Linux Network Administrator's Guide, 2nd Edition By Olaf Kirch & Terry Dawson 2nd Edition June 2000 1-56592-400-2, Order Number: 4002 506 pages, $34.95

Preface

Contents:
Purpose and Audience for This Book
Sources of Information
File System Standards
Standard Linux Base
About This Book
The Official Printed Version
Overview
Conventions Used in This Book
Submitting Changes
Acknowledgments

The Internet is now a household term in many countries. With otherwise serious people beginning to joyride along the Information Superhighway, computer networking seems to be moving toward the status of TV sets and microwave ovens. The Internet has unusually high media coverage, and social science majors are descending on Usenet newsgroups, online virtual reality environments, and the Web to conduct research on the new "Internet Culture."

Of course, networking has been around for a long time. Connecting computers to form local area networks has been common practice, even at small installations, and so have long-haul links using transmission lines provided by telecommunications companies. A rapidly growing conglomerate of world-wide networks has, however, made joining the global village a perfectly reasonable option for even small non-profit organizations of private computer users. Setting up an Internet host with mail and news capabilities offering dialup and ISDN access has become affordable, and the advent of DSL (Digital Subscriber Line) and Cable Modem technologies will doubtlessly continue this trend.

Talking about computer networks often means talking about Unix. Of course, Unix is not the only operating system with network capabilities, nor will it remain a frontrunner forever, but it has been in the networking business for a long time, and will surely continue to be for some time to come.

What makes Unix particularly interesting to private users is that there has been much activity to bring free Unix-like operating systems to the PC, such as 386BSD, FreeBSD, and Linux.

Linux is a freely distributable Unix clone for personal computers. It currently runs on a variety of machines that includes the Intel family of processors, but also Motorola 680x0 machines, such as the Commodore Amiga and Apple Macintosh; Sun SPARC and Ultra-SPARC machines; Compaq Alphas; MIPS; PowerPCs, such as the new generation of Apple Macintosh; and StrongARM, like the rebel.com Netwinder and 3Com Palm machines. Linux has been ported to some relatively obscure platforms, like the Fujitsu AP-1000 and the IBM System 3/90. Ports to other interesting architectures are currently in progress in developers' labs, and the quest to move Linux into the embedded controller space promises success.

Linux was developed by a large team of volunteers across the Internet. The project was started in 1990 by Linus Torvalds, a Finnish college student, as an operating systems course project. Since that time, Linux has snowballed into a full-featured Unix clone capable of running applications as diverse as simulation and modeling programs, word processors, speech recognition systems, World Wide Web browsers, and a horde of other software, including a variety of excellent games. A great deal of hardware is supported, and Linux contains a complete implementation of TCP/IP networking, including SLIP, PPP, firewalls, a full IPX implementation, and many features and some protocols not found in any other operating system. Linux is powerful, fast, and free, and its popularity in the world beyond the Internet is growing rapidly.

The Linux operating system itself is covered by the GNU General Public License, the same copyright license used by software developed by the Free Software Foundation. This license allows anyone to redistribute or modify the software (free of charge or for a profit) as long as all modifications and distributions are freely distributable as well. The term "free software" refers to freedom of application, not freedom of cost.

Purpose and Audience for This Book

This book was written to provide a single reference for network administration in a Linux environment. Beginners and experienced users alike should find the information they need to cover nearly all important administration activities required to manage a Linux network configuration. The possible range of topics to cover is nearly limitless, so of course it has been impossible to include everything there is to say on all subjects. We've tried to cover the most important and common ones. We've found that beginners to Linux networking, even those with no prior exposure to Unix-like operating systems, have found this book good enough to help them successfully get their Linux network configurations up and running and get them ready to learn more.

There are many books and other sources of information from which you can learn any of the topics covered in this book (with the possible exception of some of the truly Linux-specific features, such as the new Linux firewall interface, which is not well documented elsewhere) in greater depth. We've provided a bibliography for you to use when you are ready to explore more.

Sources of Information

If you are new to the world of Linux, there are a number of resources to explore and become familiar with. Having access to the Internet is helpful, but not essential.

Linux Documentation Project guides

The Linux Documentation Project is a group of volunteers who have worked to produce books (guides), HOWTO documents, and manual pages on topics ranging from installation to kernel programming. The LDP works include:

Linux Installation and Getting Started

By Matt Welsh, et al. This book describes how to obtain, install, and use Linux. It includes an introductory Unix tutorial and information on systems administration, the X Window System, and networking.

