Red Hat ENTERPRISE LINUX 3 - SECURITY GUIDE User manual

Red Hat Enterprise Linux 3
Reference Guide

Red Hat Enterprise Linux 3: Reference Guide
Copyright © 2003 by Red Hat, Inc.
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Table of Contents
Introduction.......................................................................................................................................... i
1. Changes To This Manual ....................................................................................................... i
2. Finding Appropriate Documentation ....................................................................................ii
2.1. Documentation For First-Time Linux Users..........................................................ii
2.2. For the More Experienced .................................................................................... iv
2.3. Documentation for Linux Gurus............................................................................ v
3. Document Conventions......................................................................................................... v
4. Using the Mouse .................................................................................................................vii
5. Copying and Pasting Text With X......................................................................................viii
6. More to Come ....................................................................................................................viii
6.1. We Need Feedback! ............................................................................................viii
7. Sign Up for Support...........................................................................................................viii
I. System Reference.............................................................................................................................. i
1. Boot Process, Init, and Shutdown ......................................................................................... 1
1.1. The Boot Process ................................................................................................... 1
1.2. A Detailed Look at the Boot Process..................................................................... 1
1.3. Running Additional Programs at Boot Time ......................................................... 6
1.4. SysV Init Runlevels ............................................................................................... 7
1.5. Shutting Down ....................................................................................................... 8
2. Boot Loaders....................................................................................................................... 11
2.1. Boot Loaders and System Architecture ............................................................... 11
2.2. GRUB................................................................................................................... 11
2.3. Installing GRUB .................................................................................................. 13
2.4. GRUB Terminology............................................................................................. 13
2.5. GRUB Interfaces.................................................................................................. 15
2.6. GRUB Commands ............................................................................................... 16
2.7. GRUB Menu Configuration File.......................................................................... 17
2.8. LILO .................................................................................................................... 19
2.9. The LILO Configuration File............................................................................... 20
2.10. Changing Runlevels at Boot Time ..................................................................... 22
2.11. Additional Resources ......................................................................................... 22
3. File System Structure.......................................................................................................... 25
3.1. Why Share a Common Structure?........................................................................ 25
3.2. Overview of File System Hierarchy Standard (FHS) .......................................... 25
3.3. Special File Locations Under Red Hat Enterprise Linux..................................... 29
4. The sysconfig Directory.................................................................................................. 31
4.1. Files in the /etc/sysconfig/ Directory .......................................................... 31
4.2. Directories in the /etc/sysconfig/ Directory ................................................ 42
4.3. Additional Resources........................................................................................... 43
5. The proc File System......................................................................................................... 45
5.1. A Virtual File System........................................................................................... 45
5.2. Top-level Files within the proc File System ....................................................... 46
5.3. Directories within /proc/................................................................................... 60
5.4. Using the sysctl Command .............................................................................. 76
5.5. Additional Resources........................................................................................... 77
6. Users and Groups................................................................................................................ 79
6.1. User and Group Management Tools .................................................................... 79
6.2. Standard Users ..................................................................................................... 79
6.3. Standard Groups................................................................................................... 81
6.4. User Private Groups ............................................................................................. 83
6.5. Shadow Passwords............................................................................................... 84
6.6. Additional Resources........................................................................................... 84
7. The X Window System ....................................................................................................... 87
7.1. XFree86................................................................................................................ 87

