and the mode from the configuration file referenced during boot: 
~]# sestatus | grep -i mode
Current mode:           permissive
Mode from config file:  permissive
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Note that changing the runtime enforcement does not affect the boot time configuration: 
~]# setenforce 1
~]# sestatus | grep -i mode
Current mode:           enforcing
Mode from config file:  permissive
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You can also disable enforcing mode for a single daemon. For example, if you are trying to troubleshoot the named daemon and SELinux, you can turn off enforcing for just that daemon.
Use the getsebool command to get the current status of the boolean: 
~]# getsebool named_disable_trans
named_disable_trans --> off
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Use the following command to disable enforcing mode for this daemon: 
~]# setsebool named_disable_trans 1
~]# getsebool named_disable_trans
named_disable_trans --> on
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Note

This sets the runtime value only. Use the -P option to make the change persistent across reboots.
Any *_disable_trans booleans that are set to "on" invoke the conditional that prevents the process from transitioning to the domain on execution.
Use the following command to find which of these booleans are set: 
~]# getsebool -a | grep disable.*on
httpd_disable_trans=1
mysqld_disable_trans=1
ntpd_disable_trans=1
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You can set any number of boolean values using the setsebool command: 
setsebool -P httpd_disable_trans=1 mysqld_disable_trans=1 ntpd_disable_trans=1
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You can also use togglesebool <boolean_name> to change the value of a specific boolean: 
~]# getsebool httpd_disable_trans
httpd_disable_trans --> off
~]# togglesebool httpd_disable_trans
httpd_disable_trans: active
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You can configure all of these settings using system-config-selinux. The same configuration files are used, so changes appear bidirectionally.
Changing a Runtime Boolean

Use the following procedure to change a runtime boolean using the GUI.

Note

Administrator privileges are required to perform this procedure.
  1. On the System menu, point to Administration and then click Security Level and Firewall to display the Security Level Configuration dialog box.
  2. Click the SELinux tab, and then click Modify SELinux Policy.
  3. In the selection list, click the arrow next to the Name Service entry, and select the Disable SELinux protection for named daemon check box.
  4. Click OK to apply the change. Note that it may take a short time for the policy to be reloaded.
Using the Security Level Configuration dialog box to change a runtime boolean.
View larger image
Using the Security Level Configuration dialog box to change a runtime boolean.

Figure 50.1. Using the Security Level Configuration dialog box to change a runtime boolean.

If you want to control these settings with scripts, you can use the setenforce(1), getenforce(1), and selinuxenabled(1) commands.

50.2.7. Enable or Disable SELinux
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Important

Changes you make to files while SELinux is disabled may give them an unexpected security label, and new files will not have a label. You may need to relabel part or all of the file system after re-enabling SELinux.
From the command line, you can edit the /etc/sysconfig/selinux file. This file is a symlink to /etc/selinux/config. The configuration file is self-explanatory. Changing the value of SELINUX or SELINUXTYPE changes the state of SELinux and the name of the policy to be used the next time the system boots. 
~]# cat /etc/sysconfig/selinux
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#       enforcing - SELinux security policy is enforced.
#       permissive - SELinux prints warnings instead of enforcing.
#       disabled - SELinux is fully disabled.
SELINUX=permissive
# SELINUXTYPE= type of policy in use. Possible values are:
#       targeted - Only targeted network daemons are protected.
#       strict - Full SELinux protection.
SELINUXTYPE=targeted

# SETLOCALDEFS= Check local definition changes
SETLOCALDEFS=0
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Changing the Mode of SELinux Using the GUI

Use the following procedure to change the mode of SELinux using the GUI.

Note

You need administrator privileges to perform this procedure.
  1. On the System menu, point to Administration and then click Security Level and Firewall to display the Security Level Configuration dialog box.
  2. Click the SELinux tab.
  3. In the SELinux Setting select either Disabled, Enforcing or Permissive, and then click OK.
  4. If you changed from Enabled to Disabled or vice versa, you need to restart the machine for the change to take effect.
Changes made using this dialog box are immediately reflected in /etc/sysconfig/selinux.

50.2.8. Changing the Policy
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This section provides a brief introduction to using customized policies on your system. A full discussion of this topic is beyond the scope of this document.
To load a different policy on your system, change the following line in /etc/sysconfig/selinux: 
SELINUXTYPE=<policyname>
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where <policyname> is the policy name directory under /etc/selinux/. This assumes that you have the custom policy installed. After changing the SELINUXTYPE parameter, run the following commands: 
touch /.autorelabel
reboot
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Use the following procedure to load a different policy using the system-config-selinux utility:

Note

You need administrator privileges to perform this procedure.
  1. Ensure that the complete directory structure for the required policy exists under /etc/selinux.
  2. On the System menu, point to Administration and then click Security Level and Firewall to display the Security Level Configuration dialog box.
  3. Click the SELinux tab.
  4. In the Policy Type list, select the policy that you want to load, and then click OK. This list is only visible if more than one policy is installed.
  5. Restart the machine for the change to take effect.
Using the Security Level Configuration dialog box to load a custom policy.
View larger image
Using the Security Level Configuration dialog box to load a custom policy.

Figure 50.2. Using the Security Level Configuration dialog box to load a custom policy.

You can use the mount -o context= command to set a single context for an entire file system. This might be a file system that is already mounted and that supports xattrs, or a network file system that obtains a genfs label such as cifs_t or nfs_t.
For example, if you need the Apache HTTP Server to read from a mounted directory or loopback file system, you need to set the type to httpd_sys_content_t: 
mount -t nfs -o context=system_u:object_r:httpd_sys_content_t \
	server1.example.com:/shared/scripts /var/www/cgi
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Note

When troubleshooting httpd and SELinux problems, reduce the complexity of your situation. For example, if you have the file system mounted at /mnt and then symbolically linked to /var/www/html/foo, you have two security contexts to be concerned with. Because one security context is of the object class file and the other of type lnk_file, they are treated differently by the policy and unexpected behavior may occur.
Refer to Section 49.5.5, “Assigning Categories to Files” and Section 49.5.4, “Assigning Categories to Users” for information about adding and changing the security categories of files and users.
You can use the runcon command to run a command in a specific context. This is useful for scripting or for testing policy, but care should be taken to ensure that it is implemented correctly.
For example, you could use the following command to run a script to test for mislabeled content. The arguments that appear after the command are considered to be part of the command. (In this example, ~/bin/contexttest is a user-defined script.) 
runcon -t httpd_t ~/bin/contexttest -ARG1 -ARG2
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You can also specify the entire context, as follows: 
runcon user_u:system_r:httpd_t ~/bin/contexttest
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50.2.12. Useful Commands for Scripts
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The following is a list of useful commands introduced with SELinux, and which you may find useful when writing scripts to help administer your system:
getenforce
This command returns the enforcing status of SELinux.
setenforce [ Enforcing | Permissive | 1 | 0 ]
This command controls the enforcing mode of SELinux. The option 1 or Enforcing tells SELinux to enter enforcing mode. The option 0 or Permissive tells SELinux to enter passive mode. Access violations are still logged, but not prevented.
selinuxenabled
This command exits with a status of 0 if SELinux is enabled, and 1 if SELinux is disabled. 
~]# selinuxenabled
~]# echo $?
0
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getsebool [-a] [boolean_name]
This command shows the status of all booleans (-a) or a specific boolean (<boolean_name>).
setsebool [-P] <boolean_name> value | bool1=val1 bool2=val2 ...
This command sets one or more boolean values. The -P option makes the changes persistent across reboots.
togglesebool boolean ...
This command toggles the setting of one or more booleans. This effects boolean settings in memory only; changes are not persistent across reboots.

50.2.13. Changing to a Different Role
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You use the newrole command to run a new shell with the specified type and/or role. Changing roles is typically only meaningful in the strict policy; the targeted policy is generally restricted to a single role. Changing types may be useful for testing, validation, and development purposes. 
newrole -r <role_r> -t <type_t> [-- [ARGS]...]
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The ARGS are passed directly to the shell specified in the user's entry in the /etc/passwd file.

