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Reference Guide

Red Hat Enterprise Linux 4

For Red Hat Enterprise Linux 4

Edition 4

Abstract

The \Reference Guide provides reference material for Red Hat Enterprise Linux 4, and is oriented towards system administrators with a basic understanding of the system. For information regarding the deployment, configuration, and administration of this system, refer to the Red Hat Enterprise Linux 4 System Administrator's Guide.

Introduction

Welcome to the Reference Guide.

The Reference Guide contains useful information about the Red Hat Enterprise Linux system. From fundamental concepts, such as the structure of the file system, to the finer points of system security and authentication control, we hope you find this book to be a valuable resource.

This guide is for you if you want to learn a bit more about how the Red Hat Enterprise Linux system works. Topics that you can explore within this manual include the following:

The boot process
The file system structure
The X Window System
Network services
Security tools

1. Changes To This Manual

This manual has been reorganized for clarity and updated for the latest features of Red Hat Enterprise Linux 4.5.0. Some of the changes include:

A New Samba Chapter: The new Samba chapter explains various Samba daemons and configuration options. Special thanks to John Terpstra for his hard work in helping to complete this chapter.
A New SELinux Chapter: The new SELinux chapter explains various SELinux files and configuration options. Special thanks to Karsten Wade for his hard work in helping to complete this chapter.
An Updated proc File System Chapter: The proc file system chapter includes updated information in regards to the 2.6 kernel. Special thanks to Arjan van de Ven for his hard work in helping to complete this chapter.
An Updated Network File System (NFS) Chapter: The Network File System (NFS) chapter has been revised and reorganized to include NFSv4.
An Updated The X Window System Chapter: The X Window System chapter has been revised to include information on the X11R6.8 release developed by the X.Org team.

Before reading this guide, you should be familiar with the contents of the Installation Guide concerning installation issues, the Red Hat Enterprise Linux Introduction to System Adminitration for basic administration concepts, the System Administrators Guide for general customization instructions, and the Security Guide for security related instructions. This guide contains information about topics for advanced users.

2. Finding Appropriate Documentation

You need documentation that is appropriate to your level of Linux expertise. Otherwise, you might feel overwhelmed or may not find the necessary information to answer any questions. The Reference Guide deals with the more technical aspects and options of a Red Hat Enterprise Linux system. This section helps you decide whether to look in this manual for the information you need or to consider other Red Hat Enterprise Linux manuals, including online sources, in your search.

Three different categories of people use Red Hat Enterprise Linux, and each of these categories require different sets of documentation and informative sources. To help you figure out where you should start, determine your own experience level:

New to Linux: This type of user has never used any Linux (or Linux-like) operating system before or has had only limited exposure to Linux. They may or may not have experience using other operating systems (such as Windows). Is this you? If so, skip ahead to Section 2.1, “Documentation For First-Time Linux Users”.
Some Linux Experience: This type of user has installed and successfully used Linux (but not Red Hat Enterprise Linux) before or may have equivalent experience with other Linux-like operating systems. Does this describe you? If so, turn to Section 2.2, “For the More Experienced”.
Experienced User: This type of user has installed and successfully used Red Hat Enterprise Linux before. If this describes you, turn to Section 2.3, “Documentation for Linux Gurus”.

2.1. Documentation For First-Time Linux Users

For someone new to Linux, the amount of information available on any particular subject, such as printing, starting up the system or partitioning a hard drive, can be overwhelming. It helps to initially step back and gain a decent base of information centered around how Linux works before tackling these kinds of advanced issues.

Your first goal should be to obtain some useful documentation. This cannot be stressed enough. Without documentation, you only become frustrated at your inability to get a Red Hat Enterprise Linux system working the way you want.

You should acquire the following types of Linux documentation:

A brief history of Linux — Many aspects of Linux are the way they are because of historical precedent. The Linux culture is also based on past events, needs, or requirements. A basic understanding of the history of Linux helps you figure out how to solve many potential problems before you actually see them.
An explanation of how Linux works — While delving into the most arcane aspects of the Linux kernel is not necessary, it is a good idea to know something about how Linux is put together. This is particularly important if you have been working with other operating systems, as some of the assumptions you currently hold about how computers work may not transfer from that operating system to Linux.
An introductory command overview (with examples) — This is probably the most important thing to look for in Linux documentation. The underlying design philosophy for Linux is that it is better to use many small commands connected together in different ways than it is to have a few large (and complex) commands that do the whole job themselves. Without examples that illustrate this approach to doing things, you may find yourself intimidated by the sheer number of commands available on a Red Hat Enterprise Linux system.
Keep in mind that you do not have to memorize all of the available Linux commands. Different techniques exist to help you find the specific command you need to accomplish a task. You only need to know the general way in which Linux functions, what you need to accomplish, and how to access the tool that gives you the exact instructions you need to execute the command.

The Installation Guide is an excellent reference for helping you get a Red Hat Enterprise Linux system successfully installed and initially configured. The Red Hat Enterprise Linux Introduction to System Adminitration is a great place to start for those learning the basics of system administration. Start with these books and use them to build the base of your knowledge of Red Hat Enterprise Linux. Before long, more complicated concepts begin to make sense because you already grasp the general ideas.

Beyond reading the Red Hat Enterprise Linux manuals, several other excellent documentation resources are available for little or no cost:

2.1.1. Introduction to Linux Websites

http://www.redhat.com/ — On the Red Hat website, you find links to the Linux Documentation Project (LDP), online versions of the Red Hat Enterprise Linux manuals, FAQs (Frequently Asked Questions), a database which can help you find a Linux Users Group near you, technical information in the Red Hat Support Knowledge Base, and more.
http://www.linuxheadquarters.com/ — The Linux Headquarters website features easy to follow, step-by-step guides for a variety of Linux tasks.

2.1.2. Introduction to Linux Newsgroups

You can participate in newsgroups by watching the discussions of others attempting to solve problems, or by actively asking or answering questions. Experienced Linux users are known to be extremely helpful when trying to assist new users with various Linux issues — especially if you are posing questions in the right venue. If you do not have access to a news reader application, you can access this information via the Web at http://groups.google.com/. Dozens of Linux-related newsgroups exist, including the following:

linux.help — A great place to get help from fellow Linux users.
linux.redhat — This newsgroup primarily covers Red Hat Enterprise Linux-specific issues.
linux.redhat.install — Pose installation questions to this newsgroup or search it to see how others solved similar problems.
linux.redhat.misc — Questions or requests for help that do not really fit into traditional categories go here.
linux.redhat.rpm — A good place to go if you are having trouble using RPM to accomplish particular objectives.

2.2. For the More Experienced

If you have used other Linux distributions, you probably already have a basic grasp of the most frequently used commands. You may have installed your own Linux system, and maybe you have even downloaded and built software you found on the Internet. After installing Linux, however, configuration issues can be very confusing.

The System Administrators Guide is designed to help explain the various ways a Red Hat Enterprise Linux system can be configured to meet specific objectives. Use this manual to learn about specific configuration options and how to put them into effect.

When you are installing software that is not covered in the System Administrators Guide, it is often helpful to see what other people in similar circumstances have done. HOWTO documents from the Linux Documentation Project, available at http://www.redhat.com/mirrors/LDP/HOWTO/HOWTO-INDEX/howtos.html, document particular aspects of Linux, from low-level kernel esoteric changes to using Linux for amateur radio station work.

If you are concerned with the finer points and specifics of the Red Hat Enterprise Linux system, the Reference Guide is a great resource.

If you are concerned about security issues, the Security Guide is a great resource — explaining in concise terms best strategies and practices for securing Red Hat Enterprise Linux.

2.3. Documentation for Linux Gurus

If you are concerned with the finer points and specifics of the Red Hat Enterprise Linux system, the Reference Guide is a great resource.

If you are a long-time Red Hat Enterprise Linux user, you probably already know that one of the best ways to understand a particular program is to read its source code and/or configuration files. A major advantage of Red Hat Enterprise Linux is the availability of the source code for anyone to read.

Obviously, not everyone is a programmer, so the source code may not be helpful for you. However, if you have the knowledge and skills necessary to read it, the source code holds all of the answers.

3. More to Come

The Reference Guide is part of Red Hat's commitment to provide useful and timely support to Red Hat Enterprise Linux users. Future editions feature expanded information on changes to system structure and organization, new and powerful security tools, and other resources to help you extend the power of the Red Hat Enterprise Linux system — and your ability to use it.

That is where you can help.

3.1. We Need Feedback!

If you find an error in the Reference Guide, or if you have thought of a way to make this manual better, we would love to hear from you! Please submit a report in Bugzilla (http://bugzilla.redhat.com/bugzilla/) against the component rhel-rg.

Be sure to mention the manual's identifier:

rhel-rg

If you mention the manual's identifier, we know exactly which version of the guide you have.

If you have a suggestion for improving the documentation, try to be as specific as possible when describing it. If you have found an error, please include the section number and some of the surrounding text so we can find it easily.

Part I. System Reference

To manage the system effectively, it is crucial to know about its components and how they fit together. This part outlines many important aspects of the system. It covers the boot process, the basic file system layout, the location of crucial system files and file systems, and the basic concepts behind users and groups. Additionally, the X Window System is explained in detail.

Chapter 1. Boot Process, Init, and Shutdown

An important and powerful aspect of Red Hat Enterprise Linux is the open, user-configurable method it uses for starting the operating system. Users are free to configure many aspects of the boot process, including specifying the programs launched at boot-time. Similarly, system shutdown gracefully terminates processes in an organized and configurable way, although customization of this process is rarely required.

Understanding how the boot and shutdown processes work not only allows customization, but also makes it easier to troubleshoot problems related to starting or shutting down the system.

1.1. The Boot Process

Below are the basic stages of the boot process for an x86 system:

The system BIOS checks the system and launches the first stage boot loader on the MBR of the primary hard disk.
The first stage boot loader loads itself into memory and launches the second stage boot loader from the /boot/ partition.
The second stage boot loader loads the kernel into memory, which in turn loads any necessary modules and mounts the root partition read-only.
The kernel transfers control of the boot process to the /sbin/init program.
The /sbin/init program loads all services and user-space tools, and mounts all partitions listed in /etc/fstab.
The user is presented with a login screen for the freshly booted Linux system.

Because configuration of the boot process is more common than the customization of the shutdown process, the remainder of this chapter discusses in detail how the boot process works and how it can be customized to suite specific needs.

1.2. A Detailed Look at the Boot Process

The beginning of the boot process varies depending on the hardware platform being used. However, once the kernel is found and loaded by the boot loader, the default boot process is identical across all architectures. This chapter focuses primarily on the x86 architecture.

1.2.1. The BIOS

When an x86 computer is booted, the processor looks at the end of system memory for the Basic Input/Output System or BIOS program and runs it. The BIOS controls not only the first step of the boot process, but also provides the lowest level interface to peripheral devices. For this reason it is written into read-only, permanent memory and is always available for use.

Other platforms use different programs to perform low-level tasks roughly equivalent to those of the BIOS on an x86 system. For instance, Itanium-based computers use the Extensible Firmware Interface (EFI) Shell.

Once loaded, the BIOS tests the system, looks for and checks peripherals, and then locates a valid device with which to boot the system. Usually, it checks any diskette drives and CD-ROM drives present for bootable media, then, failing that, looks to the system's hard drives. In most cases, the order of the drives searched while booting is controlled with a setting in the BIOS, and it looks on the master IDE device on the primary IDE bus. The BIOS then loads into memory whatever program is residing in the first sector of this device, called the Master Boot Record or MBR. The MBR is only 512 bytes in size and contains machine code instructions for booting the machine, called a boot loader, along with the partition table. Once the BIOS finds and loads the boot loader program into memory, it yields control of the boot process to it.

1.2.2. The Boot Loader

This section looks at the default boot loader for the x86 platform, GRUB. Depending on the system's architecture, the boot process may differ slightly. Refer to Section 1.2.2.1, “Boot Loaders for Other Architectures” for a brief overview of non-x86 boot loaders. For more information about configuring and using GRUB, see Chapter 2, The GRUB Boot Loader.

A boot loader for the x86 platform is broken into at least two stages. The first stage is a small machine code binary on the MBR. Its sole job is to locate the second stage boot loader and load the first part of it into memory.

GRUB has the advantage of being able to read ext2 and ext3 ^[1] partitions and load its configuration file — /boot/grub/grub.conf — at boot time. Refer to Section 2.7, “GRUB Menu Configuration File” for information on how to edit this file.

Note

If upgrading the kernel using the Red Hat User Agent, the boot loader configuration file is updated automatically. More information on Red Hat Network can be found online at the following URL: https://rhn.redhat.com/.

Once the second stage boot loader is in memory, it presents the user with a graphical screen showing the different operating systems or kernels it has been configured to boot. On this screen a user can use the arrow keys to choose which operating system or kernel they wish to boot and press Enter. If no key is pressed, the boot loader loads the default selection after a configurable period of time has passed.

Note

If Symmetric Multi-Processor (SMP) kernel support is installed, more than one option is presented the first time the system is booted. In this situation GRUB displays Red Hat Enterprise Linux (<kernel-version>-smp), which is the SMP kernel, and Red Hat Enterprise Linux (<kernel-version>), which is for single processors.

If any problems occur using the SMP kernel, try selecting the a non-SMP kernel upon rebooting.

Once the second stage boot loader has determined which kernel to boot, it locates the corresponding kernel binary in the /boot/ directory. The kernel binary is named using the following format — /boot/vmlinuz-<kernel-version> file (where <kernel-version> corresponds to the kernel version specified in the boot loader's settings).

For instructions on using the boot loader to supply command line arguments to the kernel, refer to Chapter 2, The GRUB Boot Loader. For information on changing the runlevel at the boot loader prompt, refer Section 2.8, “Changing Runlevels at Boot Time”.

The boot loader then places one or more appropriate initramfs images into memory. Next, the kernel decompresses these images from memory to /boot/, a RAM-based virtual file system, via cpio. The initramfs is used by the kernel to load drivers and modules necessary to boot the system. This is particularly important if SCSI hard drives are present or if the systems use the ext3 file system.

Once the kernel and the initramfs image(s) are loaded into memory, the boot loader hands control of the boot process to the kernel.

For a more detailed overview of the GRUB boot loader, refer to Chapter 2, The GRUB Boot Loader.

1.2.2.1. Boot Loaders for Other Architectures

Once the kernel loads and hands off the boot process to the init command, the same sequence of events occurs on every architecture. So the main difference between each architecture's boot process is in the application used to find and load the kernel.

For example, the Itanium architecture uses the ELILO boot loader, the IBM eServer pSeries architecture uses YABOOT, and the IBM eServer zSeries and IBM S/390 systems use the z/IPL boot loader.

Consult the Installation Guide specific to these platforms for information on configuring their boot loaders.

1.2.3. The Kernel

When the kernel is loaded, it immediately initializes and configures the computer's memory and configures the various hardware attached to the system, including all processors, I/O subsystems, and storage devices. It then looks for the compressed initramfs image(s) in a predetermined location in memory, decompresses it directly to /sysroot/, and loads all necessary drivers. Next, it initializes virtual devices related to the file system, such as LVM or software RAID, before completing the initramfs processes and freeing up all the memory the disk image once occupied.

The kernel then creates a root device, mounts the root partition read-only, and frees any unused memory.

At this point, the kernel is loaded into memory and operational. However, since there are no user applications that allow meaningful input to the system, not much can be done with the system.

To set up the user environment, the kernel executes the /sbin/init program.

1.2.4. The `/sbin/init` Program

The /sbin/init program (also called init) coordinates the rest of the boot process and configures the environment for the user.

When the init command starts, it becomes the parent or grandparent of all of the processes that start up automatically on the system. First, it runs the /etc/rc.d/rc.sysinit script, which sets the environment path, starts swap, checks the file systems, and executes all other steps required for system initialization. For example, most systems use a clock, so rc.sysinit reads the /etc/sysconfig/clock configuration file to initialize the hardware clock. Another example is if there are special serial port processes which must be initialized, rc.sysinit executes the /etc/rc.serial file.

The init command then runs the /etc/inittab script, which describes how the system should be set up in each SysV init runlevel. Runlevels are a state, or mode, defined by the services listed in the SysV /etc/rc.d/rc<x>.d/ directory, where <x> is the number of the runlevel. For more information on SysV init runlevels, refer to Section 1.4, “SysV Init Runlevels”.

Next, the init command sets the source function library, /etc/rc.d/init.d/functions, for the system, which configures how to start, kill, and determine the PID of a program.

The init program starts all of the background processes by looking in the appropriate rc directory for the runlevel specified as the default in /etc/inittab. The rc directories are numbered to correspond to the runlevel they represent. For instance, /etc/rc.d/rc5.d/ is the directory for runlevel 5.

When booting to runlevel 5, the init program looks in the /etc/rc.d/rc5.d/ directory to determine which processes to start and stop.

Below is an example listing of the /etc/rc.d/rc5.d/ directory:

K05innd -> ../init.d/innd
K05saslauthd -> ../init.d/saslauthd
K10dc_server -> ../init.d/dc_server
K10psacct -> ../init.d/psacct
K10radiusd -> ../init.d/radiusd
K12dc_client -> ../init.d/dc_client
K12FreeWnn -> ../init.d/FreeWnn
K12mailman -> ../init.d/mailman
K12mysqld -> ../init.d/mysqld
K15httpd -> ../init.d/httpd
K20netdump-server -> ../init.d/netdump-server
K20rstatd -> ../init.d/rstatd
K20rusersd -> ../init.d/rusersd
K20rwhod -> ../init.d/rwhod
K24irda -> ../init.d/irda
K25squid -> ../init.d/squid
K28amd -> ../init.d/amd
K30spamassassin -> ../init.d/spamassassin
K34dhcrelay -> ../init.d/dhcrelay
K34yppasswdd -> ../init.d/yppasswdd
K35dhcpd -> ../init.d/dhcpd
K35smb -> ../init.d/smb
K35vncserver -> ../init.d/vncserver
K36lisa -> ../init.d/lisa
K45arpwatch -> ../init.d/arpwatch
K45named -> ../init.d/named
K46radvd -> ../init.d/radvd
K50netdump -> ../init.d/netdump
K50snmpd -> ../init.d/snmpd
K50snmptrapd -> ../init.d/snmptrapd
K50tux -> ../init.d/tux
K50vsftpd -> ../init.d/vsftpd
K54dovecot -> ../init.d/dovecot
K61ldap -> ../init.d/ldap
K65kadmin -> ../init.d/kadmin
K65kprop -> ../init.d/kprop
K65krb524 -> ../init.d/krb524
K65krb5kdc -> ../init.d/krb5kdc
K70aep1000 -> ../init.d/aep1000
K70bcm5820 -> ../init.d/bcm5820
K74ypserv -> ../init.d/ypserv
K74ypxfrd -> ../init.d/ypxfrd
K85mdmpd -> ../init.d/mdmpd
K89netplugd -> ../init.d/netplugd
K99microcode_ctl -> ../init.d/microcode_ctl
S04readahead_early -> ../init.d/readahead_early
S05kudzu -> ../init.d/kudzu
S06cpuspeed -> ../init.d/cpuspeed
S08ip6tables -> ../init.d/ip6tables
S08iptables -> ../init.d/iptables
S09isdn -> ../init.d/isdn
S10network -> ../init.d/network
S12syslog -> ../init.d/syslog
S13irqbalance -> ../init.d/irqbalance
S13portmap -> ../init.d/portmap
S15mdmonitor -> ../init.d/mdmonitor
S15zebra -> ../init.d/zebra
S16bgpd -> ../init.d/bgpd
S16ospf6d -> ../init.d/ospf6d
S16ospfd -> ../init.d/ospfd
S16ripd -> ../init.d/ripd
S16ripngd -> ../init.d/ripngd
S20random -> ../init.d/random
S24pcmcia -> ../init.d/pcmcia
S25netfs -> ../init.d/netfs
S26apmd -> ../init.d/apmd
S27ypbind -> ../init.d/ypbind
S28autofs -> ../init.d/autofs
S40smartd -> ../init.d/smartd
S44acpid -> ../init.d/acpid
S54hpoj -> ../init.d/hpoj
S55cups -> ../init.d/cups
S55sshd -> ../init.d/sshd
S56rawdevices -> ../init.d/rawdevices
S56xinetd -> ../init.d/xinetd
S58ntpd -> ../init.d/ntpd
S75postgresql -> ../init.d/postgresql
S80sendmail -> ../init.d/sendmail
S85gpm -> ../init.d/gpm
S87iiim -> ../init.d/iiim
S90canna -> ../init.d/canna
S90crond -> ../init.d/crond
S90xfs -> ../init.d/xfs
S95atd -> ../init.d/atd
S96readahead -> ../init.d/readahead
S97messagebus -> ../init.d/messagebus
S97rhnsd -> ../init.d/rhnsd
S99local -> ../rc.local

As illustrated in this listing, none of the scripts that actually start and stop the services are located in the /etc/rc.d/rc5.d/ directory. Rather, all of the files in /etc/rc.d/rc5.d/ are symbolic links pointing to scripts located in the /etc/rc.d/init.d/ directory. Symbolic links are used in each of the rc directories so that the runlevels can be reconfigured by creating, modifying, and deleting the symbolic links without affecting the actual scripts they reference.

The name of each symbolic link begins with either a K or an S. The K links are processes that are killed on that runlevel, while those beginning with an S are started.

The init command first stops all of the K symbolic links in the directory by issuing the /etc/rc.d/init.d/<command> stop command, where <command> is the process to be killed. It then starts all of the S symbolic links by issuing /etc/rc.d/init.d/<command> start.

Note

After the system is finished booting, it is possible to log in as root and execute these same scripts to start and stop services. For instance, the command /etc/rc.d/init.d/httpd stop stops the Apache HTTP Server.

Each of the symbolic links are numbered to dictate start order. The order in which the services are started or stopped can be altered by changing this number. The lower the number, the earlier it is started. Symbolic links with the same number are started alphabetically.

Note

One of the last things the init program executes is the /etc/rc.d/rc.local file. This file is useful for system customization. Refer to Section 1.3, “Running Additional Programs at Boot Time” for more information about using the rc.local file.

After the init command has progressed through the appropriate rc directory for the runlevel, the /etc/inittab script forks an /sbin/mingetty process for each virtual console (login prompt) allocated to the runlevel. Runlevels 2 through 5 have all six virtual consoles, while runlevel 1 (single user mode) has one, and runlevels 0 and 6 have none. The /sbin/mingetty process opens communication pathways to tty devices^[2], sets their modes, prints the login prompt, accepts the user's username and password, and initiates the login process.

In runlevel 5, the /etc/inittab runs a script called /etc/X11/prefdm. The prefdm script executes the preferred X display manager^[3] — gdm, kdm, or xdm, depending on the contents of the /etc/sysconfig/desktop file.

Once finished, the system operates on runlevel 5 and displays a login screen.

1.3. Running Additional Programs at Boot Time

The /etc/rc.d/rc.local script is executed by the init command at boot time or when changing runlevels. Adding commands to the bottom of this script is an easy way to perform necessary tasks like starting special services or initialize devices without writing complex initialization scripts in the /etc/rc.d/init.d/ directory and creating symbolic links.

The /etc/rc.serial script is used if serial ports must be setup at boot time. This script runs setserial commands to configure the system's serial ports. Refer to the setserial man page for more information.

1.4. SysV Init Runlevels

The SysV init runlevel system provides a standard process for controlling which programs init launches or halts when initializing a runlevel. SysV init was chosen because it is easier to use and more flexible than the traditional BSD-style init process.

The configuration files for SysV init are located in the /etc/rc.d/ directory. Within this directory, are the rc, rc.local, rc.sysinit, and, optionally, the rc.serial scripts as well as the following directories:

init.d/
rc0.d/
rc1.d/
rc2.d/
rc3.d/
rc4.d/
rc5.d/
rc6.d/

The init.d/ directory contains the scripts used by the /sbin/init command when controlling services. Each of the numbered directories represent the six runlevels configured by default under Red Hat Enterprise Linux.

1.4.1. Runlevels

The idea behind SysV init runlevels revolves around the idea that different systems can be used in different ways. For example, a server runs more efficiently without the drag on system resources created by the X Window System. Or there may be times when a system administrator may need to operate the system at a lower runlevel to perform diagnostic tasks, like fixing disk corruption in runlevel 1.

The characteristics of a given runlevel determine which services are halted and started by init. For instance, runlevel 1 (single user mode) halts any network services, while runlevel 3 starts these services. By assigning specific services to be halted or started on a given runlevel, init can quickly change the mode of the machine without the user manually stopping and starting services.

The following runlevels are defined by default under Red Hat Enterprise Linux:

0 — Halt
1 — Single-user text mode
2 — Not used (user-definable)
3 — Full multi-user text mode
4 — Not used (user-definable)
5 — Full multi-user graphical mode (with an X-based login screen)
6 — Reboot

In general, users operate Red Hat Enterprise Linux at runlevel 3 or runlevel 5 — both full multi-user modes. Users sometimes customize runlevels 2 and 4 to meet specific needs, since they are not used.

The default runlevel for the system is listed in /etc/inittab. To find out the default runlevel for a system, look for the line similar to the following near the top of /etc/inittab:

id:5:initdefault:

The default runlevel listed in this example is five, as the number after the first colon indicates. To change it, edit /etc/inittab as root.

Warning

Be very careful when editing /etc/inittab. Simple typos can cause the system to become unbootable. If this happens, either use a boot diskette, enter single-user mode, or enter rescue mode to boot the computer and repair the file.

For more information on single-user and rescue mode, refer to the chapter titled Basic System Recovery in the System Administrators Guide.

It is possible to change the default runlevel at boot time by modifying the arguments passed by the boot loader to the kernel. For information on changing the runlevel at boot time, refer to Section 2.8, “Changing Runlevels at Boot Time”.

1.4.2. Runlevel Utilities

One of the best ways to configure runlevels is to use an initscript utility. These tools are designed to simplify the task of maintaining files in the SysV init directory hierarchy and relieves system administrators from having to directly manipulate the numerous symbolic links in the subdirectories of /etc/rc.d/.

Red Hat Enterprise Linux provides three such utilities:

/sbin/chkconfig — The /sbin/chkconfig utility is a simple command line tool for maintaining the /etc/rc.d/init.d/ directory hierarchy.
/usr/sbin/ntsysv — The ncurses-based /sbin/ntsysv utility provides an interactive text-based interface, which some find easier to use than chkconfig.
Services Configuration Tool — The graphical Services Configuration Tool (system-config-services) program is a flexible utility for configuring runlevels.

Refer to the chapter titled Controlling Access to Services in the System Administrators Guide for more information regarding these tools.

1.5. Shutting Down

To shut down Red Hat Enterprise Linux, the root user may issue the /sbin/shutdown command. The shutdown man page has a complete list of options, but the two most common uses are:

/sbin/shutdown -h now
/sbin/shutdown -r now

After shutting everything down, the -h option halts the machine, and the -r option reboots.

PAM console users can use the reboot and halt commands to shut down the system while in runlevels 1 through 5. For more information about PAM console users, refer to Section 16.7, “PAM and Device Ownership”.

If the computer does not power itself down, be careful not to turn off the computer until a message appears indicating that the system is halted.

Failure to wait for this message can mean that not all the hard drive partitions are unmounted, which can lead to file system corruption.

^[1] GRUB reads ext3 file systems as ext2, disregarding the journal file. Refer to the chapter titled The ext3 File System in the System Administrators Guide for more information on the ext3 file system.

^[2] Refer to Section 5.3.11, “/proc/tty/” for more information about tty devices.

^[3] Refer to Section 7.5.2, “Runlevel 5” for more information about display managers.

Chapter 2. The GRUB Boot Loader

When a computer with Red Hat Enterprise Linux is turned on, the operating system is loaded into memory by a special program called a boot loader. A boot loader usually exists on the system's primary hard drive (or other media device) and has the sole responsibility of loading the Linux kernel with its required files or (in some cases) other operating systems into memory.

2.1. Boot Loaders and System Architecture

Each architecture capable of running Red Hat Enterprise Linux uses a different boot loader. The following table lists the boot loaders available for each architecture:

Table 2.1. Boot Loaders by Architecture
Architecture	Boot Loaders
AMD® AMD64	GRUB
IBM® eServer™ iSeries™	OS/400®
IBM® eServer™ pSeries™	YABOOT
IBM® S/390®	z/IPL
IBM® eServer™ zSeries®	z/IPL
Intel® Itanium™	ELILO
x86	GRUB

This chapter discusses commands and configuration options for the GRUB boot loader included with Red Hat Enterprise Linux for the x86 architecture.

2.2. GRUB

The GNU GRand Unified Boot loader (GRUB) is a program which enables the selection of the installed operating system or kernel to be loaded at system boot time. It also allows the user to pass arguments to the kernel.

2.2.1. GRUB and the x86 Boot Process

This section discusses the specific role GRUB plays when booting an x86 system. For a look at the overall boot process, refer to Section 1.2, “A Detailed Look at the Boot Process”.

GRUB loads itself into memory in the following stages:

The Stage 1 or primary boot loader is read into memory by the BIOS from the MBR^[4]. The primary boot loader exists on less than 512 bytes of disk space within the MBR and is capable of loading either the Stage 1.5 or Stage 2 boot loader.
The Stage 1.5 boot loader is read into memory by the Stage 1 boot loader, if necessary. Some hardware requires an intermediate step to get to the Stage 2 boot loader. This is sometimes true when the /boot/ partition is above the 1024 cylinder head of the hard drive or when using LBA mode. The Stage 1.5 boot loader is found either on the /boot/ partition or on a small part of the MBR and the /boot/ partition.
The Stage 2 or secondary boot loader is read into memory. The secondary boot loader displays the GRUB menu and command environment. This interface allows the user to select which kernel or operating system to boot, pass arguments to the kernel, or look at system parameters.
The secondary boot loader reads the operating system or kernel as well as the contents of /boot/sysroot/ into memory. Once GRUB determines which operating system or kernel to start, it loads it into memory and transfers control of the machine to that operating system.

The method used to boot Red Hat Enterprise Linux is called direct loading because the boot loader loads the operating system directly. There is no intermediary between the boot loader and the kernel.

The boot process used by other operating systems may differ. For example, the Microsoft® Windows® operating system, as well as other operating systems, are loaded using chain loading. Under this method, the MBR points to the first sector of the partition holding the operating system, where it finds the files necessary to actually boot that operating system.

GRUB supports both direct and chain loading boot methods, allowing it to boot almost any operating system.

Warning

During installation, Microsoft's DOS and Windows installation programs completely overwrite the MBR, destroying any existing boot loaders. If creating a dual-boot system, it is best to install the Microsoft operating system first.

2.2.2. Features of GRUB

GRUB contains several features that make it preferable to other boot loaders available for the x86 architecture. Below is a partial list of some of the more important features:

GRUB provides a true command-based, pre-OS environment on x86 machines. This feature affords the user maximum flexibility in loading operating systems with specified options or gathering information about the system. For years, many non-x86 architectures have employed pre-OS environments that allow system booting from a command line.
GRUB supports Logical Block Addressing (LBA) mode. LBA places the addressing conversion used to find files in the hard drive's firmware, and is used on many IDE and all SCSI hard devices. Before LBA, boot loaders could encounter the 1024-cylinder BIOS limitation, where the BIOS could not find a file after the 1024 cylinder head of the disk. LBA support allows GRUB to boot operating systems from partitions beyond the 1024-cylinder limit, so long as the system BIOS supports LBA mode. Most modern BIOS revisions support LBA mode.
GRUB can read ext2 partitions. This functionality allows GRUB to access its configuration file, /boot/grub/grub.conf, every time the system boots, eliminating the need for the user to write a new version of the first stage boot loader to the MBR when configuration changes are made. The only time a user needs to reinstall GRUB on the MBR is if the physical location of the /boot/ partition is moved on the disk. For details on installing GRUB to the MBR, refer to Section 2.3, “Installing GRUB”.

2.3. Installing GRUB

If GRUB was not installed during the installation process, it can be installed afterward. Once installed, it automatically becomes the default boot loader.

Before installing GRUB, make sure to use the latest GRUB package available or use the GRUB package from the installation CD-ROMs. For instructions on installing packages, refer to the chapter titled Package Management with RPM in the System Administrators Guide.

Once the GRUB package is installed, open a root shell prompt and run the command /sbin/grub-install <location>, where <location> is the location that the GRUB Stage 1 boot loader should be installed. For example, the following command installs GRUB to the MBR of the master IDE device on the primary IDE bus:

/sbin/grub-install /dev/hda

The next time the system boots, the GRUB graphical boot loader menu appears before the kernel loads into memory.

Important

If GRUB is installed on a RAID 1 array, the system may become unbootable in the event of disk failure. An unsupported workaround is provided online at the following URL:

http://www.dur.ac.uk/a.d.stribblehill/mirrored_grub.html

2.4. GRUB Terminology

One of the most important things to understand before using GRUB is how the program refers to devices, such as hard drives and partitions. This information is particularly important when configuring GRUB to boot multiple operating systems.

2.4.1. Device Names

When referring to a specific device with GRUB, do so using the following format (note that the parentheses and comma are very important syntactically):

(<type-of-device><bios-device-number>,<partition-number>)

The <type-of-device> specifies the type of device from which GRUB boots. The two most common options are hd for a hard disk or fd for a 3.5 diskette. A lesser used device type is also available called nd for a network disk. Instructions on configuring GRUB to boot over the network are available online at http://www.gnu.org/software/grub/manual/.

The <bios-device-number> is the BIOS device number. The primary IDE hard drive is numbered 0 and a secondary IDE hard drive is numbered 1. This syntax is roughly equivalent to that used for devices by the kernel. For example, the a in hda for the kernel is analogous to the 0 in hd0 for GRUB, the b in hdb is analogous to the 1 in hd1, and so on.

The <partition-number> specifies the number of a partition on a device. Like the <bios-device-number>, most types of partitions are numbered starting at 0. However, BSD partitions are specified using letters, with a corresponding to 0, b corresponding to 1, and so on.

Note

The numbering system for devices under GRUB always begins with 0, not 1. Failing to make this distinction is one of the most common mistakes made by new users.

To give an example, if a system has more than one hard drive, GRUB refers to the first hard drive as (hd0) and the second as (hd1). Likewise, GRUB refers to the first partition on the first drive as (hd0,0) and the third partition on the second hard drive as (hd1,2).

In general the following rules apply when naming devices and partitions under GRUB:

It does not matter if system hard drives are IDE or SCSI, all hard drives begin with the letters hd. The letters fd are used to specify 3.5 diskettes.
To specify an entire device without respect to partitions, leave off the comma and the partition number. This is important when telling GRUB to configure the MBR for a particular disk. For example, (hd0) specifies the MBR on the first device and (hd3) specifies the MBR on the fourth device.
If a system has multiple drive devices, it is very important to know how the drive boot order is set in the BIOS. This is a simple task if a system has only IDE or SCSI drives, but if there is a mix of devices, it becomes critical that the type of drive with the boot partition be accessed first.

2.4.2. File Names and Blocklists

When typing commands to GRUB that reference a file, such as a menu list, it is necessary to specify an absolute file path immediately after the device and partition numbers.

The following illustrates the structure of such a command:

(<device-type><device-number>,<partition-number>)</path/to/file>

In this example, replace <device-type> with hd, fd, or nd. Replace <device-number> with the integer for the device. Replace </path/to/file> with an absolute path relative to the top-level of the device.

It is also possible to specify files to GRUB that do not actually appear in the file system, such as a chain loader that appears in the first few blocks of a partition. To load such files, provide a blocklist that specifies block by block where the file is located in the partition. Since a file is often comprised of several different sets of blocks, blocklists use a special syntax. Each block containing the file is specified by an offset number of blocks, followed by the number of blocks from that offset point. Block offsets are listed sequentially in a comma-delimited list.

The following is a sample blocklist:

0+50,100+25,200+1

This sample blocklist specifies a file that starts at the first block on the partition and uses blocks 0 through 49, 100 through 124, and 200.

Knowing how to write blocklists is useful when using GRUB to load operating systems which require chain loading. It is possible to leave off the offset number of blocks if starting at block 0. As an example, the chain loading file in the first partition of the first hard drive would have the following name:

(hd0,0)+1

The following shows the chainloader command with a similar blocklist designation at the GRUB command line after setting the correct device and partition as root:

chainloader +1

2.4.3. The Root File System and GRUB

The use of the term root file system has a different meaning in regard to GRUB. It is important to remember that GRUB's root file system has nothing to do with the Linux root file system.

The GRUB root file system is the top level of the specified device. For example, the image file (hd0,0)/grub/splash.xpm.gz is located within the /grub/ directory at the top-level (or root) of the (hd0,0) partition (which is actually the /boot/ partition for the system).

Next, the kernel command is executed with the location of the kernel file as an option. Once the Linux kernel boots, it sets up the root file system that Linux users are familiar with. The original GRUB root file system and its mounts are forgotten; they only existed to boot the kernel file.

Refer to the root and kernel commands in Section 2.6, “GRUB Commands” for more information.

2.5. GRUB Interfaces

GRUB features three interfaces which provide different levels of functionality. Each of these interfaces allows users to boot the Linux kernel or another operating system.

The interfaces are as follows:

Note

The following GRUB interfaces can only be accessed by pressing any key within the three seconds of the GRUB menu bypass screen.

Menu Interface: This is the default interface shown when GRUB is configured by the installation program. A menu of operating systems or preconfigured kernels are displayed as a list, ordered by name. Use the arrow keys to select an option other than the default selection and press the Enter key to boot it. Alternatively, a timeout period is set, after which GRUB loads the default option.
Press the e key to enter the entry editor interface or the c key to load a command line interface.
Refer to Section 2.7, “GRUB Menu Configuration File” for more information on configuring this interface.
Menu Entry Editor Interface: To access the menu entry editor, press the e key from the boot loader menu. The GRUB commands for that entry are displayed here, and users may alter these command lines before booting the operating system by adding a command line (o inserts a new line after the current line and O inserts a new line before it), editing one (e), or deleting one (d).
After all changes are made, the b key executes the commands and boots the operating system. The Esc key discards any changes and reloads the standard menu interface. The c key loads the command line interface.
Note
For information about changing runlevels using the GRUB menu entry editor, refer to Section 2.8, “Changing Runlevels at Boot Time”.
Command Line Interface: The command line interface is the most basic GRUB interface, but it is also the one that grants the most control. The command line makes it possible to type any relevant GRUB commands followed by the Enter key to execute them. This interface features some advanced shell-like features, including Tab key completion, based on context, and Ctrl key combinations when typing commands, such as Ctrl+a to move to the beginning of a line and Ctrl+e to move to the end of a line. In addition, the arrow, Home, End, and Delete keys work as they do in the bash shell.
Refer to Section 2.6, “GRUB Commands” for a list of common commands.

2.5.1. Interfaces Load Order

When GRUB loads its second stage boot loader, it first searches for its configuration file. Once found, the menu interface bypass screen is displayed. If a key is pressed within three seconds, GRUB builds a menu list and displays the menu interface. If no key is pressed, the default kernel entry in the GRUB menu is used.

If the configuration file cannot be found, or if the configuration file is unreadable, GRUB loads the command line interface, allowing the user to type commands to complete the boot process.

If the configuration file is not valid, GRUB prints out the error and asks for input. This helps the user see precisely where the problem occurred. Pressing any key reloads the menu interface, where it is then possible to edit the menu option and correct the problem based on the error reported by GRUB. If the correction fails, GRUB reports an error and reloads the menu interface.

2.6. GRUB Commands

GRUB allows a number of useful commands in its command line interface. Some of the commands accept options after their name; these options should be separated from the command and other options on that line by space characters.

The following is a list of useful commands:

boot — Boots the operating system or chain loader that was last loaded.
chainloader </path/to/file> — Loads the specified file as a chain loader. If the file is located on the first sector of the specified partition, use the blocklist notation, +1, instead of the file name.
The following is an example chainloader command:
```
chainloader +1
```
displaymem — Displays the current use of memory, based on information from the BIOS. This is useful to determine how much RAM a system has prior to booting it.
initrd </path/to/initrd> — Enables users to specify an initial RAM disk to use when booting. An initrd is necessary when the kernel needs certain modules in order to boot properly, such as when the root partition is formatted with the ext3 file system.
The following is an example initrd command:
```
initrd /initrd-2.6.8-1.523.img
```
install <stage-1> <install-disk> <stage-2> p config-file — Installs GRUB to the system MBR.
- <stage-1> — Signifies a device, partition, and file where the first boot loader image can be found, such as (hd0,0)/grub/stage1.
- <install-disk> — Specifies the disk where the stage 1 boot loader should be installed, such as (hd0).
- <stage-2> — Passes the stage 2 boot loader location to the stage 1 boot loader, such as (hd0,0)/grub/stage2.
- p <config-file> — This option tells the install command to look for the menu configuration file specified by <config-file>, such as (hd0,0)/grub/grub.conf.
Warning
The install command overwrites any information already located on the MBR.
kernel </path/to/kernel> <option-1> <option-N> ... — Specifies the kernel file to load when booting the operating system. Replace </path/to/kernel> with an absolute path from the partition specified by the root command. Replace <option-1> with options for the Linux kernel, such as root=/dev/VolGroup00/LogVol00 to specify the device on which the root partition for the system is located. Multiple options can be passed to the kernel in a space separated list.
The following is an example kernel command:
```
kernel /vmlinuz-2.6.8-1.523 ro root=/dev/VolGroup00/LogVol00
```
The option in the previous example specifies that the root file system for Linux is located on the hda5 partition.
root (<device-type><device-number>,<partition>) — Configures the root partition for GRUB, such as (hd0,0), and mounts the partition.
The following is an example root command:
```
root (hd0,0)
```
rootnoverify (<device-type><device-number>,<partition>) — Configures the root partition for GRUB, just like the root command, but does not mount the partition.

Other commands are also available; type help --all for a full list of commands. For a description of all GRUB commands, refer to the documentation available online at http://www.gnu.org/software/grub/manual/.

2.7. GRUB Menu Configuration File

The configuration file (/boot/grub/grub.conf), which is used to create the list of operating systems to boot in GRUB's menu interface, essentially allows the user to select a pre-set group of commands to execute. The commands given in Section 2.6, “GRUB Commands” can be used, as well as some special commands that are only available in the configuration file.

2.7.1. Configuration File Structure

The GRUB menu interface configuration file is /boot/grub/grub.conf. The commands to set the global preferences for the menu interface are placed at the top of the file, followed by stanzas for each operating kernel or operating system listed in the menu.

The following is a very basic GRUB menu configuration file designed to boot either Red Hat Enterprise Linux or Microsoft Windows 2000:

default=0
timeout=10
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title Red Hat Enterprise Linux AS (2.6.8-1.523)
        root (hd0,0)
        kernel /vmlinuz-2.6.8-1.523 ro root=/dev/VolGroup00/LogVol00 rhgb quiet
        initrd /initrd-2.6.8-1.523.img

# section to load Windows
title Windows
        rootnoverify (hd0,0)
        chainloader +1

This file configures GRUB to build a menu with Red Hat Enterprise Linux as the default operating system and sets it to autoboot after 10 seconds. Two sections are given, one for each operating system entry, with commands specific to the system disk partition table.

Note

Note that the default is specified as an integer. This refers to the first title line in the GRUB configuration file. For the Windows section to be set as the default in the previous example, change the default=0 to default=1.

Configuring a GRUB menu configuration file to boot multiple operating systems is beyond the scope of this chapter. Consult Section 2.9, “Additional Resources” for a list of additional resources.

2.7.2. Configuration File Directives

The following are directives commonly used in the GRUB menu configuration file:

chainloader </path/to/file> — Loads the specified file as a chain loader. Replace </path/to/file> with the absolute path to the chain loader. If the file is located on the first sector of the specified partition, use the blocklist notation, +1.
color <normal-color> <selected-color> — Allows specific colors to be used in the menu, where two colors are configured as the foreground and background. Use simple color names such as red/black. For example:
```
color red/black green/blue
```
default=<integer> — Replace <integer> with the default entry title number to be loaded if the menu interface times out.
fallback=<integer> — Replace <integer> with the entry title number to try if the first attempt fails.
hiddenmenu — Prevents the GRUB menu interface from being displayed, loading the default entry when the timeout period expires. The user can see the standard GRUB menu by pressing the Esc key.
initrd </path/to/initrd> — Enables users to specify an initial RAM disk to use when booting. Replace </path/to/initrd> with the absolute path to the initial RAM disk.
kernel </path/to/kernel> <option-1> <option-N> — Specifies the kernel file to load when booting the operating system. Replace </path/to/kernel> with an absolute path from the partition specified by the root directive. Multiple options can be passed to the kernel when it is loaded.
password=<password> — Prevents a user who does not know the password from editing the entries for this menu option.
Optionally, it is possible to specify an alternate menu configuration file after the password=<password> directive. In this case, GRUB restarts the second stage boot loader and uses the specified alternate configuration file to build the menu. If an alternate menu configuration file is left out of the command, a user who knows the password is allowed to edit the current configuration file.
For more information about securing GRUB, refer to the chapter titled Workstation Security in the Security Guide.
root (<device-type><device-number>,<partition>) — Configures the root partition for GRUB, such as (hd0,0), and mounts the partition.
rootnoverify (<device-type><device-number>,<partition>) — Configures the root partition for GRUB, just like the root command, but does not mount the partition.
timeout=<integer> — Specifies the interval, in seconds, that GRUB waits before loading the entry designated in the default command.
splashimage=<path-to-image> — Specifies the location of the splash screen image to be used when GRUB boots.
title group-title — Specifies a title to be used with a particular group of commands used to load a kernel or operating system.

To add human-readable comments to the menu configuration file, begin the line with the hash mark character (#).

2.8. Changing Runlevels at Boot Time

Under Red Hat Enterprise Linux, it is possible to change the default runlevel at boot time.

To change the runlevel of a single boot session, use the following instructions:

When the GRUB menu bypass screen appears at boot time, press any key to enter the GRUB menu (within the first three seconds).
Press the a key to append to the kernel command.
Add <space><runlevel> at the end of the boot options line to boot to the desired runlevel. For example, the following entry would initiate a boot process into runlevel 3:
```
grub append> ro root=/dev/VolGroup00/LogVol00 rhgb quiet 3
```

2.9. Additional Resources

This chapter is only intended as an introduction to GRUB. Consult the following resources to discover more about how GRUB works.

2.9.1. Installed Documentation

/usr/share/doc/grub-<version-number>/ — This directory contains good information about using and configuring GRUB, where <version-number> corresponds to the version of the GRUB package installed.
info grub — The GRUB info page contains a tutorial, a user reference manual, a programmer reference manual, and a FAQ document about GRUB and its usage.

2.9.2. Useful Websites

http://www.gnu.org/software/grub/ — The home page of the GNU GRUB project. This site contains information concerning the state of GRUB development and an FAQ.
http://www.redhat.com/mirrors/LDP/HOWTO/mini/Multiboot-with-GRUB.html — Investigates various uses for GRUB, including booting operating systems other than Linux.
http://www.linuxgazette.com/issue64/kohli.html — An introductory article discussing the configuration of GRUB on a system from scratch, including an overview of GRUB command line options.

^[4] For more on the system BIOS and the MBR, refer to Section 1.2.1, “The BIOS”.

Chapter 3. File System Structure

3.1. 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. unsharable files
Variable vs. static files

Shareable files are those that can be accessed locally and by remote hosts; unsharable 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.

3.2. 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.

3.2.1. 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/.

3.2.1.1. 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.

3.2.1.2. The `/dev/` Directory

The /dev/ directory contains file system entries which represent devices that are attached to the system. These files are essential for the system to function properly.

3.2.1.3. 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/.

The X11/ and skel/ directories are subdirectories of the /etc/ directory:

/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.

3.2.1.4. 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.

3.2.1.5. The `/media/` Directory

The /media/ directory contains subdirectories used as mount points for removeable media, such as 3.5 diskettes, CD-ROMs, and Zip disks.

3.2.1.6. The `/mnt/` Directory

The /mnt/ directory is reserved for temporarily mounted file systems, such as NFS file system mounts. For all removeable media, use the /media/ directory.

Note

This directory must not be used by installation programs.

3.2.1.7. The `/opt/` Directory

The /opt/ directory provides storage for large, static 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.

Large packages that encompass many different sub-packages, each of which accomplish a particular task, 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.

3.2.1.8. The `/proc/` Directory

The /proc/ directory contains special files that either extract information from or send information to the kernel.

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.

3.2.1.9. The `/sbin/` Directory

The /sbin/ directory stores executables used by the root user. The executables in /sbin/ are only used at boot time and 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

3.2.1.10. 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.

Note

Please be aware that data files currently located in /var/ may move to /srv/ in future releases.

3.2.1.11. 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.

To see how certain USB and FireWire devices are actually mounted, refer to the /sbin/hotplug and /sbin/udev man pages.

3.2.1.12. 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/
  |- X11R6/

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, and X11R6/ is for the X Window System (XFree86 on Red Hat Enterprise Linux).

3.2.1.13. 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.

3.2.1.14. 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.

3.3. 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 titled Package Management with RPM in the System Administrators Guide.

The /var/spool/up2date/ directory contains files used by Red Hat User Agent, 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 the documentation online at https://rhn.redhat.com/.

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 4, The sysconfig Directory for more information about what is within this directory and the role these files play in the boot process.

Finally, one more directory worth noting is the /initrd/ directory. It is empty, but is used as a critical mount point during the boot process.

Warning

Do not remove the /initrd/ directory for any reason. Removing this directory causes the system to fail to boot with a kernel panic error message.

Chapter 4. The `sysconfig` Directory

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.

4.1. Files in the `/etc/sysconfig/` Directory

The following files are normally found in the /etc/sysconfig/ directory:

amd
apmd
arpwatch
authconfig
autofs
clock
desktop
devlabel
dhcpd
exim
firstboot
gpm
harddisks
hwconf
i18n
init
ip6tables-config
iptables-config
irda
keyboard
kudzu
mouse
named
netdump
network
ntpd
pcmcia
radvd
rawdevices
samba
sendmail
selinux
spamassassin
squid
system-config-securitylevel
system-config-users
system-logviewer
tux
vncservers
xinetd

Note

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

The following sections offer descriptions of these files. Files not listed here as well as extra file options 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.

4.1.1. `/etc/sysconfig/amd`

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

4.1.2. `/etc/sysconfig/apmd`

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.

4.1.3. `/etc/sysconfig/arpwatch`

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 as well as sends any messages to the root mail queue. For more information regarding available parameters for this file, refer to the arpwatch man page.

4.1.4. `/etc/sysconfig/authconfig`

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.

4.1.5. `/etc/sysconfig/autofs`

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.

4.1.6. `/etc/sysconfig/clock`

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:
- true or yes — The ARC console's 42-year time offset is in effect. This setting is only for ARC- or AlphaBIOS-based Alpha systems.
- false or no — This value indicates that the normal UNIX epoch is in use.
SRM=<value>, where <value> is the following:
- true or yes — The SRM console's 1900 epoch is in effect. This setting is only for SRM-based Alpha systems.
- false or no — This value indicates that the normal UNIX epoch is in use.
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"
```

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).

4.1.7. `/etc/sysconfig/desktop`

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 7, The X Window System.

4.1.8. `/etc/sysconfig/devlabel`

The /etc/sysconfig/devlabel is the devlabel configuration file. It should not be modified by hand, but rather, configured using the /sbin/devlabel command.

For instructions on using the devlabel command, refer to the chapter titled User-Defined Device Names in the System Administrators Guide.

4.1.9. `/etc/sysconfig/dhcpd`

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.

4.1.10. `/etc/sysconfig/exim`

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 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.

4.1.11. `/etc/sysconfig/firstboot`

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.

4.1.12. `/etc/sysconfig/gpm`

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.

4.1.13. `/etc/sysconfig/harddisks`

The /etc/sysconfig/harddisks file tunes the hard drive(s). An administrator can also use /etc/sysconfig/hardiskhd[a-h] to configure parameters for specific drives.

Warning

Do not make changes to this file without careful consideration. By changing the default values, it is possible to corrupt all of the data on the hard drive(s).

The /etc/sysconfig/harddisks file may contain the following:

USE_DMA=1, where setting this value to 1 enables DMA. However, with some chipsets and hard drive combinations, DMA can cause data corruption. Check the hard drive documentation or with the manufacturer before enabling this option. By default, this entry is commented out, and therefore disabled.
Multiple_IO=16, where a setting of 16 allows for multiple sectors per I/O interrupt. When enabled, this feature reduces operating system overhead by 30-50%. Use with caution. By default, this entry is commented out, and therefore disabled.
EIDE_32BIT=3 enables (E)IDE 32-bit I/O support to an interface card. By default, this entry is commented out, and therefore disabled.
LOOKAHEAD=1 enables drive read-lookahead. By default, this entry is commented out, and therefore disabled.
EXTRA_PARAMS= specifies where extra parameters can be added. By default, there are no parameters listed.

4.1.14. `/etc/sysconfig/hwconf`

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.

4.1.15. `/etc/sysconfig/i18n`

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"

4.1.16. `/etc/sysconfig/init`

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.

4.1.17. `/etc/sysconfig/ip6tables-config`

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:

/sbin/service ip6tables save

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 Chapter 18, iptables.

4.1.18. `/etc/sysconfig/iptables-config`

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:

/sbin/service iptables save

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 Chapter 18, iptables.

4.1.19. `/etc/sysconfig/irda`

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.

4.1.20. `/etc/sysconfig/keyboard`

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.

4.1.21. `/etc/sysconfig/kudzu`

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.

4.1.22. `/etc/sysconfig/mouse`

The /etc/sysconfig/mouse file is used to specify information about the available mouse. The following values may be used:

FULLNAME="<value>", where "<value>" refers to the full name of the kind of mouse being used.
MOUSETYPE="<value>", where "<value>" is one of the following:
- imps2 — A generic USB wheel mouse.
- microsoft — A Microsoft™ mouse.
- mouseman — A MouseMan™ mouse.
- mousesystems — A Mouse Systems™ mouse.
- ps/2 — A PS/2 mouse.
- msbm — A Microsoft™ bus mouse.
- logibm — A Logitech™ bus mouse.
- atibm — An ATI™ bus mouse.
- logitech — A Logitech™ mouse.
- mmseries — An older MouseMan™ mouse.
- mmhittab — An mmhittab mouse.
XEMU3="<value>", where "<value>" is one of the following boolean values:
- yes — The mouse only has two buttons, but three mouse buttons should be emulated.
- no — The mouse already has three buttons.
XMOUSETYPE="<value>", where "<value>" refers to the kind of mouse used when X is running. The options here are the same as the MOUSETYPE setting in this same file.
DEVICE=<value>, where <value> is the mouse device.
A sample value, /dev/input/mice, is a symbolic link that points to the actual mouse device.

4.1.23. `/etc/sysconfig/named`

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 12, Berkeley Internet Name Domain (BIND). By default, the file contains no parameters.

4.1.24. `/etc/sysconfig/netdump`

The /etc/sysconfig/netdump file is the configuration file for the /etc/init.d/netdump service. The netdump service sends both oops data and memory dumps over the network. In general, netdump is not a required service; only run it if absolutely necessary. For more information about what parameters are available for this file, refer to the netdump man page.

4.1.25. `/etc/sysconfig/network`

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.
Note
For compatibility with older software that some users may need to install, such as trn, the /etc/HOSTNAME file should contain the same value as set here.
GATEWAY=<value>, where <value> is the IP address of the network's gateway.
GATEWAYDEV=<value>, where <value> is the gateway device, such as eth0.
NISDOMAIN=<value>, where <value> is the NIS domain name.

4.1.26. `/etc/sysconfig/ntpd`

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.

4.1.27. `/etc/sysconfig/pcmcia`

The /etc/sysconfig/pcmcia file is used to specify PCMCIA configuration information. The following values may be used:

PCMCIA=<value>, where <value> is one of the following:
- yes — PCMCIA support should be enabled.
- no — PCMCIA support should not be enabled.
PCIC=<value>, where <value> is one of the following:
- i82365 — The computer has an i82365-style PCMCIA socket chipset.
- tcic — The computer has a tcic-style PCMCIA socket chipset.
PCIC_OPTS=<value>, where <value> is the socket driver (i82365 or tcic) timing parameters.
CORE_OPTS=<value>, where <value> is the list of pcmcia_core options.
CARDMGR_OPTS=<value>, where <value> is the list of options for the PCMCIA cardmgr (such as -q for quiet mode, -m to look for loadable kernel modules in the specified directory, and so on). Read the cardmgr man page for more information.

4.1.28. `/etc/sysconfig/radvd`

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.

4.1.29. `/etc/sysconfig/rawdevices`

The /etc/sysconfig/rawdevices file is used to configure raw device bindings, such as:

/dev/raw/raw1 /dev/sda1
/dev/raw/raw2 8 5

4.1.30. `/etc/sysconfig/samba`

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.

4.1.31. `/etc/sysconfig/selinux`

The /etc/sysconfig/selinux file contains the basic configuration options for SELinux. This file is a symbolic link to /etc/selinux/config. For more information on SELinux, refer to Chapter 21, SELinux.

4.1.32. `/etc/sysconfig/sendmail`

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.

4.1.33. `/etc/sysconfig/spamassassin`

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 11.4.2.6, “Spam Filters”.

4.1.34. `/etc/sysconfig/squid`

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.

4.1.35. `/etc/sysconfig/system-config-securitylevel`

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 the chapter titled Basic Firewall Configuration in the System Administrators Guide.

4.1.36. `/etc/sysconfig/system-config-users`

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 the chapter called User and Group Configuration in the System Administrators Guide.

4.1.37. `/etc/sysconfig/system-logviewer`

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 the chapter called Log Files in the System Administrators Guide.

4.1.38. `/etc/sysconfig/tux`

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.

4.1.39. `/etc/sysconfig/vncservers`

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.

Note that when using a VNC server, communication with it is unencrypted and it should not be used on an untrusted network. For specific instructions concerning the use of SSH to secure VNC communication, read the information found online at http://www.uk.research.att.com/archive/vnc/sshvnc.html. To find out more about SSH, refer to Chapter 20, SSH Protocol in the System Administrators Guide.

4.1.40. `/etc/sysconfig/xinetd`

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 17.3, “xinetd”.

4.2. Directories in the `/etc/sysconfig/` Directory

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 the chapter called Network Configuration in the System Administrators Guide.
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 up and down network interfaces, such as ifup and ifdown.
- Scripts used to bring up and down ISDN interfaces, 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 8, Network Interfaces.
rhn/ — This directory contains the configuration files and GPG keys for Red Hat Network. No files in this directory should be edited by hand. For more information on Red Hat Network, refer to the Red Hat Network website online at https://rhn.redhat.com/.

4.3. Additional Resources

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

4.3.1. Installed Documentation

/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 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 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/”.

5.2. Top-level Files within the `proc` File System

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`

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 ?

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

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 ?

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

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)

5.2.2. `/proc/buddyinfo`

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      ...

5.2.3. `/proc/cmdline`

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

This tells us that the kernel is mounted read-only (signified by (ro)), located on the first logical volume (LogVol00) of the first volume group (/dev/VolGroup00). LogVol00 is the equivalent of a disk partition in a non-LVM system (Logical Volume Management), just as /dev/VolGroup00 is similar in concept to /dev/hda1, but much more extensible.

For more information on LVM used in Red Hat Enterprise Linux, refer to http://www.tldp.org/HOWTO/LVM-HOWTO/index.html.

Next, rhgb signals that the rhgb package has been installed, and graphical booting is supported, assuming /etc/inittab shows a default runlevel set to id:5:initdefault:.

Finally, quiet indicates all verbose kernel messages are suppressed at boot time.

5.2.4. `/proc/cpuinfo`

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

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`

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

5.2.6. `/proc/devices`

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

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:

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.
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

5.2.7. `/proc/dma`

This file contains a list of the registered ISA DMA channels in use. A sample /proc/dma files looks like the following:

 4: cascade

5.2.8. `/proc/execdomains`

This file lists the execution domains currently supported by the Linux kernel, along with the range of personalities they support.

0-0   Linux           [kernel]

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`

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

5.2.10. `/proc/filesystems`

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

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`

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

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

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`

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

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`

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

The first column gives the I/O port address range reserved for the device listed in the second column.

5.2.14. `/proc/kcore`

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`

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`

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

The first three columns measure CPU and IO utilization of the last one, five, and 10 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.

5.2.17. `/proc/locks`

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

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`

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>

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>

5.2.19. `/proc/meminfo`

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

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`

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

The first column is the minor number of each device, while the second column shows the driver in use.

5.2.21. `/proc/modules`

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

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`

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

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`

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

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

5.2.24. `/proc/partitions`

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

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`

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].
      Non-prefetchable 32 bit memory at 0xe3000000 [0xe30000ff].
  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.
      Non-prefetchable 32 bit memory at 0xdc000000 [0xdfffffff].

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:

 /sbin/lspci -vb

5.2.26. `/proc/slabinfo`

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

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`

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

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`

This file measures swap space and its utilization. For a system with only one swap partition, the output of /proc/swap may look similar to the following:

Filename                          Type        Size     Used    Priority
/dev/mapper/VolGroup00-LogVol01   partition   524280   0       -1

While some of this information can be found in other files in the /proc/ directory, /proc/swap 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`

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`

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

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`

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

This information is used for a variety of purposes, including the version data presented when a user logs in.

5.3. Directories within `/proc/`

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

5.3.1. Process Directories

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

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

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 00000000 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

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
```
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

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/`

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/`

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

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

5.3.3. `/proc/driver/`

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

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`

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 9, Network File System (NFS).