7.2. Desktop Environments and Window Managers................................................... 88
7.3. XFree86 Server Configuration Files .................................................................... 89
7.4. Fonts..................................................................................................................... 95
7.5. Runlevels and XFree86........................................................................................ 98
7.6. Additional Resources........................................................................................... 99
II. Network Services Reference...................................................................................................... 101
8. Network Interfaces............................................................................................................ 103
8.1. Network Configuration Files.............................................................................. 103
8.2. Interface Configuration Files ............................................................................. 103
8.3. Interface Control Scripts .................................................................................... 110
8.4. Network Function Files...................................................................................... 111
8.5. Additional Resources......................................................................................... 112
9. Network File System (NFS).............................................................................................. 113
9.1. How It Works ..................................................................................................... 113
9.2. Starting and Stopping NFS ................................................................................ 115
9.3. NFS Server Configuration.................................................................................. 116
9.4. NFS Client Configuration Files ......................................................................... 119
9.5. Securing NFS ..................................................................................................... 121
9.6. Additional Resources......................................................................................... 122
10. Apache HTTP Server...................................................................................................... 125
10.1. Apache HTTP Server 2.0................................................................................. 125
10.2. Migrating Apache HTTP Server 1.3 Configuration Files ................................ 126
10.3. After Installation .............................................................................................. 136
10.4. Starting and Stopping httpd........................................................................... 137
10.5. Configuration Directives in httpd.conf ....................................................... 138
10.6. Default Modules............................................................................................... 153
10.7. Adding Modules............................................................................................... 154
10.8. Virtual Hosts .................................................................................................... 155
10.9. Additional Resources ....................................................................................... 156
11. Email............................................................................................................................... 159
11.1. Email Protocols................................................................................................ 159
11.2. Email Program Classifications......................................................................... 161
11.3. Mail Transport Agents ..................................................................................... 162
11.4. Mail Delivery Agents....................................................................................... 171
11.5. Mail User Agents ............................................................................................. 176
11.6. Additional Resources ....................................................................................... 178
12. Berkeley Internet Name Domain (BIND)....................................................................... 181
12.1. Introduction to DNS......................................................................................... 181
12.2. /etc/named.conf ......................................................................................... 182
12.3. Zone Files......................................................................................................... 188
12.4. Using rndc ...................................................................................................... 193
12.5. Advanced Features of BIND............................................................................ 195
12.6. Common Mistakes to Avoid ............................................................................ 197
12.7. Additional Resources ....................................................................................... 197
13. Lightweight Directory Access Protocol (LDAP)............................................................ 201
13.1. Why Use LDAP? ............................................................................................. 201
13.2. LDAP Terminology.......................................................................................... 202
13.3. OpenLDAP Daemons and Utilities.................................................................. 202
13.4. OpenLDAP Configuration Files....................................................................... 205
13.5. The /etc/openldap/schema/ Directory..................................................... 205
13.6. OpenLDAP Setup Overview............................................................................ 206
13.7. Configuring a System to Authenticate Using OpenLDAP............................... 207
13.8. Migrating Directories from Earlier Releases ................................................... 209
13.9. Additional Resources ....................................................................................... 210
14. FTP.................................................................................................................................. 213

14.1. The File Transport Protocol ............................................................................. 213
14.2. FTP Servers...................................................................................................... 214
14.3. Files Installed with vsftpd............................................................................. 214
14.4. Starting and Stopping vsftpd......................................................................... 215
14.5. vsftpd Configuration Options........................................................................ 216
14.6. Additional Resources ....................................................................................... 224
III. Security Reference.................................................................................................................... 227
15. Pluggable Authentication Modules (PAM)..................................................................... 229
15.1. Advantages of PAM ......................................................................................... 229
15.2. PAM Configuration Files ................................................................................. 229
15.3. PAM Configuration File Format ...................................................................... 229
15.4. Sample PAM Configuration Files .................................................................... 232
15.5. Creating PAM Modules ................................................................................... 234
15.6. PAM and Administrative Credential Caching.................................................. 234
15.7. PAM and Device Ownership............................................................................ 235
15.8. Additional Resources ....................................................................................... 236
16. TCP Wrappers and xinetd ............................................................................................ 239
16.1. TCP Wrappers.................................................................................................. 239
16.2. TCP Wrappers Configuration Files.................................................................. 240
16.3. xinetd............................................................................................................. 246
16.4. xinetd Configuration Files............................................................................. 247
16.5. Additional Resources ....................................................................................... 251
17. iptables ....................................................................................................................... 253
17.1. Packet Filtering ................................................................................................ 253
17.2. Differences between iptables and ipchains ............................................. 254
17.3. Options Used within iptables Commands................................................... 255
17.4. Saving iptables Rules .................................................................................. 261
17.5. iptables Control Scripts............................................................................... 261
17.6. ip6tables and IPv6....................................................................................... 263
17.7. Additional Resources ....................................................................................... 263
18. Kerberos.......................................................................................................................... 265
18.1. What is Kerberos?............................................................................................ 265
18.2. Kerberos Terminology ..................................................................................... 266
18.3. How Kerberos Works....................................................................................... 268
18.4. Kerberos and PAM........................................................................................... 269
18.5. Configuring a Kerberos 5 Server ..................................................................... 269
18.6. Configuring a Kerberos 5 Client ...................................................................... 271
18.7. Additional Resources ....................................................................................... 272
19. SSH Protocol................................................................................................................... 275
19.1. Features of SSH ............................................................................................... 275
19.2. SSH Protocol Versions..................................................................................... 276
19.3. Event Sequence of an SSH Connection ........................................................... 276
19.4. OpenSSH Configuration Files ......................................................................... 278
19.5. More Than a Secure Shell................................................................................ 279
19.6. Requiring SSH for Remote Connections ......................................................... 280
19.7. Additional Resources ....................................................................................... 281
IV. Appendixes ................................................................................................................................ 283
A. General Parameters and Modules .................................................................................... 285
A.1. Specifying Module Parameters ......................................................................... 285
A.2. SCSI parameters................................................................................................ 285
A.3. Ethernet Parameters .......................................................................................... 286
Index................................................................................................................................................. 291
Colophon.......................................................................................................................................... 305