Linux System Administrators Guide

By Lars Wirzenius and Joanna Oja. This book is a guide to general Linux system administration and covers topics such as creating and configuring users, performing system backups, configuration of major software packages, and installing and upgrading software.

Linux System Adminstration Made Easy

By Steve Frampton. This book describes day-to-day administration and maintenance issues of relevance to Linux users.

Linux Programmers Guide

By B. Scott Burkett, Sven Goldt, John D. Harper, Sven van der Meer, and Matt Welsh. This book covers topics of interest to people who wish to develop application software for Linux.

The Linux Kernel

By David A. Rusling. This book provides an introduction to the Linux Kernel, how it is constructed, and how it works. Take a tour of your kernel.

The Linux Kernel Module Programming Guide

By Ori Pomerantz. This guide explains how to write Linux kernel modules.

More manuals are in development. For more information about the LDP you should consult their World Wide Web server at http://www.linuxdoc.org/ or one of its many mirrors.

HOWTO documents

The Linux HOWTOs are a comprehensive series of papers detailing various aspects of the system -- such as installation and configuration of the X Window System software, or how to write in assembly language programming under Linux. These are generally located in the HOWTO subdirectory of the FTP sites listed later, or they are available on the World Wide Web at one of the many Linux Documentation Project mirror sites. See the Bibliography at the end of this book, or the file HOWTO-INDEX for a list of what's available.

You might want to obtain the Installation HOWTO, which describes how to install Linux on your system; the Hardware Compatibility HOWTO, which contains a list of hardware known to work with Linux; and the Distribution HOWTO, which lists software vendors selling Linux on diskette and CD-ROM.

The bibliography of this book includes references to the HOWTO documents that are related to Linux networking.

Linux Frequently Asked Questions

The Linux Frequently Asked Questions with Answers (FAQ) contains a wide assortment of questions and answers about the system. It is a must-read for all newcomers.

Documentation Available via FTP

If you have access to anonymous FTP, you can obtain all Linux documentation listed above from various sites, including metalab.unc.edu:/pub/Linux/docs and tsx-11.mit.edu:/pub/linux/docs.

These sites are mirrored by a number of sites around the world.

Documentation Available via WWW

There are many Linux-based WWW sites available. The home site for the Linux Documentation Project can be accessed at http://www.linuxdoc.org/.

The Open Source Writers Guild (OSWG) is a project that has a scope that extends beyond Linux. The OSWG, like this book, is committed to advocating and facilitating the production of OpenSource documentation. The OSWG home site is at http://www.oswg.org:8080/oswg.

Both of these sites contain hypertext (and other) versions of many Linux related documents.

Documentation Available Commercially

A number of publishing companies and software vendors publish the works of the Linux Documentation Project. Two such vendors are:

Specialized Systems Consultants, Inc. (SSC)
http://www.ssc.com/
P.O. Box 55549 Seattle, WA 98155-0549
1-206-782-7733
1-206-782-7191 (FAX)
sales@ssc.com

and:

Linux Systems Labs
18300 Tara Drive
Clinton Township, MI 48036
1-810-987-8807
1-810-987-3562 (FAX)

Both companies sell compendiums of Linux HOWTO documents and other Linux documentation in printed and bound form.

O'Reilly & Associates publishes a series of Linux books. This one is a work of the Linux Documentation Project, but most have been independently authored. Their range includes:

Running Linux

An installation and user guide to the system describing how to get the most out of personal computing with Linux.

Learning Debian GNU/Linux

Learning Red Hat Linux

More basic than Running Linux, these books contain popular distributions on CD-ROM and offer robust directions for setting them up and using them.

Linux in a Nutshell

Another in the successful "in a Nutshell" series, this book focuses on providing a broad reference text for Linux.

Linux Journal and Linux Magazine

Linux Journal and Linux Magazine are monthly magazines for the Linux community, written and published by a number of Linux activists. They contain articles ranging from novice questions and answers to kernel programming internals. Even if you have Usenet access, these magazines are a good way to stay in touch with the Linux community.

