Network programming involves writing programs that communicate with other programs across a computer network. One program is normally called the client and the other the server. Most operating systems provide precompiled programs that communicate across a network--common examples in the TCP/IP world are Web clients (browsers) and Web servers, and the FTP and Telnet clients and servers--but this book describes how to write our own network programs.

We write network programs using an application program interface or API. We describe two APIs for network programming:

  1. sockets, sometimes called "Berkeley sockets" acknowledging their heritage from Berkeley Unix, and

  2. XTI (X/Open Transport Interface), a slight modification of the Transport Layer Interface (TLI) developed by AT&T.

All the examples in the text are from the Unix operating system, although the foundation and concepts required for network programming are, to a large degree, operating system independent. The examples are also based on the TCP/IP protocol suite, both IP versions 4 and 6.

To write network programs one must understand the underlying operating system and the underlying networking protocols. This book builds on the foundation of my other four books in these two areas, and these books are abbreviated throughout this text as follows:

This second edition of UNIX Network Programming still contains information on both Unix and the TCP/IP protocols, but many references are made to these other four texts to allow interested readers to obtain more detailed information on various topics. This is especially the case for TCPv2, which describes and presents the actual 4.4BSD implementation of the network programming functions for the sockets API (socket, bind, connect, and so on). If one understands the implementation of a feature, the use of that feature in an application makes more sense.

Changes from the First Edition

This second edition is a complete rewrite of the first edition. These changes have been driven by the feedback I have received teaching this material about once a month during 1990-1996, and by following certain Usenet newsgroups during this same time, which lets one see the topics that are continually misunderstood. The following are the major changes with this new edition:

Unfortunately, the coverage of the material from the first edition has been expanded so much that it no longer fits into a single book. Therefore at least two additional volumes are planned in the UNIX Network Programming series.

Even though most of the networking applications will be covered in Volume 3, a few special applications are covered in this volume: Ping, Traceroute, and inetd.


This text can be used as either a tutorial on network programming, or as a reference for experienced programmers. When used as a tutorial or for an introductory class on network programming, the emphasis should be on Part 2 ("Elementary Sockets," Chapters 3 through 9) followed by whatever additional topics are of interest. Part 2 covers the basic socket functions, for both TCP and UDP, along with I/O multiplexing, socket options, and basic name and address conversions. Chapter 1 should be read by all readers, especially Section 1.4, which describes some wrapper functions used throughout the text. Chapter 2 and perhaps Appendix A should be referred to as necessary, depending on the reader's background. Most of the chapters in Part 3 ("Advanced Sockets") can be read independently of the others in that part.

To aid in the use as a reference, a thorough index is provided, along with summaries on the end papers of where to find detailed descriptions of all the functions and structures. To help those reading topics in a random order, numerous references to related topics are provided throughout the text.

Although the sockets API has become the de facto standard for network programming, XTI is still used, sometimes with protocol suites other than TCP/IP. While the coverage of XTI in Part 4 is smaller than the coverage of sockets in Parts 2 and 3, much of the sockets coverage describes concepts that apply to XTI as well as sockets. For example, all of the concepts regarding the use of nonblocking I/O, broadcasting, multicasting, signal-driven I/O, out-of-band data, and threads, are the same, regardless of which API (sockets or XTI) is used. Indeed, many network programming problems are fundamentally similar, independent of whether the program is written using sockets or XTI, and there is hardly anything that can be done with one API that cannot be done with the other. The concepts are the same--just the function names and arguments change.

Source Code and Errata Availability

The source code for all the examples that appear in the book is available from The best way to learn network programming is to take these programs, modify them, and enhance them. Actually writing code of this form is the only way to reinforce the concepts and techniques. Numerous exercises are also provided at the end of each chapter, and most answers are provided in Appendix E.

A current errata for the book is also available from my home page, listed at the end of the Preface.


Supporting every author is an understanding family, or nothing would ever get written! I am grateful to my family, Sally, Bill, Ellen, and David, first for their support and understanding when I wrote my first book (the first edition of this book), and for enduring this "small" revision. Their love, support, and encouragement helped make this book possible.

Numerous reviewers provided invaluable feedback (totaling 190 printed pages or 70,000 words), catching lots of errors, pointing out areas that needed more explanation, and suggesting alternative presentations, wording, and coding: Ragnvald Blindheim, Jim Bound, Gavin Bowe, Allen Briggs, Joe Doupnik, Wu-chang Feng, Bill Fenner, Bob Friesenhahn, Andrew Gierth, Wayne Hathaway, Kent Hofer, Sugih Jamin, Scott Johnson, Rick Jones, Mukesh Kacker, Marc Lampo, Marty Leisner, Jack McCann, Craig Metz, Bob Nelson, Evi Nemeth, John C. Noble, Steve Rago, Jim Reid, Chung-Shang Shao, Ian Lance Taylor, Ron Taylor, Andreas Terzis, and Dave Thaler. A special thanks to Sugih Jamin and his students in EECS 489 ("Computer Networks") at the University of Michigan who beta tested an early draft of the manuscript during the spring of 1997.

The following people answered email questions of mine, sometimes lots of questions, which improved the accuracy and presentation of the text: Dave Butenhof, Dave Hanson, Jim Hogue, Mukesh Kacker, Brian Kernighan, Vern Paxson, Steve Rago, Dennis Ritchie, Steve Summit, Paul Vixie, John Wait, Steve Wise, and Gary Wright.

A special thanks to Larry Rafsky and the wonderful team at Gari Software for handling lots of details and for many interesting technical discussions. Thank you, Larry, for everything.

Numerous individuals and their organizations went beyond the normal call of duty to provide either a loaner system, software, or access to a system, all of which were used to test some of the examples in the text.

Many people helped with the Internet connectivity used throughout the text. My thanks once again to the National Optical Astronomy Observatories (NOAO), Sidney Wolff, Richard Wolff, and Steve Grandi, for providing access to their networks and hosts. Dave Siegel, Justis Addis, and Paul Lucchina answered many questions, Phil Kaslo and Jim Davis provided an MBone connection, Ran Atkinson and Pedro Marques provided a 6bone connection, and Craig Metz provided lots of DNS help.

The staff at Prentice Hall, especially my editor Mary Franz, along with Noreen Regina, Sophie Papanikolaou, and Eileen Clark, have been a wonderful asset to a writer. Many thanks for letting me do so many things "my way."

As usual, but contrary to popular fads, I produced camera-ready copy of the book using the wonderful Groff package written by James Clark. I typed in all 291,972 words using the vi editor, created the 201 illustrations using the gpic program (using many of Gary Wright's macros), produced the 81 tables using the gtbl program, performed all the indexing, and did the final page layout. Dave Hanson's loom program and some scripts by Gary Wright were used to include the source code in the book. A set of awk scripts written by Jon Bentley and Brian Kernighan helped in producing the final index.

I welcome electronic mail from any readers with comments, suggestions, or bug fixes.

W. Richard Stevens
Tucson, Arizona
September 1997