Introduction
Welcome to the Red Hat Enterprise Linux Reference Guide.
The Red Hat Enterprise Linux Reference Guide contains useful information about the Red Hat En-
terprise 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 3. Some of the changes include:
A New FTP Chapter
The new FTP chapter explains the FTP protocol and provides a detailed list of configuration
options for the default FTP server, vsftpd.
An Updated Network File System (NFS) Chapter
The Network File System (NFS) chapter has been completely revised and reorganized for clarity.
Also, new features, such as TCP and access control list support have been added.
An Updated Modules Appendix
The Modules appendix has been completely overhauled. Instructions for the bonding module
have been added which enable users to bind multiple network interface cards together as if they
are one.
Updated Network Interfaces Chapter
The Network Interfaces chapter has been updated to include IPSec and bonding interfaces.
An Updated Apache HTTP Server Chapter
The guide for migrating from version 1.3 to version 2.0 of the Apache HTTP Server has been
updated. The list of server configuration options has been further updated and reorganized. Spe-
cial thanks to Gary Benson and Joe Orton for their hard work on the Apache HTTP Server
migration guide.
An Updated The X Window System Chapter
The X Window System has been completely revised and reorganized for clarity. New font config-
uration instructions were added as well.

ii Introduction
Updated Users and Groups Chapter
The Users and Groups chapter has been updated.
Before reading this guide, you should be familiar with the contents of the Red Hat Enterprise Linux
Installation Guide concerning installation issues, the Red Hat Enterprise Linux Introduction to System
Administration for basic administration concepts, the Red Hat Enterprise Linux System Administration
Guide for general customization instructions, and the Red Hat Enterprise Linux Security Guide for
security related instructions. The Red Hat Enterprise Linux Reference Guide contains information
about topics for advanced users.
HTML, PDF, and RPM versions of the manuals are available on the Red Hat Enterprise Linux Docu-
mentation CD and online at http://www.redhat.com/docs/.
Note
Although this manual reflects the most current information possible, read the Red Hat Enterprise
Linux Release Notes for information that may not have been available prior to our documenta-
tion being finalized. They can be found on the Red Hat Enterprise Linux CD #1 and online at
http://www.redhat.com/docs/.
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 Red Hat
Enterprise Linux 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 infor-
mation you need or 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.

Introduction iii
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. With-
out 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 prece-
dent. The Linux culture is also based on past events, needs or requirements. A basic understanding
of the history of Linux help you figure out how to solve many potential problems before you actu-
ally 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 give you the exact instructions you need to execute the command.
The Red Hat Enterprise Linux Installation Guide is a 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 Administration is an excellent place to start for those learning the basics
of system administration. You should start with these two 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 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.

iv Introduction
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 partic-
ular objectives.
2.1.3. Beginning Linux Books
•Red Hat Linux for Dummies, 2nd Edition by Jon "maddog" Hall; IDG
•Special Edition Using Red Hat Linux by Alan Simpson, John Ray and Neal Jamison; Que
•Running Linux by Matt Welsh and Lar Kaufman; O’Reilly & Associates
•Red Hat Linux 9 Unleashed by Bill Ball and Hoyle Duff; Pearson Education
The books suggested here are excellent primary sources of information for basic knowledge about
a Red Hat Enterprise Linux system. For more in-depth information concerning the various topics
discussed throughout this book, many of the chapters list specific book titles, usually in an Additional
Resources area.
One excellent resource for aspiring system administrators is the Red Hat Enterprise Linux Introduc-
tion to System Administration. This book is designed to explain strategies for administering Red Hat
Enterprise Linux and is an excellent place to start for those learning the basics of system administra-
tion.
2.2. For the More Experienced
If you have used other Linux distributions, you probably already have a basic grasp of the most fre-
quently 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, configura-
tion issues can be very confusing.
The Red Hat Enterprise Linux System Administration 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 Red Hat Enterprise Linux System
Administration 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.