5.3.5. `/proc/ide/`

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

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

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

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

5.3.6. `/proc/irq/`

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

5.3.7. `/proc/net/`

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/`

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

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

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/`

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

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

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

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/`

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

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/`

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
```
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/`

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
```
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
```
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 that can be used at one time on the system, in pages. By default, this value is 2097152.
shmmax — Sets the largest shared memory segment size allowed by the kernel, in bytes. 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, in bytes. 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/`

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

/usr/share/doc/kernel-doc-<version>/Documentation/networking/ ip-sysctl.txt

file 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

5.3.9.5. `/proc/sys/vm/`

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/`

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/`

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

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.

5.4. Using the `sysctl` Command

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

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

use the equivalent sysctl command as follows:

sysctl -w kernel.sysrq="1" kernel.sysrq = 1

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

Below are additional sources of information about proc file system.

5.5.1. Installed Documentation

Below is a list of directories you can consult for more information about the proc file system. These documents are installed through the kernel-doc package.

/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

http://www.linuxhq.com/ — This website maintains a complete database of source, patches, and documentation for various versions of the Linux kernel.

Chapter 6. Users and Groups

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 as well as access permissions can be changed by both the root user and the owner of the file.

Red Hat Enterprise Linux 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 using ACLs, refer to the chapter titled Access Control Lists in the System Administrators Guide.

Proper management of users and groups as well as the effective management of file permissions are among the most important tasks a system administrator undertakes. For a detailed look at strategies for managing users and groups, refer to the chapter titled Managing User Accounts and Resource Access in the Red Hat Enterprise Linux Introduction to System Adminitration guide.

6.1. User and Group Management Tools

Managing users and groups can be a tedious task, but Red Hat Enterprise Linux provides tools and conventions to make their management easier.

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 the chapter titled User and Group Configuration in the System Administrators Guide.

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.

For an overview of users and group management, refer to the Red Hat Enterprise Linux Introduction to System Adminitration. For a detailed look at command line tools for managing users and groups, see the chapter titled User and Group Configuration in the System Administrators Guide.

6.2. Standard Users

Table 6.1, “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 6.3, “Standard Groups” for a listing of standard groups.

Table 6.1. 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`

6.3. Standard Groups

Table 6.2, “Standard Groups” lists the standard groups configured by an Everything installation. Groups are stored in the /etc/group file.

Table 6.2. 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

6.4. User Private Groups

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 which allow both the user and that user's group 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.

6.4.1. Group Directories

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.

Lets say, for example, that a group of people work on files in the /usr/lib/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:

 /usr/sbin/groupadd emacs

To associate the contents of the directory with the emacs group, type:

 chown -R root.emacs /usr/lib/emacs/site-lisp

Now, it is possible to add the proper users to the group with the gpasswd command:

 /usr/bin/gpasswd -a <username> emacs

To allow users to create files within the directory, use the following command:

 chmod 775 /usr/lib/emacs/site-lisp

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/lib/emacs/site-lisp

At this point, because each user's default umask is 002, all members of the emacs group can create and edit files in the /usr/lib/emacs/site-lisp/ directory without the administrator having to change file permissions every time users write new files.

6.5. Shadow Passwords

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

6.6. Additional Resources

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

6.6.1. Installed Documentation

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 7. The X Window System

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.

Various windowing environments have existed in the UNIX™ world for decades, predating many of the current mainstream operating systems. Through the years, X has become the dominant graphical environment for UNIX-like operating systems.

The graphical environment for Red Hat Enterprise Linux is supplied by the X.Org Foundation, an open source consortium 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 X11R6.8 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.

7.1. The X11R6.8 Release

Red Hat Enterprise Linux 4.5.0 uses the X11R6.8 release as the base X Window System, which includes many cutting edge X.Org technology enhancements, such as 3D hardware acceleration support, the XRender extension for anti-aliased fonts, a modular driver-based design, and support for modern video hardware and input devices.

Important

Red Hat Enterprise Linux no longer provides the XFree86™ server packages. Before upgrading to the latest version of Red Hat Enterprise Linux, be sure that the video card is compatible with the X11R6.8 release by checking the Red Hat Hardware Compatibility List located online at http://hardware.redhat.com/.

The files related to the X11R6.8 release reside primarily in two locations:

/usr/X11R6/: Contains X server and some client applications, as well as X header files, libraries, modules, and documentation.
/etc/X11/: Contains configuration files for X client and server applications. This includes configuration files for the X server itself, the fs font server, the X display managers, and many other base components.
It is important to note that the configuration file for the newer Fontconfig-based font architecture is /etc/fonts/fonts.conf (which obsoletes the /etc/X11/XftConfig file). For more on configuring and adding fonts, refer to Section 7.4, “Fonts”.

Because the X server performs advanced tasks on a wide array of hardware, it requires detailed configuration. The installation program installs and configures X automatically, unless the X11R6.8 release packages are not selected for installation. However, if the monitor or video card changes, X must to be reconfigured. The best way to do this is to use the X Configuration Tool (system-config-display).

To start the X Configuration Tool while in an active X session, go to the Main Menu Button (on the Panel) => System Settings => Display. After using the X Configuration Tool during an X session, changes takes effect after logging out and logging back in. For more about using the X Configuration Tool, refer to the chapter titled X Window System Configuration in the System Administrators Guide.

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 7.3, “X Server Configuration Files”.

7.2. Desktop Environments and Window Managers

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 advanced GUI, two main classes of X client applications must connect to the X server: a desktop environment and a window manager.

7.2.1. Desktop Environments

A desktop environment brings together assorted X clients which, when used together, 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, and 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 visa versa.

7.2.2. Window Managers

Window managers are X client programs which are either part of a desktop environment or, in some cases, standalone. 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 supports custom themes.
mwm — The Motif window manager is a basic, standalone window manager. Since it is designed to be a standalone window manager, it should not be used in conjunction with GNOME or KDE.
twm — The minimalist Tab Window Manager, which provides the most basic tool set of any of the window managers and can be used either as a standalone or with a desktop environment. It is installed as part of the X11R6.8 release.

These window managers can be run without desktop environments to gain a better sense of their differences. To do this, type the xinit -e <path-to-window-manager> command, where <path-to-window-manager> is the location of the window manager binary file. The binary file can be found by typing which <window-manager-name>, where <window-manager-name> is the name of the window manager you are querying.

7.3. X Server Configuration Files

The X server is a single binary executable (/usr/X11R6/bin/Xorg) that dynamically loads any necessary X server modules at runtime from the /usr/X11R6/lib/modules/ directory. Some of these modules are automatically loaded by the server, while others are optional and must be specified in the X server configuration file.

The X server and associated configuration files are stored in the /etc/X11/ directory. The configuration file for the X server is /etc/X11/xorg.conf. When Red Hat Enterprise Linux is installed, the configuration files for X are created using information gathered about the system hardware during the installation process.

7.3.1. `xorg.conf`

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.

7.3.1.1. The Structure

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. Within each of the sections are lines containing option names and at least one option value, sometimes surrounded with 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.

7.3.1.2. `ServerFlags`

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 7.3.1.3, “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

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.

7.3.1.3. `ServerLayout`

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 at least two input devices (a keyboard and a mouse).

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

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
```
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".
If the video card has more than one head, another Screen entry would be necessary with a different number and a different Screen section identifier.
The numbers to the right of "Screen0" give the X and Y absolute 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.
There must 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 create more than one ServerLayout section. However, the server only reads the first one to appear unless an alternate ServerLayout section is specified as a command line argument.

7.3.1.4. `Files`

The Files section sets paths for services vital to the X server, such as the font path.

The following example illustrates a typical Files section:

Section "Files"
        RgbPath      "/usr/X11R6/lib/X11/rgb"
        FontPath     "unix/:7100"
EndSection

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 7.4, “Fonts” for more information concerning X and fonts.
ModulePath — An optional parameter which specifies alternate directories which store X server modules.

7.3.1.5. `Module`

The Module section specifies which modules from the /usr/X11R6/lib/modules/ directory the X server is to load. Modules add additional functionality to the X server.

The following example illustrates a typical Module section:

Section "Module"
	Load  "dbe"
	Load  "extmod"
	Load  "fbdevhw"
	Load  "glx"
	Load  "record"
	Load  "freetype"
	Load  "type1"
	Load  "dri"
EndSection

7.3.1.6. `InputDevice`

Each InputDevice section configures one input device for the X server. Systems typically have at least two InputDevice sections, keyboard and mouse.

The following example illustrates a typical InputDevice section for a mouse:

Section "InputDevice"
	Identifier  "Mouse0"
	Driver      "mouse"
	Option	    "Protocol" "IMPS/2"
	Option	    "Device" "/dev/input/mice"
	Option	    "Emulate3Buttons" "no"
EndSection

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.
For a mouse, these options typically include:
- 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.

By default, the InputDevice section has comments to allow users to configure additional options.

7.3.1.7. `Monitor`

Each Monitor section configures one type of monitor used by the system. While one Monitor section is the minimum, additional instances may occur for each monitor type in use with the machine.

The best way to configure a monitor 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 the chapter titled X Window System Configuration in the System Administrators Guide.

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

Warning

Be careful if 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.

7.3.1.8. `Device`

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 the chapter titled X Window System Configuration in the System Administrators Guide.

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

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/X11R6/lib/X11/Cards, 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 optional entry which specifies the bus location of the video card. This option is only mandatory for systems with multiple cards.
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", which activates the Service Star energy compliance setting for the monitor.

7.3.1.9. `Screen`

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

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 a required entry.
DefaultDepth — Specifies the default color depth in bits. In the previous example, 16, which provides thousands of colors, is the default. Multiple DefaultDepth entries are permitted, but at least one is required.
SubSection "Display" — Specifies the screen modes available at a particular color depth. A Screen section may have multiple Display subsections, but at least one is required for the color depth specified in the DefaultDepth entry.
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.

7.3.1.10. `DRI`

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 is ignored unless DRI is enabled in the Module section.

The following example illustrates a typical DRI section:

Section "DRI"
        Group        0
        Mode         0666
EndSection

Since different video cards use DRI in different ways, do not alter the values for this section without first referring to http://dri.sourceforge.net/.

7.4. Fonts

Red Hat Enterprise Linux uses two methods to manage and display fonts under X. 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.

7.4.1. Fontconfig

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, and 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 Main Menu Button [on the Panel] => Preferences => Font). 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"

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.

7.4.1.1. Adding Fonts to Fontconfig

Adding new fonts to the Fontconfig subsystem is a straightforward process.

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 and default installed fonts.
To add fonts for an individual user, copy the new fonts into the .fonts/ directory in the user's home directory.
Use the fc-cache command to update the font information cache, as in the following example:
```
 fc-cache <path-to-font-directory>
```
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.

7.4.2. Core X Font System

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 7.3.1.4, “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 the chapter titled Controlling Access to Services in the System Administrators Guide.

7.4.2.1. `xfs` Configuration

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 seperate each font server in a list.
catalogue — Specifies an ordered list of font paths to use. A comma must seperate 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 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 using xfs 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.

7.4.2.2. Adding Fonts to `xfs`

To add fonts to the core X font subsystem (xfs), follow these steps:

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/ 
```
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/ 
```
Copy the new font file into the /usr/share/fonts/local/ directory

Update the font information by issuing the following command as root:

 ttmkfdir -d /usr/share/fonts/local/ -o /usr/share/fonts/local/fonts.scale

Reload the xfs font server configuration file by issuing the following command as root:
```
 service xfs reload 
```

7.5. Runlevels and X

In most cases, the default installation of Red Hat Enterprise Linux configures a machine to boot into a graphical login environment, known as runlevel 5. It is possible, however, to boot into the text-only multi-user mode called runlevel 3 and begin an X session from there.

For more information about runlevels, refer to Section 1.4, “SysV Init Runlevels”.

The following subsections review how X starts up in both runlevel 3 and runlevel 5.

7.5.1. Runlevel 3

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 7.5.2, “Runlevel 5” for more information about display managers.

When the startx command is executed, it searches for an .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 looks 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, which configures settings such as the default language.

Next, the xinitrc script tries 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.

The user is returned to a text mode user session after logging out of X from runlevel 3.

7.5.2. Runlevel 5

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 within the

/usr/share/doc/initscripts-<version-number>/ sysconfig.txt

file (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 Main Menu Button [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 continued 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.

7.6. Additional Resources

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.

7.6.1. Installed Documentation

/usr/X11R6/lib/X11/doc/README — Briefly describes the XFree86 architecture and how to get additional information about the XFree86 project as a new user.
/usr/X11R6/lib/X11/doc/RELNOTES — For advanced users that want to read about the latest features available in XFree86.
man xorg.conf — Contains information about the xorg.conf configuration files, including the meaning and syntax for the different sections within the files.
man X.Org — The primary man page for X.Org Foundation information.
man Xorg — Describes the X11R6.8 display server.

7.6.2. Useful Websites

http://www.X.org/ — Home page of the X.Org Foundation, which produces the X11R6.8 release of the X Window System. The X11R6.8 release is bundled with Red Hat Enterprise Linux to control the necessary hardware and provide a GUI environment.
http://xorg.freedesktop.org/ — Home page of the XR116.8 release, which provides binaries and documention for the X Window System.
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.
http://nexp.cs.pdx.edu/fontconfig/ — Home of the Fontconfig font subsystem for X.

Part II. Network Services Reference

It is possible to deploy a wide variety of network services under Red Hat Enterprise Linux. This part describes how network interfaces are configured as well as provides details about critical network services such as FTP, NFS, the Apache HTTP Server, Sendmail, Postfix, Exim, Fetchmail, Procmail, BIND, LDAP, and Samba.

Chapter 8. Network Interfaces

Under Red Hat Enterprise Linux, all network communications occur between configured software interfaces and physical networking devices connected to the system.

The configuration files for network interfaces, and the scripts used to activate and deactivate them, are located in the /etc/sysconfig/network-scripts/ directory. Although the number and type of interface files can differ from system to system, there are three categories of files that exist in this directory:

Interface configuration files
Interface control scripts
Network function files

The files in each of these categories work together to enable various network devices.

This chapter explores the relationship between these files and how they are used.

8.1. Network Configuration Files

Before delving into the interface configuration files, let us first itemize the primary configuration files used in network configuration. Understanding the role these files play in setting up the network stack can be helpful when customizing a Red Hat Enterprise Linux system.

The primary network configuration files are as follows:

/etc/hosts — The main purpose of this file is to resolve hostnames that cannot be resolved any other way. It can also be used to resolve hostnames on small networks with no DNS server. Regardless of the type of network the computer is on, this file should contain a line specifying the IP address of the loopback device (127.0.0.1) as localhost.localdomain. For more information, refer to the hosts man page.
/etc/resolv.conf — This file specifies the IP addresses of DNS servers and the search domain. Unless configured to do otherwise, the network initialization scripts populate this file. For more information about this file, refer to the resolv.conf man page.
/etc/sysconfig/network — Specifies routing and host information for all network interfaces. For more information about this file and the directives it accepts, refer to Section 4.1.25, “/etc/sysconfig/network”.
/etc/sysconfig/network-scripts/ifcfg-<interface-name> — For each network interface, there is a corresponding interface configuration script. Each of these files provide information specific to a particular network interface. Refer to Section 8.2, “Interface Configuration Files” for more information on this type of file and the directives it accepts.

Warning

The /etc/sysconfig/networking/ directory is used by the Network Administration Tool (system-config-network) and its contents should not be edited manually. In addition, any use of the Network Administration Tool, even launching the application, will override any directives previously set in /etc/sysconfig/network-scripts. Using only one method for network configuration is strongly encouraged, due to the risk of configuration deletion.

For more information about configuring network interfaces using the Network Administration Tool, refer to the chapter titled Network Configuration in the System Administrators Guide.

8.2. Interface Configuration Files

Interface configuration files control the software interfaces for individual network devices. As the system boots, it uses these files to determine what interfaces to bring up and how to configure them. These files are usually named ifcfg-<name>, where <name> refers to the name of the device that the configuration file controls.

8.2.1. Ethernet Interfaces

One of the most common interface files is ifcfg-eth0, which controls the first Ethernet network interface card or NIC in the system. In a system with multiple NICs, there are multiple ifcfg-eth<X> files (where <X> is a unique number corresponding to a specific interface). Because each device has its own configuration file, an administrator can control how each interface functions individually.

The following is a sample ifcfg-eth0 file for a system using a fixed IP address:

DEVICE=eth0
BOOTPROTO=none
ONBOOT=yes
NETWORK=10.0.1.0
NETMASK=255.255.255.0
IPADDR=10.0.1.27
USERCTL=no

The values required in an interface configuration file can change based on other values. For example, the ifcfg-eth0 file for an interface using DHCP looks quite a bit different because IP information is provided by the DHCP server:

DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes

The Network Administration Tool (system-config-network) is an easy way to make changes to the various network interface configuration files (refer to the chapter titled Network Configuration in the System Administrators Guide for detailed instructions on using this tool).

However, it is also possible to edit the configuration files for a given network interface manually.

Below is a listing of the configurable parameters in an Ethernet interface configuration file:

BOOTPROTO=<protocol>, where <protocol> is one of the following:
- none — No boot-time protocol should be used.
- bootp — The BOOTP protocol should be used.
- dhcp — The DHCP protocol should be used.
BROADCAST=<address>, where <address> is the broadcast address. This directive is deprecated, as the value is calculated automatically with ipcalc.
DEVICE=<name>, where <name> is the name of the physical device (except for dynamically-allocated PPP devices where it is the logical name).
DHCP_HOSTNAME — Only use this option if the DHCP server requires the client to specify a hostname before receiving an IP address. (The DHCP server daemon in Red Hat Enterprise Linux does not support this feature.)
DNS{1,2}=<address>, where <address> is a name server address to be placed in /etc/resolv.conf if the PEERDNS directive is set to yes.
ETHTOOL_OPTS=<options>, where <options> are any device-specific options supported by ethtool. For example, if you wanted to force 100Mb, full duplex:
```
ETHTOOL_OPTS="autoneg off speed 100 duplex full"
```
Note that changing speed or duplex settings almost always requires disabling autonegotiation with the autoneg off option. This needs to be stated first, as the option entries are order dependent.
GATEWAY=<address>, where <address> is the IP address of the network router or gateway device (if any).
HWADDR=<MAC-address>, where <MAC-address> is the hardware address of the Ethernet device in the form AA:BB:CC:DD:EE:FF. This directive is useful for machines with multiple NICs to ensure that the interfaces are assigned the correct device names regardless of the configured load order for each NIC's module. This directive should not be used in conjunction with MACADDR.
IPADDR=<address>, where <address> is the IP address.
MACADDR=<MAC-address>, where <MAC-address> is the hardware address of the Ethernet device in the form AA:BB:CC:DD:EE:FF. This directive is used to assign a MAC address to an interface, overriding the one assigned to the physical NIC. This directive should not be used in conjunction with HWADDR.
MASTER=<bond-interface>,where <bond-interface> is the channel bonding interface to which the interface the Ethernet interface is linked.
This directive is used in conjunction with the SLAVE directive.
Refer to Section 8.2.3, “Channel Bonding Interfaces” for more about channel bonding interfaces.
NETMASK=<mask>, where <mask> is the netmask value.
NETWORK=<address>, where <address> is the network address. This directive is deprecated, as the value is calculated automatically with ipcalc.
ONBOOT=<answer>, where <answer> is one of the following:
- yes — This device should be activated at boot-time.
- no — This device should not be activated at boot-time.
PEERDNS=<answer>, where <answer> is one of the following:
- yes — Modify /etc/resolv.conf if the DNS directive is set. If using DHCP, then yes is the default.
- no — Do not modify /etc/resolv.conf.
SLAVE=<bond-interface>,where <bond-interface> is one of the following:
- yes — This device is controlled by the channel bonding interface specified in the MASTER directive.
- no — This device is not controlled by the channel bonding interface specified in the MASTER directive.
This directive is used in conjunction with the MASTER directive.
Refer to Section 8.2.3, “Channel Bonding Interfaces” for more about channel bond interfaces.
SRCADDR=<address>, where <address> is the specified source IP address for outgoing packets.
USERCTL=<answer>, where <answer> is one of the following:
- yes — Non-root users are allowed to control this device.
- no — Non-root users are not allowed to control this device.

8.2.2. IPsec Interfaces

With Red Hat Enterprise Linux it is possible to connect to other hosts or networks using a secure IP connection, known as IPsec. For instructions on setting up IPsec using the Network Administration Tool (system-config-network), refer to the chapter titled Network Configuration in the System Administrators Guide. For instructions on setting up IPsec manually, refer to the chapter titled Virtual Private Networks in the Security Guide.

The following example shows the ifcfg file for a network-to-network IPsec connection for LAN A. The unique name to identify the connection in this example is ipsec1, so the resulting file is named /etc/sysconfig/network-scripts/ifcfg-ipsec1.

TYPE=IPsec
ONBOOT=yes
IKE_METHOD=PSK
SRCNET=192.168.1.0/24
DSTNET=192.168.2.0/24
DST=X.X.X.X

In the example above, X.X.X.X is the publicly routable IP address of the destination IPsec router.

Below is a listing of the configurable parameters for an IPsec interface:

DST=<address>, where <address> is the IP address of the IPsec destination host or router. This is used for both host-to-host and network-to-network IPsec configurations.
DSTNET=<network>, where <network> is the network address of the IPsec destination network. This is only used for network-to-network IPsec configurations.
SRC=<address>, where <address> is the IP address of the IPsec source host or router. This setting is optional and is only used for host-to-host IPsec configurations.
SRCNET=<network>, where <network> is the network address of the IPsec source network. This is only used for network-to-network IPsec configurations.
TYPE=<interface-type>, where <interface-type> is IPSEC. Both applications are part of the ipsec-tools package.

Refer to /usr/share/doc/initscripts-<version-number>/sysconfig.txt (replace <version-number> with the version of the initscripts package installed) for configuration parameters if using manual key encryption with IPsec.

The racoon IKEv1 key management daemon negotiates and configures a set of parameters for IPSec. It can use preshared keys, RSA signatures, or GSS-API. If racoon is used to automatically manage key encryption, the following options are required:

IKE_METHOD=<encryption-method>, where <encryption-method> is either PSK, X509, or GSSAPI. If PSK is specified, the IKE_PSK parameter must also be set. If X509 is specified, the IKE_CERTFILE parameter must also be set.
IKE_PSK=<shared-key>, where <shared-key> is the shared, secret value for the PSK (preshared keys) method.
IKE_CERTFILE=<cert-file>, where <cert-file> is a valid X.509 certificate file for the host.
IKE_PEER_CERTFILE=<cert-file>, where <cert-file> is a valid X.509 certificate file for the remote host.
IKE_DNSSEC=<answer>, where <answer> is yes. The racoon daemon retrieves the remote host's X.509 certificate via DNS. If a IKE_PEER_CERTFILE is specified, do not include this parameter.

For more information about the encryption algorithms available for IPsec, refer to the setkey man page. For more information about racoon, refer to the racoon and racoon.conf man pages.

8.2.3. Channel Bonding Interfaces

Red Hat Enterprise Linux allows administrators to bind multiple network interfaces 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 create a channel bonding interface, create a file in the /etc/sysconfig/network-scripts/ directory called ifcfg-bond<N>, replacing <N> with the number for the interface, such as 0.

The contents of the file can be identical to whatever type of interface that is getting bonded, such as an Ethernet interface. The only difference is that the DEVICE= directive must be bond<N>, replacing <N> with the number for the interface.

The following is a sample channel bonding configuration file:

DEVICE=bond0
BOOTPROTO=none
ONBOOT=yes
NETWORK=10.0.1.0
NETMASK=255.255.255.0
IPADDR=10.0.1.27
USERCTL=no

After the channel bonding interface is created, the network interfaces to be bound together must be configured by adding the MASTER= and SLAVE= directives to their configuration files. The configuration files for each of the channel bonded interfaces can be nearly identical.

For example, if channel bonding two Ethernet interfaces, both eth0 and eth1 may look like the following example:

DEVICE=eth<N>
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SLAVE=yes
USERCTL=no

In this example, replace <N> with the numerical value for the interface.

For a channel bonding interface to be valid, the kernel module must be loaded. To insure that the module is loaded when the channel bonding interface is brought up, add the following line to /etc/modprobe.conf:

install bond0 /sbin/modprobe bonding -o bond0

Once /etc/modprobe.conf is configured, and the channel bonding interface and network interfaces are configured, the ifup command can be used to bring up the channel bonding interface.

Important

Important aspects of the channel bonding interface are controlled through the kernel module. For more information about controlling the bonding modules, refer to Section 22.5.2, “The Channel Bonding Module”.

8.2.4. Alias and Clone Files

Two lesser-used types of interface configuration files are alias and clone files.

Alias interface configuration files, which are used to bind multiple addresses to a single interface, use the ifcfg-<if-name>:<alias-value> naming scheme.

For example, an ifcfg-eth0:0 file could be configured to specify DEVICE=eth0:0 and a static IP address of 10.0.0.2, serving as an alias of an Ethernet interface already configured to receive its IP information via DHCP in ifcfg-eth0. Under this configuration, eth0 is bound to a dynamic IP address, but the same physical network card can receive request via the fixed, 10.0.0.2 IP address.

Warning

Alias interfaces do not support DHCP.

A clone interface configuration file should use the following naming convention: ifcfg-<if-name>-<clone-name>. While an alias file allows multiple addresses for an existing interface, a clone file is used to specify additional options for an interface. For example, a standard DHCP Ethernet interface called eth0, may look similar to this:

DEVICE=eth0
ONBOOT=yes
BOOTPROTO=dhcp

Since the default value for the USERCTL directive is no if it is not specified, users cannot bring this interface up and down. To give users the ability to control the interface, create a clone by copying ifcfg-eth0 to ifcfg-eth0-user and add the following line to ifcfg-eth0-user:

USERCTL=yes

This way a user can bring up the eth0 interface using the /sbin/ifup eth0-user command because the configuration options from ifcfg-eth0 and ifcfg-eth0-user are combined. While this is a very basic example, this method can be used with a variety of options and interfaces.

The easiest way to create alias and clone interface configuration files is to use the graphical Network Administration Tool. For more on using this tool, refer to the chapter called Network Configuration in the System Administrators Guide.

8.2.5. Dialup Interfaces

If connecting to the Internet via a dialup connection, a configuration file is necessary for the interface.

PPP interface files are named using the following format ifcfg-ppp<X> (where <X> is a unique number corresponding to a specific interface).

The PPP interface configuration file is created automatically when wvdial, the Network Administration Tool or Kppp is used to create a dialup account. It is also possible to create and edit this file manually.

The following is a typical ifcfg-ppp0 file:

DEVICE=ppp0
NAME=test
WVDIALSECT=test
MODEMPORT=/dev/modem
LINESPEED=115200
PAPNAME=test
USERCTL=true
ONBOOT=no
PERSIST=no
DEFROUTE=yes
PEERDNS=yes
DEMAND=no
IDLETIMEOUT=600

Serial Line Internet Protocol (SLIP) is another dialup interface, although it is used less frequently. SLIP files have interface configuration file names such as ifcfg-sl0.

Other options, not already discussed, that may be used in these files include:

DEFROUTE=<answer>, where <answer> is one of the following:
- yes — Set this interface as the default route.
- no — Do not set this interface as the default route.
DEMAND=<answer>, where <answer> is one of the following:
- yes — This interface allows pppd to initiate a connection when someone attempts to use it.
- no — A connection must be manually established for this interface.
IDLETIMEOUT=<value>, where <value> is the number of seconds of idle activity before the interface disconnects itself.
INITSTRING=<string>, where <string> is the initialization string passed to the modem device. This option is primarily used in conjunction with SLIP interfaces.
LINESPEED=<value>, where <value> is the baud rate of the device. Possible standard values include 57600, 38400, 19200, and 9600.
MODEMPORT=<device>, where <device> is the name of the serial device that is used to establish the connection for the interface.
MTU=<value>, where <value> is the Maximum Transfer Unit (MTU) setting for the interface. The MTU refers to the largest number of bytes of data a frame can carry, not counting its header information. In some dialup situations, setting this to a value of 576 results in fewer packets dropped and a slight improvement to the throughput for a connection.
NAME=<name>, where <name> is the reference to the title given to a collection of dialup connection configurations.
PAPNAME=<name>, where <name> is the username given during the Password Authentication Protocol (PAP) exchange that occurs to allow connections to a remote system.
PERSIST=<answer>, where <answer> is one of the following:
- yes — This interface should be kept active at all times, even if deactivated after a modem hang up.
- no — This interface should not be kept active at all times.
REMIP=<address>, where <address> is the remote system's IP address. This is usually left unspecified.
WVDIALSECT=<name>, where <name> associates this interface with a dialer configuration in /etc/wvdial.conf. This file contains the phone number to be dialed and other important information for the interface.

8.2.6. Other Interfaces

Other common interface configuration files include the following:

ifcfg-lo — A local loopback interface is often used in testing, as well as being used in a variety of applications that require an IP address pointing back to the same system. Any data sent to the loopback device is immediately returned to the host's network layer.
Warning
Never edit the loopback interface script, /etc/sysconfig/network-scripts/ifcfg-lo, manually. Doing so can prevent the system from operating correctly.
ifcfg-irlan0 — An infrared interface allows information between devices, such as a laptop and a printer, to flow over an infrared link. This works in a similar way to an Ethernet device except that it commonly occurs over a peer-to-peer connection.
ifcfg-plip0 — A Parallel Line Interface Protocol (PLIP) connection works much the same way as an Ethernet device, except that it utilizes a parallel port.
ifcfg-tr0 — Token Ring topologies are not as common on Local Area Networks (LANs) as they once were, having been eclipsed by Ethernet.

8.3. Interface Control Scripts

The interface control scripts activate and deactivated system interfaces. There are two primary interface control scripts, /sbin/ifdown and /sbin/ifup, that call on control scripts located in the /etc/sysconfig/network-scripts/ directory.

The ifup and ifdown interface scripts are symbolic links to scripts in the /sbin/ directory. When either of these scripts are called, they require the value of the interface to be specified, such as:

ifup eth0

Warning

The ifup and ifdown interface scripts are the only scripts that the user should use to bring up and take down network interfaces.

The following scripts are described for reference purposes only.

Two files used to perform a variety of network initialization tasks during the process of bringing up a network interface are /etc/rc.d/init.d/functions and /etc/sysconfig/network-scripts/network-functions. Refer to Section 8.4, “Network Function Files” for more information.

After verifying that an interface has been specified and that the user executing the request is allowed to control the interface, the correct script brings the interface up or down. The following are common interface control scripts found within the /etc/sysconfig/network-scripts/ directory:

ifup-aliases — Configures IP aliases from interface configuration files when more than one IP address is associated with an interface.
ifup-ippp and ifdown-ippp — Brings ISDN interfaces up and down.
ifup-ipsec and ifdown-ipsec — Brings IPsec interfaces up and down.
ifup-ipv6 and ifdown-ipv6 — Brings IPv6 interfaces up and down.
ifup-ipx — Brings up an IPX interface.
ifup-plip — Brings up a PLIP interface.
ifup-plusb — Brings up a USB interface for network connections.
ifup-post and ifdown-post — Contains commands to be executed after an interface is brought up or down.
ifup-ppp and ifdown-ppp — Brings a PPP interface up or down.
ifup-routes — Adds static routes for a device as its interface is brought up.
ifdown-sit and ifup-sit — Contains function calls related to bringing up and down an IPv6 tunnel within an IPv4 connection.
ifup-sl and ifdown-sl — Brings a SLIP interface up or down.
ifup-wireless — Brings up a wireless interface.

Warning

Removing or modifying any scripts in the /etc/sysconfig/network-scripts/ directory can cause interface connections to act irregularly or fail. Only advanced users should modify scripts related to a network interface.

The easiest way to manipulate all network scripts simultaneously is to use the /sbin/service command on the network service (/etc/rc.d/init.d/network), as illustrated the following command:

service network <action>

In this example, <action> can be either start, stop, or restart.

To view a list of configured devices and currently active network interfaces, use the following command:

service network status

8.4. Network Function Files

Red Hat Enterprise Linux makes use of several files that contain important common functions used to bring interfaces up and down. Rather than forcing each interface control file to contain these functions, they are grouped together in a few files that are called upon when necessary.

The /etc/sysconfig/network-scripts/network-functions file contains the most commonly used IPv4 functions, which are useful to many interface control scripts. These functions include contacting running programs that have requested information about changes in an interface's status, setting hostnames, finding a gateway device, verifying whether or not if a particular device is down, and adding a default route.

As the functions required for IPv6 interfaces are different than IPv4 interfaces, a /etc/sysconfig/network-scripts/network-functions-ipv6 file exists specifically to hold this information. The functions in this file configure and delete static IPv6 routes, create and remove tunnels, add and remove IPv6 addresses to an interface, and test for the existence of an IPv6 address on an interface.

8.5. Additional Resources

The following are resources which explain more about network interfaces.

8.5.1. Installed Documentation

/usr/share/doc/initscripts-<version>/sysconfig.txt — A guide to available options for network configuration files, including IPv6 options not covered in this chapter.
/usr/share/doc/iproute-<version>/ip-cref.ps — This file contains a wealth of information about the ip command, which can be used to manipulate routing tables, among other things. Use the ggv or kghostview application to view this file.

Chapter 9. Network File System (NFS)

A Network File System (NFS) allows remote hosts to mount file systems over a network and interact with those file systems as though they are mounted locally. This enables system administrators to consolidate resources onto centralized servers on the network.

This chapter focuses on fundamental NFS concepts and supplemental information. For specific instructions regarding the configuration and operation of NFS server and client software, refer to the chapter titled Network File System (NFS) in the System Administrators Guide.

9.1. How It Works

Currently, there are three versions of NFS. NFS version 2 (NFSv2) is older and is widely supported. NFS version 3 (NFSv3) has more features, including variable size file handling and better error reporting, but is not fully compatible with NFSv2 clients. NFS version 4 (NFSv4) works through firewalls and on the Internet, no longer requires portmapper, supports ACLs, and utilizes stateful operations. Red Hat Enterprise Linux supports NFSv2, NFSv3, and NFSv4 clients, and when mounting a file system via NFS, Red Hat Enterprise Linux uses NFSv3 by default, if the server supports it.

All versions of NFS can use Transmission Control Protocol (TCP) running over an IP network, with NFSv4 requiring it. NFSv2 and NFSv3 can use the User Datagram Protocol (UDP) running over an IP network to provide a stateless network connection between the client and server.

When using NFSv2 or NFSv3 with UDP, the stateless UDP connection under normal conditions minimizes network traffic, as the NFS server sends the client a cookie after the client is authorized to access the shared volume. This cookie is a random value stored on the server's side and is passed along with RPC requests from the client. The NFS server can be restarted without affecting the clients and the cookie remains intact. However, because UDP is stateless, if the server goes down unexpectedly, UDP clients continue to saturate the network with requests for the server. For this reason, TCP is the preferred protocol when connecting to an NFS server.

NFSv4 has no interaction with portmapper, rpc.mountd, rpc.lockd, and rpc.statd, since they have been rolled into the kernel. NFSv4 listens on the well known TCP port 2049.

Note

TCP is the default transport protocol for NFS under Red Hat Enterprise Linux. Refer to the chapter titled Network File System (NFS) in the System Administrators Guide for more information about connecting to NFS servers using TCP. UDP can be used for compatibility purposes as needed, but is not recommended for wide usage.

The only time NFS performs authentication is when a client system attempts to mount the shared NFS resource. To limit access to the NFS service, TCP wrappers are used. TCP wrappers read the /etc/hosts.allow and /etc/hosts.deny files to determine if a particular client or network is permitted or denied access to the NFS service. For more information on configuring access controls with TCP wrappers, refer to Chapter 17, TCP Wrappers and xinetd.

After the client is granted access by TCP wrappers, the NFS server refers to its configuration file, /etc/exports, to determine whether the client is allowed to access any of the exported file systems. Once access is granted, all file and directory operations are available to the user.

Important

In order for NFS to work with a default installation of Red Hat Enterprise Linux with a firewall enabled, IPTables with the default TCP port 2049 must be configured. Without an IPTables configuration, NFS does not function properly.

The NFS initialization script and rpc.nfsd process now allow binding to any specified port during system start up. However, this can be error prone if the port is unavailable or conflicts with another daemon.

9.1.1. Required Services

Red Hat Enterprise Linux uses a combination of kernel-level support and daemon processes to provide NFS file sharing. NFSv2 and NFSv3 rely on Remote Procedure Calls (RPC) to encode and decode requests between clients and servers. RPC services under Linux are controlled by the portmap service. To share or mount NFS file systems, the following services work together, depending on which version of NFS is implemented:

nfs — Starts the appropriate RPC processes to service requests for shared NFS file systems.
nfslock — An optional service that starts the appropriate RPC processes to allow NFS clients to lock files on the server.
portmap — The RPC service for Linux; it responds to requests for RPC services and sets up connections to the requested RPC service. This is not used with NFSv4.

The following RPC processes facilitate NFS services:

rpc.mountd — This process receives mount requests from NFS clients and verifies the requested file system is currently exported. This process is started automatically by the nfs service and does not require user configuration. This is not used with NFSv4.
rpc.nfsd — This process is the NFS server. It works with the Linux kernel to meet the dynamic demands of NFS clients, such as providing server threads each time an NFS client connects. This process corresponds to the nfs service.
rpc.lockd — An optional process that allows NFS clients to lock files on the server. This process corresponds to the nfslock service. This is not used with NFSv4.
rpc.statd — This process implements the Network Status Monitor (NSM) RPC protocol which notifies NFS clients when an NFS server is restarted without being gracefully brought down. This process is started automatically by the nfslock service and does not require user configuration. This is not used with NFSv4.
rpc.rquotad — This process provides user quota information for remote users. This process is started automatically by the nfs service and does not require user configuration.
rpc.idmapd — This process provides NFSv4 client and server upcalls which map between on-the-wire NFSv4 names (which are strings in the form of user@domain) and local UIDs and GIDs. For idmapd to function with NFSv4, the /etc/idmapd.conf must be configured. This service is required for use with NFSv4.
rpc.svcgssd — This process is used by the NFS server to perform user authentication and is started only when SECURE_NFS=yes is set in the /etc/sysconfig/nfs file.
rpc.gssd — This process is used by the NFS server to perform user authentication and is started only when SECURE_NFS=yes is set in the /etc/sysconfig/nfs file.

9.1.2. NFS and `portmap`

Note

The following section only applies to NFSv2 or NFSv3 implementations that require the portmap service for backward compatibility.

The portmap service under Linux maps RPC requests to the correct services. RPC processes notify portmap when they start, revealing the port number they are monitoring and the RPC program numbers they expect to serve. The client system then contacts portmap on the server with a particular RPC program number. The portmap service redirects the client to the proper port number so it can communicate with the requested service.

Because RPC-based services rely on portmap to make all connections with incoming client requests, portmap must be available before any of these services start.

The portmap service uses TCP wrappers for access control, and access control rules for portmap affect all RPC-based services. Alternatively, it is possible to specify access control rules for each of the NFS RPC daemons. The man pages for rpc.mountd and rpc.statd contain information regarding the precise syntax for these rules.

9.1.2.1. Troubleshooting NFS and `portmap`

Because portmap provides coordination between RPC services and the port numbers used to communicate with them, it is useful to view the status of current RPC services using portmap when troubleshooting. The rpcinfo command shows each RPC-based service with port numbers, an RPC program number, a version number, and an IP protocol type (TCP or UDP).

To make sure the proper NFS RPC-based services are enabled for portmap, issue the following command as root:

rpcinfo -p

The following is sample output from this command:

   program vers proto   port
    100000    2   tcp    111  portmapper
    100000    2   udp    111  portmapper
    100021    1   udp  32774  nlockmgr
    100021    3   udp  32774  nlockmgr
    100021    4   udp  32774  nlockmgr
    100021    1   tcp  34437  nlockmgr
    100021    3   tcp  34437  nlockmgr
    100021    4   tcp  34437  nlockmgr
    100011    1   udp    819  rquotad
    100011    2   udp    819  rquotad
    100011    1   tcp    822  rquotad
    100011    2   tcp    822  rquotad
    100003    2   udp   2049  nfs
    100003    3   udp   2049  nfs
    100003    2   tcp   2049  nfs
    100003    3   tcp   2049  nfs
    100005    1   udp    836  mountd
    100005    1   tcp    839  mountd
    100005    2   udp    836  mountd
    100005    2   tcp    839  mountd
    100005    3   udp    836  mountd
    100005    3   tcp    839  mountd

The output from this command reveals that the correct NFS services are running. If one of the NFS services does not start up correctly, portmap is unable to map RPC requests from clients for that service to the correct port. In many cases, if NFS is not present in rpcinfo output, restarting NFS causes the service to correctly register with portmap and begin working. For instructions on starting NFS, refer to Section 9.2, “Starting and Stopping NFS”.

Other useful options are available for the rpcinfo command. Refer to the rpcinfo man page for more information.

9.2. Starting and Stopping NFS

To run an NFS server, the portmap service must be running. To verify that portmap is active, type the following command as root:

 /sbin/service portmap status

If the portmap service is running, then the nfs service can be started. To start an NFS server, as root type:

 /sbin/service nfs start

To stop the server, as root, type:

 /sbin/service nfs stop

The restart option is a shorthand way of stopping and then starting NFS. This is the most efficient way to make configuration changes take effect after editing the configuration file for NFS.

To restart the server, as root, type:

 /sbin/service nfs restart

The condrestart (conditional restart) option only starts nfs if it is currently running. This option is useful for scripts, because it does not start the daemon if it is not running.

To conditionally restart the server, as root, type:

 /sbin/service nfs condrestart

To reload the NFS server configuration file without restarting the service, as root, type:

 /sbin/service nfs reload

By default, the nfs service does not start automatically at boot time. To configure the NFS to start up at boot time, use an initscript utility, such as /sbin/chkconfig, /usr/sbin/ntsysv, or the Services Configuration Tool program. Refer to the chapter titled Controlling Access to Services in the System Administrators Guide for more information regarding these tools.

9.3. NFS Server Configuration

There are three ways to configure an NFS server under Red Hat Enterprise Linux: using the NFS Server Configuration Tool (system-config-nfs), manually editing its configuration file (/etc/exports), or using the /usr/sbin/exportfs command.

For instructions on using NFS Server Configuration Tool, refer to the chapter titled Network File System (NFS) in the System Administrators Guide. The remainder of this section discusses manually editing /etc/exports and using the /usr/sbin/exportfs command to export NFS file systems.

9.3.1. The `/etc/exports` Configuration File

The /etc/exports file controls which file systems are exported to remote hosts and specifies options. Blank lines are ignored, comments can be made by starting a line with the hash mark (#), and long lines can be wrapped with a backslash (\). Each exported file system should be on its own individual line, and any lists of authorized hosts placed after an exported file system must be separated by space characters. Options for each of the hosts must be placed in parentheses directly after the host identifier, without any spaces separating the host and the first parenthesis.

A line for an exported file system has the following structure:

<export> <host1>(<options>) <hostN>(<options>)...

In this structure, replace <export> with the directory being exported, replace <host1> with the host or network to which the export is being shared, and replace <options> with the options for that host or network. Additional hosts can be specified in a space separated list.

The following methods can be used to specify host names:

single host — Where one particular host is specified with a fully qualified domain name, hostname, or IP address.
wildcards — Where a * or ? character is used to take into account a grouping of fully qualified domain names that match a particular string of letters. Wildcards should not be used with IP addresses; however, it is possible for them to work accidentally if reverse DNS lookups fail.
Be careful when using wildcards with fully qualified domain names, as they tend to be more exact than expected. For example, the use of *.example.com as a wildcard allows sales.example.com to access an exported file system, but not bob.sales.example.com. To match both possibilities both *.example.com and *.*.example.com must be specified.
IP networks — Allows the matching of hosts based on their IP addresses within a larger network. For example, 192.168.0.0/28 allows the first 16 IP addresses, from 192.168.0.0 to 192.168.0.15, to access the exported file system, but not 192.168.0.16 and higher.
netgroups — Permits an NIS netgroup name, written as @<group-name>, to be used. This effectively puts the NIS server in charge of access control for this exported file system, where users can be added and removed from an NIS group without affecting /etc/exports.

In its simplest form, the /etc/exports file only specifies the exported directory and the hosts permitted to access it, as in the following example:

 /exported/directory bob.example.com

In the example, bob.example.com can mount /exported/directory/. Because no options are specified in this example, the following default NFS options take effect:

ro — Mounts of the exported file system are read-only. Remote hosts are not able to make changes to the data shared on the file system. To allow hosts to make changes to the file system, the read/write (rw) option must be specified.
wdelay — Causes the NFS server to delay writing to the disk if it suspects another write request is imminent. This can improve performance by reducing the number of times the disk must be accessed by separate write commands, reducing write overhead. The no_wdelay option turns off this feature, but is only available when using the sync option.
root_squash — Prevents root users connected remotely from having root privileges and assigns them the user ID for the user nfsnobody. This effectively "squashes" the power of the remote root user to the lowest local user, preventing unauthorized alteration of files on the remote server. Alternatively, the no_root_squash option turns off root squashing. To squash every remote user, including root, use the all_squash option. To specify the user and group IDs to use with remote users from a particular host, use the anonuid and anongid options, respectively. In this case, a special user account can be created for remote NFS users to share and specify (anonuid=<uid-value>,anongid=<gid-value>), where <uid-value> is the user ID number and <gid-value> is the group ID number.

Important

By default, access control lists (ACLs) are supported by NFS under Red Hat Enterprise Linux. To disable this feature, specify the no_acl option when exporting the file system. For more about this feature, refer to the chapter titled Network File System (NFS) in the System Administrators Guide.

Each default for every exported file system must be explicitly overridden. For example, if the rw option is not specified, then the exported file system is shared as read-only. The following is a sample line from /etc/exports which overrides two default options:

 /another/exported/directory 192.168.0.3(rw,sync)

In this example 192.168.0.3 can mount /another/exported/directory/ read/write and all transfers to disk are committed to the disk before the write request by the client is completed.

Additionally, other options are available where no default value is specified. These include the ability to disable sub-tree checking, allow access from insecure ports, and allow insecure file locks (necessary for certain early NFS client implementations). Refer to the exports man page for details on these lesser used options.

Warning

The format of the /etc/exports file is very precise, particularly in regards to use of the space character. Remember to always separate exported file systems from hosts and hosts from one another with a space character. However, there should be no other space characters in the file except on comment lines.

For example, the following two lines do not mean the same thing:

 /home bob.example.com(rw) /home bob.example.com (rw)

The first line allows only users from bob.example.com read/write access to the /home directory. The second line allows users from bob.example.com to mount the directory as read-only (the default), while the rest of the world can mount it read/write.

For detailed instructions on configuring an NFS server by editing /etc/exports, refer to the chapter titled Network File System (NFS) in the System Administrators Guide.

9.3.2. The `exportfs` Command

Every file system being exported to remote users via NFS, as well as the access level for those file systems, are listed in the /etc/exports file. When the nfs service starts, the /usr/sbin/exportfs command launches and reads this file, passes control to rpc.mountd (if NFSv2 or NFSv3) for the actual mounting process, then to rpc.nfsd where the file systems are then available to remote users.

When issued manually, the /usr/sbin/exportfs command allows the root user to selectively export or unexport directories without restarting the NFS service. When given the proper options, the /usr/sbin/exportfs command writes the exported file systems to /var/lib/nfs/xtab. Since rpc.mountd refers to the xtab file when deciding access privileges to a file system, changes to the list of exported file systems take effect immediately.

The following is a list of commonly used options available for /usr/sbin/exportfs:

-r — Causes all directories listed in /etc/exports to be exported by constructing a new export list in /etc/lib/nfs/xtab. This option effectively refreshes the export list with any changes that have been made to /etc/exports.
-a — Causes all directories to be exported or unexported, depending on what other options are passed to /usr/sbin/exportfs. If no other options are specified, /usr/sbin/exportfs exports all file systems specified in /etc/exports.
-o file-systems — Specifies directories to be exported that are not listed in /etc/exports. Replace file-systems with additional file systems to be exported. These file systems must be formatted in the same way they are specified in /etc/exports. Refer to Section 9.3.1, “The /etc/exports Configuration File” for more information on /etc/exports syntax. This option is often used to test an exported file system before adding it permanently to the list of file systems to be exported.
-i — Ignores /etc/exports; only options given from the command line are used to define exported file systems.
-u — Unexports all shared directories. The command /usr/sbin/exportfs -ua suspends NFS file sharing while keeping all NFS daemons up. To re-enable NFS sharing, type exportfs -r.
-v — Verbose operation, where the file systems being exported or unexported are displayed in greater detail when the exportfs command is executed.

If no options are passed to the /usr/sbin/exportfs command, it displays a list of currently exported file systems.

For more information about the /usr/sbin/exportfs command, refer to the exportfs man page.

9.3.2.1. Using `exportfs` with NFSv4

Since NFSv4 no longer utilizes the rpc.mountd protocol as was used in NFSv2 and NFSv3, the mounting of file systems has changed.

An NFSv4 client now has the ability to see all of the exports served by the NFSv4 server as a single file system, called the NFSv4 pseudo-file system. On Red Hat Enterprise Linux, the pseudo-file system is identified as a single, real file system, identified at export with the fsid=0 option.

For example, the following commands could be executed on an NFSv4 server:

mkdir /exports
mkdir /exports/opt
mkdir /exports/etc
mount --bind /usr/local/opt /exports/opt
mount --bind /usr/local/etc /exports/etc
exportfs -o fsid=0,insecure,no_subtree_check gss/krb5p:/exports
exportfs -o rw,nohide,insecure,no_subtree_check gss/krb5p:/exports/opt
exportfs -o rw,nohide,insecure,no_subtree_check gss/krb5p:/exports/etc

In this example, clients are provided with multiple file systems to mount, by using the --bind option.

9.4. NFS Client Configuration Files

NFS shares are mounted on the client side using the mount command. The format of the command is as follows:

 mount -t <nfs-type> -o <options> <host>:</remote/export> </local/directory>

Replace <nfs-type> with either nfs for NFSv2 or NFSv3 servers, or nfs4 for NFSv4 servers. Replace <options> with a comma separated list of options for the NFS file system (refer to Section 9.4.3, “Common NFS Mount Options” for details). Replace <host> with the remote host, </remote/export> with the remote directory being mounted, and </local/directory> with the local directory where the remote file system is to be mounted.

Refer to the mount man page for more details.

If accessing an NFS share by manually issuing the mount command, the file system must be remounted manually after the system is rebooted. Red Hat Enterprise Linux offers two methods for mounting remote file systems automatically at boot time: the /etc/fstab file or the autofs service.

9.4.1. `/etc/fstab`

The /etc/fstab file is referenced by the netfs service at boot time, so lines referencing NFS shares have the same effect as manually typing the mount command during the boot process.

A sample /etc/fstab line to mount an NFS export looks like the following example:

<server>:</remote/export> </local/directory> <nfs-type> <options> 0 0

Replace <server> with the hostname, IP address, or fully qualified domain name of the server exporting the file system.

Replace </remote/export> with the path to the exported directory.

Replace </local/directory> with the local file system on which the exported directory is mounted. This mount point must exist before /etc/fstab is read or the mount fails.

Replace <nfs-type> with either nfs for NFSv2 or NFSv3 servers, or nfs4 for NFSv4 servers.

Replace <options> with a comma separated list of options for the NFS file system (refer to Section 9.4.3, “Common NFS Mount Options” for details). Refer to the fstab man page for additional information.

9.4.2. `autofs`

One drawback to using /etc/fstab is that, regardless of how infrequently a user accesses the NFS mounted file system, the system must dedicate resources to keep the mounted file system in place. This is not a problem with one or two mounts, but when the system is maintaining mounts to a dozen systems at one time, overall system performance can suffer. An alternative to /etc/fstab is to use the kernel-based automount utility, which can mount and unmount NFS file systems automatically, saving resources.

The autofs service is used to control the automount command through the /etc/auto.master primary configuration file. While automount can be specified on the command line, it is more convenient to specify the mount points, hostname, exported directory, and options in a set of files rather than typing them manually.

The autofs configuration files are arranged in a parent-child relationship. The main configuration file (/etc/auto.master) lists mount points on the system that are linked to a particular map type, which takes the form of other configuration files, programs, NIS maps, and other less common mount methods. The auto.master file contains lines referring to each of these mount points, organized in the following manner:

<mount-point> <map-type>

The <mount-point> element specifies the location of the mount on the local file system. The <map-type> specifies how the mount point is mounted. The most common method for auto mounting NFS exports is to use a file as the map type for the particular mount point. The map file is usually named auto.<mount-point>, where <mount-point> is the mount point designated in auto.master. A line within map files to mount an NFS export looks like the following example:

</local/directory> -<options> <server>:</remote/export>

Replace </local/directory;> with the local file system on which the exported directory is mounted. This mount point must exist before the map file is read, else the mount fails.

Replace <options> with a comma separated list of options for the NFS file system (refer to Section 9.4.3, “Common NFS Mount Options” for details). Be sure to include the hyphen character (-) immediately before the options list.

Replace <server> with the hostname, IP address, or fully qualified domain name of the server exporting the file system.

Replace </remote/export> with the path to the exported directory.

Replace <options> with a comma separated list of options for the NFS file system (refer to Section 9.4.3, “Common NFS Mount Options” for details).

While autofs configuration files can be used for a variety of mounts to many types of devices and file systems, they are particularly useful in creating NFS mounts. For example, some organizations store a user's /home/ directory on a central server via an NFS share, then configure the auto.master file on each of the workstations to point to an auto.home file containing the specifics for how to mount the /home/ directory via NFS. This allows the user to access personal data and configuration files in their /home/ directory by logging in anywhere on the network. The auto.master file in this situation would look similar to this:

 /home /etc/auto.home

This sets up the /home/ mount point on the local system to be configured by the /etc/auto.home file, which looks similar to the example below:

 * -fstype=nfs4,soft,intr,rsize=32768,wsize=32768,nosuid server.example.com:/home

This line states that any directory a user tries to access under the local /home/ directory (due to the asterisk character) should result in an NFS mount on the server.example.com system on the mount point /home/. The mount options specify that each /home/ directory NFS mounts should use a particular collection of settings. For more information on mount options, including the ones used in this example, refer to Section 9.4.3, “Common NFS Mount Options”.

For more information about the autofs configuration files, refer to the auto.master man page.

9.4.3. Common NFS Mount Options

Beyond mounting a file system via NFS on a remote host, other options can be specified at the time of the mount to make it easier to use. These options can be used with manual mount commands, /etc/fstab settings, and autofs.

The following are options commonly used for NFS mounts:

fsid=num — Forces the file handle and file attributes settings on the wire to be num, instead of a number derived from the major and minor number of the block device on the mounted file system. The value 0 has special meaning when used with NFSv4. NFSv4 has a concept of a root of the overall exported file system. The export point exported with fsid=0 is used as this root.
hard or soft — Specifies whether the program using a file via an NFS connection should stop and wait (hard) for the server to come back online, if the host serving the exported file system is unavailable, or if it should report an error (soft).
If hard is specified, the user cannot terminate the process waiting for the NFS communication to resume unless the intr option is also specified.
If soft is specified, the user can set an additional timeo=<value> option, where <value> specifies the number of seconds to pass before the error is reported.
intr — Allows NFS requests to be interrupted if the server goes down or cannot be reached.
nfsvers=2 or nfsvers=3 — Specifies which version of the NFS protocol to use. This is useful for hosts that run multiple NFS servers. If no version is specified, NFS uses the highest supported version by the kernel and mount command. This option is not supported with NFSv4 and should not be used.
noacl — Turns off all ACL processing. This may be needed when interfacing with older versions of Red Hat Enterprise Linux, Red Hat Linux, or Solaris, since the most recent ACL technology is not compatible with older systems.
nolock — Disables file locking. This setting is occasionally required when connecting to older NFS servers.
noexec — Prevents execution of binaries on mounted file systems. This is useful if the system is mounting a non-Linux file system via NFS containing incompatible binaries.
nosuid — Disables set-user-identifier or set-group-identifier bits. This prevents remote users from gaining higher privileges by running a setuid program.
port=num — Specifies the numeric value of the NFS server port. If num is 0 (the default), then mount queries the remote host's portmapper for the port number to use. If the remote host's NFS daemon is not registered with its portmapper, the standard NFS port number of TCP 2049 is used instead.
rsize=num and wsize=num — These settings speed up NFS communication for reads (rsize) and writes (wsize) by setting a larger data block size, in bytes, to be transferred at one time. Be careful when changing these values; some older Linux kernels and network cards do not work well with larger block sizes. For NFSv2 or NFSv3, the default values for both parameters is set to 8192. For NFSv4, the default values for both parameters is set to 32768.
sec=mode — Specifies the type of security to utilize when authenticating an NFS connection.
sec=sys is the default setting, which uses local UNIX UIDs and GIDs by means of AUTH_SYS to authenticate NFS operations.
sec=krb5 uses Kerberos V5 instead of local UNIX UIDs and GIDs to authenticate users.
sec=krb5i uses Kerberos V5 for user authentication and performs integrity checking of NFS operations using secure checksums to prevent data tampering.
sec=krb5p uses Kerberos V5 for user authentication, integrity checking, and encrypts NFS traffic to prevent traffic sniffing. This is the most secure setting, but it also has the most performance overhead involved.
tcp — Specifies for the NFS mount to use the TCP protocol.
udp — Specifies for the NFS mount to use the UDP protocol.

