www.it-ebooks.info www.it-ebooks.info LINUX Network Administrator’s Guide www.it-ebooks.info Other Linux resources from O’Reilly Related titles Apache Cookbook Linux Server Security DNS and BIND Cookbook Network Troubleshooting Linux Server Cookbook Tools Linux Server Hacks Running Linux Using Samba Linux Books linux.oreilly.com is a complete catalog of O’Reilly’s books on Resource Center Linux and Unix and related technologies, including sample chapters and code examples. ONLamp.com isthe premiersiteforthe opensource web plat- form: Linux, Apache, MySQL, and either Perl, Python, or PHP. Conferences O’Reilly bringsdiverseinnovatorstogethertonurturetheideas thatsparkrevolutionaryindustries.Wespecializeindocument- ing the latest tools and systems, translating the innovator’s knowledgeintousefulskillsforthoseinthetrenches.Visitcon- ferences.oreilly.com for our upcoming events. Safari Bookshelf (safari.oreilly.com) is the premier online refer- ence library for programmers and IT professionals. Conduct searchesacrossmorethan1,000books.Subscriberscanzeroin on answers to time-critical questions in a matter of seconds. Read the books on your Bookshelf from cover to cover or sim- ply flip to the page you need. Try it today with a free trial. www.it-ebooks.info LINUX Network Administrator’s Guide THIRD EDITION Tony Bautts, Terry Dawson, and Gregor N. Purdy Beijing • Cambridge • Farnham • Köln • Paris • Sebastopol • Taipei • Tokyo www.it-ebooks.info Linux Network Administrator’s Guide, Third Edition by Tony Bautts, Terry Dawson, and Gregor N. Purdy Copyright © 2005 O’Reilly Media, Inc. All rights reserved. Copyright©1995OlafKirch.Copyright©2000TerryDawson.CopyrightonO’Reillyprintedversion © 2000 O’Reilly Media, Inc. Rights to copy the O’Reilly printed version are reserved. Printed in the United States of America. Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472. O’Reilly books may be purchased for educational, business, or sales promotional use. Online editions arealsoavailableformosttitles(safari.oreilly.com).Formoreinformation,contactourcorporate/insti- tutional sales department: (800) 998-9938 [email protected]. Editor: Andy Oram Production Editor: Adam Witwer Cover Designer: Edie Freedman Interior Designer: David Futato Printing History: January 1995: First Edition. June 2000: Second Edition. February 2005: Third Edition. Nutshell Handbook, the Nutshell Handbook logo, and the O’Reilly logo are registered trademarks of O’Reilly Media, Inc. TheLinux series designations,Linux Network Administrator’s Guide, Third Edition, images of the American West, and related trade dress are trademarks of O’Reilly Media, Inc. Manyofthedesignationsusedbymanufacturersandsellerstodistinguishtheirproductsareclaimedas trademarks. Where those designations appear in this book, and O’Reilly Media, Inc. was aware of a trademark claim, the designations have been printed in caps or initial caps. While every precaution has been taken in the preparation of this book, the publisher and authors assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 2.0 License. To view a copy of this license, visithttp://creativecommons.org/licenses/by-sa/2.0/or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. This book uses RepKover™, a durable and flexible lay-flat binding. ISBN: 0-596-00548-2 [M] [5/05] www.it-ebooks.info Table of Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix 1. Introduction to Networking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 History 1 TCP/IP Networks 2 Linux Networking 11 Maintaining Your System 13 2. Issues of TCP/IP Networking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Networking Interfaces 16 IP Addresses 17 The Internet Control Message Protocol 26 3. Configuring the Serial Hardware. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Communications Software for Modem Links 29 Accessing Serial Devices 30 Using the Configuration Utilities 34 Serial Devices and the login: Prompt 38 4. Configuring TCP/IP Networking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Understanding the /proc Filesystem 43 5. Name Service and Configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 The Resolver Library 67 How DNS Works 71 Alternatives to BIND 92 v www.it-ebooks.info 6. The Point-to-Point Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 PPP on Linux 97 Running pppd 98 Using Options Files 99 Using chat to Automate Dialing 100 IP Configuration Options 102 Link Control Options 105 General Security Considerations 107 Authentication with PPP 108 Debugging Your PPP Setup 112 More Advanced PPP Configurations 112 PPPoE Options in Linux 116 7. TCP/IP Firewall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Methods of Attack 120 What Is a Firewall? 122 What Is IP Filtering? 124 Netfilter and iptables 125 iptables Concepts 127 Setting Up Linux for Firewalling 133 Using iptables 134 The iptables Subcommands 136 Basic iptables Matches 137 A Sample Firewall Configuration 141 References 144 8. IP Accounting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 Configuring the Kernel for IP Accounting 146 Configuring IP Accounting 146 Using IP Accounting Results 151 Resetting the Counters 151 Flushing the Rule Set 152 Passive Collection of Accounting Data 152 9. IP Masquerade and Network Address Translation . . . . . . . . . . . . . . . . . . . . . 154 Side Effects and Fringe Benefits 156 Configuring the Kernel for IP Masquerade 157 Configuring IP Masquerade 157 Handling Nameserver Lookups 158 More About Network Address Translation 159 vi | Table of Contents www.it-ebooks.info 10. Important Network Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 The inetd Super Server 160 The tcpd Access Control Facility 163 The xinetd Alternative 164 The Services and Protocols Files 167 Remote Procedure Call 169 Configuring Remote Login and Execution 170 11. Administration Issues with Electronic Mail . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 What Is a Mail Message? 180 How Is Mail Delivered? 182 Email Addresses 183 How Does Mail Routing Work? 184 Mail Routing on the Internet 184 12. sendmail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 Installing the sendmail Distribution 186 sendmail Configuration Files 192 sendmail.cf Configuration Language 198 Creating a sendmail Configuration 203 sendmail Databases 210 Testing Your Configuration 222 Running sendmail 227 Tips and Tricks 228 More Information 231 13. Configuring IPv6 Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 The IPv4 Problem and Patchwork Solutions 234 IPv6 as a Solution 235 14. Configuring the Apache Web Server. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 Apache HTTPD Server—An Introduction 244 Configuring and Building Apache 244 Configuration File Options 247 VirtualHost Configuration Options 250 Apache and OpenSSL 252 Troubleshooting 256 Table of Contents | vii www.it-ebooks.info 15. IMAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 IMAP—An Introduction 258 Cyrus IMAP 263 16. Samba . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 Samba—An Introduction 266 17. OpenLDAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 Understanding LDAP 278 Obtaining OpenLDAP 280 18. Wireless Networking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294 History 294 The Standards 295 802.11b Security Concerns 296 Appendix: Example Network: The Virtual Brewery . . . . . . . . . . . . . . . . . . . . . . . . . . 309 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311 viii | Table of Contents www.it-ebooks.info