Introduction v
If you are concerned about security issues, the Red Hat Enterprise Linux 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 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. Document Conventions
When you read this manual, certain words are represented in different fonts, typefaces, sizes, and
weights. This highlighting is systematic; different words are represented in the same style to indicate
their inclusion in a specific category. The types of words that are represented this way include the
following:
command
Linux commands (and other operating system commands, when used) are represented this way.
This style should indicate to you that you can type the word or phrase on the command line
and press [Enter] to invoke a command. Sometimes a command contains words that would be
displayed in a different style on their own (such as file names). In these cases, they are considered
to be part of the command, so the entire phrase is displayed as a command. For example:
Use the cat testfile command to view the contents of a file, named testfile, in the current
working directory.
file name
File names, directory names, paths, and RPM package names are represented this way. This style
should indicate that a particular file or directory exists by that name on your system. Examples:
The .bashrc file in your home directory contains bash shell definitions and aliases for your own
use.
The /etc/fstab file contains information about different system devices and file systems.
Install the webalizer RPM if you want to use a Web server log file analysis program.
application
This style indicates that the program is an end-user application (as opposed to system software).
For example:
Use Mozilla to browse the Web.
[key]
A key on the keyboard is shown in this style. For example:
To use [Tab] completion, type in a character and then press the [Tab] key. Your terminal displays
the list of files in the directory that start with that letter.

vi Introduction
[key]-[combination]
A combination of keystrokes is represented in this way. For example:
The [Ctrl]-[Alt]-[Backspace] key combination exits your graphical session and return you to the
graphical login screen or the console.
text found on a GUI interface
A title, word, or phrase found on a GUI interface screen or window is shown in this style. Text
shown in this style is being used to identify a particular GUI screen or an element on a GUI
screen (such as text associated with a checkbox or field). Example:
Select the Require Password checkbox if you would like your screensaver to require a password
before stopping.
top level of a menu on a GUI screen or window
A word in this style indicates that the word is the top level of a pulldown menu. If you click on
the word on the GUI screen, the rest of the menu should appear. For example:
Under File on a GNOME terminal, the New Tab option allows you to open multiple shell
prompts in the same window.
If you need to type in a sequence of commands from a GUI menu, they are shown like the
following example:
Go to Main Menu Button (on the Panel) => Programming => Emacs to start the Emacs text
editor.
button on a GUI screen or window
This style indicates that the text can be found on a clickable button on a GUI screen. For example:
Click on the Back button to return to the webpage you last viewed.
computer output
Text in this style indicates text displayed to a shell prompt such as error messages and responses
to commands. For example:
The ls command displays the contents of a directory. For example:
Desktop about.html logs paulwesterberg.png
Mail backupfiles mail reports
The output returned in response to the command (in this case, the contents of the directory) is
shown in this style.
prompt
A prompt, which is a computer’s way of signifying that it is ready for you to input something, is
shown in this style. Examples:
$
#
[stephen@maturin stephen]$
leopard login:
user input
Text that the user has to type, either on the command line, or into a text box on a GUI screen, is
displayed in this style. In the following example, text is displayed in this style:
To boot your system into the text based installation program, you must type in the text com-
mand at the boot: prompt.

Introduction vii
replaceable
Text used for examples which is meant to be replaced with data provided by the user is displayed
in this style. In the following example,

version-number

is displayed in this style:
The directory for the kernel source is /usr/src/

version-number

/, where

version-number

is the version of the kernel installed on this system.
Additionally, we use several different strategies to draw your attention to certain pieces of informa-
tion. In order of how critical the information is to your system, these items are marked as note, tip,
important, caution, or a warning. For example:
Note
Remember that Linux is case sensitive. In other words, a rose is not a ROSE is not a rOsE.
Tip
The directory /usr/share/doc/ contains additional documentation for packages installed on your
system.
Important
If you modify the DHCP configuration file, the changes will not take effect until you restart the DHCP
daemon.
Caution
Do not perform routine tasks as root — use a regular user account unless you need to use the root
account for system administration tasks.
Warning
Be careful to remove only the necessary Red Hat Enterprise Linux partitions. Removing other parti-
tions could result in data loss or a corrupted system environment.