Many more options are listed on the mount and nfs man pages.

9.5. Securing NFS

NFS is well suited for sharing entire file systems with a large number of known hosts in a transparent manner. However, with ease of use comes a variety of potential security problems.

The following points should be considered when exporting NFS file systems on a server or mounting them on a client. Doing so minimizes NFS security risks and better protects data on the server.

For a concise listing of steps administrators can take to secure NFS servers, refer the the chapter titled Server Security in the Security Guide.

9.5.1. Host Access

Depending on which version of NFS you plan to implement, depends on your existing network environment, and your security concerns. The following sections explain the differences between implementing security measures with NFSv2, NFSv3, and NFSv4. If at all possible, use of NFSv4 is recommended over other versions of NFS.

9.5.1.1. Using NFSv2 or NFSv3

NFS controls who can mount an exported file system based on the host making the mount request, not the user that actually uses the file system. Hosts must be given explicit rights to mount the exported file system. Access control is not possible for users, other than through file and directory permissions. In other words, once a file system is exported via NFS, any user on any remote host connected to the NFS server can access the shared data. To limit the potential risks, administrators often allow read-only access or squash user permissions to a common user and group ID. Unfortunately, these solutions prevent the NFS share from being used in the way it was originally intended.

Additionally, if an attacker gains control of the DNS server used by the system exporting the NFS file system, the system associated with a particular hostname or fully qualified domain name can be pointed to an unauthorized machine. At this point, the unauthorized machine is the system permitted to mount the NFS share, since no username or password information is exchanged to provide additional security for the NFS mount.

Wildcards should be used sparingly when exporting directories via NFS as it is possible for the scope of the wildcard to encompass more systems than intended.

It is also possible to restrict access to the portmap service via TCP wrappers. Access to ports used by portmap, rpc.mountd, and rpc.nfsd can also be limited by creating firewall rules with iptables.

For more information on securing NFS and portmap, refer to the chapter titled Server Security in the Security Guide. Additional information about firewalls can be found in Chapter 18, iptables.

9.5.1.2. Using NFSv4

The release of NFSv4 brought a revolution to authentication and security to NFS exports. NFSv4 mandates the implementation of the RPCSEC_GSS kernel module, the Kerberos version 5 GSS-API mechanism, SPKM-3, and LIPKEY. With NFSv4, the mandatory security mechanisms are oriented towards authenticating individual users, and not client machines as used in NFSv2 and NFSv3.

Note

It is assumed that a Kerberos ticket-granting server (KDC) is installed and configured correctly, prior to configuring an NFSv4 server.

NFSv4 includes ACL support based on the Microsoft Windows NT model, not the POSIX model, because of its features and because it is widely deployed. NFSv2 and NFSv3 do not have support for native ACL attributes.

Another important security feature of NFSv4 is its removal of the rpc.mountd daemon. The rpc.mountd daemon presented possible security holes because of the way it dealt with filehandlers.

For more information on the RPCSEC_GSS framework, including how rpc.svcgssd and rpc.gssd interoperate, refer to http://www.citi.umich.edu/projects/nfsv4/gssd/.

9.5.2. File Permissions

Once the NFS file system is mounted read/write by a remote host, the only protection each shared file has is its permissions. If two users that share the same user ID value mount the same NFS file system, they can modify each others files. Additionally, anyone logged in as root on the client system can use the su - command to become a user who could access particular files via the NFS share. For more on NFS and user ID conflicts, refer to the chapter titled Managing User Accounts and Resource Access in the Red Hat Enterprise Linux Introduction to System Adminitration.

By default, access control lists (ACLs) are supported by NFS under Red Hat Enterprise Linux. It is not recommended that this feature be disabled. For more about this feature, refer to the chapter titled Network File System (NFS) in the System Administrators Guide.

The default behavior when exporting a file system via NFS is to use root squashing. This sets the user ID of anyone accessing the NFS share as the root user on their local machine to a value of the server's nfsnobody account. Never turn off root squashing.

If exporting an NFS share as read-only, consider using the all_squash option, which makes every user accessing the exported file system take the user ID of the nfsnobody user.

9.6. Additional Resources

Administering an NFS server can be a challenge. Many options, including quite a few not mentioned in this chapter, are available for exporting or mounting NFS shares. Consult the following sources for more information.

9.6.1. Installed Documentation

/usr/share/doc/nfs-utils-<version-number>/ — Replace <version-number> with the version number of the NFS package installed. This directory contains a wealth of information about the NFS implementation for Linux, including a look at various NFS configurations and their impact on file transfer performance.
man mount — Contains a comprehensive look at mount options for both NFS server and client configurations.
man fstab — Gives details for the format of the /etc/fstab file used to mount file systems at boot-time.
man nfs — Provides details on NFS-specific file system export and mount options.
man exports — Shows common options used in the /etc/exports file when exporting NFS file systems.

9.6.2. Useful Websites

http://nfs.sourceforge.net/ — The home of the Linux NFS project and a great place for project status updates.
http://www.citi.umich.edu/projects/nfsv4/linux/ — An NFSv4 for Linux 2.6 kernel resource.
http://www.nfsv4.org — The home of NFS version 4 and all related standards.
http://www.vanemery.com/Linux/NFSv4/NFSv4-no-rpcsec.html — Describes the details of NFSv4 with Fedora Core 2, which includes the 2.6 kernel.
http://www.nluug.nl/events/sane2000/papers/pawlowski.pdf — An excellent whitepaper on the features and enhancements of the NFS Version 4 protocol.

Chapter 10. Apache HTTP Server

The Apache HTTP Server is a robust, commercial-grade open source Web server developed by the Apache Software Foundation (http://www.apache.org/). Red Hat Enterprise Linux includes the Apache HTTP Server 2.0 as well as a number of server modules designed to enhance its functionality.

The default configuration file installed with the Apache HTTP Server works without alteration for most situations. This chapter outlines many of the directives found within its configuration file (/etc/httpd/conf/httpd.conf) to aid those who require a custom configuration or need to convert a configuration file from the older Apache HTTP Server 1.3 format.

Warning

If using the graphical HTTP Configuration Tool (system-config-httpd), do not hand edit the Apache HTTP Server's configuration file as the HTTP Configuration Tool regenerates this file whenever it is used.

For more information about the HTTP Configuration Tool, please refer to the chapter titled Apache HTTP Server Configuration in the System Administrators Guide.

10.1. Apache HTTP Server 2.0

There are important differences between the Apache HTTP Server 2.0 and version 1.3 (version 1.3 shipped with Red Hat Enterprise Linux 2.1 and earlier). This section reviews some of the features of Apache HTTP Server 2.0 and outlines important changes. For instructions on migrating a version 1.3 configuration file to the 2.0 format, refer to Section 10.2, “Migrating Apache HTTP Server 1.3 Configuration Files”.

10.1.1. Features of Apache HTTP Server 2.0

Apache HTTP Server 2.0 includes the following features:

Apache API — Modules utilize a more powerful set of Application Programming Interfaces (APIs).
Important
Modules built for Apache HTTP Server 1.3 do not work without being ported to the new API. If unsure whether or not a particular module has been ported, consult the developer before upgrading.
Filtering — Modules can act as content filters. Refer to Section 10.2.4, “Modules and Apache HTTP Server 2.0” for more on how filtering works.
IPv6 Support — The next generation IP addressing format is supported.
Simplified Directives — A number of confusing directives have been removed while others have been simplified. Refer to Section 10.5, “Configuration Directives in httpd.conf” for more information about specific directives.
Multilingual Error Responses — When using Server Side Include (SSI) documents, customizable error response pages can be delivered in multiple languages.

A more complete list of changes can be found online at http://httpd.apache.org/docs-2.0/.

10.1.2. Packaging Changes in Apache HTTP Server 2.0

Starting with Red Hat Enterprise Linux 3, the Apache HTTP Server packages were renamed. Also, some related packages were renamed, deprecated, or incorporated into other packages.

Below is a list of packaging changes:

The apache, apache-devel and apache-manual packages were renamed to httpd, httpd-devel and httpd-manual respectively.
The mod_dav package was incorporated into the httpd package.
The mod_put and mod_roaming packages were removed, since their functionality is a subset of that provided by mod_dav (which is now incorporated into the httpd package).
The mod_auth_any and mod_bandwidth packages were removed.
The version number for the mod_ssl package is now synchronized with the httpd package. This means that the mod_ssl package for Apache HTTP Server 2.0 has a lower version number than mod_ssl package for Apache HTTP Server 1.3.

10.1.3. File System Changes in Apache HTTP Server 2.0

The following changes to the file system layout occur when upgrading to Apache HTTP Server 2.0:

The configuration directory, /etc/httpd/conf.d/, has been added. — This directory is used to store configuration files for individually packaged modules, such as mod_ssl, mod_perl, and php. The server is instructed to load configuration files from this location by the directive Include conf.d/*.conf within the Apache HTTP Server configuration file, /etc/httpd/conf/httpd.conf.
Important
It is vital that the line specifying the new configuration directory be inserted when migrating an existing configuration.
The ab and logresolve programs have been moved. — These utility programs have been moved from the /usr/sbin/ directory and into the /usr/bin/ directory. This causes scripts with absolute paths for these binaries to fail.
The dbmmanage command has been replaced. — The dbmmanage command has been replaced by htdbm. Refer to Section 10.2.4.5, “The mod_auth_dbm and mod_auth_db Modules” for more information.
The logrotate configuration file has been renamed. — The logrotate configuration file has been renamed from /etc/logrotate.d/apache to /etc/logrotate.d/httpd.

The next section outlines how to migrate an Apache HTTP Server 1.3 configuration to the 2.0 format.

10.2. Migrating Apache HTTP Server 1.3 Configuration Files

This section details migrating an Apache HTTP Server 1.3 configuration file to be utilized by Apache HTTP Server 2.0.

If upgrading to Red Hat Enterprise Linux 4.5.0 from Red Hat Enterprise Linux 2.1, note that the new stock configuration file for the Apache HTTP Server 2.0 package is installed as /etc/httpd/conf/httpd.conf.rpmnew and the original version 1.3 httpd.conf is left untouched. It is entirely up to you whether to use the new configuration file and migrate the old settings to it, or use the existing file as a base and modify it to suit; however, some parts of the file have changed more than others and a mixed approach is generally the best. The stock configuration files for both version 1.3 and 2.0 are divided into three sections.

If the /etc/httpd/conf/httpd.conf file is a modified version of the newly installed default and a saved a copy of the original configuration file is available, it may be easiest to invoke the diff command, as in the following example (logged in as root):

diff -u httpd.conf.orig httpd.conf | less

This command highlights any modifications made. If a copy of the original file is not available, extract it from an RPM package using the rpm2cpio and cpio commands, as in the following example:

rpm2cpio apache-<version-number>.i386.rpm | cpio -i --make

In the above command, replace <version-number> with the version number for the apache package.

Finally, it is useful to know that the Apache HTTP Server has a testing mode to check for configuration errors. To use access it, type the following command:

apachectl configtest

10.2.1. Global Environment Configuration

The global environment section of the configuration file contains directives which affect the overall operation of the Apache HTTP Server, such as the number of concurrent requests it can handle and the locations of the various files. This section requires a large number of changes and should be based on the Apache HTTP Server 2.0 configuration file, while migrating the old settings into it.

10.2.1.1. Interface and Port Binding

The BindAddress and Port directives no longer exist; their functionality is now provided by a more flexible Listen directive.

If Port 80 was set in the 1.3 version configuration file, change it to Listen 80 in the 2.0 configuration file. If Port was set to some value other than 80, then append the port number to the contents of the ServerName directive.

For example, the following is a sample Apache HTTP Server 1.3 directive:

Port 123
ServerName www.example.com

To migrate this setting to Apache HTTP Server 2.0, use the following structure:

Listen 123
ServerName www.example.com:123

For more on this topic, refer to the following documentation on the Apache Software Foundation's website:

10.2.1.2. Server-Pool Size Regulation

When the Apache HTTP Server accepts requests, it dispatches child processes or threads to handle them. This group of child processes or threads is known as a server-pool. Under Apache HTTP Server 2.0, the responsibility for creating and maintaining these server-pools has been abstracted to a group of modules called Multi-Processing Modules (MPMs). Unlike other modules, only one module from the MPM group can be loaded by the Apache HTTP Server. There are three MPM modules that ship with 2.0: prefork, worker, and perchild. Currently only the prefork and worker MPMs are available, although the perchild MPM may be available at a later date.

The original Apache HTTP Server 1.3 behavior has been moved into the prefork MPM. The prefork MPM accepts the same directives as Apache HTTP Server 1.3, so the following directives may be migrated directly:

StartServers
MinSpareServers
MaxSpareServers
MaxClients
MaxRequestsPerChild

The worker MPM implements a multi-process, multi-threaded server providing greater scalability. When using this MPM, requests are handled by threads, conserving system resources and allowing large numbers of requests to be served efficiently. Although some of the directives accepted by the worker MPM are the same as those accepted by the prefork MPM, the values for those directives should not be transfered directly from an Apache HTTP Server 1.3 installation. It is best to instead use the default values as a guide, then experiment to determine what values work best.

Important

To use the worker MPM, create the file /etc/sysconfig/httpd and add the following directive:

HTTPD=/usr/sbin/httpd.worker

For more on the topic of MPMs, refer to the following documentation on the Apache Software Foundation's website:

http://httpd.apache.org/docs-2.0/mpm.html

10.2.1.3. Dynamic Shared Object (DSO) Support

There are many changes required here, and it is highly recommended that anyone trying to modify an Apache HTTP Server 1.3 configuration to suit version 2.0 (as opposed to migrating the changes into the version 2.0 configuration) copy this section from the stock Apache HTTP Server 2.0 configuration file.

Those who do not want to copy the section from the stock Apache HTTP Server 2.0 configuration should note the following:

The AddModule and ClearModuleList directives no longer exist. These directives where used to ensure that modules could be enabled in the correct order. The Apache HTTP Server 2.0 API allows modules to specify their ordering, eliminating the need for these two directives.
The order of the LoadModule lines are no longer relevant in most cases.
Many modules have been added, removed, renamed, split up, or incorporated into others.
LoadModule lines for modules packaged in their own RPMs (mod_ssl, php, mod_perl, and the like) are no longer necessary as they can be found in their relevant files within the /etc/httpd/conf.d/ directory.
The various HAVE_XXX definitions are no longer defined.

Important

If modifying the original file, note that it is of paramount importance that the httpd.conf contains the following directive:

Include conf.d/*.conf

Omission of this directive results in the failure of all modules packaged in their own RPMs (such as mod_perl, php, and mod_ssl).

10.2.1.4. Other Global Environment Changes

The following directives have been removed from Apache HTTP Server 2.0's configuration:

ServerType — The Apache HTTP Server can only be run as ServerType standalone making this directive irrelevant.
AccessConfig and ResourceConfig — These directives have been removed as they mirror the functionality of the Include directive. If the AccessConfig and ResourceConfig directives are set, replace them with Include directives.
To ensure that the files are read in the order implied by the older directives, the Include directives should be placed at the end of the httpd.conf, with the one corresponding to ResourceConfig preceding the one corresponding to AccessConfig. If using the default values, include them explicitly as conf/srm.conf and conf/access.conf files.

10.2.2. Main Server Configuration

The main server configuration section of the configuration file sets up the main server, which responds to any requests that are not handled by a virtual host defined within a <VirtualHost> container. Values here also provide defaults for any <VirtualHost> containers defined.

The directives used in this section have changed little between Apache HTTP Server 1.3 and version 2.0. If the main server configuration is heavily customized, it may be easier to modify the existing configuration file to suit Apache HTTP Server 2.0. Users with only lightly customized main server sections should migrate their changes into the default 2.0 configuration.

10.2.2.1. `UserDir` Mapping

The UserDir directive is used to enable URLs such as http://example.com/~bob/ to map to a subdirectory within the home directory of the user bob, such as /home/bob/public_html/. A side-effect of this feature allows a potential attacker to determine whether a given username is present on the system. For this reason, the default configuration for Apache HTTP Server 2.0 disables this directive.

To enable UserDir mapping, change the directive in httpd.conf from:

UserDir disable

to the following:

UserDir public_html

For more on this topic, refer to the following documentation on the Apache Software Foundation's website:

http://httpd.apache.org/docs-2.0/mod/mod_userdir.html#userdir

10.2.2.2. Logging

The following logging directives have been removed:

AgentLog
RefererLog
RefererIgnore

However, agent and referrer logs are still available using the CustomLog and LogFormat directives.

For more on this topic, refer to the following documentation on the Apache Software Foundation's website:

10.2.2.3. Directory Indexing

The deprecated FancyIndexing directive has now been removed. The same functionality is available through the FancyIndexing option within the IndexOptions directive.

The VersionSort option to the IndexOptions directive causes files containing version numbers to be sorted in a more natural way. For example, httpd-2.0.6.tar appears before httpd-2.0.36.tar in a directory index page.

The defaults for the ReadmeName and HeaderName directives have changed from README and HEADER to README.html and HEADER.html.

For more on this topic, refer to the following documentation on the Apache Software Foundation's website:

10.2.2.4. Content Negotiation

The CacheNegotiatedDocs directive now takes the argument on or off. Existing instances of CacheNegotiatedDocs should be replaced with CacheNegotiatedDocs on.

For more on this topic, refer to the following documentation on the Apache Software Foundation's website:

http://httpd.apache.org/docs-2.0/mod/mod_negotiation.html#cachenegotiateddocs

10.2.2.5. Error Documents

To use a hard-coded message with the ErrorDocument directive, the message should be enclosed in a pair of double quotation marks ", rather than just preceded by a double quotation mark as required in Apache HTTP Server 1.3.

For example, the following is a sample Apache HTTP Server 1.3 directive:

ErrorDocument 404 "The document was not found

To migrate an ErrorDocument setting to Apache HTTP Server 2.0, use the following structure:

ErrorDocument 404 "The document was not found"

Note the trailing double quote in the previous ErrorDocument directive example.

For more on this topic, refer to the following documentation on the Apache Software Foundation's website:

http://httpd.apache.org/docs-2.0/mod/core.html#errordocument

10.2.3. Virtual Host Configuration

The contents of all <VirtualHost> containers should be migrated in the same way as the main server section as described in Section 10.2.2, “Main Server Configuration”.

Important

Note that SSL/TLS virtual host configuration has been moved out of the main server configuration file and into /etc/httpd/conf.d/ssl.conf.

For more on this topic, refer to the chapter titled Apache HTTP Secure Server Configuration in the System Administrators Guide and the documentation online at the following URL:

http://httpd.apache.org/docs-2.0/vhosts/

10.2.4. Modules and Apache HTTP Server 2.0

In Apache HTTP Server 2.0, the module system has been changed to allow modules to be chained together or combined in new and interesting ways. Common Gateway Interface (CGI) scripts, for example, can generate server-parsed HTML documents which can then be processed by mod_include. This opens up a tremendous number of possibilities with regards to how modules can be combined to achieve a specific goal.

The way this works is that each request is served by exactly one handler module followed by zero or more filter modules.

Under Apache HTTP Server 1.3, for example, a Perl script would be handled in its entirety by the Perl module (mod_perl). Under Apache HTTP Server 2.0, the request is initially handled by the core module — which serves static files — and is then filtered by mod_perl.

Exactly how to use this, and all other new features of Apache HTTP Server 2.0, is beyond the scope of this document; however, the change has ramifications if the PATH_INFO directive is used for a document which is handled by a module that is now implemented as a filter, as each contains trailing path information after the true file name. The core module, which initially handles the request, does not by default understand PATH_INFO and returns 404 Not Found errors for requests that contain such information. As an alternative, use the AcceptPathInfo directive to coerce the core module into accepting requests with PATH_INFO.

The following is an example of this directive:

AcceptPathInfo on

For more on this topic, refer to the following documentation on the Apache Software Foundation's website:

10.2.4.1. The `suexec` Module

In Apache HTTP Server 2.0, the mod_suexec module uses the SuexecUserGroup directive, rather than the User and Group directives, which is used for configuring virtual hosts. The User and Group directives can still be used in general, but are deprecated for configuring virtual hosts.

For example, the following is a sample Apache HTTP Server 1.3 directive:

<VirtualHost vhost.example.com:80>
    User someone
    Group somegroup
</VirtualHost>

To migrate this setting to Apache HTTP Server 2.0, use the following structure:

<VirtualHost vhost.example.com:80>
    SuexecUserGroup someone somegroup
</VirtualHost>

10.2.4.2. The `mod_ssl` Module

The configuration for mod_ssl has been moved from the httpd.conf file into the /etc/httpd/conf.d/ssl.conf file. For this file to be loaded, and for mod_ssl to work, the statement Include conf.d/*.conf must be in the httpd.conf file as described in Section 10.2.1.3, “Dynamic Shared Object (DSO) Support”.

ServerName directives in SSL virtual hosts must explicitly specify the port number.

For example, the following is a sample Apache HTTP Server 1.3 directive:

<VirtualHost _default_:443>
    # General setup for the virtual host
    ServerName ssl.example.name
    ...
</VirtualHost>

To migrate this setting to Apache HTTP Server 2.0, use the following structure:

<VirtualHost _default_:443>
    # General setup for the virtual host
    ServerName ssl.host.name:443
    ...
</VirtualHost>

It is also important to note that both the SSLLog and SSLLogLevel directives have been removed. The mod_ssl module now obeys the ErrorLog and LogLevel directives. Refer to Section 10.5.35, “ ErrorLog ” and Section 10.5.36, “ LogLevel ” for more information about these directives.

For more on this topic, refer to the following documentation on the Apache Software Foundation's website:

10.2.4.3. The `mod_proxy` Module

Proxy access control statements are now placed inside a <Proxy> block rather than a <Directory proxy:>.

The caching functionality of the old mod_proxy has been split out into the following three modules:

mod_cache
mod_disk_cache
mod_mem_cache

These generally use directives similar to the older versions of the mod_proxy module, but it is advisable to verify each directive before migrating any cache settings.

For more on this topic, refer to the following documentation on the Apache Software Foundation's website:

http://httpd.apache.org/docs-2.0/mod/mod_proxy.html

10.2.4.4. The `mod_include` Module

The mod_include module is now implemented as a filter and is therefore enabled differently. Refer to Section 10.2.4, “Modules and Apache HTTP Server 2.0” for more about filters.

For example, the following is a sample Apache HTTP Server 1.3 directive:

AddType text/html .shtml
AddHandler server-parsed .shtml

To migrate this setting to Apache HTTP Server 2.0, use the following structure:

AddType text/html .shtml
AddOutputFilter INCLUDES .shtml

Note that the Options +Includes directive is still required for the <Directory> container or in a .htaccess file.

For more on this topic, refer to the following documentation on the Apache Software Foundation's website:

http://httpd.apache.org/docs-2.0/mod/mod_include.html

10.2.4.5. The `mod_auth_dbm` and `mod_auth_db` Modules

Apache HTTP Server 1.3 supported two authentication modules, mod_auth_db and mod_auth_dbm, which used Berkeley Databases and DBM databases respectively. These modules have been combined into a single module named mod_auth_dbm in Apache HTTP Server 2.0, which can access several different database formats. To migrate from mod_auth_db, configuration files should be adjusted by replacing AuthDBUserFile and AuthDBGroupFile with the mod_auth_dbm equivalents, AuthDBMUserFile and AuthDBMGroupFile. Also, the directive AuthDBMType DB must be added to indicate the type of database file in use.

The following example shows a sample mod_auth_db configuration for Apache HTTP Server 1.3:

<Location /private/>
  AuthType Basic
  AuthName "My Private Files"
  AuthDBUserFile /var/www/authdb
  require valid-user
</Location>

To migrate this setting to version 2.0 of Apache HTTP Server, use the following structure:

<Location /private/>
  AuthType Basic
  AuthName "My Private Files"
  AuthDBMUserFile /var/www/authdb
  AuthDBMType DB
  require valid-user
</Location>

Note that the AuthDBMUserFile directive can also be used in .htaccess files.

The dbmmanage Perl script, used to manipulate username and password databases, has been replaced by htdbm in Apache HTTP Server 2.0. The htdbm program offers equivalent functionality and, like mod_auth_dbm, can operate a variety of database formats; the -T option can be used on the command line to specify the format to use.

Table 10.1, “Migrating from dbmmanage to htdbm” shows how to migrate from a DBM-format database to htdbm format using dbmmanage.

Table 10.1. Migrating from dbmmanage to htdbm
Action	dbmmanage command (1.3)	Equivalent htdbm command (2.0)
Add user to database (using given password)	`dbmmanage authdb add username password`	`htdbm -b -TDB authdb username password`
Add user to database (prompts for password)	`dbmmanage authdb adduser username`	`htdbm -TDB authdb username`
Remove user from database	`dbmmanage authdb delete username`	`htdbm -x -TDB authdb username`
List users in database	`dbmmanage authdb view`	`htdbm -l -TDB authdb`
Verify a password	`dbmmanage authdb check username`	`htdbm -v -TDB authdb username`

The -m and -s options work with both dbmmanage and htdbm, enabling the use of the MD5 or SHA1 algorithms for hashing passwords, respectively.

When creating a new database with htdbm, the -c option must be used.

For more on this topic, refer to the following documentation on the Apache Software Foundation's website:

http://httpd.apache.org/docs-2.0/mod/mod_auth_dbm.html

10.2.4.6. The `mod_perl` Module

The configuration for mod_perl has been moved from httpd.conf into the file /etc/httpd/conf.d/perl.conf. For this file to be loaded, and hence for mod_perl to work, the statement Include conf.d/*.conf must be included in httpd.conf as described in Section 10.2.1.3, “Dynamic Shared Object (DSO) Support”.

Occurrences of Apache:: in httpd.conf must be replaced with ModPerl::. Additionally, the manner in which handlers are registered has been changed.

This is a sample Apache HTTP Server 1.3 mod_perl configuration:

<Directory /var/www/perl>
    SetHandler perl-script
    PerlHandler Apache::Registry
    Options +ExecCGI
</Directory>

This is the equivalent mod_perl for Apache HTTP Server 2.0:

<Directory /var/www/perl>
    SetHandler perl-script
    PerlResponseHandler ModPerl::Registry
    Options +ExecCGI
</Directory>

Most modules for mod_perl 1.x should work without modification with mod_perl 2.x. XS modules require recompilation and may require minor Makefile modifications.

10.2.4.7. The `mod_python` Module

Configuration for mod_python has moved from httpd.conf to the /etc/httpd/conf.d/python.conf file. For this file to be loaded, and hence for mod_python to work, the statement Include conf.d/*.conf must be in httpd.conf as described in Section 10.2.1.3, “Dynamic Shared Object (DSO) Support”.

10.2.4.8. PHP

The configuration for PHP has been moved from httpd.conf into the file /etc/httpd/conf.d/php.conf. For this file to be loaded, the statement Include conf.d/*.conf must be in httpd.conf as described in Section 10.2.1.3, “Dynamic Shared Object (DSO) Support”.

Note

Any PHP configuration directives used in Apache HTTP Server 1.3 are now fully compatible, when migrating to Apache HTTP Server 2.0 on Red Hat Enterprise Linux 4.5.0;.

In PHP version 4.2.0 and later the default set of predefined variables which are available in the global scope has changed. Individual input and server variables are, by default, no longer placed directly into the global scope. This change may cause scripts to break. Revert to the old behavior by setting register_globals to On in the file /etc/php.ini.

For more on this topic, refer to the following URL for details concerning the global scope changes:

http://www.php.net/release_4_1_0.php

10.2.4.9. The `mod_authz_ldap` Module

Red Hat Enterprise Linux ships with the mod_authz_ldap module for the Apache HTTP Server. This module uses the short form of the distinguished name for a subject and the issuer of the client SSL certificate to determine the distinguished name of the user within an LDAP directory. It is also capable of authorizing users based on attributes of that user's LDAP directory entry, determining access to assets based on the user and group privileges of the asset, and denying access for users with expired passwords. The mod_ssl module is required when using the mod_authz_ldap module.

Important

The mod_authz_ldap module does not authenticate a user to an LDAP directory using an encrypted password hash. This functionality is provided by the experimental mod_auth_ldap module. Refer to the mod_auth_ldap module documentation online at http://httpd.apache.org/docs-2.0/mod/mod_auth_ldap.html for details on the status of this module.

The /etc/httpd/conf.d/authz_ldap.conf file configures the mod_authz_ldap module.

Refer to /usr/share/doc/mod_authz_ldap-<version>/index.html (replacing <version> with the version number of the package) or http://authzldap.othello.ch/ for more information on configuring the mod_authz_ldap third party module.

10.3. After Installation

After installing the httpd package, review the Apache HTTP Server's documentation available online at http://httpd.apache.org/docs-2.0/.

The Apache HTTP Server's documentation contains a full list and complete descriptions of all configuration options. This chapter provides short descriptions of the configuration directives used by Apache HTTP Server 2.0.

The Apache HTTP Server 2.0 includes the ability to set up secure Web servers using the strong SSL encryption provided by the mod_ssl and openssl packages. When looking through the configuration files, be aware that it includes both a non-secure and a secure Web server. The secure Web server runs as a virtual host, which is configured in the /etc/httpd/conf.d/ssl.conf file. For more information about virtual hosts, refer to Section 10.8, “Virtual Hosts”. For information on configuring a secure server virtual host, refer to Section 10.8.1, “Setting Up Virtual Hosts”. For information on setting up an Apache HTTP Secure Server, refer to the chapter titled Apache HTTP Secure Server Configuration in the System Administrators Guide.

Note

Red Hat, Inc does not ship FrontPage extensions as the Microsoft™ license prohibits the inclusion of these extensions in a third party product. More information about FrontPage extensions and the Apache HTTP Server can be found online at http://www.rtr.com/fpsupport/.

10.4. Starting and Stopping `httpd`

The httpd RPM installs the /etc/init.d/httpd script, which can be accessed using the /sbin/service command.

To start the server, as root type:

service httpd start

To stop the server, as root type:

service httpd stop

The restart option is a shorthand way of stopping and then starting the Apache HTTP Server.

To restart the server, as root type:

service httpd restart

Note

If running the Apache HTTP Server as a secure server, it may be necessary to type the server password whenever using the start or restart options.

After editing the httpd.conf file, however, it is not necessary to explicitly stop and start the server. Instead, use the reload option.

To reload the server configuration file, as root type:

service httpd reload

Note

If running the Apache HTTP Server as a secure server, the server password is not required when using the reload option.

By default, the httpd service does not start automatically at boot time. To configure the httpd service to start up at boot time, use an initscript utility, such as /sbin/chkconfig, /usr/sbin/ntsysv, or the Services Configuration Tool program. Refer to the chapter titled Controlling Access to Services in System Administrators Guide for more information regarding these tools.

Note

If running the Apache HTTP Server as a secure server, the secure server's password is required after the machine boots when using an encrypted private SSL key.

For information about setting up an Apache HTTP Secure Server, refer to the chapter titled Apache HTTP Secure Server Configuration in the System Administrators Guide.

10.5. Configuration Directives in `httpd.conf`

The Apache HTTP Server configuration file is /etc/httpd/conf/httpd.conf. The httpd.conf file is well-commented and mostly self-explanatory. The default configuration works for most situations; however, it is a good idea to become familiar some of the more important configuration options.

Warning

With the release of Apache HTTP Server 2.0, many configuration options have changed. If migrating a version 1.3 configuration file to the 2.0 format, refer to Section 10.2, “Migrating Apache HTTP Server 1.3 Configuration Files”.

10.5.1. General Configuration Tips

If configuring the Apache HTTP Server, edit /etc/httpd/conf/httpd.conf and then either reload, restart, or stop and start the httpd process as outlined in Section 10.4, “Starting and Stopping httpd”.

Before editing httpd.conf, make a copy the original file. Creating a backup makes it easier to recover from mistakes made while editing the configuration file.

If a mistake is made and the Web server does not work correctly, first review recently edited passages in httpd.conf to verify there are no typos.

Next look in the Web server's error log, /var/log/httpd/error_log. The error log may not be easy to interpret, depending on your level of expertise. However, the last entries in the error log should provide useful information.

The following subsections contain a list of short descriptions for many of the directives included in httpd.conf. These descriptions are not exhaustive. For more information, refer to the Apache documentation online at http://httpd.apache.org/docs-2.0/.

For more information about mod_ssl directives, refer to the documentation online at http://httpd.apache.org/docs-2.0/mod/mod_ssl.html.

10.5.2. `ServerRoot`

The ServerRoot directive specifies the top-level directory containing website content. By default, ServerRoot is set to "/etc/httpd" for both secure and non-secure servers.

10.5.3. `PidFile`

PidFile names the file where the server records its process ID (PID). By default the PID is listed in /var/run/httpd.pid.

10.5.4. `Timeout`

Timeout defines, in seconds, the amount of time that the server waits for receipts and transmissions during communications. Timeout is set to 300 seconds by default, which is appropriate for most situations.

10.5.5. `KeepAlive`

KeepAlive sets whether the server allows more than one request per connection and can be used to prevent any one client from consuming too much of the server's resources.

By default Keepalive is set to off. If Keepalive is set to on and the server becomes very busy, the server can quickly spawn the maximum number of child processes. In this situation, the server slows down significantly. If Keepalive is enabled, it is a good idea to set the the KeepAliveTimeout low (refer to Section 10.5.7, “ KeepAliveTimeout ” for more information about the KeepAliveTimeout directive) and monitor the /var/log/httpd/error_log log file on the server. This log reports when the server is running out of child processes.

10.5.6. `MaxKeepAliveRequests`

This directive sets the maximum number of requests allowed per persistent connection. The Apache Project recommends a high setting, which improves the server's performance. MaxKeepAliveRequests is set to 100 by default, which should be appropriate for most situations.

10.5.7. `KeepAliveTimeout`

KeepAliveTimeout sets the number of seconds the server waits after a request has been served before it closes the connection. Once the server receives a request, the Timeout directive applies instead. The KeepAliveTimeout directive is set to 15 seconds by default.

10.5.8. `IfModule`

<IfModule> and </IfModule> tags create a conditional container which are only activated if the specified module is loaded. Directives within the IfModule container are processed under one of two conditions. The directives are processed if the module contained within the starting <IfModule> tag is loaded. Or, if an exclamation point ! appears before the module name, the directives are processed only if the module specified in the <IfModule> tag is not loaded.

For more information about Apache HTTP Server modules, refer to Section 10.7, “Adding Modules”.

10.5.9. MPM Specific Server-Pool Directives

As explained in Section 10.2.1.2, “Server-Pool Size Regulation”, the responsibility for managing characteristics of the server-pool falls to a module group called MPMs under Apache HTTP Server 2.0. The characteristics of the server-pool differ depending upon which MPM is used. For this reason, an IfModule container is necessary to define the server-pool for the MPM in use.

By default, Apache HTTP Server 2.0 defines the server-pool for both the prefork and worker MPMs.

The following section list directives found within the MPM-specific server-pool containers.

10.5.9.1. `StartServers`

The StartServers directive sets how many server processes are created upon startup. Since the Web server dynamically kills and creates server processes based on traffic load, it is not necessary to change this parameter. The Web server is set to start 8 server processes at startup for the prefork MPM and 2 for the worker MPM.

10.5.9.2. `MaxRequestsPerChild`

MaxRequestsPerChild sets the total number of requests each child server process serves before the child dies. The main reason for setting MaxRequestsPerChild is to avoid long-lived process induced memory leaks. The default MaxRequestsPerChild for the prefork MPM is 4000 and for the worker MPM is 0.

10.5.9.3. `MaxClients`

MaxClients sets a limit on the total number of server processes, or simultaneously connected clients, that can run at one time. The main purpose of this directive is to keep a runaway Apache HTTP Server from crashing the operating system. For busy servers this value should be set to a high value. The server's default is set to 150 regardless of the MPM in use. However, it is not recommended that the value for MaxClients exceeds 256 when using the prefork MPM.

10.5.9.4. `MinSpareServers` and `MaxSpareServers`

These values are only used with the prefork MPM. They adjust how the Apache HTTP Server dynamically adapts to the perceived load by maintaining an appropriate number of spare server processes based on the number of incoming requests. The server checks the number of servers waiting for a request and kills some if there are more than MaxSpareServers or creates some if the number of servers is less than MinSpareServers.

The default MinSpareServers value is 5; the default MaxSpareServers value is 20. These default settings should be appropriate for most situations. Be careful not to increase the MinSpareServers to a large number as doing so creates a heavy processing load on the server even when traffic is light.

10.5.9.5. `MinSpareThreads` and `MaxSpareThreads`

These values are only used with the worker MPM. They adjust how the Apache HTTP Server dynamically adapts to the perceived load by maintaining an appropriate number of spare server threads based on the number of incoming requests. The server checks the number of server threads waiting for a request and kills some if there are more than MaxSpareThreads or creates some if the number of servers is less than MinSpareThreads.

The default MinSpareThreads value is 25; the default MaxSpareThreads value is 75. These default settings should be appropriate for most situations. The value for MaxSpareThreads must be greater than or equal to the sum of MinSpareThreads and ThreadsPerChild, else the Apache HTTP Server automatically corrects it.

10.5.9.6. `ThreadsPerChild`

This value is only used with the worker MPM. It sets the number of threads within each child process. The default value for this directive is 25.

10.5.10. `Listen`

The Listen command identifies the ports on which the Web server accepts incoming requests. By default, the Apache HTTP Server is set to listen to port 80 for non-secure Web communications and (in the /etc/httpd/conf.d/ssl.conf file which defines any secure servers) to port 443 for secure Web communications.

If the Apache HTTP Server is configured to listen to a port under 1024, only the root user can start it. For port 1024 and above, httpd can be started as a regular user.

The Listen directive can also be used to specify particular IP addresses over which the server accepts connections.

10.5.11. `Include`

Include allows other configuration files to be included at runtime.

The path to these configuration files can be absolute or relative to the ServerRoot.

Important

For the server to use individually packaged modules, such as mod_ssl, mod_perl, and php, the following directive must be included in Section 1: Global Environment of httpd.conf:

Include conf.d/*.conf

10.5.12. `LoadModule`

LoadModule is used to load Dynamic Shared Object (DSO) modules. More information on the Apache HTTP Server's DSO support, including instructions for using the LoadModule directive, can be found in Section 10.7, “Adding Modules”. Note, the load order of the modules is no longer important with Apache HTTP Server 2.0. Refer to Section 10.2.1.3, “Dynamic Shared Object (DSO) Support” for more information about Apache HTTP Server 2.0 DSO support.

10.5.13. `ExtendedStatus`

The ExtendedStatus directive controls whether Apache generates basic (off) or detailed server status information (on), when the server-status handler is called. The server-status handler is called using Location tags. More information on calling server-status is included in Section 10.5.60, “ Location ”.

10.5.14. `IfDefine`

The IfDefine tags surround configuration directives that are applied if the "test" stated in the IfDefine tag is true. The directives are ignored if the test is false.

The test in the IfDefine tags is a parameter name (for example, HAVE_PERL). If the parameter is defined, meaning that it is provided as an argument to the server's start-up command, then the test is true. In this case, when the Web server is started, the test is true and the directives contained in the IfDefine tags are applied.

10.5.15. `SuexecUserGroup`

The SuexecUserGroup directive, which originates from the mod_suexec module, allows the specification of user and group execution privileges for CGI programs. Non-CGI requests are still processed with the user and group specified in the User and Group directives.

Note

The SuexecUserGroup directive replaces the Apache HTTP Server 1.3 configuration of using the User and Group directives inside the configuration of VirtualHosts sections.

10.5.16. `User`

The User directive sets the username of the server process and determines what files the server is allowed to access. Any files inaccessible to this user are also inaccessible to clients connecting to the Apache HTTP Server.

By default User is set to apache.

This directive has been deprecated for the configuration of virtual hosts.

Note

For security reasons, the Apache HTTP Server does not run as the root user.

10.5.17. `Group`

Specifies the group name of the Apache HTTP Server processes.

This directive has been deprecated for the configuration of virtual hosts.

By default, Group is set to apache.

10.5.18. `ServerAdmin`

Sets the ServerAdmin directive to the email address of the Web server administrator. This email address shows up in error messages on server-generated Web pages, so users can report a problem by sending email to the server administrator.

By default, ServerAdmin is set to root@localhost.

A common way to set up ServerAdmin is to set it to webmaster@example.com. Once set, alias webmaster to the person responsible for the Web server in /etc/aliases and run /usr/bin/newaliases.

10.5.19. `ServerName`

ServerName specifies a hostname and port number (matching the Listen directive) for the server. The ServerName does not need to match the machine's actual hostname. For example, the Web server may be www.example.com, but the server's hostname is actually foo.example.com. The value specified in ServerName must be a valid Domain Name Service (DNS) name that can be resolved by the system — do not make something up.

The following is a sample ServerName directive:

ServerName www.example.com:80

When specifying a ServerName, be sure the IP address and server name pair are included in the /etc/hosts file.

10.5.20. `UseCanonicalName`

When set to on, this directive configures the Apache HTTP Server to reference itself using the value specified in the ServerName and Port directives. When UseCanonicalName is set to off, the server instead uses the value used by the requesting client when referring to itself.

UseCanonicalName is set to off by default.

10.5.21. `DocumentRoot`

DocumentRoot is the directory which contains most of the HTML files which are served in response to requests. The default DocumentRoot, for both the non-secure and secure Web servers, is the /var/www/html directory. For example, the server might receive a request for the following document:

http://example.com/foo.html

The server looks for the following file in the default directory:

/var/www/html/foo.html

To change the DocumentRoot so that it is not shared by the secure and the non-secure Web servers, refer to Section 10.8, “Virtual Hosts”.

10.5.22. `Directory`

<Directory /path/to/directory> and </Directory> tags create a container used to enclose a group of configuration directives which apply only to a specific directory and its subdirectories. Any directive which is applicable to a directory may be used within Directory tags.

By default, very restrictive parameters are applied to the root directory (/), using the Options (refer to Section 10.5.23, “ Options ”) and AllowOverride (refer to Section 10.5.24, “ AllowOverride ”) directives. Under this configuration, any directory on the system which needs more permissive settings has to be explicitly given those settings.

In the default configuration, another Directory container is configured for the DocumentRoot which assigns less rigid parameters to the directory tree so that the Apache HTTP Server can access the files residing there.

The Directory container can be also be used to configure additional cgi-bin directories for server-side applications outside of the directory specified in the ScriptAlias directive (refer to Section 10.5.41, “ ScriptAlias ” for more information).

To accomplish this, the Directory container must set the ExecCGI option for that directory.

For example, if CGI scripts are located in /home/my_cgi_directory, add the following Directory container to the httpd.conf file:

<Directory /home/my_cgi_directory>
    Options +ExecCGI
</Directory>

Next, the AddHandler directive must be uncommented to identify files with the .cgi extension as CGI scripts. Refer to Section 10.5.56, “ AddHandler ” for instructions on setting AddHandler.

For this to work, permissions for CGI scripts, and the entire path to the scripts, must be set to 0755.

10.5.23. `Options`

The Options directive controls which server features are available in a particular directory. For example, under the restrictive parameters specified for the root directory, Options is only set to the FollowSymLinks directive. No features are enabled, except that the server is allowed to follow symbolic links in the root directory.

By default, in the DocumentRoot directory, Options is set to include Indexes and FollowSymLinks. Indexes permits the server to generate a directory listing for a directory if no DirectoryIndex (for example, index.html) is specified. FollowSymLinks allows the server to follow symbolic links in that directory.

Note

Options statements from the main server configuration section need to be replicated to each VirtualHost container individually. Refer to Section 10.5.65, “VirtualHost” for more information.

10.5.24. `AllowOverride`

The AllowOverride directive sets whether any Options can be overridden by the declarations in an .htaccess file. By default, both the root directory and the DocumentRoot are set to allow no .htaccess overrides.

10.5.25. `Order`

The Order directive controls the order in which allow and deny directives are evaluated. The server is configured to evaluate the Allow directives before the Deny directives for the DocumentRoot directory.

10.5.26. `Allow`

Allow specifies which client can access a given directory. The client can be all, a domain name, an IP address, a partial IP address, a network/netmask pair, and so on. The DocumentRoot directory is configured to Allow requests from all, meaning everyone has access.

10.5.27. `Deny`

Deny works similar to Allow, except it specifies who is denied access. The DocumentRoot is not configured to Deny requests from anyone by default.

10.5.28. `UserDir`

UserDir is the subdirectory within each user's home directory where they should place personal HTML files which are served by the Web server. This directive is set to disable by default.

The name for the subdirectory is set to public_html in the default configuration. For example, the server might receive the following request:

http://example.com/~username/foo.html

The server would look for the file:

/home/username/public_html/foo.html

In the above example, /home/username/ is the user's home directory (note that the default path to users' home directories may vary).

Make sure that the permissions on the users' home directories are set correctly. Users' home directories must be set to 0711. The read (r) and execute (x) bits must be set on the users' public_html directories (0755 also works). Files that are served in a users' public_html directories must be set to at least 0644.

10.5.29. `DirectoryIndex`

The DirectoryIndex is the default page served by the server when a user requests an index of a directory by specifying a forward slash (/) at the end of the directory name.

When a user requests the page http://example/this_directory/, they get either the DirectoryIndex page, if it exists, or a server-generated directory list. The default for DirectoryIndex is index.html and the index.html.var type map. The server tries to find either of these files and returns the first one it finds. If it does not find one of these files and Options Indexes is set for that directory, the server generates and returns a listing, in HTML format, of the subdirectories and files within the directory, unless the directory listing feature is turned off.

10.5.30. `AccessFileName`

AccessFileName names the file which the server should use for access control information in each directory. The default is .htaccess.

Immediately after the AccessFileName directive, a set of Files tags apply access control to any file beginning with a .ht. These directives deny Web access to any .htaccess files (or other files which begin with .ht) for security reasons.

10.5.31. `CacheNegotiatedDocs`

By default, the Web server asks proxy servers not to cache any documents which were negotiated on the basis of content (that is, they may change over time or because of the input from the requester). If CacheNegotiatedDocs is set to on, this function is disabled and proxy servers are allowed to cache such documents.

10.5.32. `TypesConfig`

TypesConfig names the file which sets the default list of MIME type mappings (file name extensions to content types). The default TypesConfig file is /etc/mime.types. Instead of editing /etc/mime.types, the recommended way to add MIME type mappings is to use the AddType directive.

For more information about AddType, refer to Section 10.5.55, “ AddType ”.

10.5.33. `DefaultType`

DefaultType sets a default content type for the Web server to use for documents whose MIME types cannot be determined. The default is text/plain.

10.5.34. `HostnameLookups`

HostnameLookups can be set to on, off, or double. If HostnameLookups is set to on, the server automatically resolves the IP address for each connection. Resolving the IP address means that the server makes one or more connections to a DNS server, adding processing overhead. If HostnameLookups is set to double, the server performs a double-reverse DNS look up adding even more processing overhead.

To conserve resources on the server, HostnameLookups is set to off by default.

If hostnames are required in server log files, consider running one of the many log analyzer tools that perform the DNS lookups more efficiently and in bulk when rotating the Web server log files.

10.5.35. `ErrorLog`

ErrorLog specifies the file where server errors are logged. By default, this directive is set to /var/log/httpd/error_log.

10.5.36. `LogLevel`

LogLevel sets how verbose the error messages in the error logs are. LogLevel can be set (from least verbose to most verbose) to emerg, alert, crit, error, warn, notice, info, or debug. The default LogLevel is warn.

10.5.37. `LogFormat`

The LogFormat directive configures the format of the various Web server log files. The actual LogFormat used depends on the settings given in the CustomLog directive (refer to Section 10.5.38, “ CustomLog ”).

The following are the format options if the CustomLog directive is set to combined:

%h (remote host's IP address or hostname): Lists the remote IP address of the requesting client. If HostnameLookups is set to on, the client hostname is recorded unless it is not available from DNS.
%l (rfc931): Not used. A hyphen - appears in the log file for this field.
%u (authenticated user): Lists the username of the user recorded if authentication was required. Usually, this is not used, so a hyphen - appears in the log file for this field.
%t (date): Lists the date and time of the request.
%r (request string): Lists the request string exactly as it came from the browser or client.
%s (status): Lists the HTTP status code which was returned to the client host.
%b (bytes): Lists the size of the document.
%\"%{Referer}i\" (referrer): Lists the URL of the webpage which referred the client host to Web server.
%\"%{User-Agent}i\" (user-agent): Lists the type of Web browser making the request.

10.5.38. `CustomLog`

CustomLog identifies the log file and the log file format. By default, the log is recorded to the /var/log/httpd/access_log file.

The default CustomLog format is the combined log file format, as illustrated here:

remotehost rfc931 user date "request" status bytes referrer user-agent

10.5.39. `ServerSignature`

The ServerSignature directive adds a line containing the Apache HTTP Server server version and the ServerName to any server-generated documents, such as error messages sent back to clients. ServerSignature is set to on by default.

It can also be set to off or to EMail. EMail, adds a mailto:ServerAdmin HTML tag to the signature line of auto-generated responses.

10.5.40. `Alias`

The Alias setting allows directories outside the DocumentRoot directory to be accessible. Any URL ending in the alias automatically resolves to the alias' path. By default, one alias for an icons/ directory is already set up. An icons/ directory can be accessed by the Web server, but the directory is not in the DocumentRoot.

10.5.41. `ScriptAlias`

The ScriptAlias directive defines where CGI scripts are located. Generally, it is not good practice to leave CGI scripts within the DocumentRoot, where they can potentially be viewed as text documents. For this reason, a special directory outside of the DocumentRoot directory containing server-side executables and scripts is designated by the ScriptAlias directive. This directory is known as a cgi-bin and is set to /var/www/cgi-bin/ by default.

It is possible to establish directories for storing executables outside of the cgi-bin/ directory. For instructions on doing so, refer to Section 10.5.56, “ AddHandler ” and Section 10.5.22, “ Directory ”.

10.5.42. `Redirect`

When a webpage is moved, Redirect can be used to map the file location to a new URL. The format is as follows:

Redirect /<old-path>/<file-name> http://<current-domain>/<current-path>/<file-name>

In this example, replace <old-path> with the old path information for <file-name> and <current-domain> and <current-path> with the current domain and path information for <file-name>.

In this example, any requests for <file-name> at the old location is automatically redirected to the new location.

For more advanced redirection techniques, use the mod_rewrite module included with the Apache HTTP Server. For more information about configuring the mod_rewrite module, refer to the Apache Software Foundation documentation online at http://httpd.apache.org/docs-2.0/mod/mod_rewrite.html.

10.5.43. `IndexOptions`

IndexOptions controls the appearance of server generated directing listings, by adding icons, file descriptions, and so on. If Options Indexes is set (refer to Section 10.5.23, “ Options ”), the Web server generates a directory listing when the Web server receives an HTTP request for a directory without an index.

First, the Web server looks in the requested directory for a file matching the names listed in the DirectoryIndex directive (usually, index.html). If an index.html file is not found, Apache HTTP Server creates an HTML directory listing of the requested directory. The appearance of this directory listing is controlled, in part, by the IndexOptions directive.

The default configuration turns on FancyIndexing. This means that a user can re-sort a directory listing by clicking on column headers. Another click on the same header switches from ascending to descending order. FancyIndexing also shows different icons for different files, based upon file extensions.

The AddDescription option, when used in conjunction with FancyIndexing, presents a short description for the file in server generated directory listings.

IndexOptions has a number of other parameters which can be set to control the appearance of server generated directories. The IconHeight and IconWidth parameters require the server to include HTML HEIGHT and WIDTH tags for the icons in server generated webpages. The IconsAreLinks parameter combines the graphical icon with the HTML link anchor, which contains the URL link target.

10.5.44. `AddIconByEncoding`

This directive names icons which are displayed by files with MIME encoding in server generated directory listings. For example, by default, the Web server shows the compressed.gif icon next to MIME encoded x-compress and x-gzip files in server generated directory listings.

10.5.45. `AddIconByType`

This directive names icons which are displayed next to files with MIME types in server generated directory listings. For example, the server shows the icon text.gif next to files with a mime-type of text, in server generated directory listings.

10.5.46. `AddIcon`

AddIcon specifies which icon to show in server generated directory listings for files with certain extensions. For example, the Web server is set to show the icon binary.gif for files with .bin or .exe extensions.

10.5.47. `DefaultIcon`

DefaultIcon specifies the icon displayed in server generated directory listings for files which have no other icon specified. The unknown.gif image file is the default.

10.5.48. `AddDescription`

When using FancyIndexing as an IndexOptions parameter, the AddDescription directive can be used to display user-specified descriptions for certain files or file types in a server generated directory listing. The AddDescription directive supports listing specific files, wildcard expressions, or file extensions.

10.5.49. `ReadmeName`

ReadmeName names the file which, if it exists in the directory, is appended to the end of server generated directory listings. The Web server first tries to include the file as an HTML document and then tries to include it as plain text. By default, ReadmeName is set to README.html.

10.5.50. `HeaderName`

HeaderName names the file which, if it exists in the directory, is prepended to the start of server generated directory listings. Like ReadmeName, the server tries to include it as an HTML document if possible or in plain text if not.

10.5.51. `IndexIgnore`

IndexIgnore lists file extensions, partial file names, wildcard expressions, or full file names. The Web server does not include any files which match any of those parameters in server generated directory listings.

10.5.52. `AddEncoding`

AddEncoding names file name extensions which should specify a particular encoding type. AddEncoding can also be used to instruct some browsers to uncompress certain files as they are downloaded.

10.5.53. `AddLanguage`

AddLanguage associates file name extensions with specific languages. This directive is useful for Apache HTTP Servers which serve content in multiple languages based on the client Web browser's language settings.

10.5.54. `LanguagePriority`

LanguagePriority sets precedence for different languages in case the client Web browser has no language preference set.

10.5.55. `AddType`

Use the AddType directive to define or override a default MIME type and file extension pairs. The following example directive tells the Apache HTTP Server to recognize the .tgz file extension:

AddType application/x-tar .tgz

10.5.56. `AddHandler`

AddHandler maps file extensions to specific handlers. For example, the cgi-script handler can be matched with the extension .cgi to automatically treat a file ending with .cgi as a CGI script. The following is a sample AddHandler directive for the .cgi extension.

AddHandler cgi-script .cgi

This directive enables CGIs outside of the cgi-bin to function in any directory on the server which has the ExecCGI option within the directories container. Refer to Section 10.5.22, “ Directory ” for more information about setting the ExecCGI option for a directory.

In addition to CGI scripts, the AddHandler directive is used to process server-parsed HTML and image-map files.

10.5.57. `Action`

Action specifies a MIME content type and CGI script pair, so that when a file of that media type is requested, a particular CGI script is executed.

10.5.58. `ErrorDocument`

The ErrorDocument directive associates an HTTP response code with a message or a URL to be sent back to the client. By default, the Web server outputs a simple and usually cryptic error message when an error occurs. The ErrorDocument directive forces the Web server to instead output a customized message or page.

Important

To be valid, the message must be enclosed in a pair of double quotes ".

10.5.59. `BrowserMatch`

The BrowserMatch directive allows the server to define environment variables and take appropriate actions based on the User-Agent HTTP header field — which identifies the client's Web browser type. By default, the Web server uses BrowserMatch to deny connections to specific browsers with known problems and also to disable keepalives and HTTP header flushes for browsers that are known to have problems with those actions.

10.5.60. `Location`

The <Location> and </Location> tags create a container in which access control based on URL can be specified.

For instance, to allow people connecting from within the server's domain to see status reports, use the following directives:

<Location /server-status>
    SetHandler server-status 
    Order deny,allow
    Deny from all 
    Allow from <.example.com>
</Location>

Replace <.example.com> with the second-level domain name for the Web server.

To provide server configuration reports (including installed modules and configuration directives) to requests from inside the domain, use the following directives:

<Location /server-info>
    SetHandler server-info
    Order deny,allow
    Deny from all
    Allow from <.example.com>
</Location>

Again, replace <.example.com> with the second-level domain name for the Web server.

10.5.61. `ProxyRequests`

To configure the Apache HTTP Server to function as a proxy server, remove the hash mark (#) from the beginning of the <IfModule mod_proxy.c> line, the ProxyRequests, and each line in the <Proxy> stanza. Set the ProxyRequests directive to On, and set which domains are allowed access to the server in the Allow from directive of the <Proxy> stanza.

10.5.62. `Proxy`

<Proxy *> and </Proxy> tags create a container which encloses a group of configuration directives meant to apply only to the proxy server. Many directives which are allowed within a <Directory> container may also be used within <Proxy> container.

10.5.63. Cache Directives

A number of commented cache directives are supplied by the default Apache HTTP Server configuration file. In most cases, uncommenting these lines by removing the hash mark (#) from the beginning of the line is sufficient. The following, however, is a list of some of the more important cache-related directives.

CacheEnable — Specifies whether the cache is a disk, memory, or file descriptor cache. By default CacheEnable configures a disk cache for URLs at or below /.
CacheRoot — Specifies the name of the directory containing cached files. The default CacheRoot is the /var/httpd/proxy/ directory.
CacheSize — Specifies how much space the cache can use in kilobytes. The default CacheSize is 5 KB.

The following is a list of some of the other common cache-related directives.

CacheMaxExpire — Specifies how long HTML documents are retained (without a reload from the originating Web server) in the cache. The default is 24 hours (86400 seconds).
CacheLastModifiedFactor — Specifies the creation of an expiry (expiration) date for a document which did not come from its originating server with its own expiry set. The default CacheLastModifiedFactor is set to 0.1, meaning that the expiry date for such documents equals one-tenth of the amount of time since the document was last modified.
CacheDefaultExpire — Specifies the expiry time in hours for a document that was received using a protocol that does not support expiry times. The default is set to 1 hour (3600 seconds).
NoProxy — Specifies a space-separated list of subnets, IP addresses, domains, or hosts whose content is not cached. This setting is most useful for Intranet sites.