Note

The newrole command is part of the policycoreutils-newrole package, which is required if you install the strict or MLS policy. It is not installed by default in Red Hat Enterprise Linux.

50.2.14. When to Reboot
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The primary reason for rebooting the system from an SELinux perspective is to completely relabel the file system. On occasion you might need to reboot the system to enable or disable SELinux.

50.3. Analyst Control of SELinux
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This section describes some common tasks that a security analyst might need to perform on an SELinux system.

50.3.1. Enabling Kernel Auditing
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As part of an SELinux analysis or troubleshooting exercise, you might choose to enable complete kernel-level auditing. This can be quite verbose, because it generates one or more additional audit messages for each AVC audit message. To enable this level of auditing, append the audit=1 parameter to your kernel boot line, either in the /etc/grub.conf file or on the GRUB menu at boot time.
This is an example of a full audit log entry when httpd is denied access to ~/public_html because the directory is not labeled as Web content. Notice that the time and serial number stamps in the audit(...) field are identical in each case. This makes it easier to track a specific event in the audit logs: 
Jan 15 08:03:56 hostname kernel: audit(1105805036.075:2392892): \
	avc:  denied  { getattr } for  pid=2239 exe=/usr/sbin/httpd \
	path=/home/auser/public_html dev=hdb2 ino=921135 \
	scontext=user_u:system_r:httpd_t \
	tcontext=system_u:object_r:user_home_t tclass=dir
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The following audit message tells more about the source, including the kind of system call involved, showing that httpd tried to stat the directory: 
Jan 15 08:03:56 hostname kernel: audit(1105805036.075:2392892): \
	syscall=195 exit=4294967283 a0=9ef88e0 a1=bfecc0d4 a2=a97ff4 \
	a3=bfecc0d4 items=1 pid=2239 loginuid=-1 uid=48 gid=48 euid=48 \
	suid=48 fsuid=48 egid=48 sgid=48 fsgid=48
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The following message provides more information about the target: 
Jan 15 08:03:56 hostname kernel: audit(1105805036.075:2392892): \
	item=0 name=/home/auser/public_html inode=921135 dev=00:00
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The serial number stamp is always identical for a particular audited event. The time stamp may or may not be identical.

Note

If you are using an audit daemon for troubleshooting, the daemon may capture audit messages into a location other than /var/log/messages, such as /var/log/audit/audit.log.

50.3.2. Dumping and Viewing Logs
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The Red Hat Enterprise Linux 5 implementation of SELinux routes AVC audit messages to /var/log/messages. You can use any of the standard search utilities (for example, grep), to search for lines containing avc or audit.

[22] LVM is the grouping of physical storage into virtual pools that are partitioned into logical volumes.

Chapter 51. Customizing SELinux Policy
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51.1. Introduction
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In earlier releases of Red Hat Enterprise Linux it was necessary to install the selinux-policy-targeted-sources packages and then to create a local.te file in the /etc/selinux/targeted/src/policy/domains/misc directory. You could use the audit2allow utility to translate the AVC messages into allow rules, and then rebuild and reload the policy.
The problem with this was that every time a new policy package was released it would have to execute the Makefile in order to try to keep the local policy.
In Red Hat Enterprise Linux 5, this process has been completely revised. The "sources" rpm packages have been completely removed, and policy packages are treated more like the kernel. To look at the sources used to build the policy, you need to install the source rpm, selinux-policy-XYZ.src.rpm. A further package, selinux-policy-devel, has also been added, which provides further customization functionality.

51.1.1. Modular Policy
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Red Hat Enterprise Linux introduces the concept of modular policy. This allows vendors to ship SELinux policy separately from the operating system policy. It also allows administrators to make local changes to policy without worrying about the next policy install. The most important command that was added was semodule.
semodule is the tool used to manage SELinux policy modules, including installing, upgrading, listing and removing modules. You can also use semodule to force a rebuild of policy from the module store and/or to force a reload of policy without performing any other transaction. semodule acts on module packages created by semodule_package. Conventionally, these files have a .pp suffix (policy package), although this is not mandated in any way.
51.1.1.1. Listing Policy Modules
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To list the policy modules on a system, use the semodule -l command: 
~]# semodule -l
amavis  1.1.0
ccs     1.0.0
clamav  1.1.0
dcc     1.1.0
evolution       1.1.0
iscsid  1.0.0
mozilla 1.1.0
mplayer 1.1.0
nagios  1.1.0
oddjob  1.0.1
pcscd   1.0.0
pyzor   1.1.0
razor   1.1.0
ricci   1.0.0
smartmon        1.1.0
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Note

This command does not list the base policy module, which is also installed.
The /usr/share/selinux/targeted/ directory contains a number of policy package (*.pp) files. These files are included in the selinux-policy rpm and are used to build the policy file.

51.2. Building a Local Policy Module
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The following section uses an actual example to demonstrate building a local policy module to address an issue with the current policy. This issue involves the ypbind init script, which executes the setsebool command, which in turn tries to use the terminal. This is generating the following denial: 
type=AVC msg=audit(1164222416.269:22): avc:  denied  { use } for  pid=1940 comm="setsebool" name="0" dev=devpts ino=2 \
	scontext=system_u:system_r:semanage_t:s0 tcontext=system_u:system_r:init_t:s0 tclass=fd
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Even though everything still works correctly (that is, it is not preventing any applications form running as intended), it does interrupt the normal work flow of the user. Creating a local policy module addresses this issue.
The audit2allow utility now has the ability to build policy modules. Use the following command to build a policy module based on specific contents of the audit.log file:
ausearch -m AVC --comm setsebool | audit2allow -M mysemanage
The audit2allow utility has built a type enforcement file (mysemanage.te). It then executed the checkmodule command to compile a module file (mysemanage.mod). Lastly, it uses the semodule_package command to create a policy package (mysemanage.pp). The semodule_package command combines different policy files (usually just the module and potentially a file context file) into a policy package.

51.2.2. Analyzing the Type Enforcement (TE) File
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Use the cat command to inspect the contents of the TE file: 
~]# cat mysemanag.te
module mysemanage 1.0;

require {
	class fd use;
	type init_t;
	type semanage_t;
	role system_r;
};

allow semanage_t init_t:fd use;
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The TE file is comprised of three sections. The first section is the module command, which identifies the module name and version. The module name must be unique. If you create an semanage module using the name of a pre-existing module, the system would try to replace the existing module package with the newly-created version. The last part of the module line is the version. semodule can update module packages and checks the update version against the currently installed version.
The next block of the TE file is the require block. This informs the policy loader which types, classes and roles are required in the system policy before this module can be installed. If any of these fields are undefined, the semodule command will fail.
Lastly are the allow rules. In this example, you could modify this line to dontaudit, because semodule does not need to access the file descriptor.

51.2.3. Loading the Policy Package
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The last step in the process of creating a local policy module is to load the policy package into the kernel.
Use the semodule command to load the policy package: 
~]# semodule -i mysemanage.pp
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This command recompiles the policy file and regenerates the file context file. The changes are permanent and will survive a reboot. You can also copy the policy package file (mysemanage.pp) to other machines and install it using semodule.
The audit2allow command outputs the commands it executed to create the policy package so that you can edit the TE file. This means you can add new rules as required or change the allow rule to dontaudit. You could then recompile and repackage the policy package to be installed again.
There is no limit to the number of policy packages, so you could create one for each local modification you want to make. Alternatively, you could continue to edit a single package, but you need to ensure that the "require" statements match all of the allow rules.

Chapter 52. References
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The following references are pointers to additional information that is relevant to SELinux and Red Hat Enterprise Linux but beyond the scope of this guide. Note that due to the rapid development of SELinux, some of this material may only apply to specific releases of Red Hat Enterprise Linux.

