Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION... User manual

Red Hat Linux 7.2
The Official Red Hat Linux Reference Guide

ISBN: N/A
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© 2001 Red Hat, Inc.
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ii

Contents
Red Hat Linux 7.2
Introduction................................................................................... ix
Finding Appropriate Documentation ..................................................... ix
Document Conventions.................................................................... xiii
Using the Mouse ........................................................................... xvi
Copying and Pasting Text With X......................................................... xvi
MoretoCome.............................................................................. xvi
Sign Up for Support........................................................................ xvii
Part I System Reference............................................................ 19
Chapter 1 Filesystem Structure.............................................. 21
1.1 Why Share a Common Structure?.............................................. 21
1.2 Overview of Filesystem Hierarchy Standard (FHS)........................... 21
1.3 Special Red Hat Linux File Locations .......................................... 26
Chapter 2 Users and Groups .................................................. 27
2.1 Tools for User and Group Administration....................................... 27
2.2 Standard Users ................................................................... 27
2.3 Standard Groups ................................................................. 29
2.4 User Private Groups.............................................................. 31
2.5 Shadow Utilities................................................................... 33
Chapter 3 Boot Process, Init, and Shutdown ....................... 35
3.1 Introduction........................................................................ 35
3.2 Behind the Scenes of the Boot Process ....................................... 35
3.3 Sysconfig Information............................................................ 43
3.4 Init Runlevels...................................................................... 54
3.5 Running Programs at Boot Time................................................ 56
3.6 Shutting Down .................................................................... 56
iii

3.7 Differences in the Boot Process of Other Architectures ...................... 57
Chapter 4 The /proc Filesystem.............................................. 59
4.1 A Virtual Filesystem.............................................................. 59
4.2 Top-Level Files in /proc .......................................................... 62
4.3 Directories in /proc ............................................................... 78
4.4 Using sysctl ....................................................................... 98
4.5 Additional Resources ............................................................ 98
Chapter 5 GRUB ....................................................................... 101
5.1 Defining GRUB ................................................................... 101
5.2 Terminology ....................................................................... 104
5.3 Interfaces.......................................................................... 106
5.4 Commands........................................................................ 107
5.5 The Menu Configuration File .................................................... 109
5.6 Additional Resources ............................................................ 110
Chapter 6 X Servers and Clients ............................................ 113
6.1 ThePowerofX ................................................................... 113
6.2 The XFree86 Server.............................................................. 114
6.3 Desktop Environments and Window Managers ............................... 120
6.4 Runlevels.......................................................................... 121
6.5 Fonts............................................................................... 124
6.6 Additional Resources ............................................................ 126
Part II Security Reference .......................................................... 129
Chapter 7 Pluggable Authentication Modules (PAM) .......... 131
7.1 Advantages of PAM .............................................................. 131
7.2 PAM Configuration Files ......................................................... 131
7.3 Shadow Passwords .............................................................. 137
7.4 Using rlogin, rsh, and rexec with PAM.......................................... 137
7.5 Additional Resources ............................................................ 138
iv

Chapter 8 Using Kerberos 5 on Red Hat Linux .................... 139
8.1 Why Use Kerberos?.............................................................. 139
8.2 Why Not Use Kerberos? ......................................................... 139
8.3 Kerberos Terminology............................................................ 140
8.4 How Kerberos Works............................................................. 141
8.5 Kerberos and PAM................................................................ 143
8.6 Additional Resources ............................................................ 143
Chapter 9 TCP Wrappers and xinetd ..................................... 145
9.1 Purpose of TCP Wrappers ...................................................... 145
9.2 Host-Based Access Control Lists............................................... 146
9.3 Access Control Using xinetd .................................................... 150
9.4 Additional Resources ............................................................ 155
Chapter 10 SSH Protocol........................................................... 157
10.1 Introduction........................................................................ 157
10.2 Event Sequence of an SSH Connection ....................................... 158
10.3 Layers of SSH Security .......................................................... 160
10.4 OpenSSH Configuration Files................................................... 162
10.5 More Than a Secure Shell....................................................... 163
10.6 Requiring SSH for Remote Connections....................................... 165
Chapter 11 Installing and Configuring Tripwire ..................... 167
11.1 How to Use Tripwire.............................................................. 167
11.2 Installation Instructions........................................................... 169
11.3 File Locations ..................................................................... 171
11.4 Tripwire Components............................................................. 172
11.5 Modifying the Policy File ......................................................... 172
11.6 Selecting Passphrases........................................................... 173
11.7 Initializing the Database ......................................................... 174
11.8 Running an Integrity Check...................................................... 174
11.9 Printing Reports .................................................................. 174
11.10 Updating the Database after an Integrity Check .............................. 177
11.11 Updating the Policy File.......................................................... 178
v