10.5.64. `NameVirtualHost`

The NameVirtualHost directive associates an IP address and port number, if necessary, for any name-based virtual hosts. Name-based virtual hosting allows one Apache HTTP Server to serve different domains without using multiple IP addresses.

Note

Name-based virtual hosts only work with non-secure HTTP connections. If using virtual hosts with a secure server, use IP address-based virtual hosts instead.

To enable name-based virtual hosting, uncomment the NameVirtualHost configuration directive and add the correct IP address. Then add additional VirtualHost containers for each virtual host as is necessary for your configuration.

10.5.65. `VirtualHost`

<VirtualHost> and </VirtualHost> tags create a container outlining the characteristics of a virtual host. The VirtualHost container accepts most configuration directives.

A commented VirtualHost container is provided in httpd.conf, which illustrates the minimum set of configuration directives necessary for each virtual host. Refer to Section 10.8, “Virtual Hosts” for more information about virtual hosts.

Note

The default SSL virtual host container now resides in the file /etc/httpd/conf.d/ssl.conf.

10.5.66. Configuration Directives for SSL

The directives in /etc/httpd/conf.d/ssl.conf file can be configured to enable secure Web communications using SSL and TLS.

10.5.66.1. `SetEnvIf`

SetEnvIf sets environment variables based on the headers of incoming connections. It is not solely an SSL directive, though it is present in the supplied /etc/httpd/conf.d/ssl.conf file. It's purpose in this context is to disable HTTP keepalive and to allow SSL to close the connection without a closing notification from the client browser. This setting is necessary for certain browsers that do not reliably shut down the SSL connection.

For more information on other directives within the SSL configuration file, refer to the following URLs:

http://localhost/manual/mod/mod_ssl.html
http://httpd.apache.org/docs-2.0/mod/mod_ssl.html

For information about setting up an Apache HTTP Secure Server, Refer to the chapter titled Apache HTTP Secure Server Configuration in the System Administrators Guide.

Note

In most cases, SSL directives are configured appropriately during the installation of Red Hat Enterprise Linux. Be careful when altering Apache HTTP Secure Server directives, misconfiguration can lead to security vulnerabilities.

10.6. Default Modules

The Apache HTTP Server is distributed with a number of modules. By default the following modules are installed and enabled with the httpd package in Red Hat Enterprise Linux 4.5.0;:

mod_access
mod_actions
mod_alias
mod_asis
mod_auth
mod_auth_anon
mod_auth_dbm
mod_auth_digest
mod_auth_ldap
mod_autoindex
mod_cache
mod_cern_meta
mod_cgi
mod_dav
mod_dav_fs
mod_deflate
mod_dir
mod_disk_cache
mod_env
mod_expires
mod_ext_filter
mod_file_cache
mod_headers
mod_imap
mod_include
mod_info
mod_ldap
mod_log_config
mod_logio
mod_mem_cache
mod_mime
mod_mime_magic
mod_negotiation
mod_proxy
mod_proxy_connect
mod_proxy_ftp
mod_proxy_http
mod_rewrite
mod_setenvif
mod_speling
mod_status
mod_suexec
mod_unique_id
mod_userdir
mod_usertrack
mod_vhost_alias

Additionally, the following modules are available by installing additional packages:

mod_auth_kerb
mod_auth_mysql
mod_auth_pgsql
mod_authz_ldap
mod_dav_svn
mod_jk2
mod_perl
mod_python
mod_ssl
php

10.7. Adding Modules

The Apache HTTP Server supports Dynamically Shared Objects (DSOs), or modules, which can easily be loaded at runtime as necessary.

The Apache Project provides complete DSO documentation online at http://httpd.apache.org/docs-2.0/dso.html. Or, if the http-manual package is installed, documentation about DSOs can be found online at http://localhost/manual/mod/.

For the Apache HTTP Server to use a DSO, it must be specified in a LoadModule directive within /etc/httpd/conf/httpd.conf. If the module is provided by a separate package, the line must appear within the modules configuration file in the /etc/httpd/conf.d/ directory. Refer to Section 10.5.12, “ LoadModule ” for more information.

If adding or deleting modules from http.conf, Apache HTTP Server must be reloaded or restarted, as referred to in Section 10.4, “Starting and Stopping httpd”.

If creating a new module, first install the httpd-devel package which contains the include files, the header files, as well as the APache eXtenSion (/usr/sbin/apxs) application, which uses the include files and the header files to compile DSOs.

After writing a module, use /usr/sbin/apxs to compile the module sources outside the Apache source tree. For more information about using the /usr/sbin/apxs command, refer to the the Apache documentation online at http://httpd.apache.org/docs-2.0/dso.html as well as the apxs man page.

Once compiled, put the module in the /usr/lib/httpd/modules/ directory. Then add a LoadModule line to the httpd.conf, using the following structure:

LoadModule <module-name> <path/to/module.so>

Where <module-name> is the name of the module and <path/to/module.so> is the path to the DSO.

10.8. Virtual Hosts

The Apache HTTP Server's built in virtual hosting allows the server to provide different information based on which IP address, hostname, or port is being requested. A complete guide to using virtual hosts is available online at http://httpd.apache.org/docs-2.0/vhosts/.

10.8.1. Setting Up Virtual Hosts

To create a name-based virtual host, it is best to use the virtual host container provided in httpd.conf as an example.

The virtual host example read as follows:

#NameVirtualHost *:80
#
#<VirtualHost  *:80>
#    ServerAdmin webmaster@dummy-host.example.com
#    DocumentRoot /www/docs/dummy-host.example.com
#    ServerName dummy-host.example.com
#    ErrorLog logs/dummy-host.example.com-error_log
#    CustomLog logs/dummy-host.example.com-access_log common
#</VirtualHost>

To activate name-based virtual hosting, uncomment the NameVirtualHost line by removing the hash mark (#) and replace the asterisk (*) with the IP address assigned to the machine.

Next, configure a virtual host by uncommenting and customizing the <VirtualHost> container.

On the <VirtualHost> line, change the asterisk (*) to the server's IP address. Change the ServerName to a valid DNS name assigned to the machine, and configure the other directives as necessary.

The <VirtualHost> container is highly customizable and accepts almost every directive available within the main server configuration.

Note

If configuring a virtual host to listen on a non-default port, that port must be added to the Listen directive in the global settings section of /etc/httpd/conf/httpd.conf file.

To activate a newly created virtual host, the Apache HTTP Server must be reloaded or restarted. Refer to Section 10.4, “Starting and Stopping httpd” for further instructions.

Comprehensive information about creating and configuring both name-based and IP address-based virtual hosts is provided online at http://httpd.apache.org/docs-2.0/vhosts/.

10.8.2. The Secure Web Server Virtual Host

By default, the Apache HTTP Server is configured as both a non-secure and a secure server. Both the non-secure and secure servers use the same IP address and hostname, but listen on different ports: 80 and 443 respectively. This enables both non-secure and secure communications to take place simultaneously.

One aspect of SSL enhanced HTTP transmissions is that they are more resource intensive than the standard HTTP protocol, so a secure server cannot serve as many pages per second. For this reason, it is often a good idea to minimize the information available from the secure server, especially on a high traffic website.

Important

Do not use name-based virtual hosts in conjunction with a secure Web server as the SSL handshake occurs before the HTTP request identifies the appropriate name-based virtual host. Name-based virtual hosts only work with the non-secure Web server.

The configuration directives for the secure server are contained within virtual host tags in the /etc/httpd/conf.d/ssl.conf file.

By default, both the secure and the non-secure Web servers share the same DocumentRoot. It is recommended that a different DocumentRoot be made available for the secure Web server.

To stop the non-secure Web server from accepting connections, comment out the line in httpd.conf which reads Listen 80 by placing a hash mark (#) at the beginning of the line. When finished, the line looks like the following example:

#Listen 80

For more information on configuring an SSL enhanced Web server, refer to the chapter titled Apache HTTP Secure Server Configuration in the System Administrators Guide. For advanced configuration tips, refer to the Apache Software Foundation documentation available online at the following URLs:

10.9. Additional Resources

To learn more about the Apache HTTP Server, refer to the following resources.

10.9.1. Useful Websites

http://httpd.apache.org/ — The official website for the Apache HTTP Server with documentation on all the directives and default modules.
http://www.modssl.org/ — The official website for mod_ssl.
http://www.apacheweek.com/ — A comprehensive online weekly newsletter about all things Apache.

Chapter 11. Email

The birth of electronic mail (email) occurred in the early 1960s. The mailbox was a file in a user's home directory that was readable only by that user. Primitive mail applications appended new text messages to the bottom of the file, making the user wade through the constantly growing file to find any particular message. This system was only capable of sending messages to users on the same system.

The first network transfer of an electronic mail message file took place in 1971 when a computer engineer named Ray Tomlinson sent a test message between two machines via ARPANET — the precursor to the Internet. Communication via email soon became very popular, comprising 75 percent of ARPANET's traffic in less than two years.

Today, email systems based on standardized network protocols have evolved into some of the most widely used services on the Internet. Red Hat Enterprise Linux offers many advanced applications to serve and access email.

This chapter reviews modern email protocols in use today and some of the programs designed to send and receive email.

11.1. Email Protocols

Today, email is delivered using a client/server architecture. An email message is created using a mail client program. This program then sends the message to a server. The server then forwards the message to the recipient's email server, where the message is then supplied to the recipient's email client.

To enable this process, a variety of standard network protocols allow different machines, often running different operating systems and using different email programs, to send and receive email.

The following protocols discussed are the most commonly used in the transfer of email.

11.1.1. Mail Transport Protocols

Mail delivery from a client application to the server, and from an originating server to the destination server, is handled by the Simple Mail Transfer Protocol (SMTP).

11.1.1.1. SMTP

The primary purpose of SMTP is to transfer email between mail servers. However, it is critical for email clients as well. To send email, the client sends the message to an outgoing mail server, which in turn contacts the destination mail server for delivery. For this reason, it is necessary to specify an SMTP server when configuring an email client.

Under Red Hat Enterprise Linux, a user can configure an SMTP server on the local machine to handle mail delivery. However, it is also possible to configure remote SMTP servers for outgoing mail.

One important point to make about the SMTP protocol is that it does not require authentication. This allows anyone on the Internet to send email to anyone else or even to large groups of people. It is this characteristic of SMTP that makes junk email or spam possible. Modern SMTP servers attempt to minimize this behavior by allowing only known hosts access to the SMTP server. Those servers that do not impose such restrictions are called open relay servers.

By default, Sendmail (/usr/sbin/sendmail) is the default SMTP program under Red Hat Enterprise Linux. However, a simpler mail server application called Postfix (/usr/sbin/postfix) is also available.

11.1.2. Mail Access Protocols

There are two primary protocols used by email client applications to retrieve email from mail servers: the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP).

The use of IMAP and POP is configured through dovecot; by default dovecot runs only IMAP. To configure dovecot to use POP:

Edit /etc/dovecot.conf to have the line:
```
protocols = imap imaps pop3 pop3s
```
Make that change operational for the current session by running the command:
```
service dovecot restart
```
Make that change operational after the next reboot by running the command:
```
chkconfig dovecot on
```

Unlike SMTP, both of these protocols require connecting clients to authenticate using a username and password. By default, passwords for both protocols are passed over the network unencrypted.

11.1.2.1. POP

The default POP server under Red Hat Enterprise Linux is /usr/sbin/ipop3d and is provided by the dovecot package. When using a POP server, email messages are downloaded by email client applications. By default, most POP email clients are automatically configured to delete the message on the email server after it has been successfully transferred, however this setting usually can be changed.

POP is fully compatible with important Internet messaging standards, such as Multipurpose Internet Mail Extensions (MIME), which allow for email attachments.

POP works best for users who have one system on which to read email. It also works well for users who do not have a persistent connection to the Internet or the network containing the mail server. Unfortunately for those with slow network connections, POP requires client programs upon authentication to download the entire content of each message. This can take a long time if any messages have large attachments.

The most current version of the standard POP protocol is POP3.

There are, however, a variety of lesser-used POP protocol variants:

APOP — POP3 with MDS authentication. An encoded hash of the user's password is sent from the email client to the server rather then sending an unencrypted password.
KPOP — POP3 with Kerberos authentication. Refer to Chapter 19, Kerberos for more information.
RPOP — POP3 with RPOP authentication. This uses a per-user ID, similar to a password, to authenticate POP requests. However, this ID is not encrypted, so RPOP is no more secure than standard POP.

For added security, it is possible to use Secure Socket Layer (SSL) encryption for client authentication and data transfer sessions. This can be enabled by using the ipop3s service or by using the /usr/sbin/stunnel program. Refer to Section 11.5.1, “Securing Communication” for more information.

11.1.2.2. IMAP

The default IMAP server under Red Hat Enterprise Linux is /usr/sbin/imapd and is provided by either the cyrus or dovecot packages. When using an IMAP mail server, email messages remain on the server where users can read or delete them. IMAP also allows client applications to create, rename, or delete mail directories on the server to organize and store email.

IMAP is particularly useful for those who access their email using multiple machines. The protocol is also convenient for users connecting to the mail server via a slow connection, because only the email header information is downloaded for messages until opened, saving bandwidth. The user also has the ability to delete messages without viewing or downloading them.

For convenience, IMAP client applications are capable of caching copies of messages locally, so the user can browse previously read messages when not directly connected to the IMAP server.

IMAP, like POP, is fully compatible with important Internet messaging standards, such as MIME, which allow for email attachments.

For added security, it is possible to use SSL encryption for client authentication and data transfer sessions. This can be enabled by using the imaps service, or by using the /usr/sbin/stunnel program. Refer to Section 11.5.1, “Securing Communication” for more information.

Other free, as well as commercial, IMAP clients and servers are available, many of which extend the IMAP protocol and provide additional functionality. A comprehensive list can be found online at http://www.imap.org/products/longlist.htm.

11.2. Email Program Classifications

In general, all email applications fall into at least one of three classifications. Each classification plays a specific role in the process of moving and managing email messages. While most users are only aware of the specific email program they use to receive and send messages, each one is important for ensuring that email arrives at the correct destination.

11.2.1. Mail Transfer Agent

A Mail Transfer Agent (MTA) transfers email messages between hosts using SMTP. A message may involve several MTAs as it moves to its intended destination.

While the delivery of messages between machines may seem rather straightforward, the entire process of deciding if a particular MTA can or should accept a message for delivery is quite complicated. In addition, due to problems from spam, use of a particular MTA is usually restricted by the MTA's configuration or the access configuration for the network on which the MTA resides.

Many modern email client programs can act as an MTA when sending email. However, this action should not be confused with the role of a true MTA. The sole reason email client programs are capable of sending email like an MTA is because the host running the application does not have its own MTA. This is particularly true for email client programs on non-UNIX-based operating systems. However, these client programs only send outbound messages to an MTA they are authorized to use and do not directly deliver the message to the intended recipient's email server.

Since Red Hat Enterprise Linux installs two MTAs, Sendmail and Postfix, email client programs are often not required to act as an MTA. Red Hat Enterprise Linux also includes a special purpose MTA called Fetchmail.

For more information on Sendmail, Postfix, and Fetchmail, refer to Section 11.3, “Mail Transport Agents”.

11.2.2. Mail Delivery Agent

A Mail Delivery Agent (MDA) is invoked by the MTA to file incoming email in the proper user's mailbox. In many cases, the MDA is actually a Local Delivery Agent (LDA), such as mail or Procmail.

Any program that actually handles a message for delivery to the point where it can be read by an email client application can be considered an MDA. For this reason, some MTAs (such as Sendmail and Postfix) can fill the role of an MDA when they append new email messages to a local user's mail spool file. In general, MDAs do not transport messages between systems nor do they provide a user interface; MDAs distribute and sort messages on the local machine for an email client application to access.

11.2.3. Mail User Agent

A Mail User Agent (MUA) is synonymous with an email client application. An MUA is a program that, at the very least, allows a user to read and compose email messages. Many MUAs are capable of retrieving messages via the POP or IMAP protocols, setting up mailboxes to store messages, and sending outbound messages to an MTA.

MUAs may be graphical, such as Mozilla Mail, or have a very simple, text-based interface, such as mutt.

11.3. Mail Transport Agents

Red Hat Enterprise Linux includes two primary MTAs, Sendmail and Postfix. Sendmail is configured as the default MTA, although it is easy to switch the default MTA to Postfix.

Note

For information about how to switch the default MTA from Sendmail to Postfix, refer to the chapter called Mail Transport Agent (MTA) Configuration in the System Administrators Guide.

11.3.1. Sendmail

Sendmail's core purpose, like other MTAs, is to safely transfer email among hosts, usually using the SMTP protocol. However, Sendmail is highly configurable, allowing control over almost every aspect of how email is handled, including the protocol used. Many system administrators elect to use Sendmail as their MTA due to its power and scalability.

11.3.1.1. Purpose and Limitations

It is important to be aware of what Sendmail is and what it can do, as opposed to what it is not. In these days of monolithic applications that fulfill multiple roles, Sendmail may seem like the only application needed to run an email server within an organization. Technically, this is true, as Sendmail can spool mail to each users' directory and deliver outbound mail for users. However, most users actually require much more than simple email delivery. Users usually want to interact with their email using an MUA, that uses POP or IMAP, to download their messages to their local machine. Or, they may prefer a Web interface to gain access to their mailbox. These other applications can work in conjunction with Sendmail, but they actually exist for different reasons and can operate separately from one another.

It is beyond the scope of this section to go into all that Sendmail should or could be configured to do. With literally hundreds of different options and rule sets, entire volumes have been dedicated to helping explain everything that can be done and how to fix things that go wrong. Refer to the Section 11.6, “Additional Resources” for a list of Sendmail resources.

This section reviews the files installed with Sendmail by default and reviews basic configuration changes, including how to stop unwanted email (spam) and how to extend Sendmail with the Lightweight Directory Access Protocol (LDAP).

11.3.1.2. The Default Sendmail Installation

The Sendmail executable is /usr/sbin/sendmail.

Sendmail's lengthy and detailed configuration file is /etc/mail/sendmail.cf. Avoid editing the sendmail.cf file directly. Instead, to make configuration changes to Sendmail, edit the /etc/mail/sendmail.mc file, back up the original /etc/mail/sendmail.cf, and then use the included m4 macro processor to create a new /etc/mail/sendmail.cf. More information on configuring Sendmail can be found in Section 11.3.1.3, “Common Sendmail Configuration Changes”.

Various Sendmail configuration files are installed in the /etc/mail/ directory including:

access — Specifies which systems can use Sendmail for outbound email.
domaintable — Specifies domain name mapping.
local-host-names — Specifies aliases for the host.
mailertable — Specifies instructions that override routing for particular domains.
virtusertable — Specifies a domain-specific form of aliasing, allowing multiple virtual domains to be hosted on one machine.

Several of the configuration files in /etc/mail/, such as access, domaintable, mailertable and virtusertable, must actually store their information in database files before Sendmail can use any configuration changes. To include any changes made to these configurations in their database files, run the command

makemap hash /etc/mail/<name> < /etc/mail/<name>

where <name> is replaced with the name of the configuration file to convert.

For example, to have all emails addressed to the example.com domain delivered to bob@other-example.com, add the following line to the virtusertable file:

@example.com     bob@other-example.com

To finalize the change, the virtusertable.db file must be updated using the following command as root:

makemap hash /etc/mail/virtusertable < /etc/mail/virtusertable

This creates an updated virtusertable.db file containing the new configuration.

11.3.1.3. Common Sendmail Configuration Changes

When altering the Sendmail configuration file, it is best not to edit an existing file, but to generate an entirely new /etc/mail/sendmail.cf file.

Warning

Before changing the sendmail.cf file, it is a good idea to create a backup copy.

To add the desired functionality to Sendmail, edit the /etc/mail/sendmail.mc file as the root user. When finished, use the m4 macro processor to generate a new sendmail.cf by executing the following command:

m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

By default, the m4 macro processor is installed with Sendmail but is part of the m4 package.

After creating a new /etc/mail/sendmail.cf file, restart Sendmail for the changes to take effect. The easiest way to do this is to type the following command:

/sbin/service sendmail restart

Important

The default sendmail.cf file does not allow Sendmail to accept network connections from any host other than the local computer. To configure Sendmail as a server for other clients, edit the /etc/mail/sendmail.mc file, and either change the address specified in the Addr= option of the DAEMON_OPTIONS directive from 127.0.0.1 to the IP address of an active network device or comment out the DAEMON_OPTIONS directive all together by placing dnl at the beginning of the line. When finished, regenerate /etc/mail/sendmail.cf by executing the following command:

m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

The default configuration which ships with Red Hat Enterprise Linux works for most SMTP-only sites. However, it does not work for UUCP (UNIX to UNIX Copy) sites. If using UUCP mail transfers, the /etc/mail/sendmail.mc file must be reconfigured and a new /etc/mail/sendmail.cf must be generated.

Consult the /usr/share/sendmail-cf/README file before editing any files in the directories under the /usr/share/sendmail-cf directory, as they can affect the future configuration of /etc/mail/sendmail.cf files.

11.3.1.4. Masquerading

One common Sendmail configuration is to have a single machine act as a mail gateway for all machines on the network. For instance, a company may want to have a machine called mail.example.com that handles all of their email and assigns a consistent return address to all outgoing mail.

In this situation, the Sendmail server must masquerade the machine names on the company network so that their return address is user@example.com instead of user@host.example.com.

To do this, add the following lines to /etc/mail/sendmail.mc:

FEATURE(always_add_domain)dnl
FEATURE(`masquerade_entire_domain')
FEATURE(`masquerade_envelope')
FEATURE(`allmasquerade')
MASQUERADE_AS(`bigcorp.com.')
MASQUERADE_DOMAIN(`bigcorp.com.')
MASQUERADE_AS(bigcorp.com)

After generating a new sendmail.cf using m4, this configuration makes all mail from inside the network appear as if it were sent from bigcorp.com.

11.3.1.5. Stopping Spam

Email spam can be defined as unnecessary and unwanted email received by a user who never requested the communication. It is a disruptive, costly, and widespread abuse of Internet communication standards.

Sendmail makes it relatively easy to block new spamming techniques being employed to send junk email. It even blocks many of the more usual spamming methods by default.

For example, forwarding of SMTP messages, also called relaying, has been disabled by default since Sendmail version 8.9. Before this change occurred, Sendmail directed the mail host (x.edu) to accept messages from one party (y.com) and sent them to a different party (z.net). Now, however, Sendmail must be configured to permit any domain to relay mail through the server. To configure relay domains, edit the /etc/mail/relay-domains file and restart Sendmail.

However, many times users are bombarded with spam from other servers throughout the Internet. In these instances, Sendmail's access control features available through the /etc/mail/access file can be used to prevent connections from unwanted hosts. The following example illustrates how this file can be used to both block and specifically allow access to the Sendmail server:

badspammer.com       ERROR:550 "Go away and do not spam us anymore"
tux.badspammer.com   OK
10.0                 RELAY

This example shows that any email sent from badspammer.com is blocked with a 550 RFC-821 compliant error code, with a message sent back to the spammer. Email sent from the tux.badspammer.com sub-domain, is accepted. The last line shows that any email sent from the 10.0.*.* network can be relayed through the mail server.

Because /etc/mail/access.db is a database, use makemap to activate any changes. Do this using the following command as root:

makemap hash /etc/mail/access < /etc/mail/access

This example only represents a small part of what Sendmail can do in terms of allowing or blocking access. Refer to the /usr/share/sendmail-cf/README for more information and examples.

Since Sendmail calls the Procmail MDA when delivering mail, it is also possible to use a spam filtering program, such as SpamAssassin, to identify and file spam for users. Refer to Section 11.4.2.6, “Spam Filters” for more about using SpamAssassin.

11.3.1.6. Using Sendmail with LDAP

Using the Lightweight Directory Access Protocol (LDAP) is a very quick and powerful way to find specific information about a particular user from a much larger group. For example, an LDAP server can be used to look up a particular email address from a common corporate directory by the user's last name. In this kind of implementation, LDAP is largely separate from Sendmail, with LDAP storing the hierarchical user information and Sendmail only being given the result of LDAP queries in pre-addressed email messages.

However, Sendmail supports a much greater integration with LDAP, where it uses LDAP to replace separately maintained files, such as aliases and virtusertables, on different mail servers that work together to support a medium- to enterprise-level organization. In short, LDAP abstracts the mail routing level from Sendmail and its separate configuration files to a powerful LDAP cluster that can be leveraged by many different applications.

The current version of Sendmail contains support for LDAP. To extend the Sendmail server using LDAP, first get an LDAP server, such as OpenLDAP, running and properly configured. Then edit the /etc/mail/sendmail.mc to include the following:

LDAPROUTE_DOMAIN('yourdomain.com')dnl
FEATURE('ldap_routing')dnl

Note

This is only for a very basic configuration of Sendmail with LDAP. The configuration can differ greatly from this depending on the implementation of LDAP, especially when configuring several Sendmail machines to use a common LDAP server.

Consult /usr/share/sendmail-cf/README for detailed LDAP routing configuration instructions and examples.

Next, recreate the /etc/mail/sendmail.cf file by running m4 and restarting Sendmail. Refer to Section 11.3.1.3, “Common Sendmail Configuration Changes” for instructions.

For more information on LDAP, refer to Chapter 13, Lightweight Directory Access Protocol (LDAP).

11.3.2. Postfix

Originally developed at IBM by security expert and programmer Wietse Venema, Postfix is a Sendmail-compatible MTA that is designed to be secure, fast, and easy to configure.

To improve security, Postfix uses a modular design, where small processes with limited privileges are launched by a master daemon. The smaller, less privileged processes perform very specific tasks related to the various stages of mail delivery and run in a change rooted environment to limit the effects of attacks.

Configuring Postfix to accept network connections from hosts other than the local computer takes only a few minor changes in its configuration file. Yet for those with more complex needs, Postfix provides a variety of configuration options, as well as third party add ons that make it a very versatile and full-featured MTA.

The configuration files for Postfix are human readable and support upward of 250 directives. Unlike Sendmail, no macro processing is required for changes to take effect and the majority of the most commonly used options are described in the heavily commented files.

Important

Before using Postfix, the default MTA must be switched from Sendmail to Postfix. Refer to the chapter called Mail Transport Agent (MTA) Configuration in the System Administrators Guide for further details.

11.3.2.1. The Default Postfix Installation

The Postfix executable is /usr/sbin/postfix. This daemon launches all related processes needed to handle mail delivery.

Postfix stores its configuration files in the /etc/postfix/ directory. The following is a list of the more commonly used files:

access — Used for access control, this file specifies which hosts are allowed to connect to Postfix.
aliases — A configurable list required by the mail protocol.
main.cf — The global Postfix configuration file. The majority of configuration options are specified in this file.
master.cf — Specifies how Postfix interacts with various processes to accomplish mail delivery.
transport — Maps email addresses to relay hosts.

Important

The default /etc/postfix/main.cf file does not allow Postfix to accept network connections from a host other than the local computer. For instructions on configuring Postfix as a server for other clients, refer to Section 11.3.2.2, “Basic Postfix Configuration”.

When changing some options within files in the /etc/postfix/ directory, it may be necessary to restart the postfix service for the changes to take effect. The easiest way to do this is to type the following command:

service postfix restart

11.3.2.2. Basic Postfix Configuration

By default, Postfix does not accept network connections from any host other than the local host. Perform the following steps as root to enable mail delivery for other hosts on the network:

Edit the /etc/postfix/main.cf file with a text editor, such as vi.
Uncomment the mydomain line by removing the hash mark (#), and replace domain.tld with the domain the mail server is servicing, such as example.com.
Uncomment the myorigin = $mydomain line.
Uncomment the myhostname line, and replace host.domain.tld with the hostname for the machine.
Uncomment the mydestination = $myhostname, localhost.$mydomain line.
Uncomment the mynetworks line, and replace 168.100.189.0/28 with a valid network setting for hosts that can connect to the server.
Uncomment the inet_interfaces = all line.
Restart the postfix service.

Once these steps are complete, the host accepts outside emails for delivery.

Postfix has a large assortment of configuration options. One of the best ways to learn how to configure Postfix is to read the comments within /etc/postfix/main.cf. Additional resources including information about LDAP and SpamAssassin integration are available online at http://www.postfix.org/.

11.3.3. Fetchmail

Fetchmail is an MTA which retrieves email from remote servers and delivers it to the local MTA. Many users appreciate the ability to separate the process of downloading their messages located on a remote server from the process of reading and organizing their email in an MUA. Designed with the needs of dial-up users in mind, Fetchmail connects and quickly downloads all of the email messages to the mail spool file using any number of protocols, including POP3 and IMAP. It can even forward email messages to an SMTP server, if necessary.

Fetchmail is configured for each user through the use of a .fetchmailrc file in the user's home directory.

Using preferences in the .fetchmailrc file, Fetchmail checks for email on a remote server and downloads it. It then delivers it to port 25 on the local machine, using the local MTA to place the email in the correct user's spool file. If Procmail is available, it is launched to filter the email and place it in a mailbox so that it can be read by an MUA.

11.3.3.1. Fetchmail Configuration Options

Although it is possible to pass all necessary options on the command line to check for email on a remote server when executing Fetchmail, using a .fetchmailrc file is much easier. Place any desired configuration options in the .fetchmailrc file for those options to be used each time the fetchmail command is issued. It is possible to override these at the time Fetchmail is run by specifying that option on the command line.

A user's .fetchmailrc file contains three classes of configuration options:

global options — Gives Fetchmail instructions that control the operation of the program or provide settings for every connection that checks for email.
server options — Specifies necessary information about the server being polled, such as the hostname, as well as preferences for specific email servers, such as the port to check or number of seconds to wait before timing out. These options affect every user using that server.
user options — Contains information, such as username and password, necessary to authenticate and check for email using a specified email server.

Global options appear at the top of the .fetchmailrc file, followed by one or more server options, each of which designate a different email server that Fetchmail should check. User options follow server options for each user account checking that email server. Like server options, multiple user options may be specified for use with a particular server as well as to check multiple email accounts on the same server.

Server options are called into service in the .fetchmailrc file by the use of a special option verb, poll or skip, that precedes any of the server information. The poll action tells Fetchmail to use this server option when it is run, which checks for email using the specified user options. Any server options after a skip action, however, are not checked unless this server's hostname is specified when Fetchmail is invoked. The skip option is useful when testing configurations in .fetchmailrc because it only checks skipped servers when specifically invoked, and does not affect any currently working configurations.

A sample .fetchmailrc file looks similar to the following example:

set postmaster "user1"
set bouncemail

poll pop.domain.com proto pop3
     user 'user1' there with password 'secret' is user1 here

poll mail.domain2.com
     user 'user5' there with password 'secret2' is user1 here
     user 'user7' there with password 'secret3' is user1 here

In this example, the global options specify that the user is sent email as a last resort (postmaster option) and all email errors are sent to the postmaster instead of the sender (bouncemail option). The set action tells Fetchmail that this line contains a global option. Then, two email servers are specified, one set to check using POP3, the other for trying various protocols to find one that works. Two users are checked using the second server option, but all email found for any user is sent to user1's mail spool. This allows multiple mailboxes to be checked on multiple servers, while appearing in a single MUA inbox. Each user's specific information begins with the user action.

Note

Users are not required to place their password in the .fetchmailrc file. Omitting the with password '<password>' section causes Fetchmail to ask for a password when it is launched.

Fetchmail has numerous global, server, and local options. Many of these options are rarely used or only apply to very specific situations. The fetchmail man page explains each option in detail, but the most common ones are listed here.

11.3.3.2. Global Options

Each global option should be placed on a single line after a set action.

daemon <seconds> — Specifies daemon-mode, where Fetchmail stays in the background. Replace <seconds> with the number of seconds Fetchmail is to wait before polling the server.
postmaster — Specifies a local user to send mail to in case of delivery problems.
syslog — Specifies the log file for errors and status messages. By default, this is /var/log/maillog.

11.3.3.3. Server Options

Server options must be placed on their own line in .fetchmailrc after a poll or skip action.

auth <auth-type> — Replace <auth-type> with the type of authentication to be used. By default, password authentication is used, but some protocols support other types of authentication, including kerberos_v5, kerberos_v4, and ssh. If the any authentication type is used, Fetchmail first tries methods that do not require a password, then methods that mask the password, and finally attempts to send the password unencrypted to authenticate to the server.
interval <number> — Polls the specified server every <number> of times that it checks for email on all configured servers. This option is generally used for email servers where the user rarely receives messages.
port <port-number> — Replace <port-number> with the port number. This value overrides the default port number for the specified protocol.
proto <protocol> — Replace <protocol> with the protocol, such as pop3 or imap, to use when checking for messages on the server.
timeout <seconds> — Replace <seconds> with the number of seconds of server inactivity after which Fetchmail gives up on a connection attempt. If this value is not set, a default of 300 seconds is assumed.

11.3.3.4. User Options

User options may be placed on their own lines beneath a server option or on the same line as the server option. In either case, the defined options must follow the user option (defined below).

fetchall — Orders Fetchmail to download all messages in the queue, including messages that have already been viewed. By default, Fetchmail only pulls down new messages.
fetchlimit <number> — Replace <number> with the number of messages to be retrieved before stopping.
flush — Deletes all previously viewed messages in the queue before retrieving new messages.
limit <max-number-bytes> — Replace <max-number-bytes> with the maximum size in bytes that messages are allowed to be when retrieved by Fetchmail. This option is useful with slow network links, when a large message takes too long to download.
password '<password>' — Replace <password> with the user's password.
preconnect "<command>" — Replace <command> with a command to be executed before retrieving messages for the user.
postconnect "<command>" — Replace <command> with a command to be executed after retrieving messages for the user.
ssl — Activates SSL encryption.
user "<username>" — Replace <username> with the username used by Fetchmail to retrieve messages. This option must precede all other user options.

11.3.3.5. Fetchmail Command Options

Most Fetchmail options used on the command line when executing the fetchmail command mirror the .fetchmailrc configuration options. In this way, Fetchmail may be used with or without a configuration file. These options are not used on the command line by most users because it is easier to leave them in the .fetchmailrc file.

There may be times when it is desirable to run the fetchmail command with other options for a particular purpose. It is possible to issue command options to temporarily override a .fetchmailrc setting that is causing an error, as any options specified at the command line override configuration file options.

11.3.3.6. Informational or Debugging Options

Certain options used after the fetchmail command can supply important information.

--configdump — Displays every possible option based on information from .fetchmailrc and Fetchmail defaults. No email is retrieved for any users when using this option.
-s — Executes Fetchmail in silent mode, preventing any messages, other than errors, from appearing after the fetchmail command.
-v — Executes Fetchmail in verbose mode, displaying every communication between Fetchmail and remote email servers.
-V — Displays detailed version information, lists its global options, and shows settings to be used with each user, including the email protocol and authentication method. No email is retrieved for any users when using this option.

11.3.3.7. Special Options

These options are occasionally useful for overriding defaults often found in the .fetchmailrc file.

-a — Fetchmail downloads all messages from the remote email server, whether new or previously viewed. By default, Fetchmail only downloads new messages.
-k — Fetchmail leaves the messages on the remote email server after downloading them. This option overrides the default behavior of deleting messages after downloading them.
-l <max-number-bytes> — Fetchmail does not download any messages over a particular size and leaves them on the remote email server.
--quit — Quits the Fetchmail daemon process.

More commands and .fetchmailrc options can be found in the fetchmail man page.

11.4. Mail Delivery Agents

Red Hat Enterprise Linux includes two primary MDAs, Procmail and mail. Both of the applications are considered LDAs and both move email from the MTA's spool file into the user's mailbox. However, Procmail provides a robust filtering system.

This section details only Procmail. For information on the mail command, consult its man page.

Procmail delivers and filters email as it is placed in the mail spool file of the localhost. It is powerful, gentle on system resources, and widely used. Procmail can play a critical role in delivering email to be read by email client applications.

Procmail can be invoked in several different ways. Whenever an MTA places an email into the mail spool file, Procmail is launched. Procmail then filters and files the email for the MUA and quits. Alternatively, the MUA can be configured to execute Procmail any time a message is received so that messages are moved into their correct mailboxes. By default, the presence of /etc/procmailrc or of a .procmailrc file (also called an rc file) in the user's home directory invokes Procmail whenever an MTA receives a new message.

Whether Procmail acts upon an email message depends upon whether the message matches a specified set of conditions or recipes in the rc file. If a message matches a recipe, then the email is placed in a specified file, is deleted, or is otherwise processed.

When Procmail starts, it reads the email message and separates the body from the header information. Next, Procmail looks for /etc/procmailrc and rc files in the /etc/procmailrcs directory for default, system-wide, Procmail environmental variables and recipes. Procmail then searches for a .procmailrc file in the user's home directory. Many users also create additional rc files for Procmail that are referred to within the .procmailrc file in their home directory.

By default, no system-wide rc files exist in the /etc/ directory and no .procmailrc files exist in any user's home directory. Therefore, to use Procmail, each user must construct a .procmailrc file with specific environment variables and rules.

11.4.1. Procmail Configuration

The Procmail configuration file contains important environmental variables. These variables specify things such as which messages to sort and what to do with the messages that do not match any recipes.

These environmental variables usually appear at the beginning of .procmailrc in the following format:

<env-variable>="<value>"

In this example, <env-variable> is the name of the variable and <value> defines the variable.

There are many environment variables not used by most Procmail users and many of the more important environment variables are already defined by a default value. Most of the time, the following variables are used:

DEFAULT — Sets the default mailbox where messages that do not match any recipes are placed.
The default DEFAULT value is the same as $ORGMAIL.
INCLUDERC — Specifies additional rc files containing more recipes for messages to be checked against. This breaks up the Procmail recipe lists into individual files that fulfill different roles, such as blocking spam and managing email lists, that can then be turned off or on by using comment characters in the user's .procmailrc file.
For example, lines in a user's .procmailrc file may look like this:
```
MAILDIR=$HOME/Msgs
INCLUDERC=$MAILDIR/lists.rc
INCLUDERC=$MAILDIR/spam.rc
```
If the user wants to turn off Procmail filtering of their email lists but leave spam control in place, they would comment out the first INCLUDERC line with a hash mark character (#).
LOCKSLEEP — Sets the amount of time, in seconds, between attempts by Procmail to use a particular lockfile. The default is eight seconds.
LOCKTIMEOUT — Sets the amount of time, in seconds, that must pass after a lockfile was last modified before Procmail assumes that the lockfile is old and can be deleted. The default is 1024 seconds.
LOGFILE — The file to which any Procmail information or error messages are written.
MAILDIR — Sets the current working directory for Procmail. If set, all other Procmail paths are relative to this directory.
ORGMAIL — Specifies the original mailbox, or another place to put the messages if they cannot be placed in the default or recipe-required location.
By default, a value of /var/spool/mail/$LOGNAME is used.
SUSPEND — Sets the amount of time, in seconds, that Procmail pauses if a necessary resource, such as swap space, is not available.
SWITCHRC — Allows a user to specify an external file containing additional Procmail recipes, much like the INCLUDERC option, except that recipe checking is actually stopped on the referring configuration file and only the recipes on the SWITCHRC-specified file are used.
VERBOSE — Causes Procmail to log more information. This option is useful for debugging.

Other important environmental variables are pulled from the shell, such as LOGNAME, which is the login name; HOME, which is the location of the home directory; and SHELL, which is the default shell.

A comprehensive explanation of all environments variables, as well as their default values, is available in the procmailrc man page.

11.4.2. Procmail Recipes

New users often find the construction of recipes the most difficult part of learning to use Procmail. To some extent, this is understandable, as recipes do their message matching using regular expressions, which is a particular format used to specify qualifications for a matching string. However, regular expressions are not very difficult to construct and even less difficult to understand when read. Additionally, the consistency of the way Procmail recipes are written, regardless of regular expressions, makes it easy to learn by example. To see example Procmail recipes, refer to Section 11.4.2.5, “Recipe Examples”.

Procmail recipes take the following form:

:0<flags>: <lockfile-name>

* <special-condition-character> <condition-1>
* <special-condition-character> <condition-2>
* <special-condition-character> <condition-N>

<special-action-character><action-to-perform>

The first two characters in a Procmail recipe are a colon and a zero. Various flags can be placed after the zero to control how Procmail processes the recipe. A colon after the <flags> section specifies that a lockfile is created for this message. If a lockfile is created, the name can be specified by replacing <lockfile-name>.

A recipe can contain several conditions to match against the message. If it has no conditions, every message matches the recipe. Regular expressions are placed in some conditions to facilitate message matching. If multiple conditions are used, they must all match for the action to be performed. Conditions are checked based on the flags set in the recipe's first line. Optional special characters placed after the * character can further control the condition.

The <action-to-perform> specifies the action taken when the message matches one of the conditions. There can only be one action per recipe. In many cases, the name of a mailbox is used here to direct matching messages into that file, effectively sorting the email. Special action characters may also be used before the action is specified. Refer to Section 11.4.2.4, “Special Conditions and Actions” for more information.

11.4.2.1. Delivering vs. Non-Delivering Recipes

The action used if the recipe matches a particular message determines whether it is considered a delivering or non-delivering recipe. A delivering recipe contains an action that writes the message to a file, sends the message to another program, or forwards the message to another email address. A non-delivering recipe covers any other actions, such as a nesting block. A nesting block is a set of actions, contained in braces { }, that are performed on messages which match the recipe's conditions. Nesting blocks can be nested inside one another, providing greater control for identifying and performing actions on messages.

When messages match a delivering recipe, Procmail performs the specified action and stops comparing the message against any other recipes. Messages that match non-delivering recipes continue to be compared against other recipes.

11.4.2.2. Flags

Flags are essential to determine how or if a recipe's conditions are compared to a message. The following flags are commonly used:

A — Specifies that this recipe is only used if the previous recipe without an A or a flag also matched this message.
a — Specifies that this recipe is only used if the previous recipe with an A or a flag also matched this message and was successfully completed.
B — Parses the body of the message and looks for matching conditions.
b — Uses the body in any resulting action, such as writing the message to a file or forwarding it. This is the default behavior.
c — Generates a carbon copy of the email. This is useful with delivering recipes, since the required action can be performed on the message and a copy of the message can continue being processed in the rc files.
D — Makes the egrep comparison case-sensitive. By default, the comparison process is not case-sensitive.
E — While similar to the A flag, the conditions in the recipe are only compared to the message if the immediately preceding the recipe without an E flag did not match. This is comparable to an else action.
e — The recipe is compared to the message only if the action specified in the immediately preceding recipe fails.
f — Uses the pipe as a filter.
H — Parses the header of the message and looks for matching conditions. This occurs by default.
h — Uses the header in a resulting action. This is the default behavior.
w — Tells Procmail to wait for the specified filter or program to finish, and reports whether or not it was successful before considering the message filtered.
W — Is identical to w except that "Program failure" messages are suppressed.

For a detailed list of additional flags, refer to the procmailrc man page.

11.4.2.3. Specifying a Local Lockfile

Lockfiles are very useful with Procmail to ensure that more than one process does not try to alter a message simultaneously. Specify a local lockfile by placing a colon (:) after any flags on a recipe's first line. This creates a local lockfile based on the destination file name plus whatever has been set in the LOCKEXT global environment variable.

Alternatively, specify the name of the local lockfile to be used with this recipe after the colon.

11.4.2.4. Special Conditions and Actions

Special characters used before Procmail recipe conditions and actions change the way they are interpreted.

The following characters may be used after the * character at the beginning of a recipe's condition line:

! — In the condition line, this character inverts the condition, causing a match to occur only if the condition does not match the message.
< — Checks if the message is under a specified number of bytes.
> — Checks if the message is over a specified number of bytes.

The following characters are used to perform special actions:

! — In the action line, this character tells Procmail to forward the message to the specified email addresses.
$ — Refers to a variable set earlier in the rc file. This is often used to set a common mailbox that is referred to by various recipes.
| — Starts a specified program to process the message.
{ and } — Constructs a nesting block, used to contain additional recipes to apply to matching messages.

If no special character is used at the beginning of the action line, Procmail assumes that the action line is specifying the mailbox in which to write the message.

11.4.2.5. Recipe Examples

Procmail is an extremely flexible program, but as a result of this flexibility, composing Procmail recipes from scratch can be difficult for new users.

The best way to develop the skills to build Procmail recipe conditions stems from a strong understanding of regular expressions combined with looking at many examples built by others. A thorough explanation of regular expressions is beyond the scope of this section. The structure of Procmail recipes and useful sample Procmail recipes can be found at various places on the Internet (such as http://www.iki.fi/era/procmail/links.html). The proper use and adaptation of regular expressions can be derived by viewing these recipe examples. In addition, introductory information about basic regular expression rules can be found in the grep man page.

The following simple examples demonstrate the basic structure of Procmail recipes and can provide the foundation for more intricate constructions.

A basic recipe may not even contain conditions, as is illustrated in the following example:

:0:
new-mail.spool

The first line specifies that a local lockfile is to be created but does not specify a name, so Procmail uses the destination file name and appends the value specified in the LOCKEXT environment variable. No condition is specified, so every message matches this recipe and is placed in the single spool file called new-mail.spool, located within the directory specified by the MAILDIR environment variable. An MUA can then view messages in this file.

A basic recipe, such as this, can be placed at the end of all rc files to direct messages to a default location.

The following example matched messages from a specific email address and throws them away.

:0
* ^From: spammer@domain.com
/dev/null

With this example, any messages sent by spammer@domain.com are sent to the /dev/null device, deleting them.

Warning

Be certain that rules are working as intended before sending messages to /dev/null for permanent deletion. If a recipe inadvertently catches unintended messages, and those messages disappear, it becomes difficult to troubleshoot the rule.

A better solution is to point the recipe's action to a special mailbox, which can be checked from time to time to look for false positives. Once satisfied that no messages are accidentally being matched, delete the mailbox and direct the action to send the messages to /dev/null.

The following recipe grabs email sent from a particular mailing list and places it in a specified folder.

:0:
* ^(From|CC|To).*tux-lug
tuxlug

Any messages sent from the tux-lug@domain.com mailing list are placed in the tuxlug mailbox automatically for the MUA. Note that the condition in this example matches the message if it has the mailing list's email address on the From, CC, or To lines.

Consult the many Procmail online resources available in Section 11.6, “Additional Resources” for more detailed and powerful recipes.

11.4.2.6. Spam Filters

Because it is called by Sendmail, Postfix, and Fetchmail upon receiving new emails, Procmail can be used as a powerful tool for combating spam.

This is particularly true when Procmail is used in conjunction with SpamAssassin. When used together, these two applications can quickly identify spam emails, and sort or destroy them.

SpamAssassin uses header analysis, text analysis, blacklists, a spam-tracking database, and self-learning Bayesian spam analysis to quickly and accurately identify and tag spam.

The easiest way for a local user to use SpamAssassin is to place the following line near the top of the ~/.procmailrc file:

INCLUDERC=/etc/mail/spamassassin/spamassassin-default.rc

The /etc/mail/spamassassin/spamassassin-default.rc contains a simple Procmail rule that activates SpamAssassin for all incoming email. If an email is determined to be spam, it is tagged in the header as such and the title is prepended with the following pattern:

*****SPAM*****

The message body of the email is also prepended with a running tally of what elements caused it to be diagnosed as spam.

To file email tagged as spam, a rule similar to the following can be used:

:0 Hw
* ^X-Spam-Status: Yes
spam

This rule files all email tagged in the header as spam into a mailbox called spam.

Since SpamAssassin is a Perl script, it may be necessary on busy servers to use the binary SpamAssassin daemon (spamd) and client application (spamc). Configuring SpamAssassin this way, however, requires root access to the host.

To start the spamd daemon, type the following command as root:

service spamassassin start

To start the SpamAssassin daemon when the system is booted, use an initscript utility, such as the Services Configuration Tool (system-config-services), to turn on the spamassassin service. Refer to Section 1.4.2, “Runlevel Utilities” for more information about initscript utilities.

To configure Procmail to use the SpamAssassin client application instead of the Perl script, place the following line near the top of the ~/.procmailrc file. For a system-wide configuration, place it in /etc/procmailrc:

INCLUDERC=/etc/mail/spamassassin/spamassassin-spamc.rc

11.5. Mail User Agents

There are scores of mail programs available under Red Hat Enterprise Linux. There are full-featured, graphical email client programs, such as Mozilla Mail or Ximian Evolution, as well as text-based email programs such as mutt.

The remainder of this section focuses on securing communication between the client and server.

11.5.1. Securing Communication

Popular MUAs included with Red Hat Enterprise Linux, such as Mozilla Mail, Ximian Evolution, and mutt offer SSL-encrypted email sessions.

Like any other service that flows over a network unencrypted, important email information, such as usernames, passwords, and entire messages, may be intercepted and viewed by users on the network. Additionally, since the standard POP and IMAP protocols pass authentication information unencrypted, it is possible for an attacker to gain access to user accounts by collecting usernames and passwords as they are passed over the network.

11.5.1.1. Secure Email Clients

Most Linux MUAs designed to check email on remote servers support SSL encryption. To use SSL when retrieving email, it must be enabled on both the email client and server.

SSL is easy to enable on the client-side, often done with the click of a button in the MUA's configuration window or via an option in the MUA's configuration file. Secure IMAP and POP have known port numbers (993 and 995, respectively) that the MUA uses to authenticate and download messages.

11.5.1.2. Securing Email Client Communications

Offering SSL encryption to IMAP and POP users on the email server is a simple matter.

First, create an SSL certificate. This can be done two ways: by applying to a Certificate Authority (CA) for an SSL certificate or by creating a self-signed certificate.

Warning

Self-signed certificates should be used for testing purposes only. Any server used in a production environment should use an SSL certificate granted by a CA.

To create a self-signed SSL certificate for IMAP, change to the /usr/share/ssl/certs/ directory and type the following commands as root:

rm -f imapd.pem
make imapd.pem

Answer all of the questions to complete the process.

To create a self-signed SSL certificate for POP, change to the /usr/share/ssl/certs/ directory, and type the following commands as root:

rm -f ipop3d.pem
make ipop3d.pem

Again, answer all of the questions to complete the process.

Important

Please be sure to remove the default imapd.pem and ipop3d.pem files before issuing each make command.

Once finished, execute the /sbin/service xinetd restart command to restart the xinetd daemon which controls imapd and ipop3d.

Alternatively, the stunnel command can be used as an SSL encryption wrapper around the standard, non-secure daemons, imapd or pop3d.

The stunnel program uses external OpenSSL libraries included with Red Hat Enterprise Linux to provide strong cryptography and protect the connections. It is best to apply to a CA to obtain an SSL certificate, but it is also possible to create a self-signed certificate.

To create a self-signed SSL certificate, change to the /usr/share/ssl/certs/ directory, and type the following command:

make stunnel.pem

Again, answer all of the questions to complete the process.

Once the certificate is generated, it is possible to use the stunnel command to start the imapd mail daemon using the following command:

/usr/sbin/stunnel -d 993 -l /usr/sbin/imapd imapd

Once this command is issued, it is possible to open an IMAP email client and connect to the email server using SSL encryption.

To start the pop3d using the stunnel command, type the following command:

/usr/sbin/stunnel -d 995 -l /usr/sbin/pop3d pop3d

For more information about how to use stunnel, read the stunnel man page or refer to the documents in the /usr/share/doc/stunnel-<version-number>/ directory, where <version-number> is the version number for stunnel.

11.6. Additional Resources

The following is a list of additional documentation about email applications.

11.6.1. Installed Documentation

Information on configuring Sendmail is included with the sendmail and sendmail-cf packages.
- /usr/share/sendmail-cf/README.cf — Contains information on m4, file locations for Sendmail, supported mailers, how to access enhanced features, and more.
In addition, the sendmail and aliases man pages contain helpful information covering various Sendmail options and the proper configuration of the Sendmail /etc/mail/aliases file.
/usr/share/doc/postfix-<version-number> — Contains a large amount of information about ways to configure Postfix. Replace <version-number> with the version number of Postfix.
/usr/share/doc/fetchmail-<version-number> — Contains a full list of Fetchmail features in the FEATURES file and an introductory FAQ document. Replace <version-number> with the version number of Fetchmail.
/usr/share/doc/procmail-<version-number> — Contains a README file that provides an overview of Procmail, a FEATURES file that explores every program feature, and an FAQ file with answers to many common configuration questions. Replace <version-number> with the version number of Procmail.
When learning how Procmail works and creating new recipes, the following Procmail man pages are invaluable:
- procmail — Provides an overview of how Procmail works and the steps involved with filtering email.
- procmailrc — Explains the rc file format used to construct recipes.
- procmailex — Gives a number of useful, real-world examples of Procmail recipes.
- procmailsc — Explains the weighted scoring technique used by Procmail to match a particular recipe to a message.
- /usr/share/doc/spamassassin-<version-number>/ — Contains a large amount of information pertaining to SpamAssassin. Replace <version-number> with the version number of the spamassassin package.

11.6.2. Useful Websites

http://www.redhat.com/mirrors/LDP/HOWTO/Mail-Administrator-HOWTO.html — Provides an overview of how email works, and examines possible email solutions and configurations on the client and server sides.
http://www.redhat.com/mirrors/LDP/HOWTO/Mail-User-HOWTO/ — Looks at email from the user's perspective, investigates various popular email client applications and gives an introduction to topics such as aliases, forwarding, auto-replying, mailing lists, mail filters, and spam.
http://www.redhat.com/mirrors/LDP/HOWTO/mini/Secure-POP+SSH.html — Demonstrates a way to retrieve POP email using SSH with port forwarding, so that the email passwords and messages are transferred securely.
http://www.sendmail.net/ — Contains news, interviews, and articles concerning Sendmail, including an expanded view of the many options available.
http://www.sendmail.org/ — Offers a thorough technical breakdown of Sendmail features and configuration examples.
http://www.postfix.org/ — The Postfix project home page contains a wealth of information about Postfix. The mailing list is a particularly good place to look for information.
http://catb.org/~esr/fetchmail/ — The home page for Fetchmail, featuring an online manual, and a thorough FAQ.
http://www.procmail.org/ — The home page for Procmail with links to assorted mailing lists dedicated to Procmail as well as various FAQ documents.
http://www.ling.helsinki.fi/users/reriksso/procmail/mini-faq.html — An excellent Procmail FAQ, offers troubleshooting tips, details about file locking, and the use of wildcard characters.
http://www.uwasa.fi/~ts/info/proctips.html — Contains dozens of tips that make using Procmail much easier. Includes instructions on how to test .procmailrc files and use Procmail scoring to decide if a particular action should be taken.
http://www.spamassassin.org/ — The official site of the SpamAssassin project.

Chapter 12. Berkeley Internet Name Domain (BIND)

On most modern networks, including the Internet, users locate other computers by name. This frees users from the daunting task of remembering the numerical network address of network resources. The most effective way to configure a network to allow such name-based connections is to set up a Domain Name Service (DNS) or a nameserver, which resolves hostnames on the network to numerical addresses and vice versa.

This chapter reviews the nameserver included in Red Hat Enterprise Linux, the Berkeley Internet Name Domain (BIND) DNS server, with an emphasis on the structure of its configuration files and how it may be administered both locally and remotely.

12.1. Introduction to DNS

When hosts on a network connect to one another via a hostname, also called a fully qualified domain name (FQDN), DNS is used to associate the names of machines to the IP address for the host.

Use of DNS and FQDNs also has advantages for system administrators, allowing the flexibility to change the IP address for a host without effecting name-based queries to the machine. Conversely, administrators can shuffle which machines handle a name-based query.

DNS is normally implemented using centralized servers that are authoritative for some domains and refer to other DNS servers for other domains.

When a client host requests information from a nameserver, it usually connects to port 53. The nameserver then attempts to resolve the FQDN based on its resolver library, which may contain authoritative information about the host requested or cached data from an earlier query. If the nameserver does not already have the answer in its resolver library, it queries other nameservers, called root nameservers, to determine which nameservers are authoritative for the FQDN in question. Then, with that information, it queries the authoritative nameservers to determine the IP address of the requested host. If performing a reverse lookup, the same procedure is used, except the query is made with an unknown IP address rather than a name.

12.1.1. Nameserver Zones

On the Internet, the FQDN of a host can be broken down into different sections. These sections are organized into a hierarchy (much like a tree), with a main trunk, primary branches, secondary branches, and so forth. Consider the following FQDN:

bob.sales.example.com

When looking at how an FQDN is resolved to find the IP address that relates to a particular system, read the name from right to left, with each level of the hierarchy divided by periods (.). In this example, com defines the top level domain for this FQDN. The name example is a sub-domain under com, while sales is a sub-domain under example. The name furthest to the left, bob, identifies a specific machine hostname.

Except for the hostname, each section is a called a zone, which defines a specific namespace. A namespace controls the naming of the sub-domains to its left. While this example only contains two sub-domains, a FQDN must contain at least one sub-domain but may include many more, depending upon how the namespace is organized.

Zones are defined on authoritative nameservers through the use of zone files, which describe the namespace of that zone, the mail servers to be used for a particular domain or sub-domain, and more. Zone files are stored on primary nameservers (also called master nameservers), which are truly authoritative and where changes are made to the files, and secondary nameservers (also called slave nameservers), which receive their zone files from the primary nameservers. Any nameserver can be a primary and secondary nameserver for different zones at the same time, and they may also be considered authoritative for multiple zones. It all depends on how the nameserver is configured.

12.1.2. Nameserver Types

There are four primary nameserver configuration types:

master — Stores original and authoritative zone records for a namespace, and answers queries about the namespace from other nameservers.
slave — Answers queries from other nameservers concerning namespaces for which it is considered an authority. However, slave nameservers get their namespace information from master nameservers.
caching-only — Offers name to IP resolution services but is not authoritative for any zones. Answers for all resolutions are cached in memory for a fixed period of time, which is specified by the retrieved zone record.
forwarding — Forwards requests to a specific list of nameservers for name resolution. If none of the specified nameservers can perform the resolution, the resolution fails.

A nameserver may be one or more of these types. For example, a nameserver can be a master for some zones, a slave for others, and only offer forwarding resolutions for others.

12.1.3. BIND as a Nameserver

BIND performs name resolution services through the /usr/sbin/named daemon. BIND also includes an administration utility called /usr/sbin/rndc. More information about rndc can be found in Section 12.4, “Using rndc”.

BIND stores its configuration files in the following locations:

/etc/named.conf — The configuration file for the named daemon.
/var/named/ directory — The named working directory which stores zone, statistic, and cache files.

The next few sections review the BIND configuration files in more detail.

12.2. `/etc/named.conf`

The named.conf file is a collection of statements using nested options surrounded by opening and closing ellipse characters, { }. Administrators must be careful when editing named.conf to avoid syntactical errors as many seemingly minor errors prevent the named service from starting.

