With the Internet much of a buzzword recently, and otherwise serious people joyriding along the ``Informational Superhighway,'' computer networking seems to be moving toward the status of TV sets and microwave ovens. The Internet is recently getting an unusually high media coverage, and social science majors are descending on Usenet newsgroups to conduct researches on the ``Internet Culture.'' Carrier companies are working to introduce new transmission techniques like ATM that offer many times the bandwidth the average network link of today has.
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 been long-haul links using public telephone lines. A rapidly growing conglomerate of world-wide networks has, however, made joining the global village a viable option even for small non-profit organizations of private computer users. Setting up an Internet host with mail and news capabilities offering dial-up access has become affordable, and the advent of ISDN will doubtlessly accelerate this trend.
Talking of computer networks quite frequently means talking about UNIX. Of course, UNIX is neither the only operating system with network capabilities, nor will it remain a front-runner forever, but it has been in the networking business for a long time, and will surely continue to do so for some time to come.
What makes it particularly interesting to private users is that there has been much activity to bring free UNIXoid operating systems to the PC, being 386BSD, FreeBSD--- and . However, is not UNIX. That is a registered trademark of whoever currently holds the rights to it (Univel, while I'm typing this), while is an operating system that strives to offer all functionality the POSIX standards require for UNIX-like operating systems, but is a complete re-implementation.
The kernel was written largely by Linus Torvalds, who started it as a project to get to know the Intel-i386, and to ``make MINIX better.'' MINIX was then another popular PC operating system offering vital ingredients of functionality, and was written by Prof. Andrew S.-Tanenbaum.
is covered by the GNU General Public License (GPL), which allows free distribution of the code (please read the GPL in appendix- for a definition of what ``free software'' means). Outgrowing its child's diseases, and drawing from a large and ever-growing base of free application programs, it is quickly becoming the operating system of choice for many PC owners. The kernel and C library have become that good that most standard software may be compiled with no more effort than is required on any other mainstream ish system, and a broad assortment of packaged distributions allows you to almost drop it onto your hard disk and start playing.
One of the complaints that are frequently leveled at (and free software in general) is the sorry state or complete lack of documentation. In the early days it was not unusual for a package to come with a handful of READMEs and installation notes. They gave the moderately experienced wizard enough information to successfully install and run it, but left the average newbie out in the cold.
Back in late 1992, Lars Wirzenius and Michael K.-Johnson suggested to form the Documentation Project, or LDP, which aims at providing a coherent set of manuals. Stopping short of answering questions like ``How?'', or ``Why?'', or ``What's the meaning of life, universe, and all the rest?'', these manuals attempt to cover most aspects of running and using a system users without requiring a prior degree in .
Among the achievements of the LDP are the Installation and Getting Started Guide, written by Matt Welsh, the Kernel Hacker's Guide by Michael K.-Johnson, and the manpage project coordinated by Rik Faith, which so far supplied a set of roughly 450 manual pages for most system and C-library calls. The System Administrators' Guide, written by Lars Wirzenius, is still at the Alpha stage. A User's Guide is being prepared.
This book, the Network Administrators' Guide, is part of the LDP series, too. As such, it may be copied and distributed freely under the LDP copying license which is reproduced on the second page.
However, the LDP books are not the only source of information on . At the moment, there are more than a dozen HOWTOs that are posted to comp.os.linux.announce regularly and archived at various FTP sites. HOWTOs are short documents of a few pages that give you a brief introduction into topics such as Ethernet support under , or the configuration of Usenet news software, and answer frequently asked questions. They usually provide the most accurate and up-to-date information available on the topic. A list of available HOWTOs is produced in the ``Annotated Bibliography'' toward the end of this book.
About This Book When I joined the Documentation Project in 1992, I wrote two small chapters on UUCP and smail, which I 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, I wondered aloud if it wouldn't be nice to have a Networking Guide. ``Great'', everyone said, ``I'd say, go for it!'' So I went for it, and wrote a first version of the Networking Guide, which I released in September 1993.
The new Networking Guide you are reading right now is a complete rewrite that features several new applications that have become available to users since the first release.