11.12 Tripwire and Email................................................................ 179
11.13 Additional Resources ............................................................ 180
Part III Network Services Reference.......................................... 181
Chapter 12 Network Scripts...................................................... 183
12.1 Interface Configuration Files .................................................... 183
12.2 Interface Control Scripts ......................................................... 187
12.3 Network Functions................................................................ 189
Chapter 13 Apache..................................................................... 191
13.1 Default Modules .................................................................. 192
13.2 Starting and Stopping httpd ..................................................... 193
13.3 Configuration Directives in httpd.conf .......................................... 194
13.4 Adding Modules to Your Server................................................. 215
13.5 Using Virtual Hosts............................................................... 218
13.6 Additional Resources ............................................................ 221
Chapter 14 Berkeley Internet Name Domain (BIND).............. 223
14.1 Introduction to DNS and BIND .................................................. 223
14.2 BIND Configuration Files ........................................................ 225
14.3 Using rndc......................................................................... 238
14.4 BIND Advanced Features........................................................ 241
14.5 Common Mistakes to Avoid ..................................................... 243
14.6 Additional Resources ............................................................ 244
Chapter 15 Lightweight Directory Access Protocol (LDAP). 247
15.1 What is LDAP?.................................................................... 247
15.2 Pros and Cons of LDAP ......................................................... 247
15.3 Uses for LDAP .................................................................... 248
15.4 LDAP Terminology................................................................ 249
15.5 OpenLDAP 2.0 Enhancements ................................................. 249
15.6 OpenLDAP Files.................................................................. 250
vi

15.7 OpenLDAP Daemons and Utilities.............................................. 252
15.8 Modules for Adding Extra Functionality to LDAP.............................. 253
15.9 LDAP How To: A Quick Overview............................................... 254
15.10 Configuring Your System to Authenticate Using OpenLDAP................. 254
15.11 Additional Resources ............................................................ 257
Chapter 16 Email ........................................................................ 259
16.1 Protocols .......................................................................... 259
16.2 Different Types of Email Programs ............................................. 262
16.3 Sendmail .......................................................................... 263
16.4 Fetchmail .......................................................................... 268
16.5 Procmail ........................................................................... 273
16.6 Security............................................................................ 280
16.7 Additional Resources ............................................................ 282
Chapter 17 Network File System (NFS) ................................... 285
17.1 Methodology ...................................................................... 285
17.2 NFS Server Configuration Files................................................. 287
17.3 NFS Client Configuration Files.................................................. 291
17.4 Securing NFS..................................................................... 293
17.5 Additional Resources ............................................................ 295
Chapter 18 Firewalling with iptables........................................ 297
18.1 Packet Filtering ................................................................... 297
18.2 Differences between iptables and ipchains .................................... 298
18.3 Options Used in iptables Commands........................................... 299
18.4 Storing iptables Information ..................................................... 307
18.5 Additional Resources ............................................................ 308
Part IV Appendixes....................................................................... 309
Appendix A General Parameters and Modules........................ 311
A.1 Specifying Module Parameters.................................................. 312
vii

A.2 CD-ROM Module Parameters................................................... 312
A.3 SCSI parameters ................................................................. 315
A.4 Ethernet parameters ............................................................. 319
viii