A typical named.conf file is organized similar to the following example:

<statement-1> ["<statement-1-name>"] [<statement-1-class>] {
  <option-1>;
  <option-2>;
  <option-N>;
};

<statement-2> ["<statement-2-name>"] [<statement-2-class>] {
  <option-1>;
  <option-2>;
  <option-N>;
};

<statement-N> ["<statement-N-name>"] [<statement-N-class>] {
  <option-1>;
  <option-2>;
  <option-N>;
};

12.2.1. Common Statement Types

The following types of statements are commonly used in /etc/named.conf:

12.2.1.1. `acl` Statement

The acl statement (or access control statement) defines groups of hosts which can then be permitted or denied access to the nameserver.

An acl statement takes the following form:

acl <acl-name> {
  <match-element>;
  [<match-element>; ...]
};

In this statement, replace <acl-name> with the name of the access control list and replace <match-element> with a semi-colon separated list of IP addresses. Most of the time, an individual IP address or IP network notation (such as 10.0.1.0/24) is used to identify the IP addresses within the acl statement.

The following access control lists are already defined as keywords to simplify configuration:

any — Matches every IP address.
localhost — Matches any IP address in use by the local system.
localnets — Matches any IP address on any network to which the local system is connected.
none — Matches no IP addresses.

When used in conjunction with other statements (such as the options statement), acl statements can be very useful in preventing the misuse of a BIND nameserver.

The following example defines two access control lists and uses an options statement to define how they are treated by the nameserver:

acl black-hats {
  10.0.2.0/24;
  192.168.0.0/24;
};

acl red-hats {
  10.0.1.0/24;
};

options {
  blackhole { black-hats; };
  allow-query { red-hats; };
  allow-recursion { red-hats; };
};

This example contains two access control lists, black-hats and red-hats. Hosts in the black-hats list are denied access to the nameserver, while hosts in the red-hats list are given normal access.

12.2.1.2. `include` Statement

The include statement allows files to be included in a named.conf file. In this way, sensitive configuration data (such as keys) can be placed in a separate file with restrictive permissions.

An include statement takes the following form:

include  "<file-name>"

In this statement, <file-name> is replaced with an absolute path to a file.

12.2.1.3. `options` Statement

The options statement defines global server configuration options and sets defaults for other statements. It can be used to specify the location of the named working directory, the types of queries allowed, and much more.

The options statement takes the following form:

options { 
  <option>;
	[<option>; ...] 
};

In this statement, the <option> directives are replaced with a valid option.

The following are commonly used options:

allow-query — Specifies which hosts are allowed to query this nameserver. By default, all hosts are allowed to query. An access control list, or collection of IP addresses or networks may be used here to only allow particular hosts to query the nameserver.
allow-recursion — Similar to allow-query, this option applies to recursive queries. By default, all hosts are allowed to perform recursive queries on the nameserver.
blackhole — Specifies which hosts are not allowed to query the server.
directory — Specifies the named working directory if different from the default value, /var/named/.
forward — Specifies the forwarding behavior of a forwarders directive.
The following options are accepted:
- first — Specifies that the nameservers listed in the forwarders directive be queried before named attempts to resolve the name itself.
- only — Specifies that named does not attempt name resolution itself in the event queries to nameservers specified in the forwarders directive fail.
forwarders — Specifies a list of valid IP addresses for nameservers where requests should be forwarded for resolution.
listen-on — Specifies the network interface on which named listens for queries. By default, all interfaces are used.
Using this directive on a DNS server which also acts a gateway, BIND can be configured to only answer queries that originate from one of the networks.
A listen-on directive looks like the following example:
```
options {
  listen-on { 10.0.1.1; };
};
```
In this example, only requests that arrive from the network interface serving the private network (10.0.1.1) are accepted.
notify — Controls whether named notifies the slave servers when a zone is updated. It accepts the following options:
- yes — Notifies slave servers.
- no — Does not notify slave servers.
- explicit — Only notifies slave servers specified in an also-notify list within a zone statement.
pid-file — Specifies the location of the process ID file created by named.
root-delegation-only — Turns on the enforcement of delegation properties in top-level domains (TLDs) and root zones with an optional exclude list. Delegation is the process of separating a single zone into multiple subzones. In order to create a delegated zone, items known as NS records are used. NameServer records (delegation records) announce the authoritative nameservers for a particular zone.
The following root-delegation-only example specifies an exclude list of TLDs from whom undelegated responses are expected and trusted:
```
options {
  root-delegation-only exclude { "ad"; "ar"; "biz"; "cr"; "cu"; "de"; "dm"; "id;
    "lu"; "lv"; "md"; "ms"; "museum"; "name"; "no"; "pa"; 
    "pf"; "se"; "sr"; "to"; "tw"; "us"; "uy"; };
};
```
statistics-file — Specifies an alternate location for statistics files. By default, named statistics are saved to the /var/named/named.stats file.

Dozens of other options are also available, many of which rely upon one another to work properly. Refer to the BIND 9 Administrator Reference Manual referenced in Section 12.7.1, “Installed Documentation” and the bind.conf man page for more details.

12.2.1.4. `zone` Statement

A zone statement defines the characteristics of a zone such as the location of its configuration file and zone-specific options. This statement can be used to override the global options statements.

A zone statement takes the following form:

zone <zone-name> <zone-class> {
  <zone-options>;
  [<zone-options>; ...]
};

In this statement, <zone-name> is the name of the zone, <zone-class> is the optional class of the zone, and <zone-options> is a list of options characterizing the zone.

The <zone-name> attribute for the zone statement is particularly important. It is the default value assigned for the $ORIGIN directive used within the corresponding zone file located in the /var/named/ directory. The named daemon appends the name of the zone to any non-fully qualified domain name listed in the zone file.

For example, if a zone statement defines the namespace for example.com, use example.com as the <zone-name> so it is placed at the end of hostnames within the example.com zone file.

For more information about zone files, see Section 12.3, “Zone Files”.

The most common zone statement options include the following:

allow-query — Specifies the clients that are allowed to request information about this zone. The default is to allow all query requests.
allow-transfer — Specifies the slave servers that are allowed to request a transfer of the zone's information. The default is to allow all transfer requests.
allow-update — Specifies the hosts that are allowed to dynamically update information in their zone. The default is to deny all dynamic update requests.
Be careful when allowing hosts to update information about their zone. Do not enable this option unless the host specified is completely trusted. In general, it better to have an administrator manually update the records for a zone and reload the named service.
file — Specifies the name of the file in the named working directory that contains the zone's configuration data.
masters — Specifies the IP addresses from which to request authoritative zone information and is used only if the zone is defined as type slave.
notify — Specifies whether or not named notifies the slave servers when a zone is updated. This directive accepts the following options:
- yes — Notifies slave servers.
- no — Does not notify slave servers.
- explicit — Only notifies slave servers specified in an also-notify list within a zone statement.
type — Defines the type of zone.
Below is a list of valid options:
- delegation-only — Enforces the delegation status of infrastructure zones such as COM, NET, or ORG. Any answer that is received without an explicit or implicit delegation is treated as NXDOMAIN. This option is only applicable in TLDs or root zone files used in recursive or caching implementations.
- forward — Forwards all requests for information about this zone to other nameservers.
- hint — A special type of zone used to point to the root nameservers which resolve queries when a zone is not otherwise known. No configuration beyond the default is necessary with a hint zone.
- master — Designates the nameserver as authoritative for this zone. A zone should be set as the master if the zone's configuration files reside on the system.
- slave — Designates the nameserver as a slave server for this zone. Also specifies the IP address of the master nameserver for the zone.
zone-statistics — Configures named to keep statistics concerning this zone, writing them to either the default location (/var/named/named.stats) or the file listed in the statistics-file option in the server statement. Refer to Section 12.2.2, “Other Statement Types” for more information about the server statement.

12.2.1.5. Sample `zone` Statements

Most changes to the /etc/named.conf file of a master or slave nameserver involves adding, modifying, or deleting zone statements. While these zone statements can contain many options, most nameservers require only a small subset to function efficiently. The following zone statements are very basic examples illustrating a master-slave nameserver relationship.

The following is an example of a zone statement for the primary nameserver hosting example.com (192.168.0.1):

zone "example.com" IN {
  type master;
  file "example.com.zone";
  allow-update { none; };
};

In the statement, the zone is identified as example.com, the type is set to master, and the named service is instructed to read the /var/named/example.com.zone file. It also tells named not to allow any other hosts to update.

A slave server's zone statement for example.com is slightly different from the previous example. For a slave server, the type is set to slave and in place of the allow-update line is a directive telling named the IP address of the master server.

The following is an example slave server zone statement for example.com zone:

zone "example.com" {
  type slave;
  file "example.com.zone";
  masters { 192.168.0.1; };
};

This zone statement configures named on the slave server to query the master server at the 192.168.0.1 IP address for information about the example.com zone. The information the slave server receives from the master server is saved to the /var/named/example.com.zone file.

12.2.2. Other Statement Types

The following is a list of lesser used statement types available within named.conf:

controls — Configures various security requirements necessary to use the rndc command to administer the named service.
Refer to Section 12.4.1, “Configuring /etc/named.conf” to learn more about how the controls statement is structured and available options.
key "<key-name>" — Defines a particular key by name. Keys are used to authenticate various actions, such as secure updates or the use of the rndc command. Two options are used with key:
- algorithm <algorithm-name> — The type of algorithm used, such as dsa or hmac-md5.
- secret "<key-value>" — The encrypted key.
Refer to Section 12.4.2, “Configuring /etc/rndc.conf” for instructions on how to write a key statement.
logging — Allows for the use of multiple types of logs, called channels. By using the channel option within the logging statement, a customized type of log, with its own file name (file), size limit (size), versioning (version), and level of importance (severity), can be constructed. Once a customized channel has been defined, a category option is used to categorize the channel and begin logging when named is restarted.
By default, named logs standard messages to the syslog daemon, which places them in /var/log/messages. This occurs because several standard channels are built into BIND with various severity levels, such as one that handles informational logging messages (default_syslog) and another that specifically handles debugging messages (default_debug). A default category, called default, uses the built-in channels to do normal logging without any special configuration.
Customizing the logging process can be a very detailed process and is beyond the scope of this chapter. For information on creating custom BIND logs, refer to the BIND 9 Administrator Reference Manual referenced in Section 12.7.1, “Installed Documentation”.
server — Specifies options that affect how named should respond to remote nameservers, especially in regards to notifications and zone transfers.
The transfer-format option controls whether one resource record is sent with each message (one-answer) or multiple resource records are sent with each message (many-answers). While many-answers is more efficient, only newer BIND nameservers understand it.
trusted-keys — Contains assorted public keys used for secure DNS (DNSSEC). Refer to Section 12.5.3, “Security” for more information concerning BIND security.
view "<view-name>" — Creates special views depending upon which network the host querying the nameserver is on. This allows some hosts to receive one answer regarding a zone while other hosts receive totally different information. Alternatively, certain zones may only be made available to particular trusted hosts while non-trusted hosts can only make queries for other zones.
Multiple views may be used, but their names must be unique. The match-clients option specifies the IP addresses that apply to a particular view. Any options statements may also be used within a view, overriding the global options already configured for named. Most view statements contain multiple zone statements that apply to the match-clients list. The order in which view statements are listed is important, as the first view statement that matches a particular client's IP address is used.
Refer to Section 12.5.2, “Multiple Views” for more information about the view statement.

12.2.3. Comment Tags

The following is a list of valid comment tags used within named.conf:

// — When placed at the beginning of a line, that line is ignored by named.
# — When placed at the beginning of a line, that line is ignored by named.
/* and */ — When text is enclose in these tags, the block of text is ignored by named.

12.3. Zone Files

Zone files contain information about a namespace and are stored in the named working directory, /var/named/, by default. Each zone file is named according to the file option data in the zone statement, usually in a way that relates to the domain in question and identifies the file as containing zone data, such as example.com.zone.

Each zone file may contain directives and resource records. Directives tell the nameserver to perform tasks or apply special settings to the zone. Resource records define the parameters of the zone and assign identities to individual hosts. Directives are optional, but resource records are required to provide name service to a zone.

All directives and resource records should be entered on individual lines.

Comments can be placed after semicolon characters (;) in zone files.

12.3.1. Zone File Directives

Directives begin with the dollar sign character ($) followed by the name of the directive. They usually appear at the top of the zone file.

The following are commonly used directives:

$INCLUDE — Configures named to include another zone file in this zone file at the place where the directive appears. This allows additional zone settings to be stored apart from the main zone file.
$ORIGIN — Appends the domain name to unqualified records, such as those with the hostname and nothing more.
For example, a zone file may contain the following line:
```
$ORIGIN example.com.
```
Any names used in resource records that do not end in a trailing period (.) are appended with example.com.
Note
The use of the $ORIGIN directive is unnecessary if the zone is specified in /etc/named.conf because the zone name is used as the value for the $ORIGIN directive by default.
$TTL — Sets the default Time to Live (TTL) value for the zone. This is the length of time, in seconds, a zone resource record is valid. Each resource record can contain its own TTL value, which overrides this directive.
Increasing this value allows remote nameservers to cache the zone information for a longer period of time, reducing the number of queries for the zone and lengthening the amount of time required to proliferate resource record changes.

12.3.2. Zone File Resource Records

The primary component of a zone file is its resource records.

There are many types of zone file resource records. The following are used most frequently:

A — Address record, which specifies an IP address to assign to a name, as in this example:
```
<host>     IN     A     <IP-address>
```
If the <host> value is omitted, then an A record points to a default IP address for the top of the namespace. This system is the target for all non-FQDN requests.
Consider the following A record examples for the example.com zone file:
```
             IN     A       10.0.1.3
server1      IN     A       10.0.1.5
```
Requests for example.com are pointed to 10.0.1.3, while requests for server1.example.com are pointed to 10.0.1.5.
CNAME — Canonical name record, maps one name to another. This type of record is also known as an alias record.
The next example tells named that any requests sent to the <alias-name> should point to the host, <real-name>. CNAME records are most commonly used to point to services that use a common naming scheme, such as www for Web servers.
```
<alias-name>     IN     CNAME       <real-name>
```
In the following example, an A record binds a hostname to an IP address, while a CNAME record points the commonly used www hostname to it.
```
server1      IN     A       10.0.1.5
www          IN     CNAME   server1
```
MX — Mail eXchange record, which tells where mail sent to a particular namespace controlled by this zone should go.
```
      IN     MX     <preference-value>  <email-server-name>
```
In this example, the <preference-value> allows numerical ranking of the email servers for a namespace, giving preference to some email systems over others. The MX resource record with the lowest <preference-value> is preferred over the others. However, multiple email servers can possess the same value to distribute email traffic evenly among them.
The <email-server-name> may be a hostname or FQDN.
```
      IN     MX     10     mail.example.com.
      IN     MX     20     mail2.example.com.
```
In this example, the first mail.example.com email server is preferred to the mail2.example.com email server when receiving email destined for the example.com domain.
NS — NameServer record, which announces the authoritative nameservers for a particular zone.
This is an example of an NS record:
```
      IN     NS     <nameserver-name>
```
The <nameserver-name> should be a FQDN.
Next, two nameservers are listed as authoritative for the domain. It is not important whether these nameservers are slaves or if one is a master; they are both still considered authoritative.
```
      IN     NS     dns1.example.com.
      IN     NS     dns2.example.com.
```
PTR — PoinTeR record, designed to point to another part of the namespace.
PTR records are primarily used for reverse name resolution, as they point IP addresses back to a particular name. Refer to Section 12.3.4, “Reverse Name Resolution Zone Files” for more examples of PTR records in use.
SOA — Start Of Authority resource record, proclaims important authoritative information about a namespace to the nameserver.
Located after the directives, an SOA resource record is the first resource record in a zone file.
The following example shows the basic structure of an SOA resource record:
```
@     IN     SOA    <primary-name-server>     <hostmaster-email> (
                    <serial-number>
                    <time-to-refresh>
                    <time-to-retry>
                    <time-to-expire>
                    <minimum-TTL> )
```
The @ symbol places the $ORIGIN directive (or the zone's name, if the $ORIGIN directive is not set) as the namespace being defined by this SOA resource record. The hostname of the primary nameserver that is authoritative for this domain is the <primary-name-server> directive, and the email of the person to contact about this namespace is the <hostmaster-email> directive.
The <serial-number> directive is a numerical value incremented every time the zone file is altered to indicate it is time for named to reload the zone. The <time-to-refresh> directive is the numerical value slave servers use to determine how long to wait before asking the master nameserver if any changes have been made to the zone. The <serial-number> directive is a numerical value used by the slave servers to determine if it is using outdated zone data and should therefore refresh it.
The <time-to-retry> directive is a numerical value used by slave servers to determine the length of time to wait before issuing a refresh request in the event the master nameserver is not answering. If the master has not replied to a refresh request before the amount of time specified in the <time-to-expire> directive elapses, the slave servers stop responding as an authority for requests concerning that namespace.
The <minimum-TTL> directive is the quantity of time other nameservers cache the zone's information.
When configuring BIND, all times are specified in seconds. However, it is possible to use abbreviations when specifying units of time other than seconds, such as minutes (M), hours (H), days (D), and weeks (W). The table in Table 12.1, “Seconds compared to other time units” shows an amount of time in seconds and the equivalent time in another format.
Table 12.1. Seconds compared to other time units
Seconds Other Time Units
60 1M
1800 30M
3600 1H
10800 3H
21600 6H
43200 12H
86400 1D
259200 3D
604800 1W
31536000 365D
The following example illustrates the form an SOA resource record might take when it is populated with real values.
```
@     IN     SOA    dns1.example.com.     hostmaster.example.com. (
                    2001062501 ; serial
                    21600      ; refresh after 6 hours
                    3600       ; retry after 1 hour
                    604800     ; expire after 1 week
                    86400 )    ; minimum TTL of 1 day
```

Table 12.1. Seconds compared to other time units
Seconds	Other Time Units
`60`	`1M`
`1800`	`30M`
`3600`	`1H`
`10800`	`3H`
`21600`	`6H`
`43200`	`12H`
`86400`	`1D`
`259200`	`3D`
`604800`	`1W`
`31536000`	`365D`

12.3.3. Example Zone File

Seen individually, directives and resource records can be difficult to grasp. However, when placed together in a single file, they become easier to understand.

The following example shows a very basic zone file.

$ORIGIN example.com. 
$TTL 86400 
@	IN	SOA	dns1.example.com.	hostmaster.example.com. (
			2001062501 ; serial                     
			21600      ; refresh after 6 hours                     
			3600       ; retry after 1 hour                     
			604800     ; expire after 1 week                     
			86400 )    ; minimum TTL of 1 day  
		     
		           
	IN	NS	dns1.example.com.       
	IN	NS	dns2.example.com.        
	
	
	IN	MX	10	mail.example.com.       
	IN	MX	20	mail2.example.com.        

	
dns1	IN	A	10.0.1.1
dns2	IN	A	10.0.1.2	

			       
server1	IN	A	10.0.1.5        
server2	IN	A	10.0.1.6

       
ftp	IN	A	10.0.1.3
	IN	A	10.0.1.4
	
mail	IN	CNAME	server1
mail2	IN	CNAME	server2


www	IN	CNAME	server1

In this example, standard directives and SOA values are used. The authoritative nameservers are set as dns1.example.com and dns2.example.com, which have A records that tie them to 10.0.1.2 and 10.0.1.3, respectively.

The email servers configured with the MX records point to server1 and server2 via CNAME records. Since the server1 and server2 names do not end in a trailing period (.), the $ORIGIN domain is placed after them, expanding them to server1.example.com and server2.example.com. Through the related A resource records, their IP addresses can be determined.

FTP and Web services, available at the standard ftp.example.com and www.example.com names, are pointed at the appropriate servers using CNAME records.

12.3.4. Reverse Name Resolution Zone Files

A reverse name resolution zone file is used to translate an IP address in a particular namespace into a FQDN. It looks very similar to a standard zone file, except that PTR resource records are used to link the IP addresses to a fully qualified domain name.

A PTR record looks similar to this:

<last-IP-digit>      IN     PTR    <FQDN-of-system>

The <last-IP-digit> is the last number in an IP address which points to a particular system's FQDN.

In the follow example, IP addresses 10.0.1.20 through 10.0.1.25 are pointed to corresponding FQDNs.

$ORIGIN 1.0.10.in-addr.arpa.
$TTL 86400
@     IN     SOA    dns1.example.com.     hostmaster.example.com. (
                    2001062501 ; serial
                    21600      ; refresh after 6 hours
                    3600       ; retry after 1 hour
                    604800     ; expire after 1 week
                    86400 )    ; minimum TTL of 1 day

      IN     NS     dns1.example.com.
      IN     NS     dns2.example.com.

20    IN     PTR    alice.example.com.
21    IN     PTR    betty.example.com.
22    IN     PTR    charlie.example.com.
23    IN     PTR    doug.example.com.
24    IN     PTR    ernest.example.com.
25    IN     PTR    fanny.example.com.

This zone file would be called into service with a zone statement in the named.conf file which looks similar to the following:

zone "1.0.10.in-addr.arpa" IN {
  type master;
  file "example.com.rr.zone";
  allow-update { none; };
};

There is very little difference between this example and a standard zone statement, except for the zone name. Note that a reverse name resolution zone requires the first three blocks of the IP address reversed followed by .in-addr.arpa. This allows the single block of IP numbers used in the reverse name resolution zone file to be associated with the zone.

12.4. Using `rndc`

BIND includes a utility called rndc which allows command line administration of the named daemon from the localhost or from a remote host.

In order to prevent unauthorized access to the named daemon, BIND uses a shared secret key authentication method to grant privileges to hosts. This means an identical key must be present in both /etc/named.conf and the rndc configuration file, /etc/rndc.conf.

12.4.1. Configuring `/etc/named.conf`

In order for rndc to connect to a named service, there must a controls statement in the BIND server's /etc/named.conf file.

The controls statement, shown in the following example, allows rndc to connect from the localhost.

controls {
  inet 127.0.0.1 allow { localhost; } keys { <key-name>; };
};

This statement tells named to listen on the default TCP port 953 of the loopback address and allow rndc commands coming from the localhost, if the proper key is given. The <key-name> specifies a name in the key statement within the /etc/named.conf file. The next example illustrates a sample key statement.

key "<key-name>" {
  algorithm hmac-md5;
  secret "<key-value>";
};

In this case, the <key-value> uses the HMAC-MD5 algorithm. Use the following command to generate keys using the HMAC-MD5 algorithm:

dnssec-keygen -a hmac-md5 -b <bit-length> -n HOST <key-file-name>

A key with at least a 256-bit length is a good idea. The actual key that should be placed in the <key-value> area can be found in the <key-file-name> file generated by this command.

Warning

Because /etc/named.conf is world-readable, it is a good idea to place the key statement in a separate file, readable only by root, and then use an include statement to reference it. For example:

include "/etc/rndc.key";

12.4.2. Configuring `/etc/rndc.conf`

The key is the most important statement in /etc/rndc.conf.

key "<key-name>" {
  algorithm hmac-md5;
  secret "<key-value>";
};

The <key-name> and <key-value> should be exactly the same as their settings in /etc/named.conf.

To match the keys specified in the target server's /etc/named.conf, add the following lines to /etc/rndc.conf.

options {
  default-server  localhost;
  default-key     "<key-name>";
};

This directive sets a global default key. However, the rndc configuration file can also specify different keys for different servers, as in the following example:

server localhost {
  key  "<key-name>";
};

Warning

Make sure that only the root user can read or write to the /etc/rndc.conf file.

For more information about the /etc/rndc.conf file, refer to the rndc.conf man page.

12.4.3. Command Line Options

An rndc command takes the following form:

rndc <options> <command> <command-options>

When executing rndc on a properly configured localhost, the following commands are available:

halt — Stops the named service immediately.
querylog — Logs all queries made to this nameserver.
refresh — Refreshes the nameserver's database.
reload — Reloads the zone files but keeps all other previously cached responses. This command also allows changes to zone files without losing all stored name resolutions.
If changes only affected a specific zone, reload only that specific zone by adding the name of the zone after the reload command.
stats — Dumps the current named statistics to the /var/named/named.stats file.
stop — Stops the server gracefully, saving any dynamic update and Incremental Zone Transfers (IXFR) data before exiting.

Occasionally, it may be necessary to override the default settings in the /etc/rndc.conf file. The following options are available:

-c <configuration-file> — Specifies the alternate location of a configuration file.
-p <port-number> — Specifies a port number to use for the rndc connection other than port 953, the default.
-s <server> — Specifies a server other than the default-server listed in /etc/rndc.conf.
-y <key-name> — Specifies a key other than the default-key option in /etc/rndc.conf.

Additional information about these options can be found in the rndc man page.

12.5. Advanced Features of BIND

Most BIND implementations only use named to provide name resolution services or to act as an authority for a particular domain or sub-domain. However, BIND version 9 has a number of advanced features that allow for a more secure and efficient DNS service.

Warning

Some of these advanced features, such as DNSSEC, TSIG, and IXFR (which are defined in the following section), should only be used in network environments with nameservers that support the features. If the network environment includes non-BIND or older BIND nameservers, verify that each advanced feature is supported before attempting to use it.

All of the features mentioned are discussed in greater detail in the BIND 9 Administrator Reference Manual referenced in Section 12.7.1, “Installed Documentation”.

12.5.1. DNS Protocol Enhancements

BIND supports Incremental Zone Transfers (IXFR), where a slave nameserver only downloads the updated portions of a zone modified on a master nameserver. The standard transfer process requires that the entire zone be transferred to each slave nameserver for even the smallest change. For very popular domains with very lengthy zone files and many slave nameservers, IXFR makes the notification and update process much less resource intensive.

Note that IXFR is only available when using dynamic updating to make changes to master zone records. If manually editing zone files to make changes, Automatic Zone Transfer (AXFR) is used. More information on dynamic updating is available in the BIND 9 Administrator Reference Manual. See Section 12.7.1, “Installed Documentation” for more information.

12.5.2. Multiple Views

Through the use of the view statement in named.conf, BIND can present different information depending which network a request originates from.

This is primarily used to deny sensitive DNS entries from clients outside of the local network, while allowing queries from clients inside the local network.

The view statement uses the match-clients option to match IP addresses or entire networks and give them special options and zone data.

12.5.3. Security

BIND supports a number of different methods to protect the updating and transfer of zones, on both master and slave nameservers:

DNSSEC — Short for DNS SECurity, this feature allows for zones to be cryptographically signed with a zone key.
In this way, the information about a specific zone can be verified as coming from a nameserver that has signed it with a particular private key, as long as the recipient has that nameserver's public key.
BIND version 9 also supports the SIG(0) public/private key method of message authentication.
TSIG — Short for Transaction SIGnatures, this feature allows a transfer from master to slave only after verifying that a shared secret key exists on both nameservers.
This feature strengthens the standard IP address-based method of transfer authorization. An attacker would not only need to have access to the IP address to transfer the zone, but they would also need to know the secret key.
BIND version 9 also supports TKEY, which is another shared secret key method of authorizing zone transfers.

12.5.4. IP version 6

BIND version 9 supports name service in IP version 6 (IPv6) environments through the use of A6 zone records.

If the network environment includes both IPv4 and IPv6 hosts, use the lwresd lightweight resolver daemon on all network clients. This daemon is a very efficient, caching-only nameserver which understands the new A6 and DNAME records used under IPv6. Refer to the lwresd man page for more information.

12.6. Common Mistakes to Avoid

It is very common for beginners to make mistakes when editing BIND configuration files. Be sure to avoid the following issues:

Take care to increment the serial number when editing a zone file.
If the serial number is not incremented, the master nameserver has the correct, new information, but the slave nameservers are never notified of the change and do not attempt to refresh their data of that zone.
Be careful to use ellipses and semi-colons correctly in the /etc/named.conf file.
An omitted semi-colon or unclosed ellipse section can cause named to refuse to start.
Remember to place periods (.) in zone files after all FQDNs and omit them on hostnames.
A period at the end of a domain name denotes a fully qualified domain name. If the period is omitted, then named appends the name of the zone or the $ORIGIN value to complete it.
If a firewall is blocking connections from the named program to other nameservers, edit its configuration file.
By default, BIND version 9 uses random ports above 1024 to query other nameservers. Some firewalls, however, expect all nameservers to communicate using only port 53. To force named to use port 53, add the following line to the options statement of /etc/named.conf:
```
query-source address * port 53;
```

12.7. Additional Resources

The following sources of information provide additional resources regarding BIND.

12.7.1. Installed Documentation

BIND features a full-range of installed documentation covering many different topics, each placed in its own subject directory:
- /usr/share/doc/bind-<version-number>/ — This directory lists the most recent features. Replace <version-number> with the version of bind installed on the system.
- /usr/share/doc/bind-<version-number>/arm/ — This directory contains HTML and SGML of the BIND 9 Administrator Reference Manual, which details BIND resource requirements, how to configure different types of nameservers, perform load balancing, and other advanced topics. For most new users of BIND, this is the best place to start. Replace <version-number> with the version of bind installed on the system.
- /usr/share/doc/bind-<version-number>/draft/ — This directory contains assorted technical documents that reviews issues related to DNS service and some methods proposed to address them. Replace <version-number> with the version of bind installed on the system.
- /usr/share/doc/bind-<version-number>/misc/ — This directory contains documents designed to address specific advanced issues. Users of BIND version 8 should consult the migration document for specific changes they must make when moving to BIND 9. The options file lists all of the options implemented in BIND 9 that are used in /etc/named.conf. Replace <version-number> with the version of bind installed on the system.
- /usr/share/doc/bind-<version-number>/rfc/ — This directory privides every RFC document related to BIND. Replace <version-number> with the version of bind installed on the system.
BIND related man pages — There are a number of man pages for the various applications and configuration files involved with BIND. The following lists some of the more important man pages.
Administrative Applications
man rndc — Explains the different options available when using the rndc command to control a BIND nameserver.
Server Applications
man named — Explores assorted arguments that can be used to control the BIND nameserver daemon.
man lwresd — Describes the purpose of and options available for the lightweight resolver daemon.
Configuration Files
man named.conf — A comprehensive list of options available within the named configuration file.
man rndc.conf — A comprehensive list of options available within the rndc configuration file.

12.7.2. Useful Websites

http://www.isc.org/products/BIND/ — The home page of the BIND project containing information about current releases as well as a PDF version of the BIND 9 Administrator Reference Manual.
http://www.redhat.com/mirrors/LDP/HOWTO/DNS-HOWTO.html — Covers the use of BIND as a resolving, caching nameserver and the configuration of various zone files necessary to serve as the primary nameserver for a domain.

Chapter 13. Lightweight Directory Access Protocol (LDAP)

The Lightweight Directory Access Protocol (LDAP) is a set of open protocols used to access centrally stored information over a network. It is based on the X.500 standard for directory sharing, but is less complex and resource intensive. For this reason, LDAP is sometimes referred to as "X.500 Lite." The X.500 standard is a directory that contains hierarchical and categorized information, which could include information such as names, addresses, and phone numbers.

Like X.500, LDAP organizes information in a hierarchal manner using directories. These directories can store a variety of information and can even be used in a manner similar to the Network Information Service (NIS), enabling anyone to access their account from any machine on the LDAP enabled network.

In many cases, LDAP is used as a virtual phone directory, allowing users to easily access contact information for other users. But LDAP is more flexible than a traditional phone directory, as it is capable of referring a querent to other LDAP servers throughout the world, providing an ad-hoc global repository of information. Currently, however, LDAP is more commonly used within individual organizations, like universities, government departments, and private companies.

LDAP is a client/server system. The server can use a variety of databases to store a directory, each optimized for quick and copious read operations. When an LDAP client application connects to an LDAP server, it can either query a directory or attempt to modify it. In the event of a query, the server either answers the query locally, or it can refer the querent to an LDAP server which does have the answer. If the client application is attempting to modify information within an LDAP directory, the server verifies that the user has permission to make the change and then adds or updates the information.

This chapter refers to the configuration and use of OpenLDAP 2.0, an open source implementation of the LDAPv2 and LDAPv3 protocols.

13.1. Why Use LDAP?

The main benefit of using LDAP is that information for an entire organization can be consolidated into a central repository. For example, rather than managing user lists for each group within an organization, LDAP can be used as a central directory accessible from anywhere on the network. And because LDAP supports Secure Sockets Layer (SSL) and Transport Layer Security (TLS), sensitive data can be protected from prying eyes.

LDAP also supports a number of back-end databases in which to store directories. This allows administrators the flexibility to deploy the database best suited for the type of information the server is to disseminate. Because LDAP also has a well-defined client Application Programming Interface (API), the number of LDAP-enabled applications are numerous and increasing in quantity and quality.

13.1.1. OpenLDAP Features

OpenLDAP includes a number of important features.

LDAPv3 Support — OpenLDAP supports Simple Authentication and Security Layer (SASL), Transport Layer Security (TLS), and Secure Sockets Layer (SSL), among other improvements. Many of the changes in the protocol since LDAPv2 are designed to make LDAP more secure.
IPv6 Support — OpenLDAP supports the next generation Internet Protocol version 6.
LDAP Over IPC — OpenLDAP can communicate within a system using interprocess communication (IPC). This enhances security by eliminating the need to communicate over a network.
Updated C API — Improves the way programmers can connect to and use LDAP directory servers.
LDIFv1 Support — Provides full compliance with the LDAP Data Interchange Format (LDIF) version 1.
Enhanced Stand-Alone LDAP Server — Includes an updated access control system, thread pooling, better tools, and much more.

13.2. LDAP Terminology

Any discussion of LDAP requires a basic understanding of a set of LDAP-specific terms:

entry — A single unit within an LDAP directory. Each entry is identified by its unique Distinguished Name (DN).
attributes — Information directly associated with an entry. For example, an organization could be represented as an LDAP entry. Attributes associated with the organization might include a fax number, an address, and so on. People can also be represented as entries in an LDAP directory, with common attributes such as the person's telephone number and email address.
Some attributes are required, while other attributes are optional. An objectclass definition sets which attributes are required for each entry. Objectclass definitions are found in various schema files, located in the /etc/openldap/schema/ directory. For more information, refer to Section 13.5, “The /etc/openldap/schema/ Directory”.
LDIF — The LDAP Data Interchange Format (LDIF) is an ASCII text representation of LDAP entries. Files used for importing data to LDAP servers must be in LDIF format. An LDIF entry looks similar to the following example:
```
[<id>]
dn: <distinguished name>
<attrtype>: <attrvalue>
<attrtype>: <attrvalue>
<attrtype>: <attrvalue>
```
Each entry can contain as many <attrtype>: <attrvalue> pairs as needed. A blank line indicates the end of an entry.
Warning
All <attrtype> and <attrvalue> pairs must be defined in a corresponding schema file to use this information.
Any value enclosed within a < and a > is a variable and can be set whenever a new LDAP entry is created. This rule does not apply, however, to <id>. The <id> is a number determined by the application used to edit the entry.

13.3. OpenLDAP Daemons and Utilities

The suite of OpenLDAP libraries and tools are included within the following packages:

openldap — Contains the libraries necessary to run the OpenLDAP server and client applications.
openldap-clients — Contains command line tools for viewing and modifying directories on an LDAP server.
openldap-servers — Contains the servers and other utilities necessary to configure and run an LDAP server.

There are two servers contained in the openldap-servers package: the Standalone LDAP Daemon (/usr/sbin/slapd) and the Standalone LDAP Update Replication Daemon (/usr/sbin/slurpd).

The slapd daemon is the standalone LDAP server while the slurpd daemon is used to synchronize changes from one LDAP server to other LDAP servers on the network. The slurpd daemon is only used when dealing with multiple LDAP servers.

To perform administrative tasks, the openldap-servers package installs the following utilities into the /usr/sbin/ directory:

slapadd — Adds entries from an LDIF file to an LDAP directory. For example, the command /usr/sbin/slapadd -l ldif-input reads in the LDIF file, ldif-input, containing the new entries.
Important
Only the root user may use /usr/sbin/slapadd. However, the directory server runs as the ldap user. Therefore the directory server is unable to modify any files created by slapadd. To correct this issue, after using slapadd, type the following command:
```
chown -R ldap /var/lib/ldap
```
slapcat — Pulls entries from an LDAP directory in the default format, Sleepycat Software's Berkeley DB system, and saves them in an LDIF file. For example, the command /usr/sbin/slapcat -l ldif-output outputs an LDIF file called ldif-output containing the entries from the LDAP directory.
slapindex — Re-indexes the slapd directory based on the current content. This tool should be run whenever indexing options within /etc/openldap/slapd.conf are changed.
slappasswd — Generates an encrypted user password value for use with ldapmodify or the rootpw value in the slapd configuration file, /etc/openldap/slapd.conf. Execute the /usr/sbin/slappasswd command to create the password.

Warning

You must stop slapd by issuing the /sbin/service ldap stop command before using slapadd, slapcat or slapindex. Otherwise, the integrity of the LDAP directory is at risk.

For more information on using these utilities, refer to their respective man pages.

The openldap-clients package installs tools into /usr/bin/ which are used to add, modify, and delete entries in an LDAP directory. These tools include the following:

ldapadd — Adds entries to an LDAP directory by accepting input via a file or standard input; ldapadd is actually a hard link to ldapmodify -a.
ldapdelete — Deletes entries from an LDAP directory by accepting user input at a shell prompt or via a file.
ldapmodify — Modifies entries in an LDAP directory, accepting input via a file or standard input.
ldappasswd — Sets the password for an LDAP user.
ldapsearch — Searches for entries in an LDAP directory using a shell prompt.

With the exception of ldapsearch, each of these utilities is more easily used by referencing a file containing the changes to be made rather than typing a command for each entry to be changed within an LDAP directory. The format of such a file is outlined in the man page for each utility.

13.3.1. NSS, PAM, and LDAP

In addition to the OpenLDAP packages, Red Hat Enterprise Linux includes a package called nss_ldap, which enhances LDAP's ability to integrate into both Linux and other UNIX environments.

The nss_ldap package provides the following modules:

/lib/libnss_ldap-<glibc-version>.so
/lib/security/pam_ldap.so

The nss_ldap package provides the following modules for Itanium or AMD64 architectures:

/lib64/libnss_ldap-<glibc-version>.so
/lib64/security/pam_ldap.so

The libnss_ldap-<glibc-version>.so module allows applications to look up users, groups, hosts, and other information using an LDAP directory via glibc's Nameservice Switch (NSS) interface (replace <glibc-version> with the version of libnss_ldap in use). NSS allows applications to authenticate using LDAP in conjunction with the NIS name service and flat authentication files.

The pam_ldap module allows PAM-aware applications to authenticate users using information stored in an LDAP directory. PAM-aware applications include console login, POP and IMAP mail servers, and Samba. By deploying an LDAP server on a network, all of these applications can authenticate using the same user ID and password combination, greatly simplifying administration.

For more about configuring PAM, refer to Chapter 16, Pluggable Authentication Modules (PAM) and the PAM man pages.

13.3.2. PHP4, LDAP, and the Apache HTTP Server

Red Hat Enterprise Linux includes a package containing an LDAP module for the PHP server-side scripting language.

The php-ldap package adds LDAP support to the PHP4 HTML-embedded scripting language via the /usr/lib/php4/ldap.so module. This module allows PHP4 scripts to access information stored in an LDAP directory.

Important

The mod_authz_ldap module does not authenticate a user to an LDAP directory using an encrypted password hash. This functionality is provided by the experimental mod_auth_ldap module, which is not included with Red Hat Enterprise Linux. Refer to the Apache Software Foundation website online at http://www.apache.org/ for details on the status of this module.

13.3.3. LDAP Client Applications

There are graphical LDAP clients available which support creating and modifying directories, but they are not included with Red Hat Enterprise Linux. One such application is LDAP Browser/Editor — A Java-based tool available online at http://www.iit.edu/~gawojar/ldap/.

Most other LDAP clients access directories as read-only, using them to reference, but not alter, organization-wide information. Some examples of such applications are Sendmail, Mozilla, Gnome Meeting, and Evolution.

13.4. OpenLDAP Configuration Files

OpenLDAP configuration files are installed into the /etc/openldap/ directory. The following is a brief list highlighting the most important directories and files:

/etc/openldap/ldap.conf — This is the configuration file for all client applications which use the OpenLDAP libraries such as ldapsearch, ldapadd, Sendmail, Evolution, and Gnome Meeting.
/etc/openldap/slapd.conf — This is the configuration file for the slapd daemon. Refer to Section 13.6.1, “Editing /etc/openldap/slapd.conf” for more information file.
/etc/openldap/schema/ directory — This subdirectory contains the schema used by the slapd daemon. Refer to Section 13.5, “The /etc/openldap/schema/ Directory” for more information.

Note

If the nss_ldap package is installed, it creates a file named /etc/ldap.conf. This file is used by the PAM and NSS modules supplied by the nss_ldap package. Refer to Section 13.7, “Configuring a System to Authenticate Using OpenLDAP” for more information.

13.5. The `/etc/openldap/schema/` Directory

The /etc/openldap/schema/ directory holds LDAP definitions, previously located in the slapd.at.conf and slapd.oc.conf files. The /etc/openldap/schema/redhat/ directory holds customized schemas distributed by Red Hat for Red Hat Enterprise Linux.

All attribute syntax definitions and objectclass definitions are now located in the different schema files. The various schema files are referenced in /etc/openldap/slapd.conf using include lines, as shown in this example:

include		/etc/openldap/schema/core.schema
include		/etc/openldap/schema/cosine.schema
include		/etc/openldap/schema/inetorgperson.schema
include		/etc/openldap/schema/nis.schema
include		/etc/openldap/schema/rfc822-MailMember.schema
include		/etc/openldap/schema/redhat/autofs.schema

Warning

Do not modify schema items defined in the schema files installed by OpenLDAP.

It is possible to extend the schema used by OpenLDAP to support additional attribute types and object classes using the default schema files as a guide. To do this, create a local.schema file in the /etc/openldap/schema/ directory. Reference this new schema within slapd.conf by adding the following line below the default include schema lines:

include          /etc/openldap/schema/local.schema

Next, define new attribute types and object classes within the local.schema file. Many organizations use existing attribute types from the schema files installed by default and add new object classes to the local.schema file.

Extending the schema to match certain specialized requirements is quite involved and beyond the scope of this chapter. Refer to http://www.openldap.org/doc/admin/schema.html for information.

13.6. OpenLDAP Setup Overview

This section provides a quick overview for installing and configuring an OpenLDAP directory. For more details, refer to the following URLs:

http://www.openldap.org/doc/admin/quickstart.html — The Quick-Start Guide on the OpenLDAP website.
http://www.redhat.com/mirrors/LDP/HOWTO/LDAP-HOWTO.html — The LDAP Linux HOWTO from the Linux Documentation Project, mirrored on Red Hat's website.

The basic steps for creating an LDAP server are as follows:

Install the openldap, openldap-servers, and openldap-clients RPMs.
Edit the /etc/openldap/slapd.conf file to specify the LDAP domain and server. Refer to Section 13.6.1, “Editing /etc/openldap/slapd.conf” for more information.
Start slapd with the command:
```
service ldap start
```
After configuring LDAP, use chkconfig, /usr/sbin/ntsysv, or the Services Configuration Tool to configure LDAP to start at boot time. For more information about configuring services, refer to the chapter titled Controlling Access to Services in the System Administrators Guide.
Add entries to an LDAP directory with ldapadd.
Use ldapsearch to determine if slapd is accessing the information correctly.
At this point, the LDAP directory should be functioning properly and can be configured with LDAP-enabled applications.

13.6.1. Editing `/etc/openldap/slapd.conf`

To use the slapd LDAP server, modify its configuration file, /etc/openldap/slapd.conf, to specify the correct domain and server.

The suffix line names the domain for which the LDAP server provides information and should be changed from:

suffix          "dc=your-domain,dc=com"

so that it reflects a fully qualified domain name. For example:

suffix          "dc=example,dc=com"

The rootdn entry is the Distinguished Name (DN) for a user who is unrestricted by access controls or administrative limit parameters set for operations on the LDAP directory. The rootdn user can be thought of as the root user for the LDAP directory. In the configuration file, change the rootdn line from its default value as in the following example:

rootdn          "cn=root,dc=example,dc=com"

When populating an LDAP directory over a network, change the rootpw line — replacing the default value with an encrypted password string. To create an encrypted password string, type the following command:

slappasswd

When prompted, type and then re-type a password. The program prints the resulting encrypted password to the shell prompt.

Next, copy the newly created encrypted password into the /etc/openldap/slapd.conf on one of the rootpw lines and remove the hash mark (#).

When finished, the line should look similar to the following example:

rootpw {SSHA}vv2y+i6V6esazrIv70xSSnNAJE18bb2u

Warning

LDAP passwords, including the rootpw directive specified in /etc/openldap/slapd.conf, are sent over the network unencrypted, unless TLS encryption is enabled.

To enable TLS encryption, review the comments in /etc/openldap/slapd.conf and refer to the man page for slapd.conf.

For added security, the rootpw directive should be commented out after populating the LDAP directory by preceding it with a hash mark (#).

When using the /usr/sbin/slapadd command line tool locally to populate the LDAP directory, use of the rootpw directive is not necessary.

Important

Only the root user can use /usr/sbin/slapadd. However, the directory server runs as the ldap user. Therefore, the directory server is unable to modify any files created by slapadd. To correct this issue, after using slapadd, type the following command:

chown -R ldap /var/lib/ldap

13.7. Configuring a System to Authenticate Using OpenLDAP

This section provides a brief overview of how to configure OpenLDAP user authentication. Unless you are an OpenLDAP expert, more documentation than is provided here is necessary. Refer to the references provided in Section 13.9, “Additional Resources” for more information.

Install the Necessary LDAP Package

First, make sure that the appropriate packages are installed on both the LDAP server and the LDAP client machines. The LDAP server needs the openldap-servers package.

The openldap, openldap-clients, and nss_ldap packages need to be installed on all LDAP client machines.

Edit the Configuration Files

On the server, edit the /etc/openldap/slapd.conf file on the LDAP server to make sure it matches the specifics of the organization. Refer to Section 13.6.1, “Editing /etc/openldap/slapd.conf” for instructions about editing slapd.conf.
On the client machines, both /etc/ldap.conf and /etc/openldap/ldap.conf need to contain the proper server and search base information for the organization.
To do this, run the graphical Authentication Configuration Tool (system-config-authentication) and select Enable LDAP Support under the User Information tab.
It is also possible to edit these files by hand.
On the client machines, the /etc/nsswitch.conf must be edited to use LDAP.
To do this, run the Authentication Configuration Tool (system-config-authentication) and select Enable LDAP Support under the User Information tab.
If editing /etc/nsswitch.conf by hand, add ldap to the appropriate lines.
For example:
```
passwd: files ldap
shadow: files ldap
group: files ldap
```

13.7.1. PAM and LDAP

To have standard PAM-enabled applications use LDAP for authentication, run the Authentication Configuration Tool (system-config-authentication) and select Enable LDAP Support under the the Authentication tab. For more about configuring PAM, refer to Chapter 16, Pluggable Authentication Modules (PAM) and the PAM man pages.

13.7.2. Migrating Old Authentication Information to LDAP Format

The /usr/share/openldap/migration/ directory contains a set of shell and Perl scripts for migrating authentication information into an LDAP format.

Note

Perl must be installed on the system to use these scripts.

First, modify the migrate_common.ph file so that it reflects the correct domain. The default DNS domain should be changed from its default value to something like:

$DEFAULT_MAIL_DOMAIN = "example";

The default base should also be changed, to something like:

$DEFAULT_BASE = "dc=example,dc=com";

The job of migrating a user database into a format that is LDAP readable falls to a group of migration scripts installed in the same directory. Using Table 13.1, “LDAP Migration Scripts”, decide which script to run to migrate the user database.

Run the appropriate script based on the existing name service.

The README and the migration-tools.txt files in the /usr/share/openldap/migration/ directory provide more details on how to migrate the information.

Table 13.1. LDAP Migration Scripts
Existing name service	Is LDAP running?	Script to Use
`/etc` flat files	yes	`migrate_all_online.sh`
`/etc` flat files	no	`migrate_all_offline.sh`
NetInfo	yes	`migrate_all_netinfo_online.sh`
NetInfo	no	`migrate_all_netinfo_offline.sh`
NIS (YP)	yes	`migrate_all_nis_online.sh`
NIS (YP)	no	`migrate_all_nis_offline.sh`

13.8. Migrating Directories from Earlier Releases

With Red Hat Enterprise Linux, OpenLDAP uses Sleepycat Software's Berkeley DB system as its on-disk storage format for directories. Earlier versions of OpenLDAP used GNU Database Manager (gdbm). For this reason, before upgrading an LDAP implementation to Red Hat Enterprise Linux 4.5.0, original LDAP data should first be exported before the upgrade, and then reimported afterwards. This can be achieved by performing the following steps:

Before upgrading the operating system, run the command /usr/sbin/slapcat -l ldif-output. This outputs an LDIF file called ldif-output containing the entries from the LDAP directory.
Upgrade the operating system, being careful not to reformat the partition containing the LDIF file.
Re-import the LDAP directory to the upgraded Berkeley DB format by executing the command /usr/sbin/slapadd -l ldif-output.

13.9. Additional Resources

The following resources offer additional information on LDAP. It is highly recommended that you review these, especially the OpenLDAP website and the LDAP HOWTO, before configuring LDAP on your system(s).

13.9.1. Installed Documentation

/usr/share/docs/openldap-<versionnumber>/ directory — Contains a general README document and miscellaneous information.
LDAP related man pages — There are a number of man pages for the various applications and configuration files involved with LDAP. The following is a list of some of the more important man pages.
Client Applications
man ldapadd — Describes how to add entries to an LDAP directory.
man ldapdelete — Describes how to delete entries within an LDAP directory.
man ldapmodify — Describes how to modify entries within an LDAP directory.
man ldapsearch — Describes how to search for entries within an LDAP directory.
man ldappasswd — Describes how to set or change the password of an LDAP user.
Server Applications
man slapd — Describes command line options for the LDAP server.
man slurpd — Describes command line options for the LDAP replication server.
Administrative Applications
man slapadd — Describes command line options used to add entries to a slapd database.
man slapcat — Describes command line options used to generate an LDIF file from a slapd database.
man slapindex — Describes command line options used to regenerate an index based upon the contents of a slapd database.
man slappasswd — Describes command line options used to generate user passwords for LDAP directories.
Configuration Files
man ldap.conf — Describes the format and options available within the configuration file for LDAP clients.
man slapd.conf — Describes the format and options available within the configuration file referenced by both the LDAP server applications (slapd and slurpd) and the LDAP administrative tools (slapadd, slapcat, and slapindex).

13.9.2. Useful Websites

http://www.openldap.org/ — Home of the OpenLDAP Project. This website contains a wealth of information about configuring OpenLDAP as well as a future roadmap and version changes.
http://www.redhat.com/mirrors/LDP/HOWTO/LDAP-HOWTO.html — A comprehensive, relevant, and updated LDAP HOWTO.
http://www.padl.com/ — Developers of nss_ldap and pam_ldap, among other useful LDAP tools.
http://www.kingsmountain.com/ldapRoadmap.shtml — Jeff Hodges' LDAP Road Map contains links to several useful FAQs and emerging news concerning the LDAP protocol.
http://www.newarchitectmag.com/archives/2000/05/wilcox/ — A useful look at managing groups in LDAP.
http://www.ldapman.org/articles/ — Articles that offer a good introduction to LDAP, including methods to design a directory tree and customizing directory structures.

Chapter 14. Samba

Samba is an open source implementation of the Server Message Block (SMB) protocol. It allows the networking of Microsoft Windows®, Linux, UNIX, and other operating systems together, enabling access to Windows-based file and printer shares. Samba's use of SMB allows it to appear as a Windows server to Windows clients.

14.1. Introduction to Samba

The third major release of Samba, version 3.0.0, introduced numerous improvements from prior versions, including:

The ability to join an Active Directory domain by means of LDAP and Kerberos
Built in Unicode support for internationalization
Support for Microsoft Windows XP Professional client connections to Samba servers without needing local registry hacking
Two new documents developed by the Samba.org team, which include a 400+ page reference manual, and a 300+ page implementation and integration manual. For more information about these published titles, refer to Section 14.9.3, “Related Books”.

14.1.1. Samba Features

Samba is a powerful and versatile server application. Even seasoned system administrators must know its abilities and limitations before attempting installation and configuration.

What Samba can do:

Serve directory trees and printers to Linux, UNIX, and Windows clients
Assist in network browsing (with or without NetBIOS)
Authenticate Windows domain logins
Provide Windows Internet Name Service (WINS) name server resolution
Act as a Windows NT®-style Primary Domain Controller (PDC)
Act as a Backup Domain Controller (BDC) for a Samba-based PDC
Act as an Active Directory domain member server
Join a Windows NT/2000/2003 PDC

What Samba cannot do:

Act as a BDC for a Windows PDC (and vice versa)
Act as an Active Directory domain controller

14.2. Samba Daemons and Related Services

The following is a brief introduction to the individual Samba daemons and services, as well as details on how to start and stop them.

14.2.1. Daemon Overview

Samba is comprised of three daemons (smbd, nmbd, and winbindd). Two services (smb and windbind) control how the daemons are started, stopped, and other service-related features. Each daemon is listed in detail, as well as which specific service has control over it.

14.2.1.1. The `smbd` daemon

The smbd server daemon provides file sharing and printing services to Windows clients. In addition, it is responsible for user authentication, resource locking, and data sharing through the SMB protocol. The default ports on which the server listens for SMB traffic are TCP ports 139 and 445.

The smbd daemon is controlled by the smb service.

14.2.1.2. The `nmbd` daemon

The nmbd server daemon understands and replies to NetBIOS name service requests such as those produced by SMB/CIFS in Windows-based systems. These systems include Windows 95/98/ME, Windows NT, Windows 2000, Windows XP, and LanManager clients. It also participates in the browsing protocols that make up the Windows Network Neighborhood view. The default port that the server listens to for NMB traffic is UDP port 137.

The nmbd daemon is controlled by the smb service.

14.2.1.3. The `winbindd` daemon

The winbind service resolves user and group information on a Windows NT server and makes it understandable by UNIX platforms. This is achieved by using Microsoft RPC calls, Pluggable Authentication Modules (PAM), and the Name Service Switch (NSS). This allows Windows NT domain users to appear and operate as UNIX users on a UNIX machine. Though bundled with the Samba distribution, the winbind service is controlled separately from the smb service.

The winbindd daemon is controlled by the winbind service and does not require the smb service to be started in order to operate. Because winbind is a client-side service used to connect to Windows NT based servers, further discussion of winbind is beyond the scope of this manual.

14.2.2. Starting and Stopping Samba

To start a Samba server, type the following command in a shell prompt while logged in as root:

service smb start

Important

To set up a domain member server, you must first join the domain or Active Directory using the net join command before starting the smb service.

To stop the server, type the following command in a shell prompt while logged in as root:

service smb stop

The restart option is a quick way of stopping and then starting Samba. This is the most reliable way to make configuration changes take effect after editing the configuration file for Samba. Note that the restart option starts the daemon even if it was not running originally.

To restart the server, type the following command in a shell prompt while logged in as root:

service smb restart

The condrestart (conditional restart) option only starts smb on the condition that it is currently running. This option is useful for scripts, because it does not start the daemon if it is not running.

Note

When the smb.conf file is changed, Samba automatically reloads it after a few minutes. Issuing a manual restart or reload is just as affective.

To conditionally restart the server, type the following command as root:

service smb condrestart

A manual reload of the smb.conf file can be useful in case of a failed automatic reload by the smb service. To ensure that the Samba server configuration file is reloaded without restarting the service, type the following command as root:

service smb reload

By default, the smb service does not start automatically at boot time. To configure Samba to start at boot time, use an initscript utility, such as /sbin/chkconfig, /usr/sbin/ntsysv, or the Services Configuration Tool program. Refer to the chapter titled Controlling Access to Services in the System Administrators Guide for more information regarding these tools.

14.3. Samba Server Types and the `smb.conf` File

Samba configuration is straightforward. All modifications to Samba are done in the /etc/samba/smb.conf configuration file. Although the default smb.conf file is well documented, it does not address complex topics such as LDAP, Active Directory, and the numerous domain controller implementations.

The following sections describe the different ways a Samba server can be configured. Keep in mind your needs and the changes required to the smb.conf file for a successful configuration.

14.3.1. Stand-alone Server

A stand-alone server can be a workgroup server or a member of a workgroup environment. A stand-alone server is not a domain controller and does not participate in a domain in any way. The following examples include several anonymous share-level security configurations and one user-level security configuration. For more information on share-level and user-level security modes, refer to Section 14.4, “ Samba Security Modes ”.

14.3.1.1. Anonymous Read-Only

The following smb.conf file shows a sample configuration needed to implement anonymous read-only file sharing. The security = share parameter makes a share anonymous. Note, security levels for a single Samba server cannot be mixed. The security directive is a global Samba parameter located in the [global] configuration section of the smb.conf file.

[global]
workgroup = DOCS
netbios name = DOCS_SRV
security = share

[data]
comment = Documentation Samba Server
path = /export
read only = Yes
guest only = Yes

14.3.1.2. Anonymous Read/Write

The following smb.conf file shows a sample configuration needed to implement anonymous read/write file sharing. To enable anonymous read/write file sharing, set the read only directive to no. The force user and force group directives are also added to enforce the ownership of any newly placed files specified in the share.

Note

Although having an anonymous read/write server is possible, it is not recommended. Any files placed in the share space, regardless of user, are assigned the user/group combination as specified by a generic user (force user) and group (force group) in the smb.conf file.

[global]
workgroup = DOCS
netbios name = DOCS_SRV
security = share

[data]
comment = Data
path = /export
force user = docsbot
force group = users
read only = No
guest ok = Yes

14.3.1.3. Anonymous Print Server

The following smb.conf file shows a sample configuration needed to implement an anonymous print server. Setting browseable to no as shown does not list the printer in Windows Network Neighborhood. Although hidden from browsing, configuring the printer explicitly is possible. By connecting to DOCS_SRV using NetBIOS, the client can have access to the printer if the client is also part of the DOCS workgroup. It is also assumed that the client has the correct local printer driver installed, as the use client driver directive is set to Yes. In this case, the Samba server has no responsibility for sharing printer drivers to the client.

[global]
workgroup = DOCS
netbios name = DOCS_SRV
security = share
printcap name = cups
disable spools= Yes
show add printer wizard = No
printing = cups

[printers]
comment = All Printers
path = /var/spool/samba
guest ok = Yes
printable = Yes
use client driver = Yes
browseable = Yes

14.3.1.4. Secure Read/Write File and Print Server

The following smb.conf file shows a sample configuration needed to implement a secure read/write print server. Setting the security directive to user forces Samba to authenticate client connections. Notice the [homes] share does not have a force user or force group directive as the [public] share does. The [homes] share uses the authenticated user details for any files created as opposed to the force user and force group in [public].