Books

SELinux by Example
Mayer, MacMillan, and Caplan
Prentice Hall, 2007

Tutorials and Help

Understanding and Customizing the Apache HTTP SELinux Policy
http://docs.fedoraproject.org/selinux-apache-fc3/
Tutorials and talks from Russell Coker
http://www.coker.com.au/selinux/talks/ibmtu-2004/
Generic Writing SELinux policy HOWTO
https://sourceforge.net/docman/display_doc.php?docid=21959[amp ]group_id=21266
Red Hat Knowledgebase
http://kbase.redhat.com/

General Information

NSA SELinux main website
http://www.nsa.gov/research/selinux/index.shtml
NSA SELinux FAQ
http://www.nsa.gov/research/selinux/faqs.shtml
Fedora SELinux FAQ
http://docs.fedoraproject.org/selinux-faq/
SELinux NSA's Open Source Security Enhanced Linux
http://www.oreilly.com/catalog/selinux/

Technology

An Overview of Object Classes and Permissions
http://www.tresys.com/selinux/obj_perms_help.html
Integrating Flexible Support for Security Policies into the Linux Operating System (a history of Flask implementation in Linux)
http://www.nsa.gov/research/_files/selinux/papers/freenix01/freenix01.shtml
Implementing SELinux as a Linux Security Module
http://www.nsa.gov/research/selinux/index.shtmlpapers/module-abs.cfm
A Security Policy Configuration for the Security-Enhanced Linux
http://www.nsa.gov/research/_files/selinux/papers/policy/policy.shtml

Community

SELinux community page
http://selinux.sourceforge.net
IRC
irc.freenode.net, #rhel-selinux

History

Quick history of Flask
http://www.cs.utah.edu/flux/fluke/html/flask.html
Full background on Fluke
http://www.cs.utah.edu/flux/fluke/html/index.html

Part VIII. Red Hat Training And Certification
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Red Hat courses and certifications are indisputably regarded as the best in Linux, and perhaps in all of IT. Taught entirely by experienced Red Hat experts, our certification programs measure competency on actual live systems and are in great demand by employers and IT professionals alike.
Choosing the right certification depends on your background and goals. Whether you have advanced, minimal, or no UNIX or Linux experience whatsoever, Red Hat Training has a training and certification path that is right for you.

Chapter 53. Red Hat Training and Certification
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53.1. Three Ways to Train
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Open Enrollment
Open enrollment courses are offered continually in 50+ locations across North America and 125+ locations worldwide. Red Hat courses are performance—based—students have access to at least one dedicated system, and in some courses, as many as five. Instructors are all experienced Red Hat Certified Engineers (RHCEs) who are intimately familiar with course curriculum.
Course schedules are available at http://www.redhat.com/explore/training
Onsite Training
Onsite training is delivered by Red Hat at your facility for teams of 12 to 16 people per class. Red Hat's technical staff will assist your technical staff prior to arrival to ensure the training venue is prepared to run Red Hat Enterprise Linux, Red Hat or JBoss courses, and/or Red Hat certification exams. Onsites are a great way to train large groups at once. Open enrollment can be leveraged later for incremental training.
For more information, visit http://www.redhat.com/explore/onsite
eLearning
Fully updated for Red Hat Enterprise Linux 4! No time for class? Red Hat's e—Learning titles are delivered online and cover RHCT and RHCE track skills. Our growing catalog also includes courses on the latest programming languages, scripting and ecommerce.
For course listings visit http://www.redhat.com/explore/elearning
Tailored info and offers for Microsoft® Certified Professionals looking to add a Red Hat certification to their personal portfolio.
Check it out today: http://www.redhat.com/explore/manager

Chapter 54. Certification Tracks
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Red Hat Certified Technician® (RHCT®)
Now entering its third year, Red Hat Certified Technician is the fastest-growing credential in all of Linux, with currently over 15,000 certification holders. RHCT is the best first step in establishing Linux credentials and is an ideal initial certification for those transitioning from non-UNIX®/ Linux environments.
Red Hat certifications are indisputably regarded as the best in Linux, and perhaps, according to some, in all of IT. Taught entirely by experienced Red Hat experts, our certification programs measure competency on actual live systems and are in great demand by employers and IT professionals alike.
Choosing the right certification depends on your background and goals. Whether you have advanced, minimal, or no UNIX or Linux experience whatsoever, Red Hat Training has a training and certification path that is right for you.
Red Hat Certified Engineer® (RHCE®)
Red Hat Certified Engineer began in 1999 and has been earned by more than 20,000 Linux experts. Called the "crown jewel of Linux certifications," independent surveys have ranked the RHCE program #1 in all of IT.
Red Hat Certified Security Specialist (RHCSS)
An RHCSS has RHCE security knowledge plus specialized skills in Red Hat Enterprise Linux, Red Hat Directory Server and SELinux to meet the security requirements of today's enterprise environments. RHCSS is Red Hat's newest certification, and the only one of its kind in Linux.
Red Hat Certified Architect (RHCA)
RHCEs who seek advanced training can enroll in Enterprise Architect courses and prove their competency with the newly announced Red Hat Certified Architect (RHCA) certification. RHCA is the capstone certification to Red Hat Certified Technician (RHCT) and Red Hat Certified Engineer (RHCE), the most acclaimed certifications in the Linux space.

54.1. Free Pre-assessment tests
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Test your Linux smarts and identify your Red Hat course level with our automated pre-assessment tests.
Completely free, no obligations, 10 minutes of your time. http://www.redhat.com/explore/assess

Chapter 55. RH033: Red Hat Linux Essentials
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http://www.redhat.com/training/rhce/courses/rh033.html

55.1. Course Description
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The first course for both RHCT and RHCE certification tracks, RH033 is ideal for individuals who have never used Linux or UNIX, and who have no prior command line experience in any other operating system. You are taught the basics of a Red Hat Enterprise Linux environment, and it prepares you for your future role as a system administrator.

55.1.1. Prerequisites
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User-level experience with any computer system, use of menus, use of any graphical user interface.

55.1.2. Goal
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A Red Hat Enterprise Linux power user who can be productive in using and customizing a Red Hat system for common command line processes and desktop productivity roles, and who is ready to learn system administration (RH133).

55.1.3. Audience
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Users who are new to Linux and have no prior UNIX or command line skills, who want to develop and practice the basic skills to use and control their own Red Hat Linux system.

55.1.4. Course Objectives
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  1. Understand the Linux file system
  2. Perform common file maintenance
  3. Use and customize the GNOME interface
  4. Issue essential Linux commands from the command line
  5. Perform common tasks using the GNOME GUI
  6. Open, edit, and save text documents using the vi editor
  7. File access permissions
  8. Customize X Window System
  9. Regular expression pattern matching and I/O redirection
  10. Install, upgrade, delete and query packages on your system
  11. Network utilities for the user
  12. Power user utilities

55.1.5. Follow-on Courses
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RH133 Red Hat Linux Sys. Admin.
RH253 Red Hat Linux Net. and Sec. Admin
RH300 Red Hat Linux RHCE Rapid Track
"I would enthusiastically recommend this course to anyone interested in Linux."——Mike Kimmel, ITT Systems Division
http://www.redhat.com/training/rhce/courses/rh035.html

56.1. Course Description
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Designed for Windows® professionals with no prior UNIX or Linux experience, this course teaches fundamental Red Hat Enterprise Linux system administration skills. The first day provides a conceptual and practical transition for individuals to successfully add Linux management competencies to their portfolio. The remaining four days combines with the highly-acclaimed RH033 course, immersing individuals in the basics of a Red Hat Enterprise Linux environment and preparing them for future roles as cross-platform system administrators. The course also serves as the first course in the RHCT and RHCE tracks.

56.1.1. Prerequisites
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Have experience with job tasks using Windows OS products at technician or system administrator level; experience as an IT professional; no prior UNIX or Linux experience required.

56.1.2. Goal
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A Red Hat Enterprise Linux power user familiar with common command line processes who can perform some system administration tasks using graphical tools. The individual will also be ready to develop a deeper understanding of Red Hat Enterprise Linux system administration (RH133).

