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

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
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

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

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

1.1. Why Share a Common Structure?

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.

1.2. Overview of File System Hierarchy Standard (FHS)

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

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

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

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.
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

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/
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

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

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

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

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

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

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

1.2.1.10. The /srv/ Directory

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

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

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/
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

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/
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

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/
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.

1.3. Special File Locations Under Red Hat Enterprise Linux

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

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

To display all currently attached file systems, run the mount command with no additional arguments:
mount
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
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)

2.2. Mounting a File System

To attach a certain file system, use the mount command in the following form:
mount [option] device directory
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
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

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
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”.
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.

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

2.2.2. Specifying the Mount Options

To specify additional mount options, use the command in the following form:
mount -o options
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”.
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
Note that ISO 9660 is by design a read-only file system.

2.2.3. Sharing Mounts

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
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
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
Alternatively, you can change the mount type for the selected mount point and all mount points under it:
mount --make-rshared mount_point

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
Then create its duplicate in /mnt by using the following command:
~]# mount --bind /media /mnt
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
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
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
Alternatively, you can change the mount type for the selected mount point and all mount points under it:
mount --make-rslave mount_point

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
Then create its duplicate in /mnt, but mark it as slave:
~]# mount --bind /media /mnt
~]# mount --make-slave /mnt
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
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
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
Alternatively, you can change the mount type for the selected mount point and all mount points under it:
mount --make-rprivate mount_point

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
To mark the /mnt directory as private, type:
~]# mount --make-private /mnt
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
~]#
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
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
Alternatively, you can change the mount type for the selected mount point and all mount points under it:
mount --make-runbindable mount_point

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
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

2.2.4. Moving a Mount Point

To change the directory in which a file system is mounted, use the following command:
mount --move old_directory new_directory

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
To verify the mount point has been moved, list the content of both directories:
~]# ls /mnt/userdirs
~]# ls /home
jill  joe

2.3. Unmounting a File System

To detach a previously mounted file system, use either of the following variants of the umount command:
umount directory
umount device
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
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

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

2.4. Additional Resources

The following resources provide an in-depth documentation on the subject.

2.4.1. Installed Documentation

  • 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

  • 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

The default file system is the journaling ext3 file system.

3.1. Features of ext3

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

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

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>
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
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

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
Next, change the file system type to ext2 by typing the following command as root:
tune2fs -O ^has_journal /dev/mapper/VolGroup00-LogVol02
Check the partition for errors by typing the following command as root:
e2fsck -y /dev/mapper/VolGroup00-LogVol02
Then mount the partition again as ext2 file system by typing:
mount -t ext2 /dev/mapper/VolGroup00-LogVol02 /mount/point
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
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

4.1. Features of ext4

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

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
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

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
    ~]# mke4fs -t ext4 block_device
    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
  3. Create a mount point and mount the new file system to that mount point:
    ~]# mkdir /mount/point
    ~]# mount block_device /mount/point
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
For more information about creating file systems, refer to man mkfs.ext4.

4.4. Mounting an ext4 File System

An ext4 file system can be mounted with no extra options, same as any other file system:
~]# mount block_device /mount/point
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
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
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
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

4.5. Resizing an ext4 File System

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
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

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

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

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
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

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 
Other files act as binary or Boolean switches. Typing