[global]
workgroup = DOCS
netbios name = DOCS_SRV
security = user
printcap name = cups
disable spools = Yes
show add printer wizard = No
printing = cups

[homes]
comment = Home Directories
valid users = %S
read only = No
browseable = No

[public]
comment = Data
path = /export
force user = docsbot
force group = users
guest ok = Yes

[printers]
comment = All Printers
path = /var/spool/samba
printer admin = john, ed, @admins
create mask = 0600
guest ok = Yes
printable = Yes
use client driver = Yes
browseable = Yes

14.3.2. Domain Member Server

A domain member, while similar to a stand-alone server, is logged into a domain controller (either Windows or Samba) and is subject to the domain's security rules. An example of a domain member server would be a departmental server running Samba that has a machine account on the Primary Domain Controller (PDC). All of the department's clients still authenticate with the PDC, and desktop profiles and all network policy files are included. The difference is that the departmental server has the ability to control printer and network shares.

14.3.2.1. Active Directory Domain Member Server

The following smb.conf file shows a sample configuration needed to implement an Active Directory domain member server. In this example, Samba authenticates users for services being run locally but is also a client of the Active Directory. Ensure that your kerberos realm parameter is shown in all caps (for example realm = EXAMPLE.COM). Since Windows 2000/2003 requires Kerberos for Active Directory authentication, the realm directive is required. If Active Directory and Kerberos are running on different servers, the password server directive may be required to help the distinction.

[global]
realm = EXAMPLE.COM
security = ADS
encrypt passwords = yes
# Optional. Use only if Samba cannot determine the Kerberos server automatically.
password server = kerberos.example.com

In order to join a member server to an Active Directory domain, the following steps must be completed:

Configuration of the smb.conf file on the member server
Configuration of Kerberos, including the /etc/krb5.conf file, on the member server
Creation of the machine account on the Active Directory domain server
Association of the member server to the Active Directory domain

To create the machine account and join the Windows 2000/2003 Active Directory, Kerberos must first be initialized for the member server wishing to join the Active Directory domain. To create an administrative Kerberos ticket, type the following command as root on the member server:

kinit administrator@EXAMPLE.COM

The kinit command is a Kerberos initialization script that references the Active Directory administrator account and Kerberos realm. Since Active Directory requires Kerberos tickets, kinit obtains and caches a Kerberos ticket-granting ticket for client/server authentication. For more information on Kerberos, the /etc/krb5.conf file, and the kinit command, refer to Chapter 19, Kerberos.

To join an Active Directory server (windows1.example.com), type the following command as root on the member server:

net ads join -S windows1.example.com -U administrator%password

Since the machine windows1 was automatically found in the corresponding Kerberos realm (the kinit command succeeded), the net command connects to the Active Directory server using its required administrator account and password. This creates the appropriate machine account on the Active Directory and grants permissions to the Samba domain member server to join the domain.

Note

Since security = ads and not security = user is used, a local password backend such as smbpasswd is not needed. Older clients that do not support security = ads are authenticated as if security = domain had been set. This change does not affect functionality and allows local users not previously in the domain.

14.3.2.2. Windows NT4-based Domain Member Server

The following smb.conf file shows a sample configuration needed to implement a Windows NT4-based domain member server. Becoming a member server of an NT4-based domain is similar to connecting to an Active Directory. The main difference is NT4-based domains do not use Kerberos in their authentication method, making the smb.conf file simpler. In this instance, the Samba member server serves as a pass through to the NT4-based domain server.

[global]
workgroup = DOCS
netbios name = DOCS_SRV
security = domain

[homes]
comment = Home Directories
valid users = %S
read only = No
browseable = No

[public]
comment = Data
path = /export
force user = docsbot
force group = users
guest ok = Yes

Having Samba as a domain member server can be useful in many situations. There are times where the Samba server can have other uses besides file and printer sharing. It may be beneficial to make Samba a domain member server in instances where Linux-only applications are required for use in the domain environment. Administrators appreciate keeping track of all machines in the domain, even if not Windows-based. In the event the Windows-based server hardware is deprecated, it is quite easy to modify the smb.conf file to convert the server to a Samba-based PDC. If Windows NT-based servers are upgraded to Windows 2000/2003, the smb.conf file is easily modifiable to incorporate the infrastructure change to Active Directory if needed.

Important

After configuring the smb.conf file, join the domain before starting Samba by typing the following command as root:

net rpc join -U administrator%password

Note that the -S option, which specifies the domain server hostname, does not need to be stated in the net rpc join command. Samba uses the hostname specified by the workgroup directive in the smb.conf file instead of it being stated explicitly.

14.3.3. Domain Controller

A domain controller in Windows NT is functionally similar to a Network Information Service (NIS) server in a Linux environment. Domain controllers and NIS servers both host user/group information databases as well as related services. Domain controllers are mainly used for security, including the authentication of users accessing domain resources. The service that maintains the user/group database integrity is called the Security Account Manager (SAM). The SAM database is stored differently between Windows and Linux Samba-based systems, therefore SAM replication cannot be achieved and platforms cannot be mixed in a PDC/BDC environment.

In a Samba environment, there can be only one PDC and zero or more BDCs.

Important

Samba cannot exist in a mixed Samba/Windows domain controller environment (Samba cannot be a BDC of a Windows PDC or vice versa). Alternatively, Samba PDCs and BDCs can coexist.

14.3.3.1. Primary Domain Controller (PDC) using `tdbsam`

The simplest and most common implementation of a Samba PDC uses the tdbsam password database backend. Planned to replace the aging smbpasswd backend, tdbsam has numerous improvements that are explained in more detail in Section 14.5, “ Samba Account Information Databases ”. The passdb backend directive controls which backend is to be used for the PDC.

[global]
workgroup = DOCS
netbios name = DOCS_SRV 
passdb backend = tdbsam
security = user
add user script = /usr/sbin/useradd -m %u
delete user script = /usr/sbin/userdel -r %u
add group script = /usr/sbin/groupadd %g 
delete group script = /usr/sbin/groupdel %g 
add user to group script = /usr/sbin/usermod -G %g %u
add machine script = \
 /usr/sbin/useradd -s /bin/false -d /dev/null \
 -g machines %u
# The following specifies the default logon script 
# Per user logon scripts can be specified in the user
# account using pdbedit
logon script = logon.bat
# This sets the default profile path.
# Set per user paths with pdbedit
logon path = \\%L\Profiles\%U
logon drive = H:
logon home = \\%L\%U
domain logons = Yes
os level = 35
preferred master = Yes
domain master = Yes
idmap uid = 15000-20000
idmap gid = 15000-20000

[homes]
comment = Home Directories
valid users = %S
read only = No
browseable = No
writable = Yes

[public]
comment = Data
path = /export
force user = docsbot
force group = users
guest ok = Yes

[netlogon]
comment = Network Logon Service
path = /var/lib/samba/netlogon/scripts
admin users = ed, john, sam
guest ok = No
browseable = No
writable = No

# For profiles to work, create a user directory under the
# path shown. mkdir -p /var/lib/samba/profiles/john
[Profiles]
comment = Roaming Profile Share
path = /var/lib/samba/profiles
read only = No
browseable = No
guest ok = Yes
profile acls = Yes

# Other resource shares
...
...

Note

If you need more than one domain controller or have more than 250 users, do not use a tdbsam authentication backend. LDAP is recommended in these cases.

14.3.3.2. Primary Domain Controller (PDC) using LDAP

The most powerful and versatile implementation of a Samba PDC is its ability to have an LDAP password backend. LDAP is highly scalable. LDAP database servers can be used for redundancy and fail-over by replicating to a Samba BDC. Groups of LDAP PDCs and BDCs with load balancing are ideal for an enterprise environment. On the other hand, LDAP configurations are inherently complex to setup and maintain. If SSL is to be incorporated with LDAP, the complexity instantly multiplies. Even so, with careful and precise planning, LDAP is an ideal solution for enterprise environments.

Note the passdb backend directive as well as specific LDAP suffix specifications. Although the Samba configuration for LDAP is straightforward, the installation of OpenLDAP is not trivial. LDAP should be installed and configured before any Samba configuration. Also notice that Samba and LDAP do not need to be on the same server to function. It is highly recommended to separate the two in an enterprise environment.

[global] 
workgroup = DOCS
netbios name = DOCS_SRV 
passdb backend = ldapsam:ldap://ldap.example.com
username map = /etc/samba/smbusers
security = user
add user script = /usr/sbin/useradd -m %u
delete user script = /usr/sbin/userdel -r %u
add group script = /usr/sbin/groupadd %g 
delete group script = /usr/sbin/groupdel %g 
add user to group script = /usr/sbin/usermod -G %g %u
add machine script = \
 /usr/sbin/useradd -s /bin/false -d /dev/null \
 -g machines %u
# The following specifies the default logon script 
# Per user logon scripts can be specified in the
# user account using pdbedit
logon script = scripts\logon.bat
# This sets the default profile path.
# Set per user paths with pdbedit
logon path = \\%L\Profiles\%U
logon drive = H:
logon home = \\%L\%U
domain logons = Yes
os level = 35
preferred master = Yes
domain master = Yes
ldap suffix = dc=example,dc=com
ldap machine suffix = ou=People
ldap user suffix = ou=People
ldap group suffix = ou=Group
ldap idmap suffix = ou=People
ldap admin dn = cn=Manager
ldap ssl = no
ldap passwd sync = yes
idmap uid = 15000-20000
idmap gid = 15000-20000
...

# Other resource shares
...
...

Note

Implementing LDAP in this smb.conf file assumes that a working LDAP server has been successfully installed on ldap.example.com.

14.3.3.3. Backup Domain Controller (BDC) using LDAP

A BDC is an integral part of any enterprise Samba/LDAP solution. The smb.conf files between the PDC and BDC are virtually identical except for the domain master directive. Make sure the PDC has a value of Yes and the BDC has a value of No. If you have multiple BDCs for a PDC, the os level directive is useful in setting the BDC election priority. The higher the value, the higher the server priority for connecting clients.

Note

A BDC can either use the LDAP database of the PDC or have its own LDAP database. This example uses the LDAP database of the PDC as seen in the passdb backend directive.

[global] workgroup = DOCS
netbios name = DOCS_SRV2
passdb backend = ldapsam:ldap://ldap.example.com
username map = /etc/samba/smbusers
security = user
add user script = /usr/sbin/useradd -m %u
delete user script = /usr/sbin/userdel -r %u
add group script = /usr/sbin/groupadd %g 
delete group script = /usr/sbin/groupdel %g 
add user to group script = /usr/sbin/usermod -G %g %u
add machine script = \
 /usr/sbin/useradd -s /bin/false -d /dev/null \
 -g machines %u
# The following specifies the default logon script 
# Per user logon scripts can be specified in the
# user account using pdbedit
logon script = scripts\logon.bat
# This sets the default profile path.
# Set per user paths with pdbedit
logon path = \\%L\Profiles\%U
logon drive = H:
logon home = \\%L\%U
domain logons = Yes
os level = 35
preferred master = Yes
domain master = No
ldap suffix = dc=example,dc=com
ldap machine suffix = ou=People
ldap user suffix = ou=People
ldap group suffix = ou=Group
ldap idmap suffix = ou=People
ldap admin dn = cn=Manager
ldap ssl = no
ldap passwd sync = yes
idmap uid = 15000-20000
idmap gid = 15000-20000
...

# Other resource shares
...
...

14.3.3.4. Primary Domain Controller (PDC) with Active Directory

Although it is possible for Samba to be a member of an Active Directory, it is not possible for Samba to operate as an Active Directory domain controller.

14.4. Samba Security Modes

There are only two types of security modes for Samba, share-level and user-level, which are collectively known as security levels. Share-level security can only be implemented in one way, while user-level security can be implemented in one of four different ways. The different ways of implementing a security level are called security modes.

14.4.1. User-Level Security

User-level security is the default setting for Samba. Even if the security = user directive is not listed in the smb.conf file, it is used by Samba. If the server accepts the client's username/password, the client can then mount multiple shares without specifying a password for each instance. Samba can also accept session-based username/password requests. The client maintains multiple authentication contexts by using a unique UID for each logon.

In smb.conf, the security = user directive that sets user-level security is:

[GLOBAL]
...
security = user
...

14.4.2. Share-Level Security

With share-level security, the server accepts only a password without an explicit username from the client. The server expects a password for each share, independent of the username. There have been recent reports that Microsoft Windows clients have compatibility issues with share-level security servers. Samba developers strongly discourage use of share-level security.

In smb.conf, the security = share directive that sets share-level security is:

[GLOBAL]
...
security = share
...

14.4.3. Domain Security Mode (User-Level Security)

In domain security mode, the Samba server has a machine account (domain security trust account) and causes all authentication requests to be passed through to the domain controllers. The Samba server is made into a domain member server by using the following directives in smb.conf:

[GLOBAL]
...
security = domain
workgroup = MARKETING
...

14.4.4. Active Directory Security Mode (User-Level Security)

If you have an Active Directory environment, it is possible to join the domain as a native Active Directory member. Even if a security policy restricts the use of NT-compatible authentication protocols, the Samba server can join an ADS using Kerberos. Samba in Active Directory member mode can accept Kerberos tickets.

In smb.conf, the following directives make Samba an Active Directory member server:

[GLOBAL]
...
security = ADS
realm = EXAMPLE.COM
password server = kerberos.example.com 
...

14.4.5. Server Security Mode (User-Level Security)

Server security mode was previously used when Samba was not capable of acting as a domain member server.

Note

It is highly recommended to not use this mode since there are numerous security drawbacks.

In smb.conf, the following directives enable Samba to operate in server security mode:

[GLOBAL]
...
encrypt passwords = Yes
security = server
password server = "NetBIOS_of_Domain_Controller"
...

14.5. Samba Account Information Databases

The latest release of Samba offers many new features including new password database backends not previously available. Samba version 3.0.0 fully supports all databases used in previous versions of Samba. However, although supported, many backends may not be suitable for production use.

14.5.1. Backward Compatible Backends

Plain Text: Plain text backends are nothing more than the /etc/passwd type backends. With a plain text backend, all usernames and passwords are sent unencrypted between the client and the Samba server. This method is very insecure and is not recommended for use by any means. It is possible that different Windows clients connecting to the Samba server with plain text passwords cannot support such an authentication method.
smbpasswd: A popular backend used in previous Samba packages, the smbpasswd backend utilizes a plain ASCII text layout that includes the MS Windows LanMan and NT account, and encrypted password information. The smbpasswd backend lacks the storage of the Windows NT/2000/2003 SAM extended controls. The smbpasswd backend is not recommended because it does not scale well or hold any Windows information, such as RIDs for NT-based groups. The tdbsam backend solves these issues for use in a smaller database (250 users), but is still not an enterprise-class solution.
Warning
This type of backend may be deprecated for future releases and replaced by the tdbsam backend, which does include the SAM extended controls.
ldapsam_compat: The ldapsam_compat backend allows continued OpenLDAP support for use with upgraded versions of Samba. This option is ideal for migration, but is not required. This tool will eventually be deprecated.

14.5.2. New Backends

tdbsam: The tdbsam backend provides an ideal database backend for local servers, servers that do not need built-in database replication, and servers that do not require the scalability or complexity of LDAP. The tdbsam backend includes all of the smbpasswd database information as well as the previously-excluded SAM information. The inclusion of the extended SAM data allows Samba to implement the same account and system access controls as seen with Windows NT/2000/2003-based systems.
The tdbsam backend is recommended for 250 users at most. Larger organizations should require Active Directory or LDAP integration due to scalability and possible network infrastructure concerns.
ldapsam: The ldapsam backend provides an optimal distributed account installation method for Samba. LDAP is optimal because of its ability to replicate its database to any number of servers using the OpenLDAP slurpd daemon. LDAP databases are light-weight and scalable, perfect for most organizations, especially large enterprises. LDAP is definitely the "wave of the future" with regards to Samba. Improvements to LDAP are constantly being added into Samba such as easing installation and configuration issues.
mysqlsam: The mysqlsam backend uses a MySQL-based database backend. This is useful for sites that already implement MySQL.
xmlsam: The xmlsam backend uses account and password data that are stored in an XML formatted file. This method can be useful for migration of different backend databases or backups.

14.6. Samba Network Browsing

Network browsing is a concept that enables Windows and Samba servers to appear in the Windows Network Neighborhood. Inside the Network Neighborhood, icons are represented as servers and if opened, the server's shares and printers that are available are displayed.

Network browsing capabilities require NetBIOS over TCP/IP. NetBIOS-based networking uses broadcast (UDP) messaging to accomplish browse list management. Without NetBIOS and WINS as the primary method for TCP/IP hostname resolution, other methods such as static files (/etc/hosts) or DNS, must be used.

A domain master browser collates the browse lists from local master browsers on all subnets so that browsing can occur between workgroups and subnets. Also, the domain master browser should preferably be the local master browser for its own subnet.

14.6.1. Workgroup Browsing

For each workgroup, there must be one and only one domain master browser. You can have one local master browser per subnet without a domain master browser, but this results in isolated workgroups unable to see each other. To resolve NetBIOS names in cross-subnet workgroups, WINS is required.

Note

The Domain Master Browser can be the same machine as the WINS server.

There can only be one domain master browser per workgroup name. Here is an example of the smb.conf file in which the Samba server is a domain master browser:

[global]
domain master = Yes
local master = Yes
preferred master = Yes
os level = 35

Next is an example of the smb.conf file in which the Samba server is a local master browser:

[global]
domain master = no
local master = Yes
preferred master = Yes
os level = 35

The os level directive operates as a priority system for master browsers in a subnet. Setting different values ensures master browsers do not conflict with each other for authority.

Note

Lowering the os level directive results in Samba conflicting with other master browsers on the same subnet. The higher the value, the higher the priority. The highest a Windows server can operate at is 32. This is a good way of tuning multiple local master browsers.

There are instances when a Windows NT machine on the subnet could be the local master browser. The following is an example smb.conf configuration in which the Samba server is not serving in any browsing capacity:

[global]
domain master = no
local master = no
preferred master = no
os level = 0

Warning

Having multiple local master browsers result in each server competing for browsing election requests. Make sure there is only one local master browser per subnet.

14.6.2. Domain Browsing

By default, a Windows NT PDC for a domain is also the domain master browser for that domain. A Samba server must be set up as a domain master server in this type of situation. Network browsing may fail if the Samba server is running WINS along with other domain controllers in operation.

For subnets that do not include the Windows NT PDC, a Samba server can be implemented as a local master browser. Configuring the smb.conf for a local master browser (or no browsing at all) in a domain controller environment is the same as workgroup configuration.

14.6.3. WINS (Windows Internetworking Name Server)

Either a Samba server or a Windows NT server can function as a WINS server. When a WINS server is used with NetBIOS enabled, UDP unicasts can be routed which allows name resolution across networks. Without a WINS server, the UDP broadcast is limited to the local subnet and therefore cannot be routed to other subnets, workgroups, or domains. If WINS replication is necessary, do not use Samba as your primary WINS server, as Samba does not currently support WINS replication.

In a mixed NT/2000/2003 server and Samba environment, it is recommended that you use the Microsoft WINS capabilities. In a Samba-only environment, it is recommended that you use only one Samba server for WINS.

The following is an example of the smb.conf file in which the Samba server is serving as a WINS server:

[global]
wins support = Yes

Note

All servers (including Samba) should connect to a WINS server to resolve NetBIOS names. Without WINS, browsing only occurs on the local subnet. Furthermore, even if a domain-wide list is somehow obtained, hosts are not resolvable for the client without WINS.

14.7. Samba with CUPS Printing Support

Samba allows client machines to share printers connected to the Samba server, as well as send Linux documents to Windows printer shares. Although there are other printing systems that function with Red Hat Enterprise Linux, CUPS (Common UNIX Print System) is the recommended printing system due to its close integration with Samba.

14.7.1. Simple `smb.conf` Settings

The following example shows a very basic smb.conf configuration for CUPS support:

[global]
load printers = Yes
printing = cups
printcap name = cups

[printers]
comment = All Printers
path = /var/spool/samba/print
printer = IBMInfoP
browseable = No
public = Yes
guest ok = Yes
writable = No
printable = Yes
printer admin = @ntadmins

[print$]
comment = Printer Drivers Share
path = /var/lib/samba/drivers
write list = ed, john
printer admin = ed, john

More complicated printing configurations are possible. To add additional security and privacy for printing confidential documents, users can have their own print spooler not located in a public path. If a job fails, other users would not have access to the file.

The print$ share contains printer drivers for clients to access if not available locally. The print$ share is optional and may not be required depending on the organization.

Setting browseable to Yes enables the printer to be viewed in the Windows Network Neighborhood, provided the Samba server is set up correctly in the domain/workgroup.

14.8. Samba Distribution Programs

14.8.1. `findsmb`

findsmb <subnet_broadcast_address>

The findsmb program is a Perl script which reports information about SMB-aware systems on a specific subnet. If no subnet is specified the local subnet is used. Items displayed include IP address, NetBIOS name, workgroup or domain name, operating system, and version.

The following example shows the output of executing findsmb as any valid user on a system:

~]$ findsmb
 
IP ADDR       NETBIOS NAME  WORKGROUP/OS/VERSION
------------------------------------------------------------------
10.1.59.25    VERVE         [MYGROUP] [Unix] [Samba 3.0.0-15]
10.1.59.26    STATION22     [MYGROUP] [Unix] [Samba 3.0.2-7.FC1]
10.1.56.45    TREK         +[WORKGROUP] [Windows 5.0] [Windows 2000 LAN Manager]
10.1.57.94    PIXEL         [MYGROUP] [Unix] [Samba 3.0.0-15]
10.1.57.137   MOBILE001     [WORKGROUP] [Windows 5.0] [Windows 2000 LAN Manager]
10.1.57.141   JAWS         +[KWIKIMART] [Unix] [Samba 2.2.7a-security-rollup-fix]
10.1.56.159   FRED         +[MYGROUP] [Unix] [Samba 3.0.0-14.3E]
10.1.59.192   LEGION       *[MYGROUP] [Unix] [Samba 2.2.7-security-rollup-fix]
10.1.56.205   NANCYN       +[MYGROUP] [Unix] [Samba 2.2.7a-security-rollup-fix]

14.8.2. `make_smbcodepage`

make_smbcodepage <c|d> <codepage_number> <inputfile> <outputfile>

The make_smbcodepage program compiles a binary codepage file from a text-format definition. The reverse is also allowed by decompiling a binary codepage file to a text-format definition. This obsolete program is part of the internationalization features of previous versions of Samba which are included by default with the current version of Samba.

14.8.3. `make_unicodemap`

make_unicodemap <codepage_number> <inputfile> <outputfile>

The make_unicodemap program compiles binary Unicode files from text files so Samba can display non-ASCII charactersets. This obsolete program was part of the internationalization features of previous versions of Samba which are now included with the current release of Samba.

14.8.4. `net`

net <protocol> <function> <misc_options> <target_options>

The net utility is similar to the net utility used for Windows and MS-DOS. The first argument is used to specify the protocol to use when executing a command. The <protocol> option can be ads, rap, or rpc for specifying the type of server connection. Active Directory uses ads, Win9x/NT3 uses rap, and Windows NT4/2000/2003 uses rpc. If the protocol is omitted, net automatically tries to determine it.

The following example displays a list the available shares for a host named wakko:

~]# net -l share -S wakko
Password:
 
Enumerating shared resources (exports) on remote server:
 
 
Share name   Type     Description
----------   ----     -----------
data         Disk     Wakko data share
tmp          Disk     Wakko tmp share
IPC$         IPC      IPC Service (Samba Server)
ADMIN$       IPC      IPC Service (Samba Server)

The following example displays a list of Samba users for a host named wakko:

~]# net -l user -S wakko
root password:
 
User name             Comment
-----------------------------
andriusb              Documentation
joe                   Marketing
lisa                  Sales

14.8.5. `nmblookup`

nmblookup <options> <netbios_name>

The nmblookup program resolves NetBIOS names into IP addresses. The program broadcasts its query on the local subnet until the target machine replies.

Here is an example:

~]# nmblookup trek
querying trek on 10.1.59.255
10.1.56.45 trek<00>

14.8.6. `pdbedit`

pdbedit <options>

The pdbedit program manages accounts located in the SAM database. All backends are supported including smbpasswd, LDAP, NIS+, and the tdb database library.

The following are examples of adding, deleting, and listing users:

~]# pdbedit -a kristin
new password:
retype new password:
Unix username:        kristin
NT username:
Account Flags:        [U          ]
User SID:             S-1-5-21-1210235352-3804200048-1474496110-2012
Primary Group SID:    S-1-5-21-1210235352-3804200048-1474496110-2077
Full Name:
Home Directory:       \\wakko\kristin
HomeDir Drive:
Logon Script:
Profile Path:         \\wakko\kristin\profile
Domain:               WAKKO
Account desc:
Workstations:
Munged dial:
Logon time:           0
Logoff time:          Mon, 18 Jan 2038 22:14:07 GMT
Kickoff time:         Mon, 18 Jan 2038 22:14:07 GMT
Password last set:    Thu, 29 Jan 2004 08:29:28 GMT
Password can change:  Thu, 29 Jan 2004 08:29:28 GMT
Password must change: Mon, 18 Jan 2038 22:14:07 GMT

~]# pdbedit -v -L kristin
Unix username:        kristin
NT username:
Account Flags:        [U          ]
User SID:             S-1-5-21-1210235352-3804200048-1474496110-2012
Primary Group SID:    S-1-5-21-1210235352-3804200048-1474496110-2077
Full Name:
Home Directory:       \\wakko\kristin
HomeDir Drive:
Logon Script:
Profile Path:         \\wakko\kristin\profile
Domain:               WAKKO
Account desc:
Workstations:
Munged dial:
Logon time:           0
Logoff time:          Mon, 18 Jan 2038 22:14:07 GMT
Kickoff time:         Mon, 18 Jan 2038 22:14:07 GMT
Password last set:    Thu, 29 Jan 2004 08:29:28 GMT
Password can change:  Thu, 29 Jan 2004 08:29:28 GMT
Password must change: Mon, 18 Jan 2038 22:14:07 GMT

~]# pdbedit -L
andriusb:505:
joe:503:
lisa:504:
kristin:506:

~]# pdbedit -x joe

~]# pdbedit -L
andriusb:505:
lisa:504:
kristin:506:

14.8.7. `rpcclient`

rpcclient <server> <options>

The rpcclient program issues administrative commands using Microsoft RPCs, which provide access to the Windows administration graphical user interfaces (GUIs) for systems management. This is most often used by advanced users that understand the full complexity of Microsoft RPCs.

14.8.8. `smbcacls`

smbcacls <//server/share> <filename> <options>

The smbcacls program modifies Windows ACLs on files and directories shared by the Samba server.

14.8.9. `smbclient`

smbclient <//server/share> <password> <options>

The smbclient program is a versatile UNIX client which provides functionality similar to ftp.

14.8.10. `smbcontrol`

smbcontrol -i <options>

smbcontrol <options> <destination> <messagetype> <parameters>

The smbcontrol program sends control messages to running smbd or nmbd daemons. Executing smbcontrol -i runs commands interactively until a blank line or a 'q' is entered.

14.8.11. `smbgroupedit`

smbgroupedit <options>

The smbgroupedit program maps between Linux groups and Windows groups. It also allows a Linux group to be a domain group.

14.8.12. `smbmount`

smbmount <//server/share> <mount_point> <-o options>

The smbmount program uses the low-level smbmnt program to mount an smbfs file system (Samba share). The mount -t smbfs <//server/share> <mount_point> <-o options> command also works.

For example:

~]# smbmount //wakko/html /mnt/html -o username=kristin
Password: <password>
[root@yakko /]# ls -l /mnt/html
total 0
-rwxr-xr-x    1 root     root            0 Jan 29 08:09 index.html

14.8.13. `smbpasswd`

smbpasswd <options> <username> <password>

The smbpasswd program manages encrypted passwords. This program can be run by a superuser to change any user's password as well as by an ordinary user to change their own Samba password.

14.8.14. `smbspool`

smbspool <job> <user> <title> <copies> <options> <filename>

The smbspool program is a CUPS-compatible printing interface to Samba. Although designed for use with CUPS printers, smbspool can work with non-CUPS printers as well.

14.8.15. `smbstatus`

smbstatus <options>

The smbstatus program displays the status of current connections to a Samba server.

14.8.16. `smbtar`

smbtar <options>

The smbtar program performs backup and restores of Windows-based share files and directories to a local tape archive. Though similar to the tar command, the two are not compatible.

14.8.17. `testparm`

testparm <options> <filename> <hostname IP_address>

The testparm program checks the syntax of the smb.conf file. If your smb.conf file is in the default location (/etc/samba/smb.conf) you do not need to specify the location. Specifying the hostname and IP address to the testparm program verifies that the hosts.allow and host.deny files are configured correctly. The testparm program also displays a summary of your smb.conf file and the server's role (stand-alone, domain, etc.) after testing. This is convenient when debugging as it excludes comments and concisely presents information for experienced administrators to read.

For example:

~]# testparm
Load smb config files from /etc/samba/smb.conf
Processing section "[homes]"
Processing section "[printers]"
Processing section "[tmp]"
Processing section "[html]"
Loaded services file OK.
Server role: ROLE_STANDALONE
Press enter to see a dump of your service definitions
<enter>
# Global parameters
[global]
        workgroup = MYGROUP
        server string = Samba Server
        security = SHARE
        log file = /var/log/samba/%m.log
        max log size = 50
        socket options = TCP_NODELAY SO_RCVBUF=8192 SO_SNDBUF=8192
        dns proxy = No
 
[homes]
        comment = Home Directories
        read only = No
        browseable = No
 
[printers]
        comment = All Printers
        path = /var/spool/samba
        printable = Yes
        browseable = No
 
[tmp]
        comment = Wakko tmp
        path = /tmp
        guest only = Yes
 
[html]
        comment = Wakko www
        path = /var/www/html
        force user = andriusb
        force group = users
        read only = No
        guest only = Yes

14.8.18. `testprns`

testprns <printername> <printcapname>

The testprns program checks if printername is valid and exists in the printcap. If the printcapname is not specified, the default specified in the Samba or printcap configuration files is used.

14.8.19. `wbinfo`

wbinfo <options>

The wbinfo program displays information from the winbindd daemon. The winbindd daemon must be running for wbinfo to work.

14.9. Additional Resources

The following sections give you the means to explore Samba in greater detail.

14.9.1. Installed Documentation

/usr/share/doc/samba-<version-number>/ — All additional files included with the Samba distribution. This includes all helper scripts, sample configuration files, and documentation.

14.9.2. Red Hat Documentation

System Administrators Guide; Red Hat, Inc — The Samba chapter explains how to configure a Samba server.

14.9.3. Related Books

The Official Samba-3 HOWTO-Collection by John H. Terpstra and Jelmer R. Vernooij; Prentice Hall — The official Samba-3 documentation as issued by the Samba development team. This is more of a reference guide than a step-by-step guide.
Samba-3 by Example by John H. Terpstra; Prentice Hall — This is another official release issued by the Samba development team which discusses detailed examples of OpenLDAP, DNS, DHCP, and printing configuration files. This has step-by-step related information that helps in real-world implementations.
Using Samba, 2nd Edition by Jay T's, Robert Eckstein, and David Collier-Brown; O'Reilly — A good resource for novice to advanced users, which includes comprehensive reference material.

14.9.4. Useful Websites

http://www.samba.org/ — Homepage for the Samba distribution and all official documentation created by the Samba development team. Many resources are available in HTML and PDF formats, while others are only available for purchase. Although many of these links are not Red Hat Enterprise Linux specific, some concepts may apply.
http://samba.org/samba/archives.html — Active email lists for the Samba community. Enabling digest mode is recommended due to high levels of list activity.
Samba newsgroups — Samba threaded newsgroups, such as gmane.org, that use the NNTP protocol are also available. This an alternative to receiving mailing list emails.

Chapter 15. FTP

File Transfer Protocol (FTP) is one of the oldest and most commonly used protocols found on the Internet today. Its purpose is to reliably transfer files between computer hosts on a network without requiring the user to log directly into the remote host or have knowledge of how to use the remote system. It allows users to access files on remote systems using a standard set of simple commands.

This chapter outlines the basics of the FTP protocol, as well as configuration options for the primary FTP server shipped with Red Hat Enterprise Linux, vsftpd.

15.1. The File Transport Protocol

FTP uses a client server architecture to transfer files using the TCP network protocol. Because FTP is an older protocol, it uses unencrypted username and password authentication. For this reason, it is considered an insecure protocol and should not be used unless absolutely necessary. A good substitute for FTP is sftp from the OpenSSH suite of tools. For information about configuring OpenSSH, refer to the chapter titled OpenSSH in System Administrators Guide. For more information about the SSH protocol, refer to Chapter 20, SSH Protocol.

However, because FTP is so prevalent on the Internet, it is often required to share files to the public. System administrators, therefore, should be aware of the FTP protocol's unique characteristics.

15.1.1. Multiple Ports, Multiple Modes

Unlike most protocols used on the Internet, FTP requires multiple network ports to work properly. When an FTP client application initiates a connection to an FTP server, it opens port 21 on the server — known as the command port. This port is used to issue all commands to the server. Any data requested from the server is returned to the client via a data port. The port number for data connections, and the way in which data connections are initialized, vary depending upon whether the client requests the data in active or passive mode.

The following defines these modes:

active mode: Active mode is the original method used by the FTP protocol for transferring data to the client application. When an active mode data transfer is initiated by the FTP client, the server opens a connection from port 20 on the server to the IP address and a random, unprivileged port (greater than 1024) specified by the client. This arrangement means that the client machine must be allowed to accept connections over any port above 1024. With the growth of insecure networks, such as the Internet, the use of firewalls to protect client machines is now prevalent. Because these client-side firewalls often deny incoming connections from active mode FTP servers, passive mode was devised.
passive mode: Passive mode, like active mode, is initiated by the FTP client application. When requesting data from the server, the FTP client indicates it wants to access the data in passive mode and the server provides the IP address and a random, unprivileged port (greater than 1024) on the server. The client then connects to that port on the server to download the requested information.
While passive mode resolves issues for client-side firewall interference with data connections, it can complicate administration of the server-side firewall. Limiting the range of unprivileged ports offered for passive connections in the FTP server's configuration file is one way to reduce the number of open ports on a server and simplify the task of creating firewall rules for the server. Refer to Section 15.5.8, “Network Options” for more about limiting passive ports.

15.2. FTP Servers

Red Hat Enterprise Linux ships with two different FTP servers:

Red Hat Content Accelerator — A kernel-based Web server that delivers high performance Web server and FTP services. Since speed as its primary design goal, it has limited functionality and runs only as an anonymous FTP server. For more information about configuring and administering Red Hat Content Accelerator, consult the documentation available online at http://www.redhat.com/docs/manuals/tux/.
vsftpd — A fast, secure FTP daemon which is the preferred FTP server for Red Hat Enterprise Linux. The remainder of this chapter focuses on vsftpd.

15.2.1. `vsftpd`

The Very Secure FTP Daemon (vsftpd) is designed from the ground up to be fast, stable, and, most importantly, secure. Its ability to handle large numbers of connections efficiently and securely is why vsftpd is the only stand-alone FTP distributed with Red Hat Enterprise Linux.

The security model used by vsftpd has three primary aspects:

Strong separation of privileged and non-privileged processes — Separate processes handle different tasks, and each of these processes run with the minimal privileges required for the task.
Tasks requiring elevated privileges are handled by processes with the minimal privilege necessary — By leveraging compatibilities found in the libcap library, tasks that usually require full root privileges can be executed more safely from a less privileged process.
Most processes run in a chroot jail — Whenever possible, processes are change-rooted to the directory being shared; this directory is then considered a chroot jail. For example, if the directory /var/ftp/ is the primary shared directory, vsftpd reassigns /var/ftp/ to the new root directory, known as /. This disallows any potential malicious hacker activities for any directories not contained below the new root directory.

Use of these security practices has the following effect on how vsftpd deals with requests:

The parent process runs with the least privileges required — The parent process dynamically calculates the level of privileges it requires to minimize the level of risk. Child processes handle direct interaction with the FTP clients and run with as close to no privileges as possible.
All operations requiring elevated privileges are handled by a small parent process — Much like the Apache HTTP Server, vsftpd launches unprivileged child processes to handle incoming connections. This allows the privileged, parent process to be as small as possible and handle relatively few tasks.
All requests from unprivileged child processes are distrusted by the parent process — Communication with child processes are received over a socket, and the validity of any information from child processes is checked before being acted on.
Most interaction with FTP clients is handled by unprivileged child processes in a chroot jail — Because these child processes are unprivileged and only have access to the directory being shared, any crashed processes only allows the attacker access to the shared files.

15.3. Files Installed with `vsftpd`

The vsftpd RPM installs the daemon (/usr/sbin/vsftpd), its configuration and related files, as well as FTP directories onto the system. The following is a list of the files and directories most often considered when configuring vsftpd:

/etc/rc.d/init.d/vsftpd — The initialization script (initscript) used by the /sbin/service command to start, stop, or reload vsftpd. Refer to Section 15.4, “Starting and Stopping vsftpd” for more information about using this script.
/etc/pam.d/vsftpd — The Pluggable Authentication Modules (PAM) configuration file for vsftpd. This file defines the requirements a user must meet to login to the FTP server. For more information, refer to Chapter 16, Pluggable Authentication Modules (PAM).
/etc/vsftpd/vsftpd.conf — The configuration file for vsftpd. Refer to Section 15.5, “vsftpd Configuration Options” for a list of important options contained within this file.
/etc/vsftpd.ftpusers — A list of users not allowed to log into vsftpd. By default, this list includes the root, bin, and daemon users, among others.
/etc/vsftpd.user_list — This file can be configured to either deny or allow access to the users listed, depending on whether the userlist_deny directive is set to YES (default) or NO in /etc/vsftpd/vsftpd.conf. If /etc/vsftpd.user_list is used to grant access to users, the usernames listed must not appear in /etc/vsftpd.ftpusers.
/var/ftp/ — The directory containing files served by vsftpd. It also contains the /var/ftp/pub/ directory for anonymous users. Both directories are world-readable, but writable only by the root user.

15.4. Starting and Stopping `vsftpd`

The vsftpd RPM installs the /etc/rc.d/init.d/vsftpd script, which can be accessed using the /sbin/service command.

To start the server, as root type:

service vsftpd start

To stop the server, as root type:

service vsftpd stop

The restart option is a shorthand way of stopping and then starting vsftpd. This is the most efficient way to make configuration changes take effect after editing the configuration file for vsftpd.

To restart the server, as root type:

service vsftpd restart

The condrestart (conditional restart) option only starts vsftpd if it is currently running. This option is useful for scripts, because it does not start the daemon if it is not running.

To conditionally restart the server, as root type:

service vsftpd condrestart

By default, the vsftpd service does not start automatically at boot time. To configure the vsftpd service to start at boot time, use an initscript utility, such as /sbin/chkconfig, /usr/sbin/ntsysv, or the Services Configuration Tool program. Refer to the chapter titled Controlling Access to Services in System Administrators Guide for more information regarding these tools.

15.4.1. Starting Multiple Copies of `vsftpd`

Sometimes one computer is used to serve multiple FTP domains. This is a technique called multihoming. One way to multihome using vsftpd is by running multiple copies of the daemon, each with its own configuration file.

To do this, first assign all relevant IP addresses to network devices or alias network devices on the system. Refer to the chapter titled Network Configuration in System Administrators Guide for more information about configuring network devices and device aliases. Additional information can be found about network configuration scripts in Chapter 8, Network Interfaces.

Next, the DNS server for the FTP domains must be configured to reference the correct machine. If the DNS server is running on Red Hat Enterprise Linux, refer to the chapter titled BIND Configuration in System Administrators Guide for instructions about using the Domain Name Service Configuration Tool (system-config-bind). For information about BIND and its configuration files, refer to Chapter 12, Berkeley Internet Name Domain (BIND).

For vsftpd to answer requests on different IP addresses, multiple copies of the daemon must be running. The first copy must be run using the vsftpd initscripts, as outlined in Section 15.4, “Starting and Stopping vsftpd”. This copy uses the standard configuration file, /etc/vsftpd/vsftpd.conf.

Each additional FTP site must have a configuration file with a unique name in the /etc/vsftpd/ directory, such as /etc/vsftpd/vsftpd-site-2.conf. Each configuration file must be readable and writable only by root. Within each configuration file for each FTP server listening on an IPv4 network, the following directive must be unique:

listen_address=N.N.N.N

Replace N.N.N.N with the unique IP address for the FTP site being served. If the site is using IPv6, use the listen_address6 directive instead.

Once each additional server has a configuration file, the vsftpd daemon must be launched from a root shell prompt using the following command:

vsftpd /etc/vsftpd/<configuration-file> &

In the above command, replace <configuration-file> with the unique name for the server's configuration file, such as /etc/vsftpd/vsftpd-site-2.conf.

Other directives to consider altering on a per-server basis are:

anon_root
local_root
vsftpd_log_file
xferlog_file

For a detailed list of directives available within vsftpd's configuration file, refer to Section 15.5, “vsftpd Configuration Options”.

To configure any additional servers to start automatically at boot time, add the above command to the end of the /etc/rc.local file.

15.5. `vsftpd` Configuration Options

Although vsftpd may not offer the level of customization other widely available FTP servers have, it offers enough options to fill most administrator's needs. The fact that it is not overly feature-laden limits configuration and programmatic errors.

All configuration of vsftpd is handled by its configuration file, /etc/vsftpd/vsftpd.conf. Each directive is on its own line within the file and follows the following format:

<directive>=<value>

For each directive, replace <directive> with a valid directive and <value> with a valid value.

Important

There must not be any spaces between the <directive>, equal symbol, and the <value> in a directive.

Comment lines must be preceded by a hash mark (#) and are ignored by the daemon.

For a complete list of all directives available, refer to the man page for vsftpd.conf.

Important

For an overview of ways to secure vsftpd, refer to the chapter titled Server Security in the Security Guide.

The following is a list of some of the more important directives within /etc/vsftpd/vsftpd.conf. All directives not explicitly found within vsftpd's configuration file are set to their default value.

15.5.1. Daemon Options

The following is a list of directives which control the overall behavior of the vsftpd daemon.

listen — When enabled, vsftpd runs in stand-alone mode. Red Hat Enterprise Linux sets this value to YES. This directive cannot be used in conjunction with the listen_ipv6 directive.
The default value is NO.
listen_ipv6 — When enabled, vsftpd runs in stand-alone mode, but listens only to IPv6 sockets. This directive cannot be used in conjunction with the listen directive.
The default value is NO.
session_support — When enabled, vsftpd attempts to maintain login sessions for each user through Pluggable Authentication Modules (PAM). Refer to Chapter 16, Pluggable Authentication Modules (PAM) for more information. If session logging is not necessary, disabling this option allows vsftpd to run with less processes and lower privileges.
The default value is YES.

15.5.2. Log In Options and Access Controls

The following is a list of directives which control the login behavior and access control mechanisms.

anonymous_enable — When enabled, anonymous users are allowed to log in. The usernames anonymous and ftp are accepted.
The default value is YES.
Refer to Section 15.5.3, “Anonymous User Options” for a list of directives affecting anonymous users.
banned_email_file — If the deny_email_enable directive is set to YES, this directive specifies the file containing a list of anonymous email passwords which are not permitted access to the server.
The default value is /etc/vsftpd.banned_emails.
banner_file — Specifies the file containing text displayed when a connection is established to the server. This option overrides any text specified in the ftpd_banner directive.
There is no default value for this directive.
cmds_allowed — Specifies a comma-delimited list of FTP commands allowed by the server. All other commands are rejected.
There is no default value for this directive.
deny_email_enable — When enabled, any anonymous user using email passwords specified in the /etc/vsftpd.banned_emails are denied access to the server. The name of the file referenced by this directive can be specified using the banned_email_file directive.
The default value is NO.
ftpd_banner — When enabled, the string specified within this directive is displayed when a connection is established to the server. This option can be overridden by the banner_file directive.
By default vsftpd displays its standard banner.
local_enable — When enabled, local users are allowed to log into the system.
The default value is YES.
Refer to Section 15.5.4, “Local User Options” for a list of directives affecting local users.
pam_service_name — Specifies the PAM service name for vsftpd.
The default value is ftp. Note, in Red Hat Enterprise Linux, the value is set to vsftpd.
tcp_wrappers — When enabled, TCP wrappers are used to grant access to the server. If the FTP server is configured on multiple IP addresses, the VSFTPD_LOAD_CONF option can be used to load different configuration files based on the IP address being requested by the client. For more information about TCP Wrappers, refer to Chapter 17, TCP Wrappers and xinetd.
The default value is NO. Note, in Red Hat Enterprise Linux, the value is set to YES.
userlist_deny — When used in conjunction with the userlist_enable directive and set to NO, all local users are denied access unless the username is listed in the file specified by the userlist_file directive. Because access is denied before the client is asked for a password, setting this directive to NO prevents local users from submitting unencrypted passwords over the network.
The default value is YES.
userlist_enable — When enabled, the users listed in the file specified by the userlist_file directive are denied access. Because access is denied before the client is asked for a password, users are prevented from submitting unencrypted passwords over the network.
The default value is NO, however under Red Hat Enterprise Linux the value is set to YES.
userlist_file — Specifies the file referenced by vsftpd when the userlist_enable directive is enabled.
The default value is /etc/vsftpd.user_list and is created during installation.
cmds_allowed — Specifies a comma separated list of FTP commands that the server allows. Any other commands are rejected.
There is no default value for this directive.

15.5.3. Anonymous User Options

The following lists directives which control anonymous user access to the server. To use these options, the anonymous_enable directive must be set to YES.

anon_mkdir_write_enable — When enabled in conjunction with the write_enable directive, anonymous users are allowed to create new directories within a parent directory which has write permissions.
The default value is NO.
anon_root — Specifies the directory vsftpd changes to after an anonymous user logs in.
There is no default value for this directive.
anon_upload_enable — When enabled in conjunction with the write_enable directive, anonymous users are allowed to upload files within a parent directory which has write permissions.
The default value is NO.
anon_world_readable_only — When enabled, anonymous users are only allowed to download world-readable files.
The default value is YES.
ftp_username — Specifies the local user account (listed in /etc/passwd) used for the anonymous FTP user. The home directory specified in /etc/passwd for the user is the root directory of the anonymous FTP user.
The default value is ftp.
no_anon_password — When enabled, the anonymous user is not asked for a password.
The default value is NO.
secure_email_list_enable — When enabled, only a specified list of email passwords for anonymous logins are accepted. This is a convenient way to offer limited security to public content without the need for virtual users.
Anonymous logins are prevented unless the password provided is listed in /etc/vsftpd.email_passwords. The file format is one password per line, with no trailing white spaces.
The default value is NO.

15.5.4. Local User Options

The following lists directives which characterize the way local users access the server. To use these options, the local_enable directive must be set to YES.

chmod_enable — When enabled, the FTP command SITE CHMOD is allowed for local users. This command allows the users to change the permissions on files.
The default value is YES.
chroot_list_enable — When enabled, the local users listed in the file specified in the chroot_list_file directive are placed in a chroot jail upon log in.
If enabled in conjunction with the chroot_local_user directive, the local users listed in the file specified in the chroot_list_file directive are not placed in a chroot jail upon log in.
The default value is NO.
chroot_list_file — Specifies the file containing a list of local users referenced when the chroot_list_enable directive is set to YES.
The default value is /etc/vsftpd.chroot_list.
chroot_local_user — When enabled, local users are change-rooted to their home directories after logging in.
The default value is NO.
Warning
Enabling chroot_local_user opens up a number of security issues, especially for users with upload privileges. For this reason, it is not recommended.
guest_enable — When enabled, all non-anonymous users are logged in as the user guest, which is the local user specified in the guest_username directive.
The default value is NO.
guest_username — Specifies the username the guest user is mapped to.
The default value is ftp.
local_root — Specifies the directory vsftpd changes to after a local user logs in.
There is no default value for this directive.
local_umask — Specifies the umask value for file creation. Note that the default value is in octal form (a numerical system with a base of eight), which includes a "0" prefix. Otherwise the value is treated as a base-10 integer.
The default value is 022.
passwd_chroot_enable — When enabled in conjunction with the chroot_local_user directive, vsftpd change-roots local users based on the occurrence of the /./ in the home directory field within /etc/passwd.
The default value is NO.
user_config_dir — Specifies the path to a directory containing configuration files bearing the name of local system users that contain specific setting for that user. Any directive in the user's configuration file overrides those found in /etc/vsftpd/vsftpd.conf.
There is no default value for this directive.

15.5.5. Directory Options

The following lists directives which affect directories.

dirlist_enable — When enabled, users are allowed to view directory lists.
The default value is YES.
dirmessage_enable — When enabled, a message is displayed whenever a user enters a directory with a message file. This message is found within the directory being entered. The name of this file is specified in the message_file directive and is .message by default.
The default value is NO. Note, in Red Hat Enterprise Linux, the value is set to YES.
force_dot_files — When enabled, files beginning with a dot (.) are listed in directory listings, with the exception of the . and .. files.
The default value is NO.
hide_ids — When enabled, all directory listings show ftp as the user and group for each file.
The default value is NO.
message_file — Specifies the name of the message file when using the dirmessage_enable directive.
The default value is .message.
text_userdb_names — When enabled, text usernames and group names are used in place of UID and GID entries. Enabling this option may slow performance of the server.
The default value is NO.
use_localtime — When enabled, directory listings reveal the local time for the computer instead of GMT.
The default value is NO.

15.5.6. File Transfer Options

The following lists directives which affect directories.

download_enable — When enabled, file downloads are permitted.
The default value is YES.
chown_uploads — When enabled, all files uploaded by anonymous users are owned by the user specified in the chown_username directive.
The default value is NO.
chown_username — Specifies the ownership of anonymously uploaded files if the chown_uploads directive is enabled.
The default value is root.
write_enable — When enabled, FTP commands which can change the file system are allowed, such as DELE, RNFR, and STOR.
The default value is YES.

15.5.7. Logging Options

The following lists directives which affect vsftpd's logging behavior.

dual_log_enable — When enabled in conjunction with xferlog_enable, vsftpd writes two files simultaneously: a wu-ftpd-compatible log to the file specified in the xferlog_file directive (/var/log/xferlog by default) and a standard vsftpd log file specified in the vsftpd_log_file directive (/var/log/vsftpd.log by default).
The default value is NO.
log_ftp_protocol — When enabled in conjunction with xferlog_enable and with xferlog_std_format set to NO, all FTP commands and responses are logged. This directive is useful for debugging.
The default value is NO.
syslog_enable — When enabled in conjunction with xferlog_enable, all logging normally written to the standard vsftpd log file specified in the vsftpd_log_file directive (/var/log/vsftpd.log by default) is sent to the system logger instead under the FTPD facility.
The default value is NO.
vsftpd_log_file — Specifies the vsftpd log file. For this file to be used, xferlog_enable must be enabled and xferlog_std_format must either be set to NO or, if xferlog_std_format is set to YES, dual_log_enable must be enabled. It is important to note that if syslog_enable is set to YES, the system log is used instead of the file specified in this directive.
The default value is /var/log/vsftpd.log.
xferlog_enable — When enabled, vsftpd logs connections (vsftpd format only) and file transfer information to the log file specified in the vsftpd_log_file directive (/var/log/vsftpd.log by default). If xferlog_std_format is set to YES, file transfer information is logged but connections are not, and the log file specified in xferlog_file (/var/log/xferlog by default) is used instead. It is important to note that both log files and log formats are used if dual_log_enable is set to YES.
The default value is NO. Note, in Red Hat Enterprise Linux, the value is set to YES.
xferlog_file — Specifies the wu-ftpd-compatible log file. For this file to be used, xferlog_enable must be enabled and xferlog_std_format must be set to YES. It is also used if dual_log_enable is set to YES.
The default value is /var/log/xferlog.
xferlog_std_format — When enabled in conjunction with xferlog_enable, only a wu-ftpd-compatible file transfer log is written to the file specified in the xferlog_file directive (/var/log/xferlog by default). It is important to note that this file only logs file transfers and does not log connections to the server.
The default value is NO. Note, in Red Hat Enterprise Linux, the value is set to YES.

Important

To maintain compatibility with log files written by the older wu-ftpd FTP server, the xferlog_std_format directive is set to YES under Red Hat Enterprise Linux. However, this setting means that connections to the server are not logged.

To both log connections in vsftpd format and maintain a wu-ftpd-compatible file transfer log, set dual_log_enable to YES.

If maintaining a wu-ftpd-compatible file transfer log is not important, either set xferlog_std_format to NO, comment the line with a hash mark (#), or delete the line entirely.

15.5.8. Network Options

The following lists directives which affect how vsftpd interacts with the network.

accept_timeout — Specifies the amount of time for a client using passive mode to establish a connection.
The default value is 60.
anon_max_rate — Specifies the maximum data transfer rate for anonymous users in bytes per second.
The default value is 0, which does not limit the transfer rate.
connect_from_port_20 When enabled, vsftpd runs with enough privileges to open port 20 on the server during active mode data transfers. Disabling this option allows vsftpd to run with less privileges, but may be incompatible with some FTP clients.
The default value is NO. Note, in Red Hat Enterprise Linux, the value is set to YES.
connect_timeout — Specifies the maximum amount of time a client using active mode has to respond to a data connection, in seconds.
The default value is 60.
data_connection_timeout — Specifies maximum amount of time data transfers are allowed to stall, in seconds. Once triggered, the connection to the remote client is closed.
The default value is 300.
ftp_data_port — Specifies the port used for active data connections when connect_from_port_20 is set to YES.
The default value is 20.
idle_session_timeout — Specifies the maximum amount of time between commands from a remote client. Once triggered, the connection to the remote client is closed.
The default value is 300.
listen_address — Specifies the IP address on which vsftpd listens for network connections.
There is no default value for this directive.
Note
If running multiple copies of vsftpd serving different IP addresses, the configuration file for each copy of the vsftpd daemon must have a different value for this directive. Refer to Section 15.4.1, “Starting Multiple Copies of vsftpd” for more information about multihomed FTP servers.
listen_address6 — Specifies the IPv6 address on which vsftpd listens for network connections when listen_ipv6 is set to YES.
There is no default value for this directive.
Note
If running multiple copies of vsftpd serving different IP addresses, the configuration file for each copy of the vsftpd daemon must have a different value for this directive. Refer to Section 15.4.1, “Starting Multiple Copies of vsftpd” for more information about multihomed FTP servers.
listen_port — Specifies the port on which vsftpd listens for network connections.
The default value is 21.
local_max_rate — Specifies the maximum rate data is transfered for local users logged into the server in bytes per second.
The default value is 0, which does not limit the transfer rate.
max_clients — Specifies the maximum number of simultaneous clients allowed to connect to the server when it is running in standalone mode. Any additional client connections would result in an error message.
The default value is 0, which does not limit connections.
max_per_ip — Specifies the maximum of clients allowed to connected from the same source IP address.
The default value is 0, which does not limit connections.
pasv_address — Specifies the IP address for the public facing IP address of the server for servers behind Network Address Translation (NAT) firewalls. This enables vsftpd to hand out the correct return address for passive mode connections.
There is no default value for this directive.
pasv_enable — When enabled, passive mode connects are allowed.
The default value is YES.
pasv_max_port — Specifies the highest possible port sent to the FTP clients for passive mode connections. This setting is used to limit the port range so that firewall rules are easier to create.
The default value is 0, which does not limit the highest passive port range. The value must not exceed 65535.
pasv_min_port — Specifies the lowest possible port sent to the FTP clients for passive mode connections. This setting is used to limit the port range so that firewall rules are easier to create.
The default value is 0, which does not limit the lowest passive port range. The value must not be lower 1024.
pasv_promiscuous — When enabled, data connections are not checked to make sure they are originating from the same IP address. This setting is only useful for certain types of tunneling.
Warning
Do not enable this option unless absolutely necessary as it disables an important security feature which verifies that passive mode connections originate from the same IP address as the control connection that initiates the data transfer.
The default value is NO.
port_enable — When enabled, active mode connects are allowed.
The default value is YES.

15.6. Additional Resources

For more information about vsftpd, refer to the following resources.

15.6.1. Installed Documentation

The /usr/share/doc/vsftpd-<version-number>/ directory — Replace <version-number> with the installed version of the vsftpd package. This directory contains a README with basic information about the software. The TUNING file contains basic performance tuning tips and the SECURITY/ directory contains information about the security model employed by vsftpd.
vsftpd related man pages — There are a number of man pages for the daemon and configuration files. The following lists some of the more important man pages.
Server Applications
man vsftpd — Describes available command line options for vsftpd.
Configuration Files
man vsftpd.conf — Contains a detailed list of options available within the configuration file for vsftpd.
man 5 hosts_access — Describes the format and options available within the TCP wrappers configuration files: hosts.allow and hosts.deny.

15.6.2. Useful Websites

http://vsftpd.beasts.org/ — The vsftpd project page is a great place to locate the latest documentation and to contact the author of the software.
http://slacksite.com/other/ftp.html — This website provides a concise explanation of the differences between active and passive mode FTP.
http://war.jgaa.com/ftp/?cmd=rfc — A comprehensive list of Request for Comments (RFCs) related to the FTP protocol.

Part III. Security Reference

Using secure protocols is a critical part of maintaining system integrity. This part describes critical tools used for the purpose of user authentication, network access control, and secure network communication. For more information about securing a Red Hat Enterprise Linux system, refer to the Security Guide.

Chapter 16. Pluggable Authentication Modules (PAM)

Programs which grant users access to a system verify each user's identity through a process called authentication. Historically, each such program had its own way of performing the task of authentication. Under Red Hat Enterprise Linux, many such programs are configured to use a centralized authentication mechanism called Pluggable Authentication Modules or 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. However, sometimes it may become 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 16.3, “PAM Configuration File Format” for more information).

16.1. Advantages of PAM

PAM offers the following advantages:

It provides a common authentication scheme that can be used with a wide variety of applications.
It allows a large amount of flexibility and control over authentication for both system administrators and application developers.
It allows application developers to develop programs without creating their own authentication scheme.

16.2. PAM Configuration Files

The directory /etc/pam.d/ contains the PAM configuration files for each PAM-aware application. In earlier versions of PAM, the file /etc/pam.conf was used, but this file is now deprecated and is only used if the /etc/pam.d/ directory does not exist.

16.2.1. PAM Service Files

Each PAM-aware application or service has a file within the /etc/pam.d/ directory. Each file within this directory bears the name of the service for which it controls access.

It is up to the PAM-aware program to define its service name and install 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.

16.3. PAM Configuration File Format

Each PAM configuration file contains a group of directives formatted as follows:

<module interface>  <control flag>   <module name>   <module arguments>

Each of these elements are explained in the subsequent sections.

16.3.1. Module Interface

There are four types of PAM module interfaces which correlate to different aspects of the authorization process:

auth — This module interface authenticates use. For example, it asks for 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 is expired or is allowed to log in at a particular time of day.
password — This module interface sets and verifies 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

This instructs PAM to use the pam_unix.so module's auth interface.

16.3.1.1. Stacking Module Interfaces

Module interface directives can be stacked, or placed upon one another, so that multiple modules are used together for one purpose. For this reason, the order in which the modules are listed is very important to the authentication process.