viii Introduction
4. Using the Mouse
Red Hat Enterprise Linux is designed to use a three-button mouse. If you have a two-button mouse,
you should have selected three-button emulation during the installation process. If you’re using three-
button emulation, pressing both mouse buttons at the same time equates to pressing the missing third
(middle) button.
In this document, if you are instructed to click with the mouse on something, that means click the left
mouse button. If you need to use the middle or right mouse button, that will be explicitly stated. (This
will be reversed if you’ve configured your mouse to be used by a left handed person.)
The phrase "drag and drop" may be familiar to you. If you’re instructed to drag and drop an item on
your GUI desktop, click on something and hold the mouse button down. While continuing to hold
down the mouse button, drag the item by moving the mouse to a new location. When you’ve reached
the desired location, release the mouse button to drop the item.
5. Copying and Pasting Text With X
Copying and pasting text is easy using your mouse and the X Window System. To copy text, simply
click and drag your mouse over the text to highlight it. To paste the text somewhere, click the middle
mouse button in the spot where the text should be placed.
6. More to Come
The Red Hat Enterprise Linux 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.
6.1. We Need Feedback!
If you find an error in the Red Hat Enterprise Linux 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(EN)-3-Print-RHI (2003-07-25T17:13)
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 de-
scribing it. If you have found an error, please include the section number and some of the surrounding
text so we can find it easily.
7. Sign Up for Support
If you have a variant of Red Hat Enterprise Linux 3, please remember to sign up for the benefits you
are entitled to as a Red Hat customer.
Registration enables access to the Red Hat Services you have purchased, such as technical support
and Red Hat Network. To register your product, go to:
http://www.redhat.com/apps/activate/

Introduction ix
Note
You must activate your product before attempting to connect to Red Hat Network. If your product
has not been activated, Red Hat Network rejects registration to channels to which the system is not
entitled.
Good luck, and thank you for choosing Red Hat Enterprise Linux!
The Red Hat Documentation Team

x Introduction

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.
Table of Contents
1. Boot Process, Init, and Shutdown.................................................................................................. 1
2. Boot Loaders.................................................................................................................................. 11
3. File System Structure ................................................................................................................... 25
4. The sysconfig Directory ............................................................................................................ 31
5. The proc File System.................................................................................................................... 45
6. Users and Groups.......................................................................................................................... 79
7. The X Window System ................................................................................................................. 87

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 pro-
cess, 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:
1. The system BIOS checks the system and launches the first stage boot loader on the MBR of the
primary hard disk.
2. The first stage boot loader loads itself into memory and launches the second stage boot loader
from the /boot/ partition.
3. The second stage boot loader loads the kernel into memory, which in turn loads any necessary
modules and mounts the root partition read-only.
4. The kernel transfers control of the boot process to the /sbin/init program.
5. The /sbin/init program loads all services and user-space tools, and mounts all partitions
listed in /etc/fstab.
6. 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, while Alpha systems use the SRM console.
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

2 Chapter 1. Boot Process, Init, and Shutdown
order of the drives searched while booting is controlled with a setting in 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 boot loaders for the x86 platform. 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.
When using Red Hat Enterprise Linux, two boot loaders are available: GRUB or LILO. GRUB is the
default boot loader, but LILO is available for those who require or prefer it. For more information
about configuring and using GRUB or LILO, see Chapter 2 Boot Loaders.
Both boot loaders for the x86 platform are 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 is the newer boot loader and has the advantage of being able read ext2 and ext31partitions 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.
With LILO, the second stage boot loader uses information on the MBR to determine the boot options
available to the user. This means that any time a configuration change is made or kernel is manually
upgraded, the /sbin/lilo -v -v command must be executed to write the appropriate information
to the MBR. For details about doing this, refer to Section 2.8 LILO.
Tip
If upgrading the kernel using the Red Hat Update Agent, the boot loader configuration file is up-
dated 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, there will be more than one option
present the first time the system is booted. In this situation, LILO will display linux, which is the
SMP kernel, and linux-up, which is for single processors. 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.
1. GRUB reads ext3 file systems as ext2, disregarding the journal file. Refer to the chapter titled The ext3
File System in the Red Hat Enterprise Linux System Administration Guide for more information on the ext3 file
system.