Linux Journal is the oldest magazine and is published by S.S.C. Incorporated, for which details were listed previously. You can also find the magazine on the World Wide Web at http://www.linuxjournal.com/.

Linux Magazine is a newer, independent publication. The home web site for the magazine is http://www.linuxmagazine.com/.

Linux Usenet Newsgroups

If you have access to Usenet news, the following Linux-related newsgroups are available:

comp.os.linux.announce

A moderated newsgroup containing announcements of new software, distributions, bug reports, and goings-on in the Linux community. All Linux users should read this group. Submissions may be mailed to linux-announce@news.ornl.gov.

comp.os.linux.help

General questions and answers about installing or using Linux.

comp.os.linux.admin

Discussions relating to systems administration under Linux.

comp.os.linux.networking

Discussions relating to networking with Linux.

comp.os.linux.development

Discussions about developing the Linux kernel and system itself.

comp.os.linux.misc

A catch-all newsgroup for miscellaneous discussions that don't fall under the previous categories.

There are also several newsgroups devoted to Linux in languages other than English, such as fr.comp.os.linux in French and de.comp.os.linux in German.

Linux Mailing Lists

There is a large number of specialist Linux mailing lists on which you will find many people willing to help with questions you might have.

The best-known of these are the lists hosted by Rutgers University. You may subscribe to these lists by sending an email message formatted as follows:

To: majordomo@vger.rutgers.edu
Subject: anything at all
Body:

subscribe listname

Some of the available lists related to Linux networking are:

linux-net

Discussion relating to Linux networking

linux-ppp

Discussion relating to the Linux PPP implementation

linux-kernel

Discussion relating to Linux kernel development

Online Linux Support

There are many ways of obtaining help online, where volunteers from around the world offer expertise and services to assist users with questions and problems.

The OpenProjects IRC Network is an IRC network devoted entirely to Open Projects -- Open Source and Open Hardware alike. Some of its channels are designed to provide online Linux support services. IRC stands for Internet Relay Chat, and is a network service that allows you to talk interactively on the Internet to other users. IRC networks support multiple channels on which groups of people talk. Whatever you type in a channel is seen by all other users of that channel.

There are a number of active channels on the OpenProjects IRC network where you will find users 24 hours a day, 7 days a week who are willing and able to help you solve any Linux problems you may have, or just chat. You can use this service by installing an IRC client like irc-II, connecting to servername irc.openprojects.org:6667, and joining the #linpeople channel.

Linux User Groups

Many Linux User Groups around the world offer direct support to users. Many Linux User Groups engage in activities such as installation days, talks and seminars, demonstration nights, and other completely social events. Linux User Groups are a great way of meeting other Linux users in your area. There are a number of published lists of Linux User Groups. Some of the better-known ones are:

Groups of Linux Users Everywhere

http://www.ssc.com/glue/groups/

LUG list project

http://www.nllgg.nl/lugww/

LUG registry

http://www.linux.org/users/

Obtaining Linux

There is no single distribution of the Linux software; instead, there are many distributions, such as Debian, RedHat, Caldera, Corel, SuSE, and Slackware. Each distribution contains everything you need to run a complete Linux system: the kernel, basic utilities, libraries, support files, and applications software.

Linux distributions may be obtained via a number of online sources, such as the Internet. Each of the major distributions has its own FTP and web site. Some of these sites are:

Caldera

http://www.caldera.com/ftp://ftp.caldera.com/

Corel

http://www.corel.com/ftp://ftp.corel.com/

Debian

http://www.debian.org/ftp://ftp.debian.org/

RedHat

http://www.redhat.com/ftp://ftp.redhat.com/

Slackware

http://www.slackware.com/ftp://ftp.slackware.com/

SuSE

http://www.suse.com/ftp://ftp.suse.com/

Many of the popular general FTP archive sites also mirror various Linux distributions. The best-known of these sites are:

metalab.unc.edu:/pub/Linux/distributions/
ftp.funet.fi:/pub/Linux/mirrors/
tsx-11.mit.edu:/pub/linux/distributions/
mirror.aarnet.edu.au:/pub/linux/distributions/

Many of the modern distributions can be installed directly from the Internet. There is a lot of software to download for a typical installation, though, so you'd probably want to do this only if you have a high-speed, permanent network connection, or if you just need to update an existing installation.[1]

[1] ... or you are extremely impatient and know that the 24 hours it might take to download the software from the Internet is faster than the 72 hours it might take to wait for a CD-ROM to be delivered!