56.1.3. Audience
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The typical student will be a Windows technician who prefers to manage servers using a graphic user interface. The individual will also possess a desire to effectively manage Red Hat Enterprise Linux systems and broaden their individual skill set.

56.1.4. Course Objectives
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  1. Learn to install software, configure the network, configure authentication, and install and configure various services using graphical tools
  2. Understand the Linux file system
  3. Issue essential Linux commands from the command line
  4. Understand file access permissions
  5. Customize X Window System
  6. Use regular expression pattern matching and I/O redirection

56.1.5. Follow-on Courses
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RH133 Red Hat Linux Sys. Admin. (p. 8)
RH253 Red Hat Linux Net. and Sec. Admin. (p. 9)
RH300 Red Hat Linux RHCE Rapid Track (p. 10)
"All in all I would rate this training experience as one of the best I have ever attended, and I've been in this industry for over 15 years." — Bill Legge, IT Consultant
http://www.redhat.com/training/rhce/courses/rh133.html

57.1. Course Description
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RH133 focuses on skills in systems administration on Red Hat Linux, to a level where you can attach and configure a workstation on an existing network. This 4.5-day course provides intensive hands-on training on Red Hat Enterprise Linux, and includes the RH202 RHCT Certification Lab Exam on the last day.

57.1.1. Prerequisites
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RH033 Red Hat Linux Essentials or equivalent experience with Red Hat Linux.

57.1.2. Goal
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Upon successful completion of this course, students will possess basic Linux system administrator knowledge which can be proved by passing the RHCT Exam. The exam is a performance-based lab exam that tests actual ability to install, configure, and attach a new Red Hat Linux system to an existing production network.

57.1.3. Audience
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Linux or UNIX users who understand the basics of Red Hat Linux and desire further technical training to begin the process of becoming a system administrator.

57.1.4. Course Objectives
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  1. Install Red Hat Linux interactively and with Kickstart
  2. Control common system hardware; administer Linux printing subsystem
  3. Create and maintain the Linux filesystem
  4. Perform user and group administration
  5. Integrate a workstation with an existing network
  6. Configure a workstation as a client to NIS, DNS, and DHCP services
  7. Automate tasks with at, cron, and anacron
  8. Back up filesystems to tape and tar archive
  9. Manipulate software packages with RPM
  10. Configure the X Window System and the GNOME d.e.
  11. Perform performance, memory, and process mgmt.
  12. Configure basic host security

57.1.5. Follow-on Courses
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RH253 Red Hat Linux Net. and Sec. Admin. (p. 9)
http://www.redhat.com/training/rhce/courses/rh202.html
  1. RHCT exam is included with RH133. It can also be purchased on its own for $349
  2. RHCT exams occur on the fifth day of all RH133 classes

58.1. Course Description
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The RHCT (Red Hat Certified Technician) is a hands-on, performance-based exam testing candidates actual skills in installing, configuring, and troubleshooting Red Hat Enterprise Linux. The Certification Lab Exam is bundled with RH133, but individuals who have mastered the content of RH033 and RH133 can take just the exam.

58.1.1. Prerequisites
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Candidates should consider taking RH033 and RH133 in preparation for the exam, but they are not required to take it.

59.1. Course Description
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RH253 arms students with in-depth knowledge needed to configure common Red Hat Enterprise Linux network services. Network and local security tasks are also topics of this course.

59.1.1. Prerequisites
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RH133 Red Hat Linux System Administration or equivalent experience with Red Hat Enterprise Linux, LAN/WAN fundamentals or equivalent, internetworking with TCP/IP or equivalent.

59.1.2. Goal
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Upon completion of this course, individuals can set up a Red Hat Enterprise Linux server and configure common network services and security at a basic level.

59.1.3. Audience
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Linux or UNIX system administrators who already have some real-world experience with Red Hat Enterprise Linux systems administration, want a first course in networking services and security, and want to build skills at configuring common network services and security administration using Red Hat Enterprise Linux.

59.1.4. Course Objectives
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  1. Networking services on Red Hat Linux server-side setup, configuration, and basic administration of common networking services: DNS, NIS, Apache, SMB, DHCP, Sendmail, FTP. Other common services: tftp, pppd, proxy.
  2. Introduction to security
  3. Developing a security policy
  4. Local security
  5. Files and filesystem security
  6. Password security
  7. Kernel security
  8. Basic elements of a firewall
  9. Red Hat Linux-based security tools
  10. Responding to a break-in attempt
  11. Security sources and methods
  12. Overview of OSS security tools

59.1.5. Follow-on Courses
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RH302 RHCE Certification Exam
"This course was excellent. The teacher was fantastic—his depth of knowledge is amazing."——Greg Peters, Future Networks USA
The fastest path to RHCE certification for experienced UNIX/Linux users.
http://www.redhat.com/training/rhce/courses/rh300.html

60.1. Course Description
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Five days in duration, this course provides intensive hands-on training on Red Hat Linux, and includes the RHCE Certification Exam on the last day.

60.1.1. Prerequisites
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RH033, RH133, RH253 or equivalent experience with UNIX. Please do not register for RH300 unless you are experienced with systems administration or are a power user in UNIX or Linux environments.

60.1.2. Goal
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Upon successful completion of this course, individuals will be a Red Hat Linux system administrator who has been trained and then tested using the RHCE Exam.

60.1.3. Audience
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UNIX or Linux system administrators who have significant real-world experience and who want a fast-track course to prepare for the RHCE Exam.

60.1.4. Course Objectives
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  1. Hardware and Installation (x86 architecture)
  2. Configuration and administration
  3. Alternate installation methods
  4. Kernel services and configuration
  5. Standard networking services
  6. X Window system
  7. User and host security
  8. Routers, Firewalls, Clusters and Troubleshooting

60.1.5. Follow-on Courses
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Enterprise Architect curriculum and RHCA certification

Chapter 61. RH302 RHCE EXAM
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  1. RHCE exams are included with RH300. It can also be purchased on its own.
  2. RHCE exams occur on the fifth day of all RH300 classes
http://www.redhat.com/training/rhce/courses/rhexam.html

61.1. Course Description
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RHCE stands apart from many other certification programs in the IT sector because of its emphasis on hands-on, performance-based testing of actual skills in Red Hat Linux installation, configuration, debugging, and setup of key networking services.

61.1.1. Prerequisites
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See RH300 course prerequisites. For further information, please refer to the RHCE Exam Prep Guide: www.redhat.com/training/rhce/examprep.html

61.1.2. Content
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  1. Section I: Troubleshooting and System Maintenance (2.5 hrs)
  2. Section II: Installation and Configuration (3 hrs.)
"Seriously, this was an outstanding class. I feel very well prepared for the test tomorrow." — Logan Ingalls, Web developer, Texterity Inc., USA
Security for the most commonly deployed services.
http://www.redhat.com/training/architect/courses/rhs333.html

62.1. Course Description
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Red Hat Enterprise Linux has gained considerable momentum as the operating system of choice for deploying network services such as web, ftp, email, and file sharing. Red Hat's RHCE curriculum provides training in deploying these services and on the essential elements of securing them.

62.1.1. Prerequisites
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RH253, RH300, or RHCE certification or equivalent work experience is required for this course. Course participants should already know the essential elements of how to configure the services covered, as this course will be focusing on more advanced topics from the outset.

62.1.2. Goal
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This class advances beyond the essential security coverage offered in the RHCE curriculum and delves deeper into the security features, capabilities, and risks associated with the most commonly deployed services.

62.1.3. Audience
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The audience for this course includes system administrators, consultants, and other IT professionals responsible for the planning, implementation, and maintenance of network servers. While the emphasis is on running these services on Red Hat Enterprise Linux, and the content and labs will assume its use, system administrators and others using proprietary forms of UNIX may also find many elements of this course relevant.