The book is organized roughly in 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. We then slowly work our way up from configuring TCP/IP at the device level 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. The remainder of the book is dedicated to two major applications that run on top of both TCP/IP and UUCP: electronic mail and news.
The email part features an introduction of the more intimate parts of mail transport and routing, and the myriads of addressing schemes you may be confronted with. It describes the configuration and management of smail, a mail transport agent commonly used on smaller mail hubs, and sendmail, which is for people who have to do more complicated routing, or have to handle a large volume of mail. The sendmail chapter has been written and contributed by Vince Skahan.
The news part attempts to give you an overview of how Usenet news works, covers C-news, the most widely used news transport software at the moment, and the use of NNTP to provide newsreading access to a local network. The book closes with a short chapter on the care and feeding of the most popular newsreaders on .
The Official Printed Version In autumn 1993, Andy Oram, who has been around the LDP mailing list from almost the very beginning, asked me about publishing my book at O'Reilly and Associates. I was excited about this; I had never imagined my book being that successful. We finally agreed that O'Reilly would produce an enhanced Official Printed Version of the Networking Guide with me, while I 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 LaTeXsource distributed on the network (or the pre-formatted DVI or PostScript versions, for that matter), and print it out. Or you can purchase the official printed version from O'Reilly, which will be available some time later this year.
Then, why 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 herself? Or is there any difference between these versions?
The answers are ``it depends,'' ``no, definitely not,'' and ``yes and no.'' O'Reilly and Associates do take a risk in publishing the Networking Guide, but I hope it will finally pay off for them. If it does, I believe this project can serve as an example how the free software world and companies can cooperate to produce something both benefit from. In my view, the great service O'Reilly is doing to the Linux community (apart from the book being readily available in your local bookstore) is that it may help Linux being recognized as something to be taken seriously: a viable and useful alternative to commercial PC UNIX operating systems.
Why are they publishing it? The reason is that 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, the level of detail, and the 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 your own.
So what about the differences between the printed version and the online one? Andy Oram has made great efforts at transforming my early ramblings into something actually worth printing. (He has also been reviewing the other books put out by the Linux Documentation Project, trying to contribute whatever professional skills he can to the Linux community.)
Since Andy started reviewing the Networking Guide and editing the copies I sent him, the book has improved vastly over what it was half a year ago. It would be nowhere close to where it is now without his contributions.
The same is true of Stephen Spainhour, who has been copy-editing the book for almost a month to get it into the shape you can 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 is different: On one hand, people at O'Reilly have put a lot of work into the look and feel, producing a much more pleasant layout than you could ever get out of standard LaTeX. Among other things, Chris Reilley has nicely redone all the figures from the additional network version, and added quite a couple of extra figures. He has done a great job at actually visualizing what I originally meant my amateurish XFIG drawings to convey.
All his edits have been fed back into online version, as will any changes that will be made to the Networking Guide during the copy-editing phase at O'Reilly. So there will be no difference in content. Still, the O'Reilly version will be different: On one hand, people at O'Reilly are putting a lot of work into the look and feel, producing a much more pleasant layout than you could ever get out of standard LaTeX. On the other hand, it will feature a couple of enhancements like an improved index, and better and more figures.
More Information If you follow the instructions in this book, and something does not work nevertheless, please be patient. Some of your problems may be due to stupid mistakes on my part, but may also be caused by changes in the networking software. Therefore, you should probably ask on comp.os.linux.help first. There's a good chance that you are not alone with your problems, so that 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 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, is ``work in progress''.
Another good place to inform yourself about current development is the Networking HOWTO. It is maintained by Terry Dawson. It is posted to comp.os.linux.announce once a month, and contains the most up-to-date information. The current version can also be obtained (among others) from tsx-11.mit.edu, in /pub/linux/doc. For problems you can't solve in any other way, you may also contact the author of this book at the address given in the preface. However, please, refrain from asking developers for help. They are already devoting a major part of their spare time to anyway, and occasionally even have a life beyond the net:-)
On the Authors Olaf has been a UNIX user and part-time administrator for a couple of years while he was studying mathematics. At the moment, he's working as a UNIX programmer and is writing a book. One of his favorite sports is doing things with sed that other people would reach for their perl interpreter for. He has about as much fun with this as with mountain hiking with a backpack and a tent.