Linux may be purchased on CD-ROM from an increasing number of software vendors. If your local computer store doesn't have it, perhaps you should ask them to stock it! Most of the popular distributions can be obtained on CD-ROM. Some vendors produce products containing multiple CD-ROMs, each of which provides a different Linux distribution. This is an ideal way to try a number of different distributions before you settle on your favorite one.

File System Standards

In the past, one of the problems that afflicted Linux distributions, as well as the packages of software running on Linux, was the lack of a single accepted filesystem layout. This resulted in incompatibilities between different packages, and confronted users and administrators with the task of locating various files and programs.

To improve this situation, in August 1993, several people formed the Linux File System Standard Group (FSSTND). After six months of discussion, the group created a draft that presents a coherent file sytem structure and defines the location of the most essential programs and configuration files.

This standard was supposed to have been implemented by most major Linux distributions and packages. It is a little unfortunate that, while most distributions have made some attempt to work toward the FSSTND, there is a very small number of distributions that has actually adopted it fully. Throughout this book, we will assume that any files discussed reside in the location specified by the standard; alternative locations will be mentioned only when there is a long tradition that conflicts with this specification.

The Linux FSSTND continued to develop, but was replaced by the Linux File Hierarchy Standard (FHS) in 1997. The FHS addresses the multi-architecture issues that the FSSTND did not. The FHS can be obtained from the Linux documentation directory of all major Linux FTP sites and their mirrors, or at its home site at http://www.pathname.com/fhs/. Daniel Quinlan, the coordinator of the FHS group, can be reached at quinlan@transmeta.com.

Standard Linux Base

The vast number of different Linux distributions, while providing lots of healthy choice for Linux users, has created a problem for software developers -- particularly developers of non-free software.

Each distribution packages and supplies certain base libraries, configuration tools, system applications, and configuration files. Unfortunately, differences in their versions, names, and locations make it very difficult to know what will exist on any distribution. This makes it hard to develop binary applications that will work reliably on all Linux distribution bases.

To help overcome this problem, a new project sprang up called the "Linux Standard Base." It aims to describe a standard base distribution that complying distributions will use. If a developer designs an application to work against the standard base platform, the application will work, and be portable to, any complying Linux distribution.

You can find information on the status of the Linux Standard Base project at its home web site at http://www.linuxbase.org/.

If you're concerned about interoperability, particularly of software from commercial vendors, you should ensure that your Linux distribution is making an effort to participate in the standardization project.

About This Book

When Olaf joined the Linux Documentation Project in 1992, he wrote two small chapters on UUCP and smail, which he meant to contribute to the System Administrator's Guide. Development of TCP/IP networking was just beginning, and when those "small chapters" started to grow, he wondered aloud whether it would be nice to have a Networking Guide. "Great!" everyone said. "Go for it!" So he went for it and wrote the first version of the Networking Guide, which was released in September 1993.

Olaf continued work on the Networking Guide and eventually produced a much enhanced version of the guide. Vince Skahan contributed the original sendmail mail chapter, which was completely replaced in this edition because of a new interface to the sendmail configuration.

The version of the guide that you are reading now is a revision and update prompted by O'Reilly & Associates and undertaken by Terry Dawson.[2] Terry has been an amateur radio operator for over 20 years and has worked in the telecommunications industry for over 15 of those. He was co-author of the original NET-FAQ, and has since authored and maintained various networking-related HOWTO documents. Terry has always been an enthusiastic supporter of the Network Administrators Guide project, and added a few new chapters to this version describing features of Linux networking that have been developed since the first edition, plus a bunch of changes to bring the rest of the book up to date.

[2] Terry Dawson can be reached at terry@linux.org.au.

The exim chapter was contributed by Philip Hazel,[3] who is a lead developer and maintainer of the package.

[3] Philip Hazel can be reached at ph10@cus.cam.ac.uk.