62.1.4. Course Objectives
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  1. Mastering basic service security
  2. Understanding cryptography
  3. Logging system activity
  4. Securing BIND and DNS
  5. Network user authentication security
  6. Improving NFS security
  7. The secure shell: OpenSSH
  8. Securing email with Sendmail and Postfix
  9. Managing FTP access
  10. Apache security
  11. Basics of intrusion response

62.1.5. Follow-on Courses
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RH401 Red Hat Enterprise Deployment and System Mgmt. RH423 Red Hat Enterprise Directory Services and Authentication RH436 Red Hat Enterprise Storage Mgmt. RH442 Red Hat Enterprise System Monitoring and Performance Tuning
Manage Red Hat Enterprise Linux deployments.
http://www.redhat.com/training/architect/courses/rh401.html

63.1. Course Description
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RH401 is a four-day intensive hands-on lab course in skills and methods critical to large-scale deployment and management of mission-critical Red Hat Enterprise Linux systems, including failover and load-balancing, CVS for system administrators, RPM rebuilding, and performance tuning for specific applications.

63.1.1. Prerequisites
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RH253 at a minimum, RHCE certification preferred, or comparable skills and knowledge. All prospective course participants without RHCE certification are encouraged to verify skills with Red Hat's free online pre—assessment tests. Note: Persons should not enroll in RH401 without meeting the above prerequisites.
All prospective course participants who do not possess RHCE certification are strongly advised to contact Red Hat Global Learning Services for a skills assessment when they enroll.

63.1.2. Goal
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RH401 trains senior system administrators to manage large numbers of Enterprise Linux servers in a variety of roles, and/or manage them for mission—critical applications that require failover and load-balancing. Further, RH401 is benchmarked on expert—level competencies in managing operating systems for enterprise roles—the course teaches how to implement and manage enterprise Red Hat Enterprise Linux deployments efficiently and effectively in ways that make the entire enterprise deployment manageable by a team.

63.1.3. Audience
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Senior Red Hat Enterprise Linux system administrators and other IT professionals working in enterprise environments and mission-critical systems.

63.1.4. Course Objectives
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  1. Configuration management using CVS
  2. Construction of custom RPM packages
  3. Software management with Red Hat Network Proxy Server
  4. Assembling a host provisioning and management system
  5. Performance tuning and analysis
  6. High-availability network load-balancing clusters
  7. High-availability application failover clusters

63.1.5. Follow-on Courses
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RHS333 Enterprise Security: Securing Network Services
RH423 Red Hat Enterprise Directory Services and Authentication
RH436 Red Hat Enterprise Storage Mgmt.
RH442 Red Hat Enterprise System Monitoring and Performance Tuning
"After taking RH401 I am completely confident that I can implement enterprise—scale high—availability solutions end-to-end."——Barry Brimer, Bunge North America
Manage and deploy directory services for Red Hat Enterprise Linux systems.
http://www.redhat.com/training/architect/courses/rh423.html

64.1. Course Description
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RH423 is an intensive course that provides four days of instruction and labs on cross-platform integration of directory services to provide authentication or information service across the enterprise.

64.1.1. Prerequisites
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RH253 at a minimum, RHCE certification preferred, or comparable skills and knowledge. All prospective course participants without RHCE certification are encouraged to verify skills with Red Hat's free online pre—assessment tests. Note: Persons should not enroll in RH423 without meeting the above prerequisites. All prospective course participants who do not possess RHCE certification are strongly advised to contact Red Hat Global Learning Services for a skills assessment when they enroll.

64.1.2. Goal
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RH423 trains senior system administrators to manage and deploy directory services on and for Red Hat Enterprise Linux systems. Gaining an understanding of the basic concepts, configuration, and management of LDAP—based services is central to this course. Students will integrate standard network clients and services with the directory service in order to take advantage of its capabilities. We will also look at PAM, the Pluggable Authentication Modules system, and how it is integrated with services that require authentication and authorization.

64.1.3. Audience
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Senior Red Hat Enterprise Linux system administrators and other IT professionals working in enterprise environments and mission-critical systems.

64.1.4. Course Objectives
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  1. Basic LDAP concepts
  2. How to configure and manage an OpenLDAP server
  3. Using LDAP as a "white pages" directory service
  4. Using LDAP for user authentication and management
  5. Integrating multiple LDAP servers

64.1.5. Follow-on Courses
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RHS333 Enterprise Security: Securing Network Services
RH401 Red Hat Enterprise Deployment and Systems Management
RH436 Red Hat Enterprise Storage Mgmt. (p. 16)
RH442 Red Hat Enterprise System Monitoring and Performance Tuning

Chapter 65. SELinux Courses
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http://www.redhat.com/training/security/courses/rhs427.html
1-day rapid intro to SELinux, how it operates within the Red Hat targeted policy, and the tools available for working with this powerful capability. RHS427 constitutes the first day of RH429.

65.1.1. Audience
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Computer security specialists and others responsible for implementing security policies on a Linux computer. RHS429 requires RHCE or comparable knowledge.

65.1.2. Course Summary
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Among the most significant features of Red Hat Enterprise Linux is SELinux (Security Enhanced Linux), a powerful, kernel-level security layer that provides fine-grained control over what users and processes may access and do on a system. By default, SELinux is enabled on Red Hat Enterprise Linux systems, enforcing a set of mandatory access controls that Red Hat calls the targeted policy. These access controls substantially enhance the security of the network services they target, but can sometimes affect the behavior of third-party applications and scripts that worked on previous versions of Red Hat Enterprise Linux.
http://www.redhat.com/training/security/courses/rhs429.html
Among the most significant features of Red Hat Enterprise Linux is SELinux (Security Enhanced Linux), a powerful, kernel-level security layer that provides fine-grained control over what users and processes may access and execute on a system. RHS429 introduces advanced system administrators, security administrators, and applications programmers to SELinux policy writing. Participants in this course will learn how SELinux works; how to manage SELinux; and how to write an SELinux policy.
Deploy and manage Red Hat's cluster file system technology.
Equipment-intensive:
  1. five servers
  2. storage array
http://www.redhat.com/training/architect/courses/rh436.html

66.1. Course Description
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RH436 provides intensive hands-on experience with the emerging Shared Storage technology delivered by Red Hat Global File System (GFS). This four-day course focuses on the implementation of native Red Hat Enterprise Linux technologies included in Red Hat Cluster Suite and GFS.

66.1.1. Prerequisites
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RH253 at a minimum, RHCE certification preferred, or comparable skills and knowledge. All prospective course participants without RHCE certification are encouraged to verify skills with Red Hat's free online pre—assessment tests.

66.1.2. Goal
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This course is designed to train people with RHCE-level competency on skills required to deploy and manage highly available storage data to the mission-critical enterprise computing environment. Complementing skills gained in RH401, this course delivers extensive hands-on training with the cluster file system, GFS.

66.1.3. Audience
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Senior Red Hat Enterprise Linux system administrators and other IT professionals working in enterprise environments and mission-critical systems.

66.1.4. Course Objectives
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  1. Review Red Hat Enterprise Linux storage management technologies
  2. Data storage design: Data sharing
  3. Cluster Suite overview
  4. Global File System (GFS) overview
  5. GFS management
  6. Modify the online GFS environment: Managing data capacity
  7. Monitor GFS
  8. Implement GFS modifications
  9. Migrating Cluster Suite NFS from DAS to GFS
  10. Re-visit Cluster Suite using GFS

66.1.5. Follow-on Courses
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RHS333 Enterprise Security: Securing Network Services
RH401 Red Hat Enterprise Deployment and Systems Management
RH423 Red Hat Enterprise Directory Services and Authentication
RH442 Red Hat Enterprise System Monitoring and Performance Tuning
"The class gave me a chance to use some of the latest Linux tools, and was a reminder of the benefits of using Linux for high-availability systems."——Paul W. Frields, FBI — Operational Technology Division Quantico, VA, USA
Performance tuning and capacity planning for Red Hat Enterprise Linux
http://www.redhat.com/training/architect/courses/rh442.html

67.1. Course Description
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RH442 is an advanced four-day hands-on lab course covering system architecture, performance characteristics, monitoring, benchmarking, and network performance tuning.