Section 0.1:Finding Appropriate Documentation ix
Introduction
Welcome to the Official Red Hat Linux Reference Guide.
The Official Red Hat Linux Reference Guide contains useful information about your Red Hat Linux
system. From fundamental concepts, such as the structure of the Red Hat Linux filesystem, to the finer
points of system security and authentication control, we hope you will find this book to be a valuable
resource.
This guide is for you if you want to learn a bit more about how your Red Hat Linux system works.
Topics that you will explore include the following:
•Booting Red Hat Linux — Information about runlevels, rc.d directories, and how to start your
favorite applications at boot time.
•The
/proc filesystem — Provides you with the kernel’s view of the system.
•The GRUB boot loader — A behind the scenes look at GRUB and how it boots your system.
•System and network security — Find out the most common methods used by attackers to com-
promise your system and how to prevent security problems.
•Apache directives — A look at the default Apache configuration and the options available.
Before reading this guide, you should be familiar with the contents of the Official Red Hat Linux
Installation Guide concerning installation issues, the Official Red Hat Linux Getting Started Guide for
basic Linux concepts and the Official Red Hat Linux Customization Guide for general customization
instructions. The Official Red Hat Linux Reference Guide contains information about advanced topics
that may not affect every user, depending upon how they use their Red Hat Linux system.
HTML and PDF versions of all Official Red HatLinux manuals are available onlineat http://www.red-
hat.com/support/manuals.
Finding Appropriate Documentation
You need documentation that is appropriate to your level of Linux expertise. Otherwise, you might
feel overwhelmed or not find the necessary information to answer your questions. The Official Red
Hat Linux Reference Guide deals with the more technical aspects and options of your Red Hat Linux
system. This section will help you decide whether to look in this manual for the information you need
or consider other Red Hat Linux manuals, including online sources, in your search.
Three different categories of people use Red Hat Linux, and each of these categories require differ-
ent sets of documentation and informatative sources. To help you figure out where you should start,
determine your own experience level:
New to Linux

x Introduction
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 Documentation For First-Time
Linux Users.
Some Linux Experience
This type of user has installed and successfully used Linux (but not Red Hat Linux) before or
may have equivalent experience with other Linux-like operating systems. Does this describe
you? If so, turn to For the More Experienced.
Experienced User
This type of user has installed and successfully used Red Hat Linux before. If this describes
you, turn to Documentation for Linux Gurus.
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 your 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 will only become frustrated at your inability to get your Red Hat 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 will help 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’s 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

Section 0.1:Finding Appropriate Documentation xi
approach to doing things, you may find yourself intimidated by the sheer number of commands
available on your Red Hat 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 need
only know the general way in which Linux functions, what you need to accomplish, and how to
access the tool that will give you the exact instructions you need to execute the command.
The Official Red Hat Linux Installation Guide is a excellent reference for helping you get your Red
Hat Linux system successfully installed and initially configured. The Official Red Hat Linux Getting
Started Guide covers the history of Linux, basic system commands, GNOME, KDE, RPM, and many
other fundamental concepts. You should start with these two books and use them to build the base of
your Red Hat Linux knowledge. Before long, more complicated concepts will begin to make sense
because you already grasp the general ideas.
Beyond reading Red Hat Linux manuals, several other excellent documentation resources are available
for little or no cost:
Introduction to Linux Websites
• http://www.redhat.com — On the Red Hat website, you will find links to the Linux Documen-
tation Project (LDP), online versions of the Red Hat Linux manuals, FAQs (Frequently Asked
Questions), a database which can help you find a Linux Users Group near you, technical informa-
tion 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.
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 your
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://www.deja.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 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.

xii Introduction
• linux.redhat.rpm — A good place to go if you are having trouble using RPM to accomplish
particular objectives.
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 7 Unleashed by William Ball and David Pitts; Sams
The books suggested here are excellent primary sources of information for basic knowledge about
a Red Hat 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.
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 Official Red Hat Linux Customization Guide is designed to help explain the various ways your
Red Hat 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 Official Red Hat Linux Customization
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/mir-
rors/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.
Documentation for Linux Gurus
If you are a long-time Red Hat Linux user, you probably already know that one of the best ways to un-
derstand a particular program is to read its source code and/or configuration files. A major advantage
of Red Hat Linux is the availability of the source code for anyone to read.
Obviously, not everyone is a C 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.