The book is organized roughly along the sequence of steps you have to take to configure your system for networking. It starts by discussing basic concepts of networks, and TCP/IP-based networks in particular. It then slowly works its way up from configuring TCP/IP at the device level to firewall, accounting, and masquerade configuration, to the setup of common applications such as rlogin and friends, the Network File System, and the Network Information System. This is followed by a chapter on how to set up your machine as a UUCP node. Most of the remaining sections is dedicated to two major applications that run on top of TCP/IP and UUCP: electronic mail and news. A special chapter has been devoted to the IPX protocol and the NCP filesystem, because these are used in many corporate environments where Linux is finding a home.

The email part features an introduction to the more intimate parts of mail transport and routing, and the myriad of addressing schemes you may be confronted with. It describes the configuration and management of exim, a mail transport agent ideal for use in most situations not requiring UUCP, and sendmail, which is for people who have to do more complicated routing involving UUCP.

The news part gives you an overview of how Usenet news works. It covers INN and C News, the two most widely used news transport software packages at the moment, and the use of NNTP to provide newsreading access to a local network. The book closes with a chapter on the care and feeding of the most popular newsreaders on Linux.

Of course, a book can never exhaustively answer all questions you might have. So if you follow the instructions in this book and something still does not work, please be patient. Some of your problems may be due to mistakes on our part (see the section ", later in this Preface), but they also may be caused by changes in the networking software. Therefore, you should check the listed information resources first. There's a good chance that you are not alone with your problems, so a fix or at least a proposed workaround is likely to be known. If you have the opportunity, you should also try to get the latest kernel and network release from one of the Linux FTP sites or a BBS near you. Many problems are caused by software from different stages of development, which fail to work together properly. After all, Linux is a "work in progress."

The Official Printed Version

In Autumn 1993, Andy Oram, who had been around the LDP mailing list from almost the very beginning, asked Olaf about publishing this book at O'Reilly & Associates. He was excited about this book, never having imagined that it would become this successful. He and Andy finally agreed that O'Reilly would produce an enhanced Official Printed Version of the Networking Guide, while Olaf retained the original copyright so that the source of the book could be freely distributed. This means that you can choose freely: you can get the various free forms of the document from your nearest Linux Documentation Project mirror site and print it out, or you can purchase the official printed version from O'Reilly.

Why, then, would you want to pay money for something you can get for free? Is Tim O'Reilly out of his mind for publishing something everyone can print and even sell themselves?[4] Is there any difference between these versions?

[4] Note that while you are allowed to print out the online version, you may not run the O'Reilly book through a photocopier, much less sell any of its (hypothetical) copies.

The answers are "it depends," "no, definitely not," and "yes and no." O'Reilly & Associates does take a risk in publishing the Networking Guide, and it seems to have paid off for them (they've asked us to do it again). We believe this project serves as a fine example of how the free software world and companies can cooperate to produce something both can benefit from. In our view, the great service O'Reilly is providing to the Linux community (apart from the book becoming readily available in your local bookstore) is that it has helped Linux become recognized as something to be taken seriously: a viable and useful alternative to other commercial operating systems. It's a sad technical bookstore that doesn't have at least one shelf stacked with O'Reilly Linux books.

Why are they publishing it? They see it as their kind of book. It's what they'd hope to produce if they contracted with an author to write about Linux. The pace, level of detail, and style fit in well with their other offerings.

The point of the LDP license is to make sure no one gets shut out. Other people can print out copies of this book, and no one will blame you if you get one of these copies. But if you haven't gotten a chance to see the O'Reilly version, try to get to a bookstore or look at a friend's copy. We think you'll like what you see, and will want to buy it for yourself.

So what about the differences between the printed and online versions? Andy Oram has made great efforts at transforming our ramblings into something actually worth printing. (He has also reviewed a few other books produced by the Linux Documentation Project, contributing whatever professional skills he can to the Linux community.)

Since Andy started reviewing the Networking Guide and editing the copies sent to him, the book has improved vastly from its original form, and with every round of submission and feedback it improves again. The opportunity to take advantage of a professional editor's skill is one not to be wasted. In many ways, Andy's contribution has been as important as that of the authors. The same is also true of the copyeditors, who got the book into the shape you see now. All these edits have been fed back into the online version, so there is no difference in content.

Still, the O'Reilly version will be different. It will be professionally bound, and while you may go to the trouble to print the free version, it is unlikely that you will get the same quality result, and even then it is more unlikely that you'll do it for the price. Secondly, our amateurish attempts at illustration will have been replaced with nicely redone figures by O'Reilly's professional artists. Indexers have generated an improved index, which makes locating information in the book a much simpler process. If this book is something you intend to read from start to finish, you should consider reading the official printed version.