67.1.1. Prerequisites
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RHCT at a minimum, RHCE certification recommended, or comparable skills and knowledge. All prospective course participants without RHCE certification are encouraged to verify skills with Red Hat's free online pre—assessment tests.

67.1.2. Goal
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RH442 is designed to teach the methodology of performance tuning and capacity planning for Red Hat Enterprise Linux. This class will cover:
  1. A discussion of system architecture with an emphasis on understanding the implications of system architecture on system performance
  2. Methods for testing the effects of performance adjustments (benchmarking)
  3. Open source benchmarking utilities
  4. Methods for analyzing system performance and networking performance
  5. Tuning configurations for specific application loads

67.1.3. Audience
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RH442 is aimed at senior Red Hat Enterprise Linux system administrators and other IT professionals working in enterprise environments and mission-critical systems.

67.1.4. Course Objectives
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  1. Overview of system components and architecture as they relate to system performance
  2. Translating manufacturers' hardware specifications into useful information
  3. Using standard monitoring tools effectively to gather and analyze trend information
  4. Gathering performance-related data with SNMP
  5. Using open source benchmarking utilities
  6. Network performance tuning
  7. Application performance tuning considerations
  8. Tuning for specific configurations

67.1.5. Follow-on Courses
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RHS333 Enterprise Security: Securing Network Services
RH401 Red Hat Enterprise Deployment and Systems Management
RH423 Red Hat Enterprise Directory Services and Authentication
RH436 Red Hat Enterprise Storage Mgmt.
http://www.redhat.com/training/developer/courses/rhd143.html
An intensive hands-on course designed to rapidly train staff in key skills for developing applications and programs on Red Hat Enterprise Linux. This five-day course provides hands-on training, concepts, demonstrations, with emphasis on realistic labs and programming exercises. Upon completion of the course, students will have learned and practiced the essential skills required to develop programs for Linux systems.

68.2. RHD221 Red Hat Linux Device Drivers
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http://www.redhat.com/training/developer/courses/rhd221.html
This course is designed to teach experienced programmers how to develop device drivers for Linux systems. Upon completion of the course, students will understand the Linux architecture, hardware and memory management, modularization, and the layout of the kernel source, and will have practiced key concepts and skills for development of character, block, and network drivers.

68.3. RHD236 Red Hat Linux Kernel Internals
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http://www.redhat.com/training/developer/courses/rhd236.html
This course is designed to provide a detailed examination of the Linux kernel architecture, including process scheduling, memory management, file systems, and driving peripheral devices. This five-day course provides hands-on training, concepts, and demonstrations, with emphasis on realistic labs and programming exercises.
http://www.redhat.com/training/developer/courses/rhd256.html
A four-day developer course for experienced programmers who are already familiar with development on a UNIX-like system and want to develop new applications as well as port existing applications to Red Hat Enterprise Linux.

Chapter 69. JBoss Courses
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69.1. RHD161 JBoss and EJB3 for Java
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http://www.redhat.com/training/jboss/courses/rhd161.html
Developers JBoss and EJB3 for Java Developers is targeted toward proficient Java developers who wish to extend their knowledge to EJB3 and J2EE middleware programming using the JBoss Application Server. This class is an in-depth introduction to EJB3 and J2EE using the JBoss Application Server. It provides a hands-on approach to EJB3 and J2EE application development, deployment and the tools necessary to facilitate both processes.

69.1.1. Prerequisites
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Basic Java programming skills and knowledge of OOAD concepts are required. The student must have practical knowledge of, and/or experience with, the following:
  1. The object-oriented concepts of inheritance, polymorphism and encapsulation
  2. Java syntax, specifically for data types, variables, operators, statements and control flow
  3. Writing Java classes as well as using Java interfaces and abstract classes

69.2. RHD163 JBoss for Web Developers
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http://www.redhat.com/training/jboss/courses/rhd163.html
JBoss for Web Developers focuses on web tier technologies in the JBoss Enterprise Middleware System (JEMS) product stack. We cover details on JBoss Portal, how to create and deploy portlets, integrating portlets with other web tier frameworks such as JavaServer Faces JSF) and configuring and tuning the Tomcat web container embedded in JBoss Application Server. Familiarity with JSP and Servlet development and related specification is heavily recommended. No previous experience with Portlets or JSF is required.

69.2.1. Prerequisites
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The prerequisite skills for this class are basic J2EE Web Container (Servlet/JSP) programming skills and some experience with J2EE Web-based and multi-tier application deployments on the JBoss Application Server in conjunction with the Tomcat container (whether embedded with Apache or integrated with the JBoss Application server). The student should have development experience with the following technologies:
  1. JNDI
  2. The Servlet 2.3/2.4 API
  3. The JSP 2.0 API
  4. J2EE application development and deployment on the JBoss Application Server
  5. Deployment of a Web Application on embedded (stand alone) Tomcat or on integrated Tomcat (JBossWeb)
  6. A working knowledge of JDBC and EJB2.1 or EJB3.0
while not a prerequisite, is helpful.

69.3. RHD167: JBOSS - HIBERNATE ESSENTIALS
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http://www.redhat.com/training/jboss/courses/rhd167.html

69.3.1. Prerequisites
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  1. An understanding of the relational persistence model
  2. Competency with the Java language
  3. Knowledge of OOAD concepts
  4. Familiarity with the UML
  5. Experience with a dialect of SQL
  6. Using the JDK and creating the necessary environment for compilation and execution of a Java executable from the command line
  7. An understanding of JDB
No prior knowledge of J2EE or Hibernate is required. This training is based on Hibernate 3.2 series.

69.3.2. Course Summary
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Hibernate Essentials is targeted toward Java developers who must become competent with the Hibernate or the Java Persistence API object/relational persistence and query service implementation. The primary audience is intended to be Java developers who work with SQL-based database systems or database developers who are looking for an introduction to object-oriented software development. Database administrators who are interested in how ORM may affect performance and how to tune the performance of the SQL database management system and persistence layer will also find this course of value. This course covers the JBoss, Inc. implementation of the JSR-220 sub-specification for Java Persistence and it covers the foundational APIs of version 3.x of the JBoss, Inc. Hibernate product, or simply, Hibernate 3.

69.4. RHD267: JBOSS - ADVANCED HIBERNATE
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http://www.redhat.com/training/jboss/courses/rhd267.html
JBoss Advanced Hibernate training is targeted toward Java developers who wish to extract the full power of the Hibernate O/R Mapping framework. The primary target audience consists of Java developers who work with SQL-based database systems, database developers who are looking for an introduction to object-oriented software development and database administrators interested in how ORM affects performance and how to tune the performance of the SQL database management system and persistence layer. The training covers the new Hibernate 3 features.

69.4.1. Prerequisites
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The prerequisite skills for this class are the following:
  1. Basic Hibernate knowledge.
  2. Competency with the Java language
  3. Knowledge of OOAD concepts
  4. Familiarity with the UML
  5. Experience with a dialect of SQL
  6. Using the JDK and creating the necessary environment for compilation and execution of a Java executable from the command line.
  7. Experience with, or comprehensive knowledge of JNDI and JDBC.
  8. Entity EJB2.1 or EJB3.0 knowledge, while not a prerequisite, is helpful.
  9. Prior reading of the book Hibernate in Action, by Christian Bauer and Gavin King (published by Manning) is recommended.
"The best part of the Advanced Hibernate course was networking with fellow engineers that had problems similar to my own, and working with a knowledgeable instructor to solve them."--Mike Pasternak, Consulting Engineer, United Switch & Signal

69.5. RHD261:JBOSS for advanced J2EE developers
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http://www.redhat.com/training/jboss/courses/rhd261.html
JBoss for Advanced J2EE Developers is targeted toward J2EE professionals who wish to take advantage of the JBoss Application Server internal architecture to enhance the functionality and performance of J2EE applications on the JBoss Application Server. This course covers topics such as JMX and those beyond the J2EE specification such as Microkernel architecture, Security, Clustering, and Fine Tuning.