Stacking makes it very easy for an administrator to require specific conditions to exist before allowing the user to authenticate. For example, rlogin normally uses five stacked auth modules, as seen in its PAM configuration file:

auth       required     pam_nologin.so
auth       required     pam_securetty.so
auth       required     pam_env.so
auth       sufficient   pam_rhosts_auth.so
auth       required     pam_stack.so service=system-auth

Before someone is allowed to use rlogin, PAM verifies that the /etc/nologin file does not exist, that they are not trying to log in remotely as a root user over a network connection, and that any environmental variables can be loaded. Then, if a successful rhosts authentication is performed, the connection is allowed. If the rhosts authentication fails, then standard password authentication is performed.

16.3.2. Control Flag

All PAM modules generate a success or failure result when called. Control flags tell PAM what do with the result. Since modules can be stacked in a particular order, control flags decide 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 a required module result fails, the user is not notified until results on all modules referencing that interface are completed.
requisite — The module result must be successful for authentication to continue. However, if a requisite module result fails, the user is notified immediately with a message reflecting the first failed required or requisite module.
sufficient — The module result is ignored if it fails. However, if a sufficient flagged module result is successful and no required flagged modules above it 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 there are no other modules referencing the interface.

Important

The order in which required modules are called is not critical. The sufficient and requisite control flags cause order to become important.

A newer control flag syntax which allows for more precise control is now available for PAM. Please see the PAM documentation located in the /usr/share/doc/pam-<version-number>/ directory for information on this new syntax (where <version-number> is the version number for PAM).

16.3.3. Module Name

The module name provides PAM the name of the pluggable module containing the specified module interface. Under older versions of Red Hat Enterprise Linux, the full path to the module was provided within the PAM configuration file, such as /lib/security/pam_stack.so. However, since the advent of multilib systems, which store 64-bit PAM modules within 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.

16.3.4. Module Arguments

PAM uses arguments to pass information to a pluggable module during authentication for some modules.

For example, the pam_userdb.so module uses secrets 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.

A typical pam_userdb.so line within a PAM configuration file looks like this:

auth      required  pam_userdb.so db=<path-to-file>

In the previous example, replace <path-to-file> with the full path to the Berkeley DB database file.

Invalid arguments are ignored and do not otherwise affect the success or failure of the PAM module. However, most modules report errors to the /var/log/messages file.

16.4. Sample PAM Configuration Files

Below is a sample PAM application configuration file:

#%PAM-1.0
auth      required  pam_securetty.so
auth      required  pam_unix.so shadow 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

The first line is a comment as denoted 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 makes sure 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.

auth      required  pam_unix.so shadow 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. The pam_unix.so module automatically detects and uses shadow passwords to authenticate users. Refer to Section 6.5, “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 verifies whether the file /etc/nologin exists. If nologin does exist 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 component 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 grace period allowed.

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. If it fails this test the first time, it gives the user two more chances to create a strong password, as specified in the retry=3 argument.

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 component of the pam_unix.so module to do so. This only happens if the auth portion of the pam_unix.so module has determined that the password needs to be changed.

The argument shadow tells 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 tells 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 specifies that the session component of the pam_unix.so module manages the session. This module logs the username and the service type to /var/log/messages at the beginning and end of each session. It can be supplemented by stacking it with other session modules for more functionality.

The next sample configuration file illustrates auth module stacking for the rlogin program.

#%PAM-1.0
auth      required    pam_nologin.so
auth      required    pam_securetty.so
auth      required    pam_env.so
auth      sufficient  pam_rhosts_auth.so
auth      required    pam_stack.so service=system-auth

First, pam_nologin.so checks to see if /etc/nologin exists. If it does, no one can log in except for root.

auth      required    pam_securetty.so

The pam_securetty.so module prevents the root user from logging in on insecure terminals. This effectively disallows all root rlogin attempts due to the application's limited security safeguards.

Note

To log in remotely as the root user, use OpenSSH instead. For more information, refer to Chapter 20, SSH Protocol.

auth      required    pam_env.so

This line loads the pam_env.so module, which sets the environmental variables specified in /etc/security/pam_env.conf.

auth      sufficient  pam_rhosts_auth.so

The pam_rhosts_auth.so module authenticates the user using .rhosts in the user's home directory. If this succeeds, PAM immediately considers the authentication to have succeeded. If pam_rhosts_auth.so fails to authenticate the user, the authentication attempt is ignored.

auth      required    pam_stack.so service=system-auth

If the pam_rhosts_auth.so module fails to successfully authenticate the user, the pam_stack.so module performs normal password authentication.

The argument service=system-auth indicates that the user must now pass through the PAM configuration for system authentication as found in /etc/pam.d/system-auth.

Note

To prevent PAM from prompting for a password when the securetty result fails, change the pam_securetty.so module from required to requisite.

16.5. Creating PAM Modules

New PAM modules can be added at any time for PAM-aware applications to use. For example, if a developer invents a one-time-password creation method and writes 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).

16.6. PAM and Administrative Credential Caching

A variety of graphical administrative tools under Red Hat Enterprise Linux give users elevated privileges for up to five minutes via 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.

Under the PAM timestamp scheme, the graphical administrative application prompts the user for the root password when it is launched. Once authenticated, the pam_timestamp.so module creates a timestamp file within the /var/run/sudo/ directory by default. If the timestamp file already exists, other 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.

The existence of the timestamp file is denoted by an authentication icon in the notification area of the panel. Below is an illustration of the authentication icon:

Figure 16.1. The Authentication Icon

16.6.1. Removing the Timestamp File

It is recommended that before walking away from a console where a PAM timestamp is active, the timestamp file be destroyed. To do this from within a graphical environment, click on the authentication icon on the panel. When a dialog box appears, click on the Forget Authorization button.

Figure 16.2. Authentication Icon Dialog

If logged into a system remotely using ssh, use the /sbin/pam_timestamp_check -k root command to destroy the timestamp file.

Note

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 command. Do not log in as root to issue this command.

For information about destroying the timestamp file using pam_timestamp_check, refer to the pam_timestamp_check man page.

16.6.2. Common `pam_timestamp` Directives

The pam_timestamp.so module accepts several directives. Below are the two most commonly used options:

timestamp_timeout — Specifies the number of seconds the during which the timestamp file is valid (in seconds). The default value is 300 seconds (five minutes).
timestampdir — Specifies the directory in which the timestamp file is stored. The default value is /var/run/sudo/.

For more information about controlling the pam_timestamp.so module, refer to Section 16.8.1, “Installed Documentation”.

16.7. PAM and Device Ownership

Red Hat Enterprise Linux allows the first user to log in on the physical console of the machine the ability to manipulate some devices and perform some tasks normally reserved for the root user. This is controlled by a PAM module called pam_console.so.

16.7.1. Device Ownership

When a user logs into a Red Hat Enterprise Linux system, the pam_console.so module is called by login or the graphical login programs, gdm and kdm. If this user is the first user to log in at the physical console — called 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. Once the 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 allows a local user to manipulate these devices without attaining root access, thus simplifying common tasks for the console user.

By modifying the file /etc/security/console.perms, the administrator can edit the list of devices controlled by pam_console.so.

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 within 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

Doing 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]*

16.7.2. Application Access

The console user is also allowed access to certain programs with a file bearing the command name in the /etc/security/console.apps/ directory.

One notable group of applications the console user has access to are three programs which shut off or reboot the system. These are:

/sbin/halt
/sbin/reboot
/sbin/poweroff

Because these are PAM-aware applications, they call the pam_console.so module as a requirement for use.

For more information, refer to the Section 16.8.1, “Installed Documentation”.

16.8. Additional Resources

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.

16.8.1. Installed Documentation

PAM related man pages — There are a number of man pages 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
man pam — Good introductory information on PAM, including the structure and purpose of the PAM configuration files. Note that although this man page talks about the /etc/pam.conf file, the actual configuration files for PAM under Red Hat Enterprise Linux are in the /etc/pam.d/ directory.
man 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.
man console.apps — Describes the format and options available within /etc/security/console.apps the configuration file which defines which applications are accessible by the console user assigned by PAM.
man console.perms — Describes the format and options available within /etc/security/console.perms, the configuration file for the console user permissions assigned by PAM.
man 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 (replace <version-number> with the version number of PAM).
/usr/share/doc/pam-<version-number>/txts/README.pam_timestamp — Contains information about the pam_timestamp.so PAM module (replace <version-number> with the version number of PAM).

16.8.2. Useful Websites

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.

Chapter 17. TCP Wrappers and `xinetd`

Controlling access to network services is one of the most important security tasks facing a server administrator. Red Hat Enterprise Linux provides several tools which do just that. For instance, 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 17.1, “Access Control to Network Services” is a basic illustration of how these tools work together to protect network services.

Figure 17.1. 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. For a discussion of using firewalls with iptables, refer to Chapter 18, iptables.

17.1. TCP Wrappers

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 hosts access files (/etc/hosts.allow and /etc/hosts.deny) to determine whether or not the client host is allowed to connect. In most cases, it then uses the syslog daemon (syslogd) to write the name of the requesting host and the requested service to /var/log/secure or /var/log/messages.

If a client host is allowed to connect, TCP wrappers release control of the connection to the requested service and do not interfere further with communication between the client host and the server.

In addition to access control and logging, TCP wrappers can activate 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 against 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 against libwrap.a, type the following command as the root user:

ldd binary-name | grep libwrap

Replace <binary-name> with the name of the network service binary.

If a prompt is returned, then the network service is not linked against libwrap.a.

17.1.1. Advantages of TCP Wrappers

TCP wrappers provide the following advantages over other network service control techniques:

Transparency to both the client host 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 configuration files for simpler management.

17.2. TCP Wrappers Configuration Files

To determine if a client machine 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 client request is received by a TCP wrapped service, it takes the following basic steps:

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.
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, access to the service is granted.

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. 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

17.2.1. Formatting Access Rules

The format for both /etc/hosts.allow and /etc/hosts.deny are identical. Any blank lines or lines that start with a hash mark (#) are ignored, and each rule must be on its own line.

Each rule uses the following basic format to control access to network services:

<daemon list>: <client list> [: <option>: <option>: ...]

<daemon list> — A comma separated list of process names (not service names) or the ALL wildcard (refer to Section 17.2.1.1, “Wildcards”). The daemon list also accepts operators (refer to Section 17.2.1.4, “Operators”) to allow greater flexibility.
<client list> — A comma separated list of hostnames, host IP addresses, special patterns (refer to Section 17.2.1.2, “Patterns”), or special wildcards (refer to Section 17.2.1.1, “Wildcards”) which identify the hosts effected by the rule. The client list also accepts operators listed in Section 17.2.1.4, “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 (refer to Section 17.2.2.4, “Expansions”), launch shell commands, allow or deny access, and alter logging behavior (refer to Section 17.2.2, “Option Fields”).

The following is a basic sample hosts access rule:

vsftpd : .example.com

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

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 (which logs the attempt to a special 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. For a more detailed look at available options, refer to Section 17.2.2, “Option Fields”.

17.2.1.1. Wildcards

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 may be used:

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 as a disruption in name resolution may prevent legitimate users from gaining access to a service.

17.2.1.2. Patterns

Patterns can be used in the client list field of access rules to more precisely specify groups of client hosts.

The following is a list of the most common accepted patterns for a client list entry:

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
```
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.
```
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
```
Important
When working in the IPv4 address space, the address/prefix length (prefixlen) pair declarations 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
```
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
```
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
```

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 in DNS service prevents even authorized users from using network services.

It is, therefore, best to use IP addresses whenever possible.

17.2.1.3. Portmap and TCP Wrappers

When creating access control rules for portmap, do not use hostnames as portmap's implementation of TCP wrappers does not support host look ups. For this reason, only use IP addresses or the keyword ALL when specifying hosts in hosts.allow or hosts.deny.

In addition, changes to portmap access control rules may not take affect immediately without restarting the portmap service.

Widely used services, such as NIS and NFS, depend on portmap to operate, so be aware of these limitations.

17.2.1.4. Operators

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

In the 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.

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.

17.2.2. Option Fields

In addition to basic rules allowing and denying access, the Red Hat Enterprise Linux implementation of TCP wrappers supports extensions to the access control language through option fields. By using option fields within hosts access rules, administrators can accomplish a variety of tasks such as altering log behavior, consolidating access control, and launching shell commands.

17.2.2.1. Logging

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

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

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.

17.2.2.2. Access Control

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 instance, 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

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 consider this an easier way of organizing access rules.

17.2.2.3. Shell Commands

Option fields allow access rules to launch shell commands through the following two directives:

spawn — Launches a shell command as a child process. This option 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
```
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 via the echo command:
```
vsftpd : .example.com \
: twist /bin/echo "421 Bad hacker, go away!"
```

For more information about shell command options, refer to the hosts_options man page.

17.2.2.4. Expansions

Expansions, when used in conjunction with the spawn and twist directives, provide information about the client, server, and processes involved.

Below is a list of supported expansions:

%a — Supplies the client's IP address.
%A — Supplies the server's IP address.
%c — Supplies a variety of client information, such as the username and hostname, or the username and IP address.
%d — Supplies the daemon process name.
%h — Supplies the client's hostname (or IP address, if the hostname is unavailable).
%H — Supplies the server's hostname (or IP address, if the hostname is unavailable).
%n — Supplies the client's hostname. If unavailable, unknown is printed. If the client's hostname and host address do not match, paranoid is printed.
%N — Supplies the server's hostname. If unavailable, unknown is printed. If the server's hostname and host address do not match, paranoid is printed.
%p — Supplies the daemon process ID.
%s —Supplies various types of server information, such as the daemon process and the host or IP address of the server.
%u — Supplies 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

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!"

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.

For additional information about TCP wrappers, refer to Section 17.5, “Additional Resources”. For more information about how to secure TCP wrappers, refer to the chapter titled Server Security in the Security Guide.

17.3. `xinetd`

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 host 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 and that the service is not consuming more than its alloted amount of resources or is in breach of any defined rules. It then starts an instance of the requested service and passes control of the connection to it. Once the connection is established, xinetd does not interfere further with communication between the client host and the server.

17.4. `xinetd` Configuration Files

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.

17.4.1. The `/etc/xinetd.conf` File

The /etc/xinetd.conf file contains general configuration settings which effect every service under xinetd's control. It is read once when the xinetd service is started, so for configuration changes to take effect, the administrator must restart the xinetd service. Below 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

These lines control the following aspects of xinetd:

instances — Sets the maximum number of requests xinetd can handle at once.
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 if the connection is successful. By default, the remote host's IP address and the process ID of server processing the request are recorded.
log_on_failure — Configures xinetd to log if there is a connection failure or if the connection is not allowed.
cps — Configures xinetd to allow no more than 25 connections per second to any given service. If this limit is reached, 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 17.4.2, “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 log files. For this reason, more information may appear in a given service's log than the /etc/xinetd.conf file may indicate. Refer to Section 17.4.3.1, “Logging Options” for additional information.

17.4.2. The `/etc/xinetd.d/` Directory

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 effect other services.

To gain an understanding of how these files are structured, consider the /etc/xinetd.d/telnet file:

service telnet
{
        flags           = REUSE
        socket_type     = stream
        wait            = no
        user            = root
        server          = /usr/sbin/in.telnetd
        log_on_failure  += USERID
        disable         = yes
}

These lines control various aspects of the telnet service:

service — Defines the service name, usually one 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.
socket_type — Sets the network socket type to stream.
wait — Defines whether the service is single-threaded (yes) or multi-threaded (no).
user — Defines what user ID the process runs under.
server — Defines the binary executable to be launched.
log_on_failure — Defines logging parameters for log_on_failure in addition to those already defined in xinetd.conf.
disable — Defines whether the service is active.

17.4.3. Altering `xinetd` Configuration Files

There are a large assortment of directives available for xinetd protected services. This section highlights some of the more commonly used options.

17.4.3.1. Logging Options

The following logging options are available for both /etc/xinetd.conf and the service-specific configuration files within the /etc/xinetd.d/ directory.

Below 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 and log_on_success).

For a complete list of logging options, refer to the xinetd.conf man page.

17.4.3.2. Access Control Options

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. Information concerning the use of TCP wrappers hosts access control files can be found in Section 17.2, “TCP Wrappers Configuration 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 within 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/sbin/in.telnetd
        log_on_failure  += USERID
        no_access       = 10.0.1.0/24
        log_on_success  += PID HOST EXIT
        access_times    = 09:45-16:15
}

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 a message stating the following message:

Connection closed by foreign host.

In addition, their login attempts are logged in /var/log/secure as follows:

May 15 17:38:49 boo xinetd[16252]: START: telnet pid=16256 from=10.0.1.2
May 15 17:38:49 boo xinetd[16256]: FAIL: telnet address from=10.0.1.2
May 15 17:38:49 boo xinetd[16252]: EXIT: telnet status=0 pid=16256

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 order of operations followed by xinetd when a client requests a connection:

The xinetd daemon accesses the TCP wrappers hosts access rules through a libwrap.a library call. If a deny rule matches the client host, the connection is dropped. If an allow rule matches the client host, the connection is passed on to xinetd.
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 host the connection is dropped. Otherwise, xinetd starts an instance of the requested service and passes control of the connection to it.

Important

Care should be taken when using TCP wrappers access controls in conjunction with xinetd access controls. Misconfiguration can cause undesired effects.

17.4.3.3. Binding and Redirection Options

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. Once configured, the bind option only allows requests for the proper IP address to access the service. In this way, different services can be bound to different network interfaces based on need.

This is particularly useful for systems with multiple network adapters or with multiple IP addresses configured. On such a system, insecure services, like Telnet, can be configured to listen only on the interface connected to a private network and not to the interface connected with 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 different IP address on the same machine, shift the request to a totally different system and port number, or any combination of these options. In this way, a user connecting to certain service on a system may be rerouted to another system with no 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.

But 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 specially 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/sbin/in.telnetd
        log_on_success		+= DURATION USERID
        log_on_failure		+= USERID
        bind                    = 123.123.123.123
        redirect                = 10.0.1.13 23
}

The bind and redirect options in this file ensures 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 send 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.

17.4.3.4. Resource Management Options

The xinetd daemon can add a basic level of protection from a Denial of Service (DoS) attacks. Below 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 of connections per second. This directive takes two integer arguments separated by white space. The first is the maximum number of connections allowed to the service per second. The second is the number of seconds xinetd must wait before re-enabling the service. 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.
max_load — Defines the CPU usage threshold for a service. It accepts a floating point number argument.

There are more resource management options available for xinetd. Refer to the chapter titled Server Security in the Security Guide for more information, as well as the xinetd.conf man page.

17.5. Additional Resources

Additional information concerning TCP wrappers and xinetd is available from system documentation and on the Internet.

17.5.1. Installed Documentation

The bundled documentation on your system is a good place to start looking for additional TCP Wrappers, xinetd, and access control configuration options.

/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 — There are a number of man pages for the various applications and configuration files involved with TCP wrappers and xinetd. The following lists some of the more important man pages.
Server Applications
man xinetd — The man page for the xinetd super service daemon.
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.

17.5.2. Useful Websites

http://www.xinetd.org/ — The home of xinetd, containing sample configuration files, a full listing of features, and an informative FAQ.
http://www.macsecurity.org/resources/xinetd/tutorial.shtml — A thorough tutorial that discusses many different ways to tweak default xinetd configuration files to meet specific security goals.

Chapter 18. `iptables`

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 introduction of the 2.4 kernel brought with it 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.

For instructions on constructing iptables rules or setting up a firewall based on these rules, refer to Section 18.7, “Additional Resources”.

Warning

The default firewall mechanism under the 2.4 and newer kernels is iptables, but iptables cannot be used if ipchains are 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.

18.1. Packet Filtering

The Linux kernel has the built-in ability to filter packets, allowing some of them to be received by or pass through the system while stopping others. The kernel's netfilter has three built-in tables or rules lists. They are 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.

Note

In addition to these built in tables, specialized tables can be created and stored in the /lib/modules/<kernel-version>/kernel/net/ipv4/netfilter/ directory (where <kernel-version> corresponds to the version kernel number).

Each table has a group of built-in chains which correspond to the actions performed on the packet by the 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

Do not use fully qualified domain names in firewall rules that are saved in the /etc/sysconfig/iptables or /etc/sysconfig/ip6tables files. In the following example: iptables -A FORWARD -s example.com -i eth0 -j DROP example.com is invalid because the iptables service starts before any DNS related services at boot time, which results in an error. Only IP addresses are valid in creating firewall rules.

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.

18.2. Differences between `iptables` and `ipchains`

At first glance, ipchains and iptables appear to be quite similar. Both methods of packet filtering use chains of rules operating within the Linux kernel to decide what to do with packets that match the specified rule or set of rules. However, iptables offers a more extensible way of filtering packets, giving the administrator a greater amount of control without building a great deal of complexity into the system.

Specifically, users comfortable with ipchains should be aware of the following significant differences between ipchains and iptables before attempting to use iptables:

Under iptables, each filtered packet is processed using rules from only one chain rather than multiple chains. For instance, a FORWARD packet coming into a system using ipchains would have to go through the INPUT, FORWARD, and OUTPUT chains to move along 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. For this reason, it is important to place the rule designed to catch a particular packet within the rule 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 under iptables.
Order matters when placing options in a rule. With ipchains, the order of the rule options does not matter. The iptables command uses stricter syntax. In iptables commands, the protocol (ICMP, TCP, or UDP) must be specified before the source or destination ports.
When specifying network interfaces to be used with a rule, you must only use incoming interfaces (-i option) with INPUT or FORWARD chains and outgoing interfaces (-o option) with FORWARD or OUTPUT chains. This is necessary because 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, given that iptables is a fundamentally rewritten network filter. For more specific information, refer to the Linux Packet Filtering HOWTO referenced in Section 18.7, “Additional Resources”.

18.3. Options Used within `iptables` Commands

Rules for filtering packets are put in place 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.

For more information on specific options which address these aspects of a packet, refer to Section 18.3.4, “iptables Match Options” and Section 18.3.5, “Target Options”.

The options used with given 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.

18.3.1. Structure of `iptables` Options

Many iptables commands have the following structure:

iptables [-t <table-name>] <command> <chain-name> <parameter-1> \
         <option-1> <parameter-n> <option-n>

The <table-name> option allows the user to select a table other than the default filter table to use with the command. The <command> option dictates a specific action to perform, such as appending or deleting the rule specified by the <chain-name> option. Following the <chain-name> are pairs of parameters and options that define what happens when a packet matches the rule.

When looking at the structure of an iptables command, it is important to remember that, unlike most other commands, the length and complexity of an iptables command can change based on its purpose. A command to remove a rule from a chain can be very short, while a command designed to filter packets from a particular subnet using a variety of specific parameters and options can be rather lengthy. When creating iptables commands, it is helpful to recognize that some parameters and options may create the need for other parameters and options to further specify the previous option's request. To construct a valid rule, this must continue until every parameter and option that requires another set of options is satisfied.

Type iptables -h to view a comprehensive list of iptables command structures.

18.3.2. Command Options

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 iptables rule to the end of the specified chain. This is the command used to add a rule when rule order in the chain does not matter.
-C — Checks a particular rule before adding it to the user-specified chain. This command can help you construct complicated iptables rules by prompting you for additional parameters and options.
-D — Deletes a rule in a particular chain by number (such as 5 for the fifth rule in a chain). You can also type the entire rule, and iptables deletes the rule in the chain that matches it.
-E — Renames a user-defined chain. This does not affect the structure of the table.
-F — Flushes the selected chain, which effectively deletes every rule in the 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 — Inserts a rule in a chain at a point specified by a user-defined integer value. If no number is specified, iptables places the command at the top of the chain.
Warning
Be aware when using the -A or -I option that the order of the rules within a chain are important for determining which rules apply to which packets.
-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>
```
Additional options for the -L command option, which provide rule numbers and allow more verbose rule descriptions, are described in Section 18.3.6, “Listing Options”.
-N — Creates a new chain with a user-specified name.
-P — Sets the default policy for the specified chain, so that when packets traverse an entire chain without matching a rule, they are sent on 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. Deleting a built-in chain for any table is not allowed.
-Z — Zeros the byte and packet counters in all chains for a table.

18.3.3. `iptables` Parameter Options

Once certain iptables commands are specified, including those used to add, append, delete, insert, or replace rules within a particular chain, parameters are required to construct a packet filtering rule.

-c — Resets the counters for a particular rule. This parameter accepts the PKTS and BYTES options to specify what 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.
By using the exclamation point character (!) option after this parameter, only unfragmented packets are matched.
-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. Valid targets to use after the -j option include standard options (ACCEPT, DROP, QUEUE, and RETURN) as well as extended options that are available through modules loaded by default with the Red Hat Enterprise Linux iptables RPM package, such as LOG, MARK, and REJECT, among others. Refer to the iptables man page for more information about these and other targets.
It is also possible to direct a packet matching this 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. However, the counter for this rule increases by one.
-o — Sets the outgoing network interface for a rule and may only be used with OUTPUT and FORWARD chains in the filter table, and the POSTROUTING chain in the nat and mangle tables. This parameter's options are the same as those of the incoming network interface parameter (-i).
-p — Sets the IP protocol for the rule, which can be either icmp, tcp, udp, or all, to match every supported protocol. In addition, any protocols listed in /etc/protocols may also be used. If this option is omitted when creating a rule, the all option is the default.
-s — Sets the source for a particular packet using the same syntax as the destination (-d) parameter.

18.3.4. `iptables` Match Options

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 tcp <protocol-name> (where <protocol-name> is the target protocol), makes options for the specified protocol available.

18.3.4.1. TCP Protocol

These match options are available for the TCP protocol (-p tcp):

--dport — Sets the destination port for the packet. Use either a network service name (such as www or smtp), port number, or range of port numbers to configure this option. 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.
To specify a range of port numbers, separate the two numbers with a colon (:), such as -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 which do not use that network service or port.
--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. Placing an exclamation point character (!) as a flag after the --syn option causes all non-SYN packets to be matched.
--tcp-flags — Allows TCP packets with specific set bits, or flags, to match a rule. The --tcp-flags match option accepts two parameters. The first parameter is the mask, which sets the flags to be examined in the packet. The second parameter refers to the flag that must be set to match.
The possible flags are:
- ACK
- FIN
- PSH
- RST
- SYN
- URG
- ALL
- NONE
For example, an iptables rule which contains -p tcp --tcp-flags ACK,FIN,SYN SYN only matches TCP packets that have the SYN flag set and the ACK and FIN flags unset.
Using the exclamation point character (!) after --tcp-flags reverses 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 (!).

18.3.4.2. UDP Protocol

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.

18.3.4.3. ICMP Protocol

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.

18.3.4.4. Additional Match Option Modules

Additional match options are also available through modules loaded by the iptables command. To use a match option module, load the module by name using the -m option, such as -m <module-name> (replacing <module-name> with the name of the module).

A large number of modules are available by default. It is even possible to create modules that 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. This is especially beneficial when used in conjunction with the LOG target as it can prevent a flood of matching packets from filling up the system log with repetitive messages or using up system resources. Refer to Section 18.3.5, “Target Options” for more information about the LOG target.
The limit module enables the following options:
- --limit — Sets the number of matches for a particular range of time, specified with a number and time modifier arranged in a <number>/<time> format. For example, using --limit 5/hour only lets a rule match 5 times in a single hour.
  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 should be used in conjunction with the --limit option, and it accepts a number to set the burst threshold.
  If no number is specified, only five packets are initially able to match the rule.
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.
  - 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.
  - RELATED — The matching packet is starting a new connection related in some way to an existing connection.
  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.

To view other match options available through modules, refer to the iptables man page.

18.3.5. Target Options

Once a packet has matched a particular rule, the rule can direct the packet to a number of different targets that decide its fate and, possibly, take additional actions. 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> — Replace <user-defined-chain> with the name of a user-defined chain within the table. This target passes the packet to the target chain.
ACCEPT — Allows the packet to successfully move on to its destination or 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 decides what action to take.

In addition to these standard targets, various other targets may be used with extensions called target modules. For more information about match option modules, refer to Section 18.3.4.4, “Additional Match Option Modules”.

There are many extended target modules, most of which only apply to specific tables or situations. A couple of the most popular target modules included by default in Red Hat Enterprise Linux are:

LOG — Logs all packets that match this rule. Since 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. A list of priority levels can be found within the syslog.conf man page.
- --log-ip-options — Logs any options set in the header of a 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.
- --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 sent back with the error packet. The message port-unreachable is the default <type> error given if no other option is used. For a full list of <type> options, refer to the iptables man page.

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.

18.3.6. Listing Options

The default list command, iptables -L, 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 seen, 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 seen by a particular chain or rule may be abbreviated using K (thousands), M (millions), and G (billions) at the end of the number. 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 — Specifies a table name.

18.4. Saving `iptables` Rules

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 system reboot, they need to be saved. To do this, log in as root and type:

 /sbin/service iptables save

This executes the iptables initscript, 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.

Important

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

18.5. `iptables` Control Scripts

There are two basic methods for controlling iptables under Red Hat Enterprise Linux:

Security Level Configuration Tool (system-config-securitylevel) — A graphical interface for creating, activating, and saving basic firewall rules. For more information about how to use this tool, refer to the chapter titled Basic Firewall Configuration in the System Administrators Guide.
/sbin/service iptables <option> — A command issued by the root user capable of activating, deactivating, and performing other functions of iptables via its initscript. Replace <option> in the command with one of the following directives:
- start — If a firewall is configured (meaning /etc/sysconfig/iptables exists), all running iptables are stopped completely and then started using the /sbin/iptables-restore command. The start directive only works if the ipchains kernel module is not loaded.
- 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 within 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 18.5.1, “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. The restart directive only works if the ipchains kernel module is not loaded.
  If the IPTABLES_SAVE_ON_RESTART directive within 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 18.5.1, “iptables Control Scripts Configuration File” for more information about the iptables-config file.
- status — Prints to the shell prompt the status of the firewall and a list of all active rules. If no firewall rules are loaded or configured, it indicates this fact.
  A listing of active rules containing IP addresses within rule lists unless the default value for IPTABLES_STATUS_NUMERIC is changed to no within the /etc/sysconfig/iptables-config configuration file. This change would revert status output to domain and hostname information. Refer to Section 18.5.1, “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.
- save — Saves firewall rules to /etc/sysconfig/iptables using iptables-save. Refer to Section 18.4, “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 18.6, “ip6tables and IPv6”.

18.5.1. `iptables` Control Scripts Configuration File

The behavior of the iptables initscripts is controlled by the /etc/sysconfig/iptables-config configuration file. The following is a list of directives contained within 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 a status output 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.

18.6. `ip6tables` and IPv6

If the iptables-ipv6 package is installed, netfilter under 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.

Saved rules for ip6tables are stored in the /etc/sysconfig/ip6tables file. Old rules saved by the ip6tables initscripts are saved in the /etc/sysconfig/ip6tables.save file.

The configuration file for ip6tables initscript is /etc/sysconfig/ip6tables-config and the names for each directive vary slightly. For instance, the iptables-config directive IPTABLES_MODULES is IP6TABLES_MODULES in the ip6tables-config file.

18.7. Additional Resources

Refer to the following sources for additional information on packet filtering with iptables.

Security Guide; Red Hat, Inc — Contains a chapter about the role of firewalls within an overall security strategy as well as strategies for constructing firewall rules.
System Administrators Guide; Red Hat, Inc — Contains a chapter about configuring firewalls using Security Level Configuration Tool.

18.7.1. Installed Documentation

man iptables — Contains a description of iptables as well as a comprehensive list of targets, options, and match extensions.

18.7.2. Useful Websites

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 — A basic and general look at the way packets move through the Linux kernel, plus an introduction to constructing basic iptables commands.
http://www.redhat.com/support/resources/networking/firewall.html — This webpage links to a variety of update-to-date packet filter resources.

Chapter 19. Kerberos

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. Moreover, 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 under the 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.

19.1. What is Kerberos?

Kerberos, a network authentication protocol created by MIT, uses symmetric-key cryptography^[5] to authenticate users to network services — eliminating the need to send passwords over the network. When users authenticate to network services using Kerberos, unauthorized users attempting to gather passwords by monitoring network traffic are effectively thwarted.

19.1.1. Advantages of Kerberos

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, once a network is connected to the Internet, it can no longer be assumed that the network is 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 sent in this manner, 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 packet sniffers would otherwise pose on a network.

19.1.2. Disadvantages of Kerberos

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. For more information, refer to question number 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. For more information about this issue, refer to Section 19.4, “Kerberos and PAM”.
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 sent 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 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 side 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. Once Kerberos is used on the network, any unencrypted passwords transferred to a non-kerberized 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 kerberized versions of all client/server applications which send unencrypted passwords or not use any such client/server applications at all.

19.2. Kerberos Terminology

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 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. While more secure than unencrypted data, it is fairly 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: A text generated number used to ensure that transmitted data has not been tampered with.
key: Data used when encrypting or decrypting other data. Encrypted data cannot be decrypted without the proper key or extremely good guessing.
key distribution center (KDC): A service that issues Kerberos tickets, 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). For more information about using the kinit command, refer to its man page.
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.

19.3. How Kerberos Works

Kerberos differs from username/password authentication methods because instead of authenticating each user to each network service, it uses symmetric encryption and a trusted third party, a KDC, to authenticate users to a suite of network services. Once a user authenticates to the KDC, it sends a ticket specific to that session back the user's machine and any kerberized services look for the ticket on the user's machine rather than asking the user to authenticate using a password.

When a user on a kerberized network logs in to their workstation, their principal is sent to the KDC in a request for a TGT from AS. This request can be sent by the login program so that it is transparent to the user or can be sent by the kinit program after the user logs in.

The KDC 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 machine 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 sent over the network.

The TGT is set to expire after a certain period of time (usually ten hours) and 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. Once the TGT is issued, the user does not have to re-enter their password until the TGT expires or they logout and login 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 any time any user on the network authenticates against a non-kerberized service by sending a password in plain text. Use of non-kerberized services is discouraged. Such services include Telnet and FTP. Use of other encrypted protocols, such as SSH or SSL secured services, however, is acceptable, though not ideal.

This is only a broad overview of how Kerberos authentication works. Those seeking a more in-depth look at Kerberos authentication should refer to Section 19.7, “Additional Resources”.

Note

Kerberos depends on certain network services to work correctly. First, Kerberos requires approximate clock synchronization between the machines on the network. Therefore, a clock synchronization program should be set up for the network, such as ntpd. For more about configuring ntpd, refer to /usr/share/doc/ntp-<version-number>/index.htm for details on setting up Network Time Protocol servers (replace <version-number> with the version number of the ntp package installed on the system).

Also, since certain aspects of Kerberos rely on the Domain Name Service (DNS), be sure that the DNS entries and hosts on the network are all properly configured. Refer to the Kerberos V5 System Administrator's Guide, provided in PostScript and HTML formats in /usr/share/doc/krb5-server-<version-number> for more information (replace <version-number> with the version number of the krb5-server package installed on the system).

19.4. Kerberos and PAM

Currently, kerberized services do not make use of Pluggable Authentication Modules (PAM) — kerberized servers 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 like login and gdm to authenticate users as well as obtain initial credentials using their passwords. If access to network servers is always performed using kerberized services or services that use GSS-API, such as IMAP, the network can be considered reasonably safe.

Note

Administrators should be careful to not 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 using their Kerberos passwords over Telnet.

19.5. Configuring a Kerberos 5 Server

When setting up Kerberos, install the server first. If it is necessary to set up slave servers, the details of setting up relationships between master and slave servers are covered in the Kerberos 5 Installation Guide located in the /usr/share/doc/krb5-server-<version-number> directory (replace <version-number> with the version number of the krb5-server package installed on the system).

To configure a basic Kerberos server, follow these steps:

Be sure that clock synchronization and DNS are functioning on all client and server machines before configuring Kerberos 5. Pay particular attention to time synchronization between the Kerberos server and its clients. If the server and client clocks are different by more than five minutes (this default amount is configurable in Kerberos 5), Kerberos clients can not authenticate to the server. This clock 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.htm for details about how to set up Network Time Protocol servers and http://www.ntp.org for additional information about NTP.
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.
If a graphical user interface is required to administrate Kerberos, install the gnome-kerberos package. It contains krb5, a GUI tool for managing tickets.
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.
Create the database using the kdb5_util utility from a shell prompt:
```
/usr/kerberos/sbin/kdb5_util create -s
```
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.
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  *
```
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.
Once kadmind is 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. For this reason, 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"
```

Start Kerberos using the following commands:

service krb5kdc start
service kadmin start
service krb524 start

Add principals for the users using the addprinc command with kadmin. kadmin and kadmin.local are command line interfaces to the KDC. As such, many commands are available after launching the kadmin program. Refer to the kadmin man page for more information.
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.

19.6. Configuring a Kerberos 5 Client

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. Kerberized versions of rsh and rlogin also requires some configuration changes.

Be sure that time synchronization is in place between the Kerberos client and the KDC. Refer to Section 19.5, “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.
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).
Before a workstation in the realm can allow users to connect using kerberized rsh and rlogin, that workstation must have the xinetd package installed and have its own host principal in the Kerberos database. The kshd and klogind server programs also need access to the keys for their service's principal.
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
```
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
```
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:
- 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).
  The dovecot package also contains an IMAP server alternative to cyrus-imap, which is also included with Red Hat Enterprise Linux, but does not support GSS-API and Kerberos to date.
- CVS — To use a kerberized CVS server, gserver uses a principal with a root of cvs and is otherwise identical to the CVS pserver.
For details about how to enable services, refer to the chapter titled Controlling Access to Services in the System Administrators Guide.

19.7. Additional Resources

For more information about Kerberos, refer to the following resources.

19.7.1. Installed Documentation

The /usr/share/doc/krb5-server-<version-number>/ directory — The Kerberos V5 Installation Guide and the Kerberos V5 System Administrator's Guide in PostScript and HTML formats. The krb5-server package must be installed.
The /usr/share/doc/krb5-workstation-<version-number>/ directory — The Kerberos V5 UNIX User's Guide in PostScript and HTML formats. The krb5-workstation package must be installed.
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.

19.7.2. Useful Websites

http://web.mit.edu/kerberos/www/ — Kerberos: The Network Authentication Protocol webpage from MIT.
http://www.nrl.navy.mil/CCS/people/kenh/kerberos-faq.html — The Kerberos Frequently Asked Questions (FAQ).
ftp://athena-dist.mit.edu/pub/kerberos/doc/usenix.PS — The PostScript version of Kerberos: An Authentication Service for Open Network Systems by Jennifer G. Steiner, Clifford Neuman, and Jeffrey I. Schiller. This document is the original paper describing Kerberos.
http://web.mit.edu/kerberos/www/dialogue.html — Designing an Authentication System: a Dialogue in Four Scenes originally by Bill Bryant in 1988, modified by Theodore Ts'o in 1997. This document is a conversation between two developers who are thinking through the creation of a Kerberos-style authentication system. The conversational style of the discussion make this a good starting place for people who are completely unfamiliar with Kerberos.
http://www.ornl.gov/~jar/HowToKerb.html — How to Kerberize your site is a good reference for kerberizing a network.
http://www.networkcomputing.com/netdesign/kerb1.html — Kerberos Network Design Manual is a thorough overview of the Kerberos system.

^[5] A system where both the client and the server share a common key that is used to encrypt and decrypt network communication.

Chapter 20. SSH Protocol

SSH™ (or Secure SHell) is a protocol which facilitates secure communications between two systems using a client/server architecture and allows users to log into server host systems remotely. Unlike other remote communication protocols, such as FTP or Telnet, SSH encrypts the login session, making it impossible for intruders to collect unencrypted passwords.

SSH is designed to replace older, less secure terminal applications used to log into remote hosts, such as telnet or rsh. A related program called scp replaces older programs designed to copy files between hosts, such as rcp. Because these older applications do not encrypt passwords transmitted between the client and the server, avoid them whenever possible. Using secure methods to log into remote systems decreases the risks for both the client system and the remote host.

20.1. Features of SSH

The SSH protocol provides the following safeguards:

After an initial connection, the client can verify that it is connecting to the same server it had connected to previously.
The client transmits its authentication information to the server using strong, 128-bit encryption.
All data sent and received during a session is transferred using 128-bit encryption, making intercepted transmissions extremely difficult to decrypt and read.
The client can forward X11^[6] applications from the server. This technique, called X11 forwarding, provides a secure means to use graphical applications over a network.

Because the SSH protocol encrypts everything it sends and receives, it can be used to secure otherwise insecure protocols. Using a technique called port forwarding, an SSH server can become a conduit to securing otherwise insecure protocols, like POP, and increasing overall system and data security.

Red Hat Enterprise Linux includes the general OpenSSH package (openssh) as well as the OpenSSH server (openssh-server) and client (openssh-clients) packages. Refer to the chapter titled OpenSSH in the System Administrators Guide for instructions on installing and deploying OpenSSH. Note, the OpenSSH packages require the OpenSSL package (openssl) which installs several important cryptographic libraries, enabling OpenSSH to provide encrypted communications.

20.1.1. Why Use SSH?

Nefarious computer users have a variety of tools at their disposal enabling them to disrupt, intercept, and re-route network traffic in an effort to gain access to a system. In general terms, these threats can be categorized as follows:

Interception of communication between two systems — In this scenario, the attacker can be somewhere on the network between the communicating entities, copying any information passed between them. The attacker may intercept and keep the information, or alter the information and send it on to the intended recipient.
This attack can be mounted through the use of a packet sniffer — a common network utility.
Impersonation of a particular host — Using this strategy, an attacker's system is configured to pose as the intended recipient of a transmission. If this strategy works, the user's system remains unaware that it is communicating with the wrong host.
This attack can be mounted through techniques known as DNS poisoning^[7] or IP spoofing^[8].

Both techniques intercept potentially sensitive information and, if the interception is made for hostile reasons, the results can be disastrous.

If SSH is used for remote shell login and file copying, these security threats can be greatly diminished. This is because the SSH client and server use digital signatures to verify their identity. Additionally, all communication between the client and server systems is encrypted. Attempts to spoof the identity of either side of a communication does not work, since each packet is encrypted using a key known only by the local and remote systems.

20.2. SSH Protocol Versions

The SSH protocol allows any client and server programs built to the protocol's specifications to communicate securely and to be used interchangeably.

Two varieties of SSH (version 1 and version 2) currently exist. SSH version 1 makes use of several patented encryption algorithms (however, some of these patents have expired) and is vulnerable to a well known security exploit that allows an attacker to insert data into the communication stream. The OpenSSH suite under Red Hat Enterprise Linux uses SSH version 2 which has an enhanced key exchange algorithm not vulnerable to the exploit in version 1. However, the OpenSSH suite does support version 1 connections.

Important

It is recommended that only SSH version 2-compatible servers and clients are used whenever possible.

20.3. Event Sequence of an SSH Connection

The following series of events help protect the integrity of SSH communication between two hosts.

A cryptographic handshake is made so that the client can verify that it is communicating with the correct server.
The transport layer of the connection between the client and remote host is encrypted using a symmetric cipher.
The client authenticates itself to the server.
The remote client interacts with the remote host over the encrypted connection.

20.3.1. Transport Layer

The primary role of the transport layer is to facilitate safe and secure communication between the two hosts at the time of authentication and during subsequent communication. The transport layer accomplishes this by handling the encryption and decryption of data, and by providing integrity protection of data packets as they are sent and received. The transport layer also provides compression, speeding the transfer of information.

Once an SSH client contacts a server, key information is exchanged so that the two systems can correctly construct the transport layer. The following steps occur during this exchange:

Keys are exchanged
The public key encryption algorithm is determined
The symmetric encryption algorithm is determined
The message authentication algorithm is determined
The hash algorithm is determined

During the key exchange, the server identifies itself to the client with a unique host key. If the client has never communicated with this particular server before, the server's host key is unknown to the client and it does not connect. OpenSSH gets around this problem by accepting the server's host key after the user is notified and verifies the acceptance of the new host key. In subsequent connections, the server's host key is checked against the saved version on the client, providing confidence that the client is indeed communicating with the intended server. If, in the future, the host key no longer matches, the user must remove the client's saved version before a connection can occur.

Warning

It is possible for an attacker to masquerade as an SSH server during the initial contact since the local system does not know the difference between the intended server and a false one set up by an attacker. To help prevent this, verify the integrity of a new SSH server by contacting the server administrator before connecting for the first time or in the event of a host key mismatch.

SSH is designed to work with almost any kind of public key algorithm or encoding format. After an initial key exchange creates a hash value used for exchanges and a shared secret value, the two systems immediately begin calculating new keys and algorithms to protect authentication and future data sent over the connection.

After a certain amount of data has been transmitted using a given key and algorithm (the exact amount depends on the SSH implementation), another key exchange occurs, generating another set of hash values and a new shared secret value. Even if an attacker is able to determine the hash and shared secret value, this information is only useful for a limited period of time.

20.3.2. Authentication

Once the transport layer has constructed a secure tunnel to pass information between the two systems, the server tells the client the different authentication methods supported, such as using a private key-encoded signature or typing a password. The client then tries to authenticate itself to the server using one of these supported methods.

SSH servers and clients can be configured to allow different types of authentication, which gives each side the optimal amount of control. The server can decide which encryption methods it supports based on its security model, and the client can choose the order of authentication methods to attempt from the available options. Thanks to the secure nature of the SSH transport layer, even seemingly insecure authentication methods, such as a host and password-based authentication, are safe to use.

20.3.3. Channels

After a successful authentication over the SSH transport layer, multiple channels are opened via a technique called multiplexing^[9]. Each of these channels handles communication for different terminal sessions and for forwarded X11 sessions.

Both clients and servers can create a new channel. Each channel is then assigned a different number on each end of the connection. When the client attempts to open a new channel, the clients sends the channel number along with the request. This information is stored by the server and is used to direct communication to that channel. This is done so that different types of sessions do not affect one another and so that when a given session ends, its channel can be closed without disrupting the primary SSH connection.

Channels also support flow-control, which allows them to send and receive data in an orderly fashion. In this way, data is not sent over the channel until the client receives a message that the channel is open.

The client and server negotiate the characteristics of each channel automatically, depending on the type of service the client requests and the way the user is connected to the network. This allows great flexibility in handling different types of remote connections without having to change the basic infrastructure of the protocol.

20.4. OpenSSH Configuration Files

OpenSSH has two different sets of configuration files: one for client programs (ssh, scp, and sftp) and one for the server daemon (sshd).

System-wide SSH configuration information is stored in the /etc/ssh/ directory:

moduli — Contains Diffie-Hellman groups used for the Diffie-Hellman key exchange which is critical for constructing a secure transport layer. When keys are exchanged at the beginning of an SSH session, a shared, secret value is created which cannot be determined by either party alone. This value is then used to provide host authentication.
ssh_config — The system-wide default SSH client configuration file. It is overridden if one is also present in the user's home directory (~/.ssh/config).
sshd_config — The configuration file for the sshd daemon.
ssh_host_dsa_key — The DSA private key used by the sshd daemon.
ssh_host_dsa_key.pub — The DSA public key used by the sshd daemon.
ssh_host_key — The RSA private key used by the sshd daemon for version 1 of the SSH protocol.
ssh_host_key.pub — The RSA public key used by the sshd daemon for version 1 of the SSH protocol.
ssh_host_rsa_key — The RSA private key used by the sshd daemon for version 2 of the SSH protocol.
ssh_host_rsa_key.pub — The RSA public key used by the sshd for version 2 of the SSH protocol.

User-specific SSH configuration information is stored in the user's home directory within the ~/.ssh/ directory:

authorized_keys — This file holds a list of authorized public keys for servers. When the client connects to a server, the server authenticates the client by checking its signed public key stored within this file.
id_dsa — Contains the DSA private key of the user.
id_dsa.pub — The DSA public key of the user.
id_rsa — The RSA private key used by ssh for version 2 of the SSH protocol.
id_rsa.pub — The RSA public key used by ssh for version 2 of the SSH protocol
identity — The RSA private key used by ssh for version 1 of the SSH protocol.
identity.pub — The RSA public key used by ssh for version 1 of the SSH protocol.
known_hosts — This file contains DSA host keys of SSH servers accessed by the user. This file is very important for ensuring that the SSH client is connecting the correct SSH server.
Important
If an SSH server's host key has changed, the client notifys the user that the connection cannot proceed until the server's host key is deleted from the known_hosts file using a text editor. Before doing this, however, contact the system administrator of the SSH server to verify the server is not compromised.

Refer to the ssh_config and sshd_config man pages for information concerning the various directives available in the SSH configuration files.

20.5. More Than a Secure Shell

A secure command line interface is just the beginning of the many ways SSH can be used. Given the proper amount of bandwidth, X11 sessions can be directed over an SSH channel. Or, by using TCP/IP forwarding, previously insecure port connections between systems can be mapped to specific SSH channels.

20.5.1. X11 Forwarding

Opening an X11 session over an established SSH connection is as easy as running an X program on a local machine. When an X program is run from the secure shell prompt, the SSH client and server create a new secure channel, and the X program data is sent over that channel to the client machine transparently.

X11 forwarding can be very useful. For example, X11 forwarding can be used to create a secure, interactive session with up2date. To do this, connect to the server using ssh and type:

up2date &

After supplying the root password for the server, you will be allowed to safely update the remote system.

20.5.2. Port Forwarding

SSH can secure otherwise insecure TCP/IP protocols via port forwarding. When using this technique, the SSH server becomes an encrypted conduit to the SSH client.

Port forwarding works by mapping a local port on the client to a remote port on the server. SSH can map any port from the server to any port on the client; port numbers do not need to match for this technique to work.

To create a TCP/IP port forwarding channel which listens for connections on the localhost, use the following command:

ssh -L local-port:remote-hostname:remote-port username@hostname

Note

Setting up port forwarding to listen on ports below 1024 requires root level access.

To check email on a server called mail.example.com using POP3 through an encrypted connection, use the following command:

ssh -L 1100:mail.example.com:110 mail.example.com

Once the port forwarding channel is in place between the client machine and the mail server, direct a POP3 mail client to use port 1100 on the localhost to check for new mail. Any requests sent to port 1100 on the client system are directed securely to the mail.example.com server.

If mail.example.com is not running an SSH server, but another machine on the same network is, SSH can still be used to secure part of the connection. However, a slightly different command is necessary:

ssh -L 1100:mail.example.com:110 other.example.com

In this example, POP3 requests from port 1100 on the client machine are forwarded through the SSH connection on port 22 to the SSH server, other.example.com. Then, other.example.com connects to port 110 on mail.example.com to check for new mail. Note, when using this technique only the connection between the client system and other.example.com SSH server is secure.

Port forwarding can also be used to get information securely through network firewalls. If the firewall is configured to allow SSH traffic via its standard port (22) but blocks access to other ports, a connection between two hosts using the blocked ports is still possible by redirecting their communication over an established SSH connection.

Note

Using port forwarding to forward connections in this manner allows any user on the client system to connect to that service. If the client system becomes compromised, the attacker also has access to forwarded services.

System administrators concerned about port forwarding can disable this functionality on the server by specifying a No parameter for the AllowTcpForwarding line in /etc/ssh/sshd_config and restarting the sshd service.

20.6. Requiring SSH for Remote Connections

For SSH to be truly effective, using insecure connection protocols, such as Telnet and FTP, should be prohibited. Otherwise, a user's password may be protected using SSH for one session, only to be captured later while logging in using Telnet.

Some services to disable include:

telnet
rsh
rlogin
vsftpd

To disable insecure connection methods to the system, use the command line program chkconfig, the ncurses-based program /usr/sbin/ntsysv, or the Services Configuration Tool (system-config-services) graphical application. All of these tools require root level access.

For more information on runlevels and configuring services with chkconfig, /usr/sbin/ntsysv, and the Services Configuration Tool, refer to the chapter titled Controlling Access to Services in the System Administrators Guide.

20.7. Additional Resources

For more information about SSH, refer to the following resources.

20.7.1. Installed Documentation

The /usr/share/doc/openssh-<version-number>/ directory — Replace <version-number> with the installed version of the OpenSSH package. This directory contains a README with basic information about the OpenSSH project and a file called RFC.nroff with general information about the SSH protocol.
SSH related man pages — There are a number of man pages for the various applications and configuration files involved with SSH. The following is a list of some of the more important man pages.
Client Applications
man ssh — Describes how to use this command to connect to an SSH server.
man scp — Describes how to use this command to copy files to and from an SSH server.
man sftp — Describes how to use this command to interactively copy files to and from and SSH server.
Server Applications
man sshd — Describes available command line options for the SSH server.
Configuration Files
man ssh_config — Describes the format and options available within the configuration file for SSH clients.
man sshd_config — Describes the format and options available within the configuration file for the SSH server.

20.7.2. Useful Websites

http://www.openssh.com — The OpenSSH FAQ page, bug reports, mailing lists, project goals, and a more technical explanation of the security features.
http://www.openssl.org — The OpenSSL FAQ page, mailing lists, and a description of the project goal.
http://www.freessh.org — SSH client software for other platforms.

^[6] X11 refers to the X11R6.7 windowing display system, traditionally referred to as the X Window System or X. Red Hat Enterprise Linux includes XFree86, an open source X Window System.

^[7] DNS poisoning occurs when an intruder cracks a DNS server, pointing client systems to a maliciously duplicated host.

^[8] IP spoofing occurs when an intruder sends network packets which falsely appear to be from a trusted host on the network.

^[9] A multiplexed connection consists of several signals being sent over a shared, common medium. With SSH, different channels are sent over a common secure connection.

Chapter 21. SELinux

Security-Enhanced Linux, or SELinux, is a security architecture integrated into the current 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.

21.1. Introduction to SELinux

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 an SELinux 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 subjects and objects using a security policy that specifies how strict or lenient a given Red Hat Enterprise Linux installation should be.

For the most part, SELinux is almost completely invisible to system users. Only system administrators must worry about how strict a policy to implement for their server environment. The policy can be as strict or lenient as needed, and is very finely detailed. This detail gives the SELinux kernel complete, granular control over the entire system.

When a subject such as an application attempts to access an object such as 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. Subjects and objects gain their security context from installed policy, which also provides the information to populate the security server's matrix.

In addition to running in an enforcing mode, SELinux can run in a permissive mode, where the AVC is checked and denials are logged, but SELinux does not enforce the policy.

For more information about how SELinux works, refer to Section 21.3, “ Additional Resources ”.

21.2. Files Related to SELinux

The following sections describe SELinux configuration files and related file systems.

21.2.1. The `/selinux/` Pseudo-File System

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.

In most cases, administrators and users do not need to manipulate this component compared to other SELinux files and directories.

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

For example, running the cat command on the enforce file reveals either a 1 for enforcing mode or 0 for permissive mode.

21.2.2. SELinux Configuration Files

The following sections describe SELinux configuration and policy files, and related file systems located in the /etc/ directory.

21.2.2.1. The `/etc/sysconfig/selinux` Configuration File

There are two ways to configure SELinux under Red Hat Enterprise Linux: using the Security Level Configuration Tool (system-config-securitylevel), 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 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 will be logged, as subjects will be able to continue with actions otherwise denied in enforcing mode. For example, traversing a directory tree will produce multiple avc: denied messages for every directory level read, where a kernel in enforcing mode 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 cause the file system to no longer have the proper security context as defined by the policy. Running fixfiles relabel prior to enabling SELinux will relabel the file system so that SELinux works properly when enabled. For more information, refer to the fixfiles(8) manpage.
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 spaces.
SELINUXTYPE=<targeted|strict> — Specifies which policy is currently being enforced by SELinux.
- 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.
  The policy files for these daemons can be found in /etc/selinux/targeted/src/policy/domains/program and 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 Security Level Configuration Tool (system-config-securitylevel). Switching a Boolean value for a targeted daemon disables the policy transition for the daemon, which prevents, for example, init from transitioning dhcpd from the unconfined_t domain to the domain specified in dhcpd.te. The domain unconfined_t allows subjects and objects with that security context to run under standard Linux security.
- strict — Full SELinux protection, for all daemons. Security contexts are defined for all subjects and objects, and every single action is processed by the policy enforcement server.

21.2.2.2. The `/etc/selinux/` Directory

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

The two subdirectories, strict/ and targeted/, are the specific directories where the policy files of the same name (i.e., strict and targeted) are contained.

For more information on SELinux policy and policy configuration, refer to the rhel-selg.

21.2.3. SELinux Utilities

The following are some of the most commonly used SELinux utilities:

/usr/bin/setenforce — Modifies in real-time the mode SELinux is running. By executing setenforce 1, SELinux is put in enforcing mode. By executing setenforce 0, SELinux is put in permissive mode. To actually disable SELinux, you need to either set the parameter in /etc/sysconfig/selinux or pass the parameter selinux=0 to the kernel, either in /etc/grub.conf or at boot time.
/usr/bin/sestatus -v — Gets the detailed status of a system running SELinux. The following example shows an excerpt of sestatus output:
```
SELinux status:         enabled
SELinuxfs mount:        /selinux
Current mode:           enforcing
Policy version:         18
```
/usr/bin/newrole — Runs a new shell in a new context, or role. Policy must allow the transition to the new role.
/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.

For more information on all binary utilities available, refer to the setools or policycoreutils package contents by running rpm -ql <package-name>, where <package-name> is the name of the specific package.

21.3. Additional Resources

The following sections give you the means to explore SELinux in greater detail.

21.3.1. Installed Documentation

/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.

21.3.2. Red Hat Documentation

Red Hat SELinux Guide; — Explains what SELinux is and explains how to work with SELinux.

21.3.3. Useful Websites

http://www.nsa.gov/selinux/ — Homepage for the NSA SELinux development team. Many resources are available in HTML and PDF formats. Although many of these links are not Red Hat Enterprise Linux specific, some concepts may apply.
http://fedora.redhat.com/docs/ — Homepage for the Fedora documentation project, which contains Fedora Core specific materials that may be more timely, since the release cycle is much shorter.
http://selinux.sourceforge.net — Homepage for the SELinux community.

Part IV. Appendixes

Chapter 22. General Parameters and Modules

This chapter is provided to illustrate some of the possible parameters available for common hardware device drivers^[10], 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.

22.1. Kernel Module Utilities

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

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

/sbin/modprobe e100

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:

/sbin/modprobe -v e100

Output similar to the following is displayed:

/sbin/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_'

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

/sbin/rmmod e100

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:

/sbin/modinfo [options] <module>

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).

22.2. Persistent Module Loading

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

Note

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

22.3. Specifying Module Parameters

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, paramater options, and aliases.

22.4. Storage parameters

Table 22.1. 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 registratons.
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"

22.5. Ethernet Parameters

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 paramaters can be viewed using the modinfo command.

Note

For information about using these tools, consult the man pages for ethtool, mii-tool, and modinfo.

Table 22.2. 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)

22.5.1. Using Multiple Ethernet Cards

It is possible to use multiple Ethernet cards on a single machine. For each card there must be an alias and, possibly, options lines for each card in /etc/modprobe.conf.