Vince Skahan has been administering large numbers of UNIX systems since 1987 and currently runs sendmail+IDA on approximately 300 UNIX workstations for over 2000 users. He admits to losing considerable sleep from editing quite a few sendmail.cf files `the hard way' before discovering sendmail+IDA in 1990. He also admits to anxiously awaiting the delivery of the first perl-based version of sendmail for even more obscure fun...
Olaf can be reached at the following address:
Vince can be reached at:
We are open to your questions, comments, postcards, etc. However, we ask you not to telephone us unless it's really important.
Thanks Olaf says: This book owes very much to the numerous people who took the time to proof-read it and helped iron out many mistakes, both technical and grammatical (never knew that there's such a thing as a dangling participle). The most vigorous among them was Andy Oram at O'Reilly and Associates.
I also owe many thanks to the people at O'Reilly I've had the pleasure to work with: Stephen Spainhour, who has been copy-editing the book for almost a month to get it into the shape you can see now; Chris Reilley, who has done all the figures; Edie Freeman and Jennifer Niederst, who designed the cover, the internal layout, and the use of old woodcuts as a visual theme (an idea suggested by Lar Kaufman); Barbara Yoder, (how would I describe her job?); and, finally, Tim O'Reilly for the courage of taking up such a project.
I am 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. I also wish to thank all those who read the first version of the Networking Guide and sent me corrections and suggestions. You can find 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 me with the critical Internet connectivity.
I would also like to thank the following groups and companies who printed the first edition of the Networking Guide and have donated money either to me, or to the Documentation Project as a whole.
Vince says: Thanks go to Neil Rickert and Paul Pomes for lots of help over the years regarding the care and feeding of sendmail+IDA and to Rich Braun for doing the initial port of sendmail+IDA to . The biggest thanks by far go to my wife Susan for all the support on this and other projects.
Typographical Conventions In writing this book, a number of typographical conventions were employed to mark shell commands, variable arguments, etc. They are explained below.
When reading the texinfo version of this document, you will notice that, for example, there is no difference between Typewiter and Typewriter Slanted. This is due to deficiencies in the LaTeX to TeXinfo converter, as well as the limitations of an ASCII terminal. We are still working on this...
The Documentation Project
The Documentation Project, or LDP, is a loose team of writers, proof-readers, and editors who are working together to provide complete documentation for the operating system. The overall coordinator of the project is Matt Welsh, who is heavily aided by Lars Wirzenius and Michael-K. Johnson.
This manual is one in a set of several being distributed by the LDP, including a Linux Users' Guide, System Administrators' Guide, Network Administrators' Guide, and Kernel Hackers' Guide. These manuals are all available in LaTeX source format, .dvi format, and postscript output by anonymous FTP from nic.funet.fi, in the directory /pub/OS/Linux/doc/doc-project, and from tsx-11.mit.edu, in the directory /pub/linux/docs/guides.
We encourage anyone with a penchant for writing or editing to join us in improving Linux documentation. If you have Internet e-mail access, you can join the DOC channel of the Linux-Activists mailing list by sending mail to
with the line
in the header or as the first line of the message body. An empty mail without the additional header line will make the mail-server return a help message. To leave the channel, send a message to the same address, including the line
Throughout the past, one of the problems that afflicted distributions as well as separate packages was that there was no single accepted file system layout. This resulted in incompatibilities between different packages, and confronted users and administrators alike with the task to locate various files and programs.
To improve this situation, in August 1993, several people formed the File System Standard Group, or FSSTND Group for short, coordinated by Daniel Quinlan. After six months of discussion, the group presented a draft that presents a coherent file system structure and defines the location of most essential programs and configuration files.
This standard is supposed to be implemented by most major distributions and packages. Throughout this book, we will therefore assume that any files discussed reside in the location specified by the standard; only where there is a long tradition that conflicts with this specification will alternative locations be mentioned.
The File System Standard can be obtained from all major FTP sites and their mirrors; for instance, you can find it on sunsite.unc.edu below /pub/linux/docs. Daniel Quinlan, the coordinator of the FSSTND group can be reached at email@example.com.