69.5.1. Prerequisites
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It is highly recommended that students either complete the JBoss for Java Developers course OR take the Middleware Placement Exam prior to registering for the JBoss for Advanced J2EE Developers course. The developer should have practical experience with each of the following topics:
  1. JNDI
  2. JDBC
  3. Servlets and JSPs
  4. Enterprise Java Beans
  5. JMS
  6. The J2EE Security Model
  7. J2EE application development and deployment on the JBoss Application
  8. Experience with ANT and XDoclet or similar technologies.
While prior knowledge of JMX is helpful, it is not required. This training is based on the 4.x series of the JBoss Application Server.
"I thought the training materials were well-organized, including both the handbook and the labs. The instructor frequently asked for feedback on material and pace. It was apparent that he cared about our understanding of the material."--Jeremy Prellwitz, SiRAS.com, USA

69.6. RH336: JBOSS for Administrators
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http://www.redhat.com/training/jboss/courses/rh336.html

69.6.1. Prerequisites
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Basic working knowledge of the Windows or Linux (Unix-based) operating system. The student must have experience with the following:
  1. Creating directories, files and modifying access rights to the file store
  2. Installing a JDK
  3. Configuring environment variables, such as JAVA_HOME, for an Operating system
  4. Launching Java applications and executing an OS-dependent script that launches a Java application.
  5. Creating and expanding a Java archive file (the jar utility)
No prior knowledge of J2EE or the JBoss Application Server is required. Some familiarity with supporting Java applications with XML configurations, however, is strongly recommended.

69.6.2. Course Summary
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JBoss for Administrators is targeted toward application support individuals, such as system administrators, configuration management and quality assurance personnel who wish to become proficient in configuring and administrating the JBoss application server (3.2.x and 4.x series) and the applications deployed on the application server.
"The JBoss for Administrators course was a great balance of both lecture and labs. It is always nice to have hands on knowledge of the topics to make them seem more real and applicable."——Thomas Skowronek, Palm Harbor Homes, USA

69.7. RHD439: JBoss Clustering
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http://www.redhat.com/training/jboss/courses/rhd439.html
Clustering is a 4-day training focusing on high availability services of JBoss Enterprise Middleware System (JEMS). You will learn how JBoss Application Server leverages JGroups and JBoss Cache for replication and fail-over, how to configure, tune and implement JGroups protocol stacks, how to leverage JBoss Cache in your own middleware application implementation and how to use and configure mod_jk for HTTP load balancing. We will also cover in some detail JBoss Application Server high availability services such as HA-JNDI, HA-JMS and HA-singleton.

69.7.1. Prerequisites
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Completion of the JBoss for Advanced J2EE Developers course is strongly recommended before taking this course. It is also strongly recommended that the student has at minimum 18 month practical development experience using J2EE and other Java middleware technologies, and it is suggested that the student have some practical experience with JBoss Application Server. Solid Java programming experience (minimum 3 years) is required and understanding of basic TCP/IP topics is necessary.
The student must have the following skills:
  1. JTA, Transactions, Java concurrency
  2. EJB 2.1, JMS, reliable messaging technologies
  3. Previous experience with Apache httpd and some exposure to mod_jk and/or mod_proxy
  4. Familiar with JBoss AS microkernel and JMX
  5. Familiarity with TCP/IP, UDP, Multicasting
"The JBoss for Administrators course was very informative. Our instructor did a great job at answering our questions (some very specific to the student) while maintaining the course direction. I am very excited about applying what I have learned in the course."——Andy Beier, Arizona Statue University, USA

69.8. RHD449: JBoss jBPM
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http://www.redhat.com/training/jboss/courses/rhd449.html

69.8.1. Description
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JBoss jBPM training is targeted for system architects and developers who work closely with business analysts and are responsible for bringing business processes into J2EE environment using jBPM as a BPM engine. In addition, The JBoss jBPM training will provide students with a thorough understanding of the BPM landscape, types of engines and positioning of the buzzwords.
Students will acquire practical hands on expertise and will be ready to start developing business processes with JBoss jBPM after the course. Another goal of the training is to provide a thorough preparation for comparing workflow engines.

69.8.2. Prerequisites
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  1. The student must have previous experience developing an Hibernate application. The student must know how to configure a simple Session Factory for Hibernate, utilize a Hibernate Session and transactional demarcation and how to perform basic queries on Hibernate objects.
  2. Competency with Java application development.
  3. Previous exposure to the concepts of workflow and business process modeling (BPM) is not required
  4. Experience with JBoss Eclipse or the Eclipse IDE with the JBoss plugin is recommended but not required
  5. Basic notions of JUnit test framework is recommended.

69.9. RHD451 JBoss Rules
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http://www.redhat.com/training/jboss/courses/rhd451.html
The course covers the core engine for Drools 3 (JBoss Rules 3.0), as well as the various techniques and languages that can be used to manage business rules, and how the rule engine may be embedded in J2SE and J2EE applications. This course will be a complimentary course to any future courses on rule management using future releases of Jboss Rules.

69.9.1. Prerequisites
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  1. Basic Java competency
  2. Some understanding of what constitutes an inferencing rule engine versus a scripting engine
  3. Viewing of the Jboss Rules webinars and demos is recommended but not required
  4. Java EE specific experience is not required for the course, but students who need to know how to integrate with Java EE will need the appropriate experience