For additional information about using multiple Ethernet cards, refer to the Linux Ethernet-HOWTO online at http://www.redhat.com/mirrors/LDP/HOWTO/Ethernet-HOWTO.html.

22.5.2. The Channel Bonding Module

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:

Add the following line to /etc/modprobe.conf:
alias bond<N> bonding
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.
Configure a channel bonding interface as outlined in Section 8.2.3, “Channel Bonding Interfaces”.
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 22.5.2.1, “bonding Module Directives” for a listing of available options.
After testing, place preferred module options in /etc/modprobe.conf.

22.5.2.1. `bonding` Module Directives

Before finalizing the settings for the bonding module, it is a good idea to test which settings work best. To do this, open a shell prompt as root and type:

tail -f /var/log/messages

Open another shell prompt and use the /sbin/insmod command to load the bonding module with different parameters while observing the kernel messages for errors.

The /sbin/insmod command is issued in the following format:

/sbin/insmod bond<N> <parameter=value>

Replace <N> with the number for the bonding interface. Replace <parameter=value> with a space separated list of desired parameters for the interface.

Once satisfied that there are no errors and after verifying the performance of the bonding interface, add the appropriate bonding module parameters to /etc/modprobe.conf.

The following is a list of available parameters for the bonding module:

mode= — Specifies one of four policies allowed for the bonding module. Acceptable values for this parameter are:
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.
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.
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.
3 — Sets a broadcast policy for fault tolerance. All transmissions are sent on all slave interfaces.
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.
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.
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.
miimon= — 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:"
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
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.
downdelay= — 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.
updelay= — 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.
arp_interval= — Specifies (in milliseconds) how often ARP monitoring occurs.
If using this setting while in mode 0 or 2 (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= — 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.
primary= — 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.

Important

It is essential that either the arp_interval and arp_ip_target or miimon parameters are specified. Failure to due so can cause degradation of network performance in the event a link fails.

Refer to the following file for more information (note that you must have the kernel-doc package installed to read this file):

/usr/share/doc/kernel-doc-<kernel-version>/Documentation/networking/bonding.txt

for detailed instructions regarding bonding interfaces.

22.6. Additional Resources

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

22.6.1. Installed Documentation

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.

22.6.2. Useful Websites

http://tldp.org/HOWTO/Module-HOWTO/ — Linux Loadable Kernel Module HOWTO from the Linux Documentation Project.

^[10] A driver is software which enables Linux to use a particular hardware device. Without a driver, the kernel cannot communicate with attached devices.

Appendix A. Revision History

Revision History

Revision 4-9.33.402 Fri Oct 25 2013 Rüdiger Landmann

Rebuild with Publican 4.0.0

Revision 4-9.33 2012-07-22 Anthony Towns

Rebuild for Publican 3.0

Revision 3-0 Wed Feb 16 2011 Jaromír Hradílek

Corrected a typing error in the vsftpd configuration.

Corrected several formatting issues.

Re-included missing images.

Revision 2-0 Wed Nov 26 2008 Don Domingo

Removed computeroutput tags nested inside screens, was causing ugly one-liner screens.

Index

Symbols

.fetchmailrc, Fetchmail Configuration Options

global options, Global Options
server options, Server Options
user options, User Options

.procmailrc, Procmail Configuration

/boot/ directory, The /boot/ Directory

/etc/named.conf (see BIND)

/etc/pam.conf, PAM Configuration Files

(see also PAM)

/etc/pam.d, PAM Configuration Files

(see also PAM)

/etc/sysconfig/ directory (see sysconfig directory)

/lib/security/, PAM Configuration Files

(see also PAM)

/lib64/security/, PAM Configuration Files

(see also PAM)

/proc/ directory (see proc file system)

A

aboot, Boot Loaders for Other Architectures

access control, TCP Wrappers and xinetd

AccessFileName

Apache configuration directive, AccessFileName

Action

Apache configuration directive, Action

AddDescription

Apache configuration directive, AddDescription

AddEncoding

Apache configuration directive, AddEncoding

AddHandler

Apache configuration directive, AddHandler

AddIcon

Apache configuration directive, AddIcon

AddIconByEncoding

Apache configuration directive, AddIconByEncoding

AddIconByType

Apache configuration directive, AddIconByType

AddLanguage

Apache configuration directive, AddLanguage

AddType

Apache configuration directive, AddType

Alias

Apache configuration directive, Alias

Allow

Apache configuration directive, Allow

AllowOverride

Apache configuration directive, AllowOverride

Apache (see Apache HTTP Server)

Apache HTTP Server

1.3

migration to 2.0, Migrating Apache HTTP Server 1.3 Configuration Files

2.0

features of, Features of Apache HTTP Server 2.0
file system changes, File System Changes in Apache HTTP Server 2.0
migration from 1.3, Migrating Apache HTTP Server 1.3 Configuration Files
MPM specific directives, MPM Specific Server-Pool Directives
packaging changes, Packaging Changes in Apache HTTP Server 2.0

additional resources, Additional Resources

related books, Related Books
useful websites, Useful Websites

configuration, Configuration Directives in httpd.conf

introducing, Apache HTTP Server

log files

/var/log/httpd/error_log, Configuration Directives in httpd.conf
combined log file format, LogFormat , CustomLog
format of, LogFormat
troubleshooting with, Configuration Directives in httpd.conf, KeepAlive
using log analyzer tools with, HostnameLookups

migration to 2.0, Migrating Apache HTTP Server 1.3 Configuration Files

bind addresses and ports, Interface and Port Binding
content negotiation, Content Negotiation
directory indexing, Directory Indexing
DSO Support, Dynamic Shared Object (DSO) Support
error documents, Error Documents
LDAP, The mod_authz_ldap Module
logging, Logging
module system changes, Modules and Apache HTTP Server 2.0
mod_auth_db, The mod_auth_dbm and mod_auth_db Modules
mod_auth_dbm, The mod_auth_dbm and mod_auth_db Modules
mod_include, The mod_include Module
mod_perl, The mod_perl Module
mod_proxy, The mod_proxy Module
mod_ssl, The mod_ssl Module
PHP, PHP
removed directives, Other Global Environment Changes
server-pool size, Server-Pool Size Regulation
SuexecUserGroup, The suexec Module, SuexecUserGroup
UserDir directive, UserDir Mapping
virtual host configuration, Virtual Host Configuration

Multi-Processing Modules

activating worker MPM, Server-Pool Size Regulation
prefork, Server-Pool Size Regulation
worker, Server-Pool Size Regulation

reloading, Starting and Stopping httpd

restarting, Starting and Stopping httpd

running without security, Virtual Hosts

server status reports, Location

starting, Starting and Stopping httpd

stopping, Starting and Stopping httpd

troubleshooting, Configuration Directives in httpd.conf

Apache HTTP Server modules, Default Modules

APXS Apache utility, Adding Modules

Authentication Configuration Tool

and LDAP, Configuring a System to Authenticate Using OpenLDAP, PAM and LDAP

autofs, autofs

(see also NFS)

B

Basic Input/Output System (see BIOS)

Berkeley Internet Name Domain (see BIND)

BIND

additional resources, Additional Resources

installed documentation, Installed Documentation
useful websites, Useful Websites

common mistakes, Common Mistakes to Avoid

configuration files

/etc/named.conf, BIND as a Nameserver, /etc/named.conf
/var/named/ directory, BIND as a Nameserver
zone files, Zone Files

configuration of

reverse name resolution, Reverse Name Resolution Zone Files
zone file directives, Zone File Directives
zone file examples, Example Zone File
zone file resource records, Zone File Resource Records
zone statements sample, Sample zone Statements

features, Advanced Features of BIND

DNS enhancements, DNS Protocol Enhancements
IPv6, IP version 6
multiple views, Multiple Views
security, Security

introducing, Berkeley Internet Name Domain (BIND), Introduction to DNS

named daemon, BIND as a Nameserver

nameserver

definition of, Introduction to DNS

nameserver types

caching-only, Nameserver Types
forwarding, Nameserver Types
master, Nameserver Types
slave, Nameserver Types

rndc program, Using rndc

/etc/rndc.conf, Configuring /etc/rndc.conf
command line options, Command Line Options
configuring keys, Configuring /etc/rndc.conf
configuring named to use, Configuring /etc/named.conf

root nameserver

definition of, Introduction to DNS

zones

definition of, Nameserver Zones

bind

additional resources

related books, Related Books

BIOS

definition of, The BIOS

(see also boot process)

block devices, /proc/devices

(see also /proc/devices)
definition of, /proc/devices

boot loaders, GRUB

(see also GRUB)

definition of, The GRUB Boot Loader

types of

ELILO, Boot Loaders and System Architecture
GRUB, Boot Loaders and System Architecture
OS/400, Boot Loaders and System Architecture
YABOOT, Boot Loaders and System Architecture
z/IPL, Boot Loaders and System Architecture

boot process, Boot Process, Init, and Shutdown, A Detailed Look at the Boot Process

(see also boot loaders)

chain loading, GRUB and the x86 Boot Process

direct loading, GRUB and the x86 Boot Process

for x86, A Detailed Look at the Boot Process

stages of, The Boot Process, A Detailed Look at the Boot Process

/sbin/init command, The /sbin/init Program
BIOS, The BIOS
boot loader, The Boot Loader
EFI shell, The BIOS
kernel, The Kernel

BrowserMatch

Apache configuration directive, BrowserMatch

C

cache directives for Apache, Cache Directives

CacheNegotiatedDocs

Apache configuration directive, CacheNegotiatedDocs

caching-only nameserver (see BIND)

CGI scripts

allowing execution outside cgi-bin, Directory
outside the ScriptAlias, AddHandler

channel bonding

interface

configuration of, Channel Bonding Interfaces

module configuration, The Channel Bonding Module

module directives, bonding Module Directives

character devices, /proc/devices

(see also /proc/devices)
definition of, /proc/devices

chkconfig, Runlevel Utilities

(see also services)

configuration

Apache HTTP Server, Configuration Directives in httpd.conf
virtual hosts, Virtual Hosts

configuration directives, Apache, General Configuration Tips

AccessFileName, AccessFileName

Action, Action

AddDescription, AddDescription

AddEncoding, AddEncoding

AddHandler, AddHandler

AddIcon, AddIcon

AddIconByEncoding, AddIconByEncoding

AddIconByType, AddIconByType

AddLanguage, AddLanguage

AddType, AddType

Alias, Alias

Allow, Allow

AllowOverride, AllowOverride

BrowserMatch, BrowserMatch

CacheNegotiatedDocs, CacheNegotiatedDocs

CustomLog, CustomLog

DefaultIcon, DefaultIcon

DefaultType, DefaultType

Deny, Deny

Directory, Directory

DirectoryIndex, DirectoryIndex

DocumentRoot, DocumentRoot

ErrorDocument, ErrorDocument

ErrorLog, ErrorLog

ExtendedStatus, ExtendedStatus

for cache functionality, Cache Directives

Group, Group

HeaderName, HeaderName

HostnameLookups, HostnameLookups

IfDefine, IfDefine

IfModule, IfModule

Include, Include

IndexIgnore, IndexIgnore

IndexOptions, IndexOptions

KeepAlive, KeepAlive

(see also KeepAliveTimeout)
troubleshooting, KeepAlive

KeepAliveTimeout, KeepAliveTimeout

LanguagePriority, LanguagePriority

Listen, Listen

LoadModule, LoadModule

Location, Location

LogFormat

format options, LogFormat

LogLevel, LogLevel

MaxClients, MaxClients

MaxKeepAliveRequests, MaxKeepAliveRequests

MaxRequestsPerChild, MaxRequestsPerChild

MaxSpareServers, MinSpareServers and MaxSpareServers

MaxSpareThreads, MinSpareThreads and MaxSpareThreads

MinSpareServers, MinSpareServers and MaxSpareServers

MinSpareThreads, MinSpareThreads and MaxSpareThreads

NameVirtualHost, NameVirtualHost

Options, Options

Order, Order

PidFile, PidFile

Proxy, Proxy

ProxyRequests, ProxyRequests

ReadmeName, ReadmeName

Redirect, Redirect

ScriptAlias, ScriptAlias

ServerAdmin, ServerAdmin

ServerName, ServerName

ServerRoot, ServerRoot

ServerSignature, ServerSignature

SetEnvIf, SetEnvIf

SSL configuration, Configuration Directives for SSL

StartServers, StartServers

SuexecUserGroup, The suexec Module, SuexecUserGroup

ThreadsPerChild, ThreadsPerChild

Timeout, Timeout

TypesConfig, TypesConfig

UseCanonicalName, UseCanonicalName

User, User

UserDir, UserDir

VirtualHost, VirtualHost

CustomLog

Apache configuration directive, CustomLog

D

DefaultIcon

Apache configuration directive, DefaultIcon

DefaultType

Apache configuration directive, DefaultType

Denial of Service

prevention using xinetd, Resource Management Options

(see also xinetd)

Denial of Service attack, /proc/sys/net/

(see also /proc/sys/net/ directory)
definition of, /proc/sys/net/

Deny

Apache configuration directive, Deny

desktop environments (see X)

dev directory, The /dev/ Directory

devices, local

ownership of, PAM and Device Ownership

(see also PAM)

directories

/boot/, The /boot/ Directory
/dev/, The /dev/ Directory
/etc/, The /etc/ Directory
/lib/, The /lib/ Directory
/media/, The /media/ Directory
/mnt/, The /mnt/ Directory
/opt/, The /opt/ Directory
/proc/, The /proc/ Directory
/sbin/, The /sbin/ Directory
/srv/, The /srv/ Directory
/sys/, The /sys/ Directory
/usr/, The /usr/ Directory
/usr/local/, The /usr/local/ Directory
/var/, The /var/ Directory

Directory

Apache configuration directive, Directory

DirectoryIndex

Apache configuration directive, DirectoryIndex

display managers (see X)

DNS, Introduction to DNS

(see also BIND)
introducing, Introduction to DNS

documentation

experienced user, For the More Experienced

finding appropriate, Finding Appropriate Documentation

first-time users, Documentation For First-Time Linux Users

newsgroups, Introduction to Linux Newsgroups
websites, Introduction to Linux Websites

guru, Documentation for Linux Gurus

DocumentRoot

Apache configuration directive, DocumentRoot
changing, Virtual Hosts
changing shared, The Secure Web Server Virtual Host

DoS (see Denial of Service)

DoS attack (see Denial of Service attack)

drivers (see kernel modules)

DSOs

loading, Adding Modules

E

EFI shell

definition of, The BIOS

(see also boot process)

ELILO, Boot Loaders for Other Architectures, Boot Loaders and System Architecture

(see also boot loaders)

email

additional resources, Additional Resources

installed documentation, Installed Documentation
related books, Related Books
useful websites, Useful Websites

Fetchmail, Fetchmail

history of, Email

Postfix, Postfix

Procmail, Mail Delivery Agents

program classifications, Email Program Classifications

protocols, Email Protocols

IMAP, IMAP
POP, POP
SMTP, SMTP

security, Securing Communication

clients, Secure Email Clients
servers, Securing Email Client Communications

Sendmail, Sendmail

spam

filtering out, Spam Filters

types

Mail Delivery Agent, Mail Delivery Agent
Mail Transfer Agent, Mail Transfer Agent
Mail User Agent, Mail User Agent

epoch, /proc/stat

(see also /proc/stat)
definition of, /proc/stat

ErrorDocument

Apache configuration directive, ErrorDocument

ErrorLog

Apache configuration directive, ErrorLog

etc directory, The /etc/ Directory

Ethernet (see network)

Ethernet modules (see kernel modules)

exec-shield

enabling, /proc/sys/kernel/
introducing, /proc/sys/kernel/

execution domains, /proc/execdomains

(see also /proc/execdomains)
definition of, /proc/execdomains

ExtendedStatus

Apache configuration directive, ExtendedStatus

Extensible Firmware Interface shell (see EFI shell)

F

feedback

contact information, We Need Feedback!

Fetchmail, Fetchmail

additional resources, Additional Resources

command options, Fetchmail Command Options

informational, Informational or Debugging Options
special, Special Options

configuration options, Fetchmail Configuration Options

global options, Global Options
server options, Server Options
user options, User Options

FHS, Overview of File System Hierarchy Standard (FHS), FHS Organization

(see also file system)

file system

FHS standard, FHS Organization
hierarchy, Overview of File System Hierarchy Standard (FHS)
organization, FHS Organization
structure, File System Structure
virtual (see proc file system)

files, proc file system

changing, Changing Virtual Files, Using the sysctl Command
viewing, Viewing Virtual Files, Using the sysctl Command

findsmb program, findsmb

forwarding nameserver (see BIND)

frame buffer device, /proc/fb

(see also /proc/fb)

FrontPage, After Installation

fstab, /etc/fstab

(see also NFS)

FTP, FTP

(see also vsftpd)

active mode, Multiple Ports, Multiple Modes

command port, Multiple Ports, Multiple Modes

data port, Multiple Ports, Multiple Modes

definition of, FTP

introducing, The File Transport Protocol

passive mode, Multiple Ports, Multiple Modes

server software

Red Hat Content Accelerator, FTP Servers
vsftpd, FTP Servers

G

GNOME, Desktop Environments

(see also X)

Group

Apache configuration directive, Group

groups

additional resources, Additional Resources

installed documentation, Installed Documentation
related books, Related Books

GID, Users and Groups

introducing, Users and Groups

shared directories, Group Directories

standard, Standard Groups

tools for management of

groupadd, User and Group Management Tools, User Private Groups
system-config-users, User Private Groups
User Manager, User and Group Management Tools

user private, User Private Groups

GRUB, The Boot Loader, Boot Loaders and System Architecture

(see also boot loaders)

additional resources, Additional Resources

installed documentation, Installed Documentation
related books, Related Books
useful websites, Useful Websites

boot process, GRUB and the x86 Boot Process

Changing Runlevels at Boot Time, Changing Runlevels at Boot Time

changing runlevels with, GRUB Interfaces

commands, GRUB Commands

configuration file

/boot/grub/grub.conf, Configuration File Structure
structure, Configuration File Structure

definition of, GRUB

features, Features of GRUB

installing, Installing GRUB

interfaces, GRUB Interfaces

command line, GRUB Interfaces
menu, GRUB Interfaces
menu entry editor, GRUB Interfaces
order of, Interfaces Load Order

menu configuration file, GRUB Menu Configuration File

directives, Configuration File Directives

role in boot process, The Boot Loader

terminology, GRUB Terminology

devices, Device Names
files, File Names and Blocklists
root file system, The Root File System and GRUB

grub.conf, Configuration File Structure

(see also GRUB)

H

halt, Shutting Down

(see also shutdown)

HeaderName

Apache configuration directive, HeaderName

hierarchy, file system, Overview of File System Hierarchy Standard (FHS)

HostnameLookups

Apache configuration directive, HostnameLookups

hosts access files (see TCP wrappers)

hosts.allow (see TCP wrappers)

hosts.deny (see TCP wrappers)

httpd.conf (see configuration directives, Apache)

hugepages

configuration of, /proc/sys/vm/

I

IfDefine

Apache configuration directive, IfDefine

ifdown, Interface Control Scripts

IfModule

Apache configuration directive, IfModule

ifup, Interface Control Scripts

Include

Apache configuration directive, Include

IndexIgnore

Apache configuration directive, IndexIgnore

IndexOptions

Apache configuration directive, IndexOptions

init command, The /sbin/init Program

(see also boot process)

configuration files

/etc/inittab, SysV Init Runlevels

role in boot process, The /sbin/init Program

(see also boot process)

runlevels

directories for, SysV Init Runlevels

runlevels accessed by, Runlevels

SysV init

definition of, SysV Init Runlevels

initrd directory, Special File Locations Under Red Hat Enterprise Linux

insmod , Kernel Module Utilities

introduction, Introduction

ip6tables

control scripts

panic, iptables Control Scripts
restart, iptables Control Scripts
save, iptables Control Scripts
start, iptables Control Scripts
status, iptables Control Scripts
stop, iptables Control Scripts

introducing, ip6tables and IPv6

ipchains (see iptables)

IPsec (see network)

iptables

/sbin/iptables-restore, Saving iptables Rules

/sbin/iptables-save, Saving iptables Rules

additional resources, Additional Resources

installed documentation, Installed Documentation
useful websites, Useful Websites

chains

target, Packet Filtering

compared with ipchains, Differences between iptables and ipchains

configuration files

/etc/sysconfig/iptables, Saving iptables Rules
/etc/sysconfig/iptables-config, iptables Control Scripts Configuration File
/etc/sysconfig/iptables.save, Saving iptables Rules

control scripts

panic, iptables Control Scripts
restart, iptables Control Scripts
save, Saving iptables Rules, iptables Control Scripts
start, iptables Control Scripts
status, iptables Control Scripts
stop, iptables Control Scripts

match options, iptables Match Options

modules, Additional Match Option Modules

options, Options Used within iptables Commands

commands, Command Options
listing, Listing Options
parameters, iptables Parameter Options
structure of, Structure of iptables Options
target, Target Options

overview of, iptables

packet filtering basics, Packet Filtering

protocols

ICMP, ICMP Protocol
TCP, TCP Protocol
UDP, UDP Protocol

rules list, Packet Filtering

saving rules, Saving iptables Rules

tables, Packet Filtering

K

KDE, Desktop Environments

(see also X)

KeepAlive

Apache configuration directive, KeepAlive

KeepAliveTimeout

Apache configuration directive, KeepAliveTimeout

Kerberos

additional resources, Additional Resources

installed documentation, Installed Documentation
useful websites, Useful Websites

advantages of, Advantages of Kerberos

and PAM, Kerberos and PAM

Authentication Server (AS), How Kerberos Works

clients set up, Configuring a Kerberos 5 Client

definition of, Kerberos

disadvantages of, Disadvantages of Kerberos

how it works, How Kerberos Works

Key Distribution Center (KDC), How Kerberos Works

server set up, Configuring a Kerberos 5 Server

terminology, Kerberos Terminology

Ticket-granting Server (TGS), How Kerberos Works

Ticket-granting Ticket (TGT), How Kerberos Works

kernel

role in boot process, The Kernel

kernel modules

/etc/rc.modules, Persistent Module Loading

Ethernet modules

parameters, Ethernet Parameters
supporting multiple cards, Using Multiple Ethernet Cards

introducing, General Parameters and Modules

listing, Kernel Module Utilities

loading, Kernel Module Utilities

module parameters

specifying, Specifying Module Parameters

persistent loading, Persistent Module Loading

SCSI modules

parameters, Storage parameters

types of, General Parameters and Modules

unload, Kernel Module Utilities

kwin, Window Managers

(see also X)

L

LanguagePriority

Apache configuration directive, LanguagePriority

LDAP

additional resources, Additional Resources

installed documentation, Installed Documentation
related books, Related Books
useful websites, Useful Websites

advantages of, Why Use LDAP?

applications

ldapadd, OpenLDAP Daemons and Utilities
ldapdelete, OpenLDAP Daemons and Utilities
ldapmodify, OpenLDAP Daemons and Utilities
ldappasswd, OpenLDAP Daemons and Utilities
ldapsearch, OpenLDAP Daemons and Utilities
OpenLDAP suite, OpenLDAP Daemons and Utilities
slapadd, OpenLDAP Daemons and Utilities
slapcat, OpenLDAP Daemons and Utilities
slapd, OpenLDAP Daemons and Utilities
slapindex, OpenLDAP Daemons and Utilities
slappasswd, OpenLDAP Daemons and Utilities
slurpd, OpenLDAP Daemons and Utilities
utilities, OpenLDAP Daemons and Utilities

authentication using, Configuring a System to Authenticate Using OpenLDAP

Authentication Configuration Tool, Configuring a System to Authenticate Using OpenLDAP
editing /etc/ldap.conf, Configuring a System to Authenticate Using OpenLDAP
editing /etc/nsswitch.conf, Configuring a System to Authenticate Using OpenLDAP
editing /etc/openldap/ldap.conf, Configuring a System to Authenticate Using OpenLDAP
editing slapd.conf, Configuring a System to Authenticate Using OpenLDAP
packages, Configuring a System to Authenticate Using OpenLDAP
PAM, PAM and LDAP
setting up clients, Configuring a System to Authenticate Using OpenLDAP

client applications, LDAP Client Applications

configuration files

/etc/ldap.conf, OpenLDAP Configuration Files
/etc/openldap/ldap.conf, OpenLDAP Configuration Files
/etc/openldap/schema/ directory, OpenLDAP Configuration Files, The /etc/openldap/schema/ Directory
/etc/openldap/slapd.conf, OpenLDAP Configuration Files, Editing /etc/openldap/slapd.conf

daemons, OpenLDAP Daemons and Utilities

definition of, Lightweight Directory Access Protocol (LDAP)

LDAPv2, Lightweight Directory Access Protocol (LDAP)

LDAPv3, Lightweight Directory Access Protocol (LDAP)

LDIF

format of, LDAP Terminology

OpenLDAP features, OpenLDAP Features

setting up, OpenLDAP Setup Overview

migrating older directories, Migrating Directories from Earlier Releases

terminology, LDAP Terminology

upgrading directories, Migrating Directories from Earlier Releases

using with Apache HTTP Server, PHP4, LDAP, and the Apache HTTP Server

using with NSS, NSS, PAM, and LDAP

using with PAM, NSS, PAM, and LDAP

using with PHP4, PHP4, LDAP, and the Apache HTTP Server

ldapadd command, OpenLDAP Daemons and Utilities

(see also LDAP)

ldapdelete command, OpenLDAP Daemons and Utilities

(see also LDAP)

ldapmodify command, OpenLDAP Daemons and Utilities

(see also LDAP)

ldappasswd command, OpenLDAP Daemons and Utilities

(see also LDAP)

ldapsearch command, OpenLDAP Daemons and Utilities

(see also LDAP)

lib directory, The /lib/ Directory

Lightweight Directory Access Protocol (see LDAP)

LILO, The Boot Loader

(see also boot loaders)
role in boot process, The Boot Loader

Listen

Apache configuration directive, Listen

LoadModule

Apache configuration directive, LoadModule

Location

Apache configuration directive, Location

LogFormat

Apache configuration directive, LogFormat

LogLevel

Apache configuration directive, LogLevel

lsmod , Kernel Module Utilities

lspci, /proc/pci

M

Mail Delivery Agent (see email)

Mail Transfer Agent (see email)

Mail User Agent (see email)

make_smbcodepage program, make_smbcodepage

make_unicodemap program, make_unicodemap

Master Boot Record (see MBR)

master nameserver (see BIND)

MaxClients

Apache configuration directive, MaxClients

MaxKeepAliveRequests

Apache configuration directive, MaxKeepAliveRequests

MaxRequestsPerChild

Apache configuration directive, MaxRequestsPerChild

MaxSpareServers

Apache configuration directive, MinSpareServers and MaxSpareServers

MaxSpareThreads

Apache configuration directive, MinSpareThreads and MaxSpareThreads

MBR

definition of, A Detailed Look at the Boot Process, The BIOS

(see also boot loaders)
(see also boot process)

MDA (see Mail Delivery Agent)

media directory, The /media/ Directory

metacity, Window Managers

(see also X)

MinSpareServers

Apache configuration directive, MinSpareServers and MaxSpareServers

MinSpareThreads

Apache configuration directive, MinSpareThreads and MaxSpareThreads

mnt directory, The /mnt/ Directory

modprobe , Kernel Module Utilities

module parameters (see kernel modules)

modules (see kernel modules)

Apache

loading, Adding Modules
the own, Adding Modules

default, Default Modules

MTA (see Mail Transfer Agent)

MUA (see Mail User Agent)

mwm, Window Managers

(see also X)

N

named daemon (see BIND)

named.conf (see BIND)

nameserver (see BIND)

NameVirtualHost

Apache configuration directive, NameVirtualHost

net program, net

netfilter (see iptables)

network

additional resources, Additional Resources

commands

/sbin/ifdown, Interface Control Scripts
/sbin/ifup, Interface Control Scripts
/sbin/service network, Interface Control Scripts

configuration, Interface Configuration Files

functions, Network Function Files

interfaces, Interface Configuration Files

alias, Alias and Clone Files
channel bonding, Channel Bonding Interfaces
clone, Alias and Clone Files
dialup, Dialup Interfaces
Ethernet, Ethernet Interfaces
IPsec, IPsec Interfaces

scripts, Network Interfaces

Network File System (see NFS)

NFS

additional resources, Additional Resources

installed documentation, Installed Documentation
related books, Related Books
useful websites, Useful Websites

client

/etc/fstab, /etc/fstab
autofs, autofs
configuration, NFS Client Configuration Files
mount options, Common NFS Mount Options

condrestart, Starting and Stopping NFS

how it works, How It Works

introducing, Network File System (NFS)

portmap, NFS and portmap

reloading, Starting and Stopping NFS

required services, Required Services

restarting, Starting and Stopping NFS

security, Securing NFS

file permissions, File Permissions
host access, Host Access
NFSv2/NFSv3 host access, Using NFSv2 or NFSv3
NFSv4 host access, Using NFSv4

server configuration, NFS Server Configuration

/etc/exports, The /etc/exports Configuration File
exportfs command, The exportfs Command
exportfs command with NFSv4, Using exportfs with NFSv4

starting, Starting and Stopping NFS

status, Starting and Stopping NFS

stopping, Starting and Stopping NFS

TCP, How It Works

UDP, How It Works

NIC modules (see kernel modules)

nmblookup program, nmblookup

non-secure Web server

disabling, The Secure Web Server Virtual Host

ntsysv, Runlevel Utilities

(see also services)

O

objects, dynamically shared (see DSOs)

OpenLDAP (see LDAP)

OpenSSH, Features of SSH

(see also SSH)
configuration files for, OpenSSH Configuration Files

opt directory, The /opt/ Directory

Options

Apache configuration directive, Options

Order

Apache configuration directive, Order

OS/400, Boot Loaders and System Architecture

(see also boot loaders)

P

packet filtering (see iptables)

PAM

additional resources, Additional Resources

installed documentation, Installed Documentation
useful websites, Useful Websites

advantages of, Advantages of PAM

configuration files, PAM Configuration Files

control flags, Control Flag

definition of, Pluggable Authentication Modules (PAM)

Kerberos and, Kerberos and PAM

modules, Module Interface

arguments, Module Arguments
components, Module Interface
creating, Creating PAM Modules
interfaces, Module Interface
location of, Module Name
stacking, Stacking Module Interfaces, Sample PAM Configuration Files

pam_console

definition of, PAM and Device Ownership

pam_timestamp

authentication icon and, PAM and Administrative Credential Caching
definition of, PAM and Administrative Credential Caching
destroying timestamps, Removing the Timestamp File
directives, Common pam_timestamp Directives

pam_timestamp_check

destroying timestamp using, Removing the Timestamp File

sample configuration files, Sample PAM Configuration Files

service files, PAM Service Files

shadow passwords, Sample PAM Configuration Files

pam_console (see PAM)

pam_timestamp (see PAM)

pam_timestamp_check (see PAM)

password, Sample PAM Configuration Files

(see also PAM)
shadow passwords, Sample PAM Configuration Files

passwords

shadow, Shadow Passwords

pdbedit program, pdbedit

PidFile

Apache configuration directive, PidFile

Pluggable Authentication Modules (see PAM)

portmap, NFS and portmap

(see also NFS)
NFS, Troubleshooting NFS and portmap
rpcinfo, Troubleshooting NFS and portmap
status, Starting and Stopping NFS

Postfix, Postfix

default installation, The Default Postfix Installation

prefdm (see X)

proc directory, The /proc/ Directory

proc file system

/proc/apm, /proc/apm

/proc/buddyinfo, /proc/buddyinfo

/proc/bus/ directory, /proc/bus/

/proc/cmdline, /proc/cmdline

/proc/cpuinfo, /proc/cpuinfo

/proc/crypto, /proc/crypto

/proc/devices

block devices, /proc/devices
character devices, /proc/devices

/proc/dma, /proc/dma

/proc/driver/ directory, /proc/driver/

/proc/execdomains, /proc/execdomains

/proc/fb, /proc/fb

/proc/filesystems, /proc/filesystems

/proc/fs/ directory, /proc/fs

/proc/ide/ directory, /proc/ide/

device directories, Device Directories

/proc/interrupts, /proc/interrupts

/proc/iomem, /proc/iomem

/proc/ioports, /proc/ioports

/proc/irq/ directory, /proc/irq/

/proc/kcore, /proc/kcore

/proc/kmsg, /proc/kmsg

/proc/loadavg, /proc/loadavg

/proc/locks, /proc/locks

/proc/mdstat, /proc/mdstat

/proc/meminfo, /proc/meminfo

/proc/misc, /proc/misc

/proc/modules, /proc/modules

/proc/mounts, /proc/mounts

/proc/mtrr, /proc/mtrr

/proc/net/ directory, /proc/net/

/proc/partitions, /proc/partitions

/proc/pci

viewing using lspci, /proc/pci

/proc/scsi/ directory, /proc/scsi/

/proc/self/ directory, /proc/self/

/proc/slabinfo, /proc/slabinfo

/proc/stat, /proc/stat

/proc/swaps, /proc/swaps

/proc/sys/ directory, /proc/sys/, Using the sysctl Command

(see also sysctl)
/proc/sys/dev/ directory, /proc/sys/dev/
/proc/sys/fs/ directory, /proc/sys/fs/
/proc/sys/kernel/ directory, /proc/sys/kernel/
/proc/sys/kernel/exec-shield, /proc/sys/kernel/
/proc/sys/kernel/sysrq (see system request key)
/proc/sys/net/ directory, /proc/sys/net/
/proc/sys/vm/ directory, /proc/sys/vm/

/proc/sysrq-trigger, /proc/sysrq-trigger

/proc/sysvipc/ directory, /proc/sysvipc/

/proc/tty/ directory, /proc/tty/

/proc/uptime, /proc/uptime

/proc/version, /proc/version

additional resources, Additional Resources

installed documentation, Installed Documentation
useful websites, Useful Websites

changing files within, Changing Virtual Files, /proc/sys/, Using the sysctl Command

files within, top-level, Top-level Files within the proc File System

introduced, The proc File System

process directories, Process Directories

subdirectories within, Directories within /proc/

viewing files within, Viewing Virtual Files

Procmail, Mail Delivery Agents

additional resources, Additional Resources

configuration, Procmail Configuration

recipes, Procmail Recipes

delivering, Delivering vs. Non-Delivering Recipes
examples, Recipe Examples
flags, Flags
local lockfiles, Specifying a Local Lockfile
non-delivering, Delivering vs. Non-Delivering Recipes
SpamAssassin, Spam Filters
special actions, Special Conditions and Actions
special conditions, Special Conditions and Actions

programs

running at boot time, Running Additional Programs at Boot Time

Proxy

Apache configuration directive, Proxy

proxy server, ProxyRequests , Cache Directives

ProxyRequests

Apache configuration directive, ProxyRequests

public_html directories, UserDir

R

rc.local

modifying, Running Additional Programs at Boot Time

rc.serial, Running Additional Programs at Boot Time

(see also setserial command)

ReadmeName

Apache configuration directive, ReadmeName

Red Hat Enterprise Linux-specific file locations

/etc/sysconfig/, Special File Locations Under Red Hat Enterprise Linux

(see also sysconfig directory)

/var/lib/rpm/, Special File Locations Under Red Hat Enterprise Linux

/var/spool/up2date, Special File Locations Under Red Hat Enterprise Linux

Redirect

Apache configuration directive, Redirect

rmmod , Kernel Module Utilities

root nameserver (see BIND)

rpcclient program, rpcclient

rpcinfo, Troubleshooting NFS and portmap

runlevels (see init command)

changing with GRUB, GRUB Interfaces

configuration of, Runlevel Utilities

(see also services)

S

Samba (see Samba)

Abilities, Samba Features

Account Information Databases, Samba Account Information Databases

ldapsam, New Backends
ldapsam_compat, Backward Compatible Backends
mysqlsam, New Backends
Plain Text, Backward Compatible Backends
smbpasswd, Backward Compatible Backends
tdbsam, New Backends
xmlsam, New Backends

Additional Resources, Additional Resources

installed documentation, Installed Documentation
Red Hat resources, Red Hat Documentation
related books, Related Books
useful websites, Useful Websites

Backward Compatible Database Backends, Backward Compatible Backends

Browsing, Samba Network Browsing

CUPS Printing Support, Samba with CUPS Printing Support

CUPS smb.conf, Simple smb.conf Settings

daemon, Samba Daemons and Related Services

nmbd, The nmbd daemon
overview, Daemon Overview
smbd, The smbd daemon
winbindd, The winbindd daemon

Introduction, Introduction to Samba

Network Browsing, Samba Network Browsing

Domain Browsing, Domain Browsing
WINS, WINS (Windows Internetworking Name Server)
Workgroup Browsing, Workgroup Browsing

New Database Backends, New Backends

Programs, Samba Distribution Programs

findsmb, findsmb
make_smbcodepage, make_smbcodepage
make_unicodemap, make_unicodemap
net, net
nmblookup, nmblookup
pdbedit, pdbedit
rpcclient, rpcclient
smbcacls, smbcacls
smbclient, smbclient
smbcontrol, smbcontrol
smbgroupedit, smbgroupedit
smbmount, smbmount
smbpasswd, smbpasswd
smbspool, smbspool
smbstatus, smbstatus
smbtar, smbtar
testparm, testparm
testprns, testprns
wbinfo, wbinfo

Reference, Samba

Security Modes, Samba Security Modes

Active Directory Security Mode, Active Directory Security Mode (User-Level Security)
Domain Security Mode, Domain Security Mode (User-Level Security)
Server Security Mode, Server Security Mode (User-Level Security)
Share-Level Security, Share-Level Security
User Level Security, User-Level Security

Server Types, Samba Server Types and the smb.conf File

server types

Domain Controller, Domain Controller
Domain Member, Domain Member Server
Stand Alone, Stand-alone Server

service

conditional restarting, Starting and Stopping Samba
reloading, Starting and Stopping Samba
restarting, Starting and Stopping Samba
starting, Starting and Stopping Samba
stopping, Starting and Stopping Samba

smb.conf, Samba Server Types and the smb.conf File

Active Directory Member Server example, Active Directory Domain Member Server
Anonymous Print Server example, Anonymous Print Server
Anonymous Read Only example, Anonymous Read-Only
Anonymous Read/Write example, Anonymous Read/Write
BDC using LDAP, Backup Domain Controller (BDC) using LDAP
NT4-style Domain Member example, Windows NT4-based Domain Member Server
PDC using Active Directory, Primary Domain Controller (PDC) with Active Directory
PDC using LDAP, Primary Domain Controller (PDC) using LDAP
PDC using tdbsam, Primary Domain Controller (PDC) using tdbsam
Secure File and Print Server example, Secure Read/Write File and Print Server

WINS, WINS (Windows Internetworking Name Server)

sbin directory, The /sbin/ Directory

ScriptAlias

Apache configuration directive, ScriptAlias

SCSI modules (see kernel modules)

security

running Apache without, Virtual Hosts

SELinux, SELinux

additional resources, Additional Resources

documentation, Red Hat Documentation
installed documentation, Installed Documentation
websites, Useful Websites

introduction, Introduction to SELinux

related files, Files Related to SELinux

/etc/selinux/ Directory, The /etc/selinux/ Directory
/etc/sysconfig/selinux, The /etc/sysconfig/selinux Configuration File
/selinux/ pseudo-file system, The /selinux/ Pseudo-File System
configuration, SELinux Configuration Files
utilities, SELinux Utilities

Sendmail, Sendmail

additional resources, Additional Resources
aliases, Masquerading
common configuration changes, Common Sendmail Configuration Changes
default installation, The Default Sendmail Installation
LDAP and, Using Sendmail with LDAP
limitations, Purpose and Limitations
masquerading, Masquerading
purpose, Purpose and Limitations
spam, Stopping Spam
with UUCP, Common Sendmail Configuration Changes

serial ports (see setserial command)

server side includes, Options , AddType

ServerAdmin

Apache configuration directive, ServerAdmin

ServerName

Apache configuration directive, ServerName

ServerRoot

Apache configuration directive, ServerRoot

ServerSignature

Apache configuration directive, ServerSignature

services

configuring with chkconfig, Runlevel Utilities
configuring with ntsysv, Runlevel Utilities
configuring with Services Configuration Tool, Runlevel Utilities

Services Configuration Tool, Runlevel Utilities

(see also services)

SetEnvIf

Apache configuration directive, SetEnvIf

setserial command

configuring, Running Additional Programs at Boot Time

shadow (see password)

shadow passwords

overview of, Shadow Passwords

shutdown, Shutting Down

(see also halt)

slab pools (see /proc/slabinfo)

slapadd command, OpenLDAP Daemons and Utilities

(see also LDAP)

slapcat command, OpenLDAP Daemons and Utilities

(see also LDAP)

slapd command, OpenLDAP Daemons and Utilities

(see also LDAP)

slapindex command, OpenLDAP Daemons and Utilities

(see also LDAP)

slappasswd command, OpenLDAP Daemons and Utilities

(see also LDAP)

slave nameserver (see BIND)

slurpd command, OpenLDAP Daemons and Utilities

(see also LDAP)

smbcacls program, smbcacls

smbclient program, smbclient

smbcontrol program, smbcontrol

smbgroupedit program, smbgroupedit

smbmount program, smbmount

smbpasswd program, smbpasswd

smbspool program, smbspool

smbstatus program, smbstatus

smbtar program, smbtar

SpamAssassin

using with Procmail, Spam Filters

srv directory, The /srv/ Directory

SSH protocol, SSH Protocol

additional resources, Additional Resources

installed documentation, Installed Documentation
related books, Related Books
useful websites, Useful Websites

authentication, Authentication

configuration files, OpenSSH Configuration Files

connection sequence, Event Sequence of an SSH Connection

features of, Features of SSH

insecure protocols and, Requiring SSH for Remote Connections

layers of

channels, Channels
transport layer, Transport Layer

port forwarding, Port Forwarding

requiring for remote login, Requiring SSH for Remote Connections

security risks, Why Use SSH?

version 1, SSH Protocol Versions

version 2, SSH Protocol Versions

X11 forwarding, X11 Forwarding

SSL configuration, Configuration Directives for SSL

StartServers

Apache configuration directive, StartServers

startx, Runlevel 3 (see X)

(see also X)

stunnel, Securing Email Client Communications

SuexecUserGroup

Apache configuration directive, The suexec Module, SuexecUserGroup

sys directory, The /sys/ Directory

sysconfig directory, Special File Locations Under Red Hat Enterprise Linux

/etc/sysconfig/amd, /etc/sysconfig/amd

/etc/sysconfig/apm-scripts/ directory, Directories in the /etc/sysconfig/ Directory

/etc/sysconfig/apmd, /etc/sysconfig/apmd

/etc/sysconfig/arpwatch, /etc/sysconfig/arpwatch

/etc/sysconfig/authconfig, /etc/sysconfig/authconfig

/etc/sysconfig/autofs, /etc/sysconfig/autofs

/etc/sysconfig/clock, /etc/sysconfig/clock

/etc/sysconfig/desktop, /etc/sysconfig/desktop

/etc/sysconfig/devlabel, /etc/sysconfig/devlabel

/etc/sysconfig/dhcpd, /etc/sysconfig/dhcpd

/etc/sysconfig/exim, /etc/sysconfig/exim

/etc/sysconfig/firstboot, /etc/sysconfig/firstboot

/etc/sysconfig/gpm, /etc/sysconfig/gpm

/etc/sysconfig/harddisks, /etc/sysconfig/harddisks

/etc/sysconfig/hwconf, /etc/sysconfig/hwconf

/etc/sysconfig/init, /etc/sysconfig/init

/etc/sysconfig/ip6tables-config, /etc/sysconfig/ip6tables-config

/etc/sysconfig/iptables, Saving iptables Rules

/etc/sysconfig/iptables-config, /etc/sysconfig/iptables-config

/etc/sysconfig/irda, /etc/sysconfig/irda

/etc/sysconfig/keyboard, /etc/sysconfig/keyboard

/etc/sysconfig/kudzu, /etc/sysconfig/kudzu

/etc/sysconfig/mouse, /etc/sysconfig/mouse

/etc/sysconfig/named, /etc/sysconfig/named

/etc/sysconfig/netdump, /etc/sysconfig/netdump

/etc/sysconfig/network, /etc/sysconfig/network

/etc/sysconfig/network-scripts/ directory, Network Interfaces

/etc/sysconfig/ntpd, /etc/sysconfig/ntpd

/etc/sysconfig/pcmcia, /etc/sysconfig/pcmcia

/etc/sysconfig/radvd, /etc/sysconfig/radvd

/etc/sysconfig/rawdevices, /etc/sysconfig/rawdevices

/etc/sysconfig/samba, /etc/sysconfig/samba

/etc/sysconfig/selinux, /etc/sysconfig/selinux

/etc/sysconfig/sendmail, /etc/sysconfig/sendmail

/etc/sysconfig/spamassassin, /etc/sysconfig/spamassassin

/etc/sysconfig/squid, /etc/sysconfig/squid

/etc/sysconfig/system-config-securitylevel , /etc/sysconfig/system-config-securitylevel

/etc/sysconfig/system-config-users, /etc/sysconfig/system-config-users

/etc/sysconfig/system-logviewer, /etc/sysconfig/system-logviewer

/etc/sysconfig/tux, /etc/sysconfig/tux

/etc/sysconfig/vncservers, /etc/sysconfig/vncservers

/etc/sysconfig/xinetd, /etc/sysconfig/xinetd

additional information about, The sysconfig Directory

additional resources, Additional Resources

installed documentation, Installed Documentation

directories in, Directories in the /etc/sysconfig/ Directory

files found in, Files in the /etc/sysconfig/ Directory

sysctl

configuring with /etc/sysctl.conf, Using the sysctl Command
controlling /proc/sys/, Using the sysctl Command

SysRq (see system request key)

system request key

enabling, /proc/sys/

System Request Key

definition of, /proc/sys/
setting timing for, /proc/sys/kernel/

SysV init (see init command)

T

TCP wrappers, xinetd

(see also xinetd)

additional resources, Additional Resources

installed documentation, Installed Documentation
related books, Related Books
useful websites, Useful Websites

advantages of, Advantages of TCP Wrappers

configuration files

/etc/hosts.allow, TCP Wrappers, TCP Wrappers Configuration Files
/etc/hosts.deny, TCP Wrappers, TCP Wrappers Configuration Files
access control option, Access Control
expansions, Expansions
formatting rules within, Formatting Access Rules
hosts access files, TCP Wrappers Configuration Files
log option, Logging
operators, Operators
option fields, Option Fields
patterns, Patterns
shell command option, Shell Commands
spawn option, Shell Commands
twist option, Shell Commands
wildcards, Wildcards

definition of, TCP Wrappers

introducing, TCP Wrappers and xinetd

testparm program, testparm

testprns program, testprns

ThreadsPerChild

Apache configuration directive, ThreadsPerChild

Timeout

Apache configuration directive, Timeout

TLB cache (see hugepages)

troubleshooting

error log, ErrorLog

twm, Window Managers

(see also X)

TypesConfig

Apache configuration directive, TypesConfig

U

UseCanonicalName

Apache configuration directive, UseCanonicalName

User

Apache configuration directive, User

user private groups (see groups)

and shared directories, Group Directories

UserDir

Apache configuration directive, UserDir

users

/etc/passwd, Standard Users

additional resources, Additional Resources

installed documentation, Installed Documentation
related books, Related Books

introducing, Users and Groups

personal HTML directories, UserDir

standard, Standard Users

tools for management of

User Manager, User and Group Management Tools
useradd, User and Group Management Tools

UID, Users and Groups

usr directory, The /usr/ Directory

usr/local/ directory, The /usr/local/ Directory

V

var directory, The /var/ Directory

var/lib/rpm/ directory, Special File Locations Under Red Hat Enterprise Linux

var/spool/up2date/ directory, Special File Locations Under Red Hat Enterprise Linux

virtual file system (see proc file system)

virtual files (see proc file system)

virtual hosts

configuring, Virtual Hosts
Listen command, Setting Up Virtual Hosts
name-based, Virtual Hosts
Options, Options
server side includes, AddType

VirtualHost

Apache configuration directive, VirtualHost

vsftpd, FTP Servers

(see also FTP)

additional resources, Additional Resources

installed documentation, Installed Documentation
related books, Related Books
useful websites, Useful Websites

condrestart, Starting and Stopping vsftpd

configuration file

/etc/vsftpd/vsftpd.conf, vsftpd Configuration Options
access controls, Log In Options and Access Controls
anonymous user options, Anonymous User Options
daemon options, Daemon Options
directory options, Directory Options
file transfer options, File Transfer Options
format of, vsftpd Configuration Options
local user options, Local User Options
logging options, Logging Options
login options, Log In Options and Access Controls
network options, Network Options

multihome configuration, Starting Multiple Copies of vsftpd

restarting, Starting and Stopping vsftpd

RPM

files installed by, Files Installed with vsftpd

security features, FTP Servers

starting, Starting and Stopping vsftpd

starting multiple copies of, Starting Multiple Copies of vsftpd

status, Starting and Stopping vsftpd

stopping, Starting and Stopping vsftpd

W

wbinfo program, wbinfo

webmaster

email address for, ServerAdmin

window managers (see X)

X

X

/etc/X11/xorg.conf

boolean values for, The Structure
Device, Device
DRI, DRI
Files section, Files
InputDevice section, InputDevice
introducing, xorg.conf
Module section, Module
Monitor, Monitor
Screen, Screen
Section tag, The Structure
ServerFlags section, ServerFlags
ServerLayout section, ServerLayout
structure of, The Structure

additional resources, Additional Resources

installed documentation, Installed Documentation
related books, Related Books
useful websites, Useful Websites

configuration files

/etc/X11/ directory, X Server Configuration Files
/etc/X11/xorg.conf, xorg.conf
options within, X Server Configuration Files
server options, xorg.conf

desktop environments

GNOME, Desktop Environments
KDE, Desktop Environments

display managers

configuration of preferred, Runlevel 5
definition of, Runlevel 5
GNOME, Runlevel 5
KDE, Runlevel 5
prefdm script, Runlevel 5
xdm, Runlevel 5

fonts

core X font subsystem, Core X Font System
Fontconfig, Fontconfig
Fontconfig, adding fonts to, Adding Fonts to Fontconfig
FreeType, Fontconfig
introducing, Fonts
X Font Server, Core X Font System
X Render Extension, Fontconfig
xfs, Core X Font System
xfs configuration, xfs Configuration
xfs, adding fonts to, Adding Fonts to xfs
Xft, Fontconfig

introducing, The X Window System

runlevels

3, Runlevel 3
5, Runlevel 5

runlevels and, Runlevels and X

utilities

system-config-display, The X11R6.8 Release

window managers

kwin, Window Managers
metacity, Window Managers
mwm, Window Managers
twm, Window Managers

X clients, The X Window System, Desktop Environments and Window Managers

desktop environments, Desktop Environments
startx command, Runlevel 3
window managers, Window Managers
xinit command, Runlevel 3

X server, The X Window System

features of, The X11R6.8 Release

X Window System (see X)

X.500 (see LDAP)

X.500 Lite (see LDAP)

xinetd, xinetd

(see also TCP wrappers)

additional resources

installed documentation, Installed Documentation
related books, Related Books
useful websites, Useful Websites

configuration files, xinetd Configuration Files

/etc/xinetd.conf, The /etc/xinetd.conf File
/etc/xinetd.d/ directory, The /etc/xinetd.d/ Directory
access control options, Access Control Options
binding options, Binding and Redirection Options
logging options, The /etc/xinetd.conf File, The /etc/xinetd.d/ Directory, Logging Options
redirection options, Binding and Redirection Options
resource management options, Resource Management Options

DoS attacks and, Resource Management Options

introducing, TCP Wrappers and xinetd, xinetd

relationship with TCP wrappers, Access Control Options

xinit (see X)

Xorg (see Xorg)

Y

YABOOT, Boot Loaders and System Architecture

(see also boot loaders)

Z

z/IPL, Boot Loaders and System Architecture

(see also boot loaders)

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Reference Guide

For Red Hat Enterprise Linux 4

Introduction

1. Changes To This Manual

2. Finding Appropriate Documentation

2.1. Documentation For First-Time Linux Users

2.1.1. Introduction to Linux Websites

2.1.2. Introduction to Linux Newsgroups

2.2. For the More Experienced

2.3. Documentation for Linux Gurus

3. More to Come

3.1. We Need Feedback!

Part I. System Reference

Chapter 1. Boot Process, Init, and Shutdown

1.1. The Boot Process

1.2. A Detailed Look at the Boot Process

1.2.1. The BIOS

1.2.2. The Boot Loader

1.2.2.1. Boot Loaders for Other Architectures

1.2.3. The Kernel

1.2.4. The /sbin/init Program

1.3. Running Additional Programs at Boot Time

1.4. SysV Init Runlevels

1.4.1. Runlevels

1.4.2. Runlevel Utilities

1.5. Shutting Down

Chapter 2. The GRUB Boot Loader

2.1. Boot Loaders and System Architecture

2.2. GRUB

2.2.1. GRUB and the x86 Boot Process

2.2.2. Features of GRUB

2.3. Installing GRUB

2.4. GRUB Terminology

2.4.1. Device Names

2.4.2. File Names and Blocklists

2.4.3. The Root File System and GRUB

2.5. GRUB Interfaces

2.5.1. Interfaces Load Order

2.6. GRUB Commands

2.7. GRUB Menu Configuration File

2.7.1. Configuration File Structure

2.7.2. Configuration File Directives

2.8. Changing Runlevels at Boot Time

2.9. Additional Resources

2.9.1. Installed Documentation

2.9.2. Useful Websites

2.9.3. Related Books

Chapter 3. File System Structure

3.1. Why Share a Common Structure?

3.2. Overview of File System Hierarchy Standard (FHS)

3.2.1. FHS Organization

3.2.1.1. The /boot/ Directory

3.2.1.2. The /dev/ Directory

3.2.1.3. The /etc/ Directory

3.2.1.4. The /lib/ Directory

3.2.1.5. The /media/ Directory

3.2.1.6. The /mnt/ Directory

3.2.1.7. The /opt/ Directory

3.2.1.8. The /proc/ Directory

3.2.1.9. The /sbin/ Directory

3.2.1.10. The /srv/ Directory

3.2.1.11. The /sys/ Directory

3.2.1.12. The /usr/ Directory

3.2.1.13. The /usr/local/ Directory

3.2.1.14. The /var/ Directory

3.3. Special File Locations Under Red Hat Enterprise Linux

Chapter 4. The sysconfig Directory

4.1. Files in the /etc/sysconfig/ Directory

4.1.1. /etc/sysconfig/amd

4.1.2. /etc/sysconfig/apmd

4.1.3. /etc/sysconfig/arpwatch

4.1.4. /etc/sysconfig/authconfig

4.1.5. /etc/sysconfig/autofs

4.1.6. /etc/sysconfig/clock

4.1.7. /etc/sysconfig/desktop

4.1.8. /etc/sysconfig/devlabel

4.1.9. /etc/sysconfig/dhcpd

4.1.10. /etc/sysconfig/exim

4.1.11. /etc/sysconfig/firstboot

1.2.4. The `/sbin/init` Program

3.2.1.1. The `/boot/` Directory

3.2.1.2. The `/dev/` Directory

3.2.1.3. The `/etc/` Directory

3.2.1.4. The `/lib/` Directory

3.2.1.5. The `/media/` Directory

3.2.1.6. The `/mnt/` Directory

3.2.1.7. The `/opt/` Directory

3.2.1.8. The `/proc/` Directory

3.2.1.9. The `/sbin/` Directory

3.2.1.10. The `/srv/` Directory

3.2.1.11. The `/sys/` Directory

3.2.1.12. The `/usr/` Directory

3.2.1.13. The `/usr/local/` Directory

3.2.1.14. The `/var/` Directory

Chapter 4. The `sysconfig` Directory

4.1. Files in the `/etc/sysconfig/` Directory

4.1.1. `/etc/sysconfig/amd`

4.1.2. `/etc/sysconfig/apmd`

4.1.3. `/etc/sysconfig/arpwatch`

4.1.4. `/etc/sysconfig/authconfig`

4.1.5. `/etc/sysconfig/autofs`

4.1.6. `/etc/sysconfig/clock`

4.1.7. `/etc/sysconfig/desktop`

4.1.8. `/etc/sysconfig/devlabel`

4.1.9. `/etc/sysconfig/dhcpd`

4.1.10. `/etc/sysconfig/exim`

4.1.11. `/etc/sysconfig/firstboot`

4.1.12. `/etc/sysconfig/gpm`

4.1.13. `/etc/sysconfig/harddisks`

4.1.14. `/etc/sysconfig/hwconf`

4.1.15. `/etc/sysconfig/i18n`

4.1.16. `/etc/sysconfig/init`

4.1.17. `/etc/sysconfig/ip6tables-config`

4.1.18. `/etc/sysconfig/iptables-config`

4.1.19. `/etc/sysconfig/irda`

4.1.20. `/etc/sysconfig/keyboard`

4.1.21. `/etc/sysconfig/kudzu`

4.1.22. `/etc/sysconfig/mouse`

4.1.23. `/etc/sysconfig/named`

4.1.24. `/etc/sysconfig/netdump`

4.1.25. `/etc/sysconfig/network`

4.1.26. `/etc/sysconfig/ntpd`

4.1.27. `/etc/sysconfig/pcmcia`

4.1.28. `/etc/sysconfig/radvd`

4.1.29. `/etc/sysconfig/rawdevices`

4.1.30. `/etc/sysconfig/samba`

4.1.31. `/etc/sysconfig/selinux`

4.1.32. `/etc/sysconfig/sendmail`

4.1.33. `/etc/sysconfig/spamassassin`

4.1.34. `/etc/sysconfig/squid`

4.1.35. `/etc/sysconfig/system-config-securitylevel`

4.1.36. `/etc/sysconfig/system-config-users`

4.1.37. `/etc/sysconfig/system-logviewer`

4.1.38. `/etc/sysconfig/tux`

4.1.39. `/etc/sysconfig/vncservers`

4.1.40. `/etc/sysconfig/xinetd`

4.2. Directories in the `/etc/sysconfig/` Directory

Chapter 5. The `proc` File System

5.2. Top-level Files within the `proc` File System

5.2.1. `/proc/apm`

5.2.2. `/proc/buddyinfo`

5.2.3. `/proc/cmdline`

5.2.4. `/proc/cpuinfo`

5.2.5. `/proc/crypto`

5.2.6. `/proc/devices`

5.2.7. `/proc/dma`

5.2.8. `/proc/execdomains`

5.2.9. `/proc/fb`

5.2.10. `/proc/filesystems`

5.2.11. `/proc/interrupts`

5.2.12. `/proc/iomem`

5.2.13. `/proc/ioports`

5.2.14. `/proc/kcore`

5.2.15. `/proc/kmsg`

5.2.16. `/proc/loadavg`

5.2.17. `/proc/locks`

5.2.18. `/proc/mdstat`

5.2.19. `/proc/meminfo`

5.2.20. `/proc/misc`