Section 0.2:Document Conventions xiii
Document Conventions
When you read this manual, you will see that certain words are represented in different fonts, type-
faces, 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 in 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 (e.g., filenames). In these cases, they are considered
to be part of the command, so the entire phrase will be 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.
filename
Filenames, 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 Red Hat Linux
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 filesystems.
The /usr/share/doc directory contains documentation for various programs.
Install the webalizer RPM if you want to use a Web server log file analysis program.
application
This style should indicate to you that the program named is an end-user application (as opposed
to system software). For example:
Use Netscape Navigator 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 will
display the list of files in the directory that start with that letter.
[key]-[combination]
A combination of keystrokes is represented in this way. For example:

xiv Introduction
The [Ctrl]-[Alt]-[Backspace] key combination will restart the X Window System.
text found on a GUI interface
A title, word or phrase found on a GUI interface screen or window will be shown in this style.
When you see text shown in this style, it is being used to identify a particular GUI screen or an
element on a GUI screen (e.g., text associated with a checkbox or field). Examples:
On the GNOME Control Center screen, you can customize your GNOME window manager.
Selectthe Require Passwordcheckbox if you wouldlikeyour screensaver torequire apassword
before stopping.
top level of a menu on a GUI screen or window
When you see a word in this style, it 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 Settings on a GNOME terminal, you will see the following menu items: Preferences,
Reset Terminal,Reset and Clear, and Color selector.
If you need to type in a sequence of commands from a GUI menu, they will be shown like the
following example:
Click on Programs=>Applications=>Emacs to start the Emacs text editor.
button on a GUI screen or window
This style indicates that the text will be found on a clickable button on a GUI screen. For ex-
ample:
Click on the Back button to return to the Web page you last viewed.
computer output
When you see text in this style, it indicates text displayed by the computer on the command line.
You will see responses to commands you typed in, error messages and interactive prompts for
your input during scripts or programs shown this way. For example:
Use the ls to display the contents of a directory:
$ls
Desktop axhome logs paulwesterberg.gif
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

Section 0.2:Document Conventions xv
A prompt, which is a computer’s way of signifying that it is ready for you to input something,
will be 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 will need to type in the text
command at the boot: prompt.
Another example, with the word root displayed as something the user needs to type in:
If you need to log in as root when you first log into your system, and you are using the graphical
login screen, at the Login prompt, type root. At the Password prompt, type in the root
password.
glossary entry
A word that appears in the glossary will be shown in the body of the document in this style. For
example:
The lpd daemon handles printing requests.
In this case, the style of the word daemon should indicate to you that a definition of the term is
available in the glossary.
Additionally, we use several differentstrategies to draw your attention to certain pieces of information.
In order of how critical the information is to your system, these items will be marked as a note, a
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.

xvi Introduction
CAUTION
Do not do routine tasks as root — use a regular user account unless you need
to use the root account to administer your system.
WARNING
If you choose not to partition manually, a server installation will remove
all existing partitions on all installed hard drives. Do not choose this
installation class unless you are sure you have no data you need to save.
Using the Mouse
Red Hat 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 em-
ulation, 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.
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.
More to Come
The Official Red Hat Linux Reference Guide is part of Red Hat’s commitment to provide useful and
timely support to Red Hat Linux users. Future editions will feature expanded information on changes

Section 0.6:Sign Up for Support xvii
to system structure and organization, new and powerful security tools, and other resources to help you
extend the power of your Red Hat Linux system — and your ability to use it.
That’s where you can help.
We Need Feedback!
If you find an error in the Official Red Hat Linux Reference Guide, or if you have thought of a
way to make this manual better, we’d love to hear from you! Please submit a report in Bugzilla (
http://bugzilla.redhat.com/bugzilla) against the component rhl-rg.
Be sure to mention the manual’s identifier:
rhl-rg(EN)-7.2-Print-RHI (2001-08-30T12:53-0400)
If you mention the manual’s identifier, we will 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.
Sign Up for Support
If you have an official edition of Red Hat Linux 7.2, please remember to sign up for the benefits you
are entitled to as a Red Hat customer.
You will be entitled to any or all of the following benefits, depending upon the Official Red Hat Linux
product you purchased:
• Official Red Hat support —Get help with your installation questionsfrom Red Hat, Inc.’s support
team.
• Red Hat Network — Easily update your packages and receivesecurity notices that are customized
for your system. Go to http://rhn.redhat.com for more details.
•Under the Brim: The Official Red Hat E-Newsletter — Every month, get the latest news and
product information directly from Red Hat.
To sign up, go to http://www.redhat.com/apps/activate/. You will find your Product ID on a black,
red, and white card in your Official Red Hat Linux box.
To read more about technical support for Official Red Hat Linux, refer to the Getting TechnicalSupport
Appendix in the Official Red Hat Linux Installation Guide.
Good luck, and thank you for choosing Red Hat Linux!

xviii Introduction
The Red Hat Documentation Team

Part I System Reference