Overview

Chapter 1, Introduction to Networking, discusses the history of Linux and covers basic networking information on UUCP, TCP/IP, various protocols, hardware, and security. The next few chapters deal with configuring Linux for TCP/IP networking and running some major applications. We examine IP a little more closely in Chapter 2, Issues of TCP/IP Networking, before getting our hands dirty with file editing and the like. If you already know how IP routing works and how address resolution is performed, you can skip this chapter.

Chapter 3, Configuring the Networking Hardware, deals with very basic configuration issues, such as building a kernel and setting up your Ethernet card. The configuration of your serial ports is covered separately in Chapter 4, Configuring the Serial Hardware, because the discussion does not apply to TCP/IP networking only, but is also relevant for UUCP.

Chapter 5, Configuring TCP/IP Networking, helps you set up your machine for TCP/IP networking. It contains installation hints for standalone hosts with loopback enabled only, and hosts connected to an Ethernet. It also introduces you to a few useful tools you can use to test and debug your setup. Chapter 6, Name Service and Resolver Configuration, discusses how to configure hostname resolution and explains how to set up a name server.

Chapter 7, Serial Line IP, explains how to establish SLIP connections and gives a detailed reference for dip, a tool that allows you to automate most of the necessary steps. Chapter 8, The Point-to-Point Protocol, covers PPP and pppd, the PPP daemon.

Chapter 9, TCP/IP Firewall, extends our discussion on network security and describes the Linux TCP/IP firewall and its configuration tools: ipfwadm, ipchains, and iptables. IP firewalling provides a means of controlling who can access your network and hosts very precisely.

Chapter 10, IP Accounting, explains how to configure IP Accounting in Linux so you can keep track of how much traffic is going where and who is generating it.

Chapter 11, IP Masquerade and Network Address Translation, covers a feature of the Linux networking software called IP masquerade, which allows whole IP networks to connect to and use the Internet through a single IP address, hiding internal systems from outsiders in the process.

Chapter 12, Important Network Features, gives a short introduction to setting up some of the most important network applications, such as rlogin, ssh, etc. This chapter also covers how services are managed by the inetd superuser, and how you may restrict certain security-relevant services to a set of trusted hosts.

Chapter 13, The Network Information System, and Chapter 14, The Network File System, discuss NIS and NFS. NIS is a tool used to distribute administative information, such as user passwords in a local area network. NFS allows you to share filesystems between several hosts in your network.

In Chapter 15, IPX and the NCP Filesystem, we discuss the IPX protocol and the NCP filesystem. These allow Linux to be integrated into a Novell NetWare environment, sharing files and printers with non-Linux machines.

Chapter 16, Managing Taylor UUCP, gives you an extensive introduction to the administration of Taylor UUCP, a free implementation of the UUCP suite.

The remainder of the book is taken up by a detailed tour of electronic mail and Usenet news. Chapter 17, Electronic Mail, introduces you to the central concepts of electronic mail, like what a mail address looks like, and how the mail handling system manages to get your message to the recipient.

Chapter 18, Sendmail, and Chapter 19, Getting Exim Up and Running, cover the configuration of sendmail and exim, two mail transport agents you can use for Linux. This book explains both of them, because exim is easier to install for the beginner, while sendmail provides support for UUCP.

Chapter 20, Netnews, through Chapter 23, Internet News, explain the way news is managed in Usenet and how you install and use C News, nntpd, and INN: three popular software packages for managing Usenet news. After the brief introduction in Chapter 20, you can read Chapter 21, C News, if you want to transfer news using C News, a traditional service generally used with UUCP. The following chapters discuss more modern alternatives to C News that use the Internet-based protocol NNTP (Network News Transfer Protocol). Chapter 22, NNTP and the nntpd Daemon covers how to set up a simple NNTP daemon, nntpd, to provide news reading access for a local network, while Chapter 23 describes a more robust server for more extensive NetNews transfers, the InterNet News daemon (INN). And finally, Chapter 24, Newsreader Configuration, shows you how to configure and maintain various newsreaders.

Conventions Used in This Book

All examples presented in this book assume you are using a sh compatible shell. The bash shell is sh compatible and is the standard shell of all Linux distributions. If you happen to be a csh user, you will have to make appropriate adjustments.