Appendix A. Revision History
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Revision History
Revision 11-1Tue 30 Jun 2015Barbora Ančincová
Updated the book with information about the POODLE vulnerability (CVE-2014-3566).
Revision 11-0Fri 12 Sep 2014Barbora Ančincová
Resolve BZ#1121893: RHEL 5 Deployment Guide - Bonding Options - regarding max_bonds and debug options.
Resolve BZ#1104152: Insecure way of generating swapfile is proposed in our documentation.
Revision 10-0Tue 01 Oct 2013Jaromír Hradílek
Resolve BZ#853938: RFE: The proc File System: Document restricting access to /proc/<PID>.
Resolve BZ#826891: RFE: Yum: Provide a procedure to upgrade systems with "yum update" using RHEL Installation ISO image.
Resolve BZ#961815: Document migration mysql5.1->mysql5.5.
Revision 9-6Tue Jan 08 2013Jaromír Hradílek
Resolve BZ#810514: RFE: Document the Ext4 File System.
Resolve BZ#216687: RFE: Postfix - a standard, FHS-compliant place for virtual user mailboxes.
Resolve BZ#816177: RFE: Postfix MySQL map support.
Resolve BZ#810512: MinorMod: Automated Tasks: default log rotation cronjob causes problems in a RHEV shared storage environment.
Resolve BZ#840000: Kdump hangs on CCISS module loading on RHEL 5.8 - HP Proliant DL380 G6 (Smartarray 410i).
Resolve BZ#852604: Kdump failed with intel_iommu=on.
Resolve BZ#847292: MajorMod: Network Interfaces: Static routes and default gateway are interface-specific?
Resolve BZ#821302: Documentation of NFS restrictions for secure nfs mounts work over TCP/UDP.
Resolve BZ#852372: Deployment Guide References $ISA in PAM sections.
Resolve BZ#713417: /root is labelled system_u:object_r:default_t:s0 after switching to MLS.
Resolve BZ#821225: No information about, amount of RAM to reserve for kdump kernel in the documentation.
Revision 8-0Tue Feb 21 2012Jaromír Hradílek
Resolve BZ#749948: [Release Notes and Deployment Guide] Migration tooling from RHN Classic to Cert-based RHN for RHEL 5.
Resolve BZ#718608: MinorMod: FTP: Missing text fragment in vsftpd configuration documentation.
Resolve BZ#720387: MinorMod: The proc File System: Illogical parameter description.
Resolve BZ#720860: Update Deployment (Guide) in RHEL5 Build Tree.
Resolve BZ#760925: MinorMod: Network File System: Severely suboptimal timeo option in NFS mount examples (for TCP).
Resolve BZ#784754: MinorMod: Network Interfaces: typo - wrong tense in 15.3. Interface Control Scripts.
Resolve BZ#740916: MinorMod: The kdump Crash Recovery Service: Incorrect description of the crashkernel parameter.
Resolve BZ#767105: incorrect default action in kdump part.
Resolve BZ#714080: debug option for bonding can not be used in BONDING_OPTS in /etc/sysconfig/network-scripts/ifcfg-bondX.
Resolve BZ#769776: Documentation needs to be updated for "default shell" option in /etc/kdump.conf.
Resolve BZ#781441: /etc/securetty documentation is incorrect [rhel-5.7].
Revision 7-0Thu Jul 21 2011Jaromír Hradílek
Resolve BZ#720382: MinorMod: Network Interfaces: LINKDELAY parameter needs to be added to "Interface Configuration Files".
Resolve BZ#632028: MajorMod: Redundant Array of Independent Disks (RAID): Document mdadm Usage.
Resolve BZ#720009: MinorMod: LVM: Update screenshots in the "Manual LVM Partitioning" section.
Resolve BZ#711162: MinorMod: Network Interfaces: Incorrect static routes configuration.
Resolve BZ#707238: broadcast is calculated with ipcalc, not ifcalc.
Resolve BZ#678316: HOTPLUG network config file option is not documented.
Resolve BZ#562018: Ch.4 Redundant Array of Independent Disks (RAID) - screenshots need updating.
Resolve BZ#485033: iptables -p ALL --dport not allowed according to man 8 iptables.
Revision 6-0Thu Jan 13 2011Jaromír Hradílek
Resolve BZ#249485: 'fsid=num' is listed under NFS client options, but it is a server-only option.
Resolve BZ#253659: additional commands required when adding machines to domain.
Resolve BZ#453242: guide does not tell you which packages you need to run an NFS server.
Resolve BZ#504250: cell should have newline characters, it shouldn't be all on one line.
Resolve BZ#520650: /proc/loadavg documentation error.
Resolve BZ#584075: vsftp typo for text_userdb_names.
Resolve BZ#625384: bonding configuration SLAVE=bond0 is invalid.
Resolve BZ#644617: misspelled word.
Resolve BZ#645123: spelling Errors in Deployment Guide II.
Resolve BZ#595366: RFE: document Shared Subtrees.
Revision 5-0Thu July 30 2010Douglas Silas
Resolve BZ#239313: document oom_adj and oom_score.
Resolve BZ#526502: correct quotaon instructions with proper, safe operating procedures.
Resolve BZ#551367: correct SELinux dhcpd_disable_trans description.
Resolve BZ#521215: clarify NFS interaction with portmapper, rpc.mountd, rpc.lockd and rpc.statd.
Resolve BZ#453875: various OpenSSH chapter corrections.
Resolve BZ#455162: correct zone example configuration file, description.
Resolve BZ#460767: make it a proper daemon.
Resolve BZ#600702: correct directories used for SSL key generation.
Revision 4-2Wed Sep 30 2009Douglas Silas, Jaromír Hradílek, Martin Prpic
Change heading titles to correspond with actual headings used in 'man rpm'.
Resolve BZ#499053: /usr/sbin/racoon is correct install path.
Remove any mention of 'pkgpolicy' in /etc/yum.conf as per BZ#237773.
Resolve BZ#455162: correct example zone file with regard to records, description.
Resolve BZ#510851: /proc/cmdline has confusing descriptions of sample output.
Resolve BZ#510847: page with multiple footnotes formatted incorrectly in online PDF.
Resolve BZ#214326: more detailed usage info concerning vsftpd banners and secueerity.
Resolve BZ#241314: formatting problems in screen elements.
Resolve BZ#466239: postfix connect-from-remote-host configuration fix.
Revision 4-1Mon Sep 14 2009Douglas Silas
Resolve BZ#214326: Server Security FTP Banner instructions: questions re: vsftpd.conf.
Resolve BZ#466239: insert line into Postfix config file to allow connecting remotely.
Resolve BZ#499053: path for racoon daemon is /usr/sbin/racoon, not /sbin/racoon.
Resolve BZ#510847: missing footnotes in PDF output.
Resolve BZ#510851: rewrite /proc/cmdline minor section to make more sense.
Resolve BZ#515613: correct location of RHEL5 GPG keys and key details.
Resolve BZ#523070: various minor fixes; --redhatprovides to rpm -q --whatprovides.
Revision 4-0Wed Sep 02 2009Douglas Silas
Resolve BZ#492539: "This directive is useful..." to "This directive must be used in machines containing more than one NIC to ensure...".
Resolve BZ#241314: re: kernel-pae and hugemem support on RHEL 4 and 5.
Resolve BZ#453071: incorrect tag use led to config files and other screen elements being displayed on single lines.
Resolve BZ#507987: clarify and correct statements about partitions being in use while resizing or removing.
Resolve BZ#462550: recommended amount of swap space, according to http://kbase.redhat.com/faq/docs/DOC-15252.
Resolve BZ#466239: line omitted from Postfix configuration meant connecting remotely failed
Resolving other MODIFIED BZs (fixed previously): 468483, 480324, 481246, 481247, 438823, 454841, 485187, 429989, 452065, 453466.
Revision 3-0Wed Jan 28 2009Michael Hideo Smith
Resolves: #460981
Changing 64GB *tested* support to support for 16GB.

Appendix B. Colophon
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The manuals are written in DocBook XML v4.3 format.
Garrett LeSage created the admonition graphics (note, tip, important, caution, and warning). They may be freely redistributed with the Red Hat documentation.
Contributing Writers: John Ha (System Administration, Filesystems, Kernel), Joshua Wulf (Installation and Booting), Brian Cleary (Virtualization), David O'Brien (Security and SELinux), Michael Hideo (System Administration), Don Domingo (System Administration), Michael Behm (System Administration), Paul Kennedy (Storage), Melissa Goldin (Red Hat Network)
Honoring those who have gone before: Sandra Moore, Edward C. Bailey, Karsten Wade, Mark Johnson, Andrius Benokraitis, Lucy Ringland
Honoring engineering efforts: Jeffrey Fearn
Technical Editing: Michael Behm
Graphic Artist: Andrew Fitzsimon
The Red Hat Localization Team consists of the following people:
  • East Asian Languages
    • Simplified Chinese
      • Tony Tongjie Fu
      • Simon Xi Huang
      • Leah Wei Liu
      • Sarah Saiying Wang
    • Traditional Chinese
      • Chester Cheng
      • Terry Chuang
      • Ben Hung-Pin Wu
    • Japanese
      • Kiyoto Hashida
      • Junko Ito
      • Noriko Mizumoto
      • Takuro Nagamoto
    • Korean
      • Eun-ju Kim
      • Michelle Kim
  • Latin Languages
    • French
      • Jean-Paul Aubry
      • Fabien Decroux
      • Myriam Malga
      • Audrey Simons
      • Corina Roe
    • German
      • Jasna Dimanoski
      • Verena Furhuer
      • Bernd Groh
      • Daniela Kugelmann
      • Timo Trinks
    • Italian
      • Francesco Valente
    • Brazilian Portuguese
      • Glaucia de Freitas
      • Leticia de Lima
      • David Barzilay
    • Spanish
      • Angela Garcia
      • Gladys Guerrero
      • Yelitza Louze
      • Manuel Ospina
    • Russian
      • Yuliya Poyarkova
  • Indic Languages
    • Bengali
      • Runa Bhattacharjee
    • Gujarati
      • Ankitkumar Rameshchandra Patel
      • Sweta Kothari
    • Hindi
      • Rajesh Ranjan
    • Malayalam
      • Ani Peter
    • Marathi
      • Sandeep Shedmake
    • Punjabi
      • Amanpreet Singh Alam
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    • Tamil
      • I Felix
      • N Jayaradha

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This document is licensed by Red Hat under the Creative Commons Attribution-ShareAlike 3.0 Unported License. If you distribute this document, or a modified version of it, you must provide attribution to Red Hat, Inc. and provide a link to the original. If the document is modified, all Red Hat trademarks must be removed.
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