The following is a list of the typographical conventions used in this book:

Italic

Used for file and directory names, program and command names, command-line options, email addresses and pathnames, URLs, and for emphasizing new terms.

Boldface

Used for machine names, hostnames, site names, usernames and IDs, and for occasional emphasis.

Constant Width

Used in examples to show the contents of code files or the output from commands and to indicate environment variables and keywords that appear in code.

Constant Width Italic

Used to indicate variable options, keywords, or text that the user is to replace with an actual value.

Constant Width Bold

Used in examples to show commands or other text that should be typed literally by the user.

WARNING: Text appearing in this manner offers a warning. You can make a mistake here that hurts your system or is hard to recover from.

Submitting Changes

We have tested and verified the information in this book to the best of our ability, but you may find that features have changed (or even that we have made mistakes!). Please let us know about any errors you find, as well as your suggestions for future editions, by writing to:

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Acknowledgments

This edition of the Networking Guide owes almost everything to the outstanding work of Olaf and Vince. It is difficult to appreciate the effort that goes into researching and writing a book of this nature until you've had a chance to work on one yourself. Updating the book was a challenging task, but with an excellent base to work from, it was an enjoyable one.

This book owes very much to the numerous people who took the time to proof-read it and help iron out many mistakes, both technical and grammatical (never knew that there was such a thing as a dangling participle). Phil Hughes, John Macdonald, and Erik Ratcliffe all provided very helpful (and on the whole, quite consistent) feedback on the content of the book.

We also owe many thanks to the people at O'Reilly we've had the pleasure to work with: Sarah Jane Shangraw, who got the book into the shape you can see now; Maureen Dempsey, who copyedited the text; Rob Romano, Rhon Porter, and Chris Reilley, who created all the figures; Hanna Dyer, who designed the cover; Alicia Cech, David Futato, and Jennifer Niedherst for the internal layout; Lars Kaufman for suggesting old woodcuts as a visual theme; Judy Hoer for the index; and finally, Tim O'Reilly for the courage to take up such a project.

We are greatly indebted to Andres Sepúlveda, Wolfgang Michaelis, Michael K. Johnson, and all developers who spared the time to check the information provided in the Networking Guide. Phil Hughes, John MacDonald, and Eric Ratcliffe contributed invaluable comments on the second edition. We also wish to thank all those who read the first version of the Networking Guide and sent corrections and suggestions. You can find a hopefully complete list of contributors in the file Thanks in the online distribution. Finally, this book would not have been possible without the support of Holger Grothe, who provided Olaf with the Internet connectivity he needed to make the original version happen.

Olaf would also like to thank the following groups and companies that printed the first edition of the Networking Guide and have donated money either to him or to the Linux Documentation Project as a whole: Linux Support Team, Erlangen, Germany; S.u.S.E. GmbH, Fuerth, Germany; and Linux System Labs, Inc., Clinton Twp., United States, RedHat Software, North Carolina, United States.

Terry thanks his wife, Maggie, who patiently supported him throughout his participation in the project despite the challenges presented by the birth of their first child, Jack. Additionally, he thanks the many people of the Linux community who either nurtured or suffered him to the point at which he could actually take part and actively contribute. "I'll help you if you promise to help someone else in return."

The Hall of Fame

Besides those we have already mentioned, a large number of people have contributed to the Networking Guide, by reviewing it and sending us corrections and suggestions. We are very grateful.

Here is a list of those whose contributions left a trace in our mail folders.

Al Longyear, Alan Cox, Andres Sepúlveda, Ben Cooper, Cameron Spitzer, Colin McCormack, D.J. Roberts, Emilio Lopes, Fred N. van Kempen, Gert Doering, Greg Hankins, Heiko Eissfeldt, J.P. Szikora, Johannes Stille, Karl Eichwalder, Les Johnson, Ludger Kunz, Marc van Diest, Michael K. Johnson, Michael Nebel, Michael Wing, Mitch D'Souza, Paul Gortmaker, Peter Brouwer, Peter Eriksson, Phil Hughes, Raul Deluth Miller, Rich Braun, Rick Sladkey, Ronald Aarts, Swen Thüemmler, Terry Dawson, Thomas Quinot, and Yury Shevchuk.

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