Linux Sendmail Administration (Craig Hunt Linux Library)

478 Pages·2001·1.93 MB·English

by  Craig Hunt

Linux Sendmail
Administration
Craig Hunt
SYBEX®
Linux Sendmail Administration
Craig Hunt
San Francisco Paris Düsseldorf Soest London
Associate Publisher: Dick Staron
Contracts and Licensing Manager: Kristine O’Callaghan
Acquisitions and Developmental Editors: Maureen Adams, Tom Cirtin
Editor: Suzanne Goraj
Production Editor: Liz Burke
Technical Editors: Randolph Russell, James Eric Gunnett
Book Designer: Bill Gibson
Electronic Publishing Specialist: Nila Nichols
Proofreaders: Jennifer Campbell, Nelson Kim, Yariv Rabinovitch, Nanette Duffy, Nancy Riddiough, Laurie
O’Connell, Andrea Fox
Indexer: Nancy Guenther
Cover Designer: Ingalls & Associates
Cover Illustrator: Ingalls & Associates
Copyright © 2001 SYBEX Inc., 1151 Marina Village Parkway, Alameda, CA 94501. World rights reserved.
No part of this publication may be stored in a retrieval system, transmitted, or reproduced in any way, includ-
ing but not limited to photocopy, photograph, magnetic, or other record, without the prior agreement and
written permission of the publisher.
Library of Congress Card Number: 2001087202
ISBN: 0-7821-2737-1
SYBEX and the SYBEX logo are either registered trademarks or trademarks of SYBEX Inc. in the United
States and/or other countries.
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TRADEMARKS: SYBEX has attempted throughout this book to distinguish proprietary trademarks from
descriptive terms by following the capitalization style used by the manufacturer.
The author and publisher have made their best efforts to prepare this book, and the content is based upon
final release software whenever possible. Portions of the manuscript may be based upon pre-release versions
supplied by software manufacturer(s). The author and the publisher make no representation or warranties of
any kind with regard to the completeness or accuracy of the contents herein and accept no liability of any kind
including but not limited to performance, merchantability, fitness for any particular purpose, or any losses or
damages of any kind caused or alleged to be caused directly or indirectly from this book.
Manufactured in the United States of America
10 9 8 7 6 5 4 3 2 1
To Sara, David, and Rebecca, who make me proud every day.
Foreword
You may already be familiar with the Craig Hunt Linux Library. If you are, you know it
is a library of books for professional system administrators that focuses directly on Linux.
The reason for creating such a high quality library is simple: Linux and the professionals
who administer Linux systems deserve it. The goal of the library is to provide highly tech-
nical books that are clear, accurate, and complete.
Creating comprehensive, concise books that focus on only Linux and that have a consis-
tent structure has had a serendipitous side effect. These books tell the story of the under-
lying technology, whether it is DNS, Samba, or Sendmail, in a clear and organized
manner. This turns out to be particularly important for Sendmail. Sendmail is an essential
component of every Linux distribution. Yet a fog of confusion has surrounded Sendmail
and particularly Sendmail configuration. Books about Sendmail have done little to alle-
viate this situation. Some books become so enmeshed in the minutiae of Sendmail con-
figuration syntax that they become little more than giant reference books that are about
as useful as reading a dictionary. Others are too superficial; they lack details needed to
help the professional system administrator. What is needed is a balance between enough
detail and too much detail.
Linux Sendmail Administration cuts through the fog by presenting the story of Sendmail
in a clear, organized manner. Reference material is where it should be, in an appendix.
The content of the book respects the reader’s technical skills, providing all of the infor-
mation you need in a form that you can use. At last! A true Sendmail tutorial.
Craig Hunt
December 2000
Acknowledgments
I have now written my third book for Sybex, which, frankly, I never thought would hap-
pen. I thought writing Linux Network Servers 24seven, my first book for Sybex, would
be a one-shot deal. But then came the opportunity to write a series of books all focused
on Linux. As much as I love writing, I love writing about Linux even more. To add to the
joy of this project, the people at Sybex have been wonderful. Like the other books, this
one has been written with the support of some excellent people.
I have been surprised by the consistent quality of the people I work with because the cast
of characters has changed. It is perfectly normal for different books to have different edi-
tors, but fate has been a bigger player in these changes than management. Guy Hart-
Davis, the Associate Publisher who first listened to my proposals for this library, inherited
a large home in England and went off to be “Lord of the Manor.” (Like everyone else at
Sybex, I’m dying to go to England to visit him.) By great good luck, Neil Edde took over
as Associate Publisher for the Linux Library. Neil is the person who introduced me to
Sybex. He was the first person to hear my ideas about the Linux Library and to encourage
me to propose them to Sybex. I couldn’t have a better publisher than Neil.
Maureen Adams, who started as the Acquisition Editor for this series, has been promoted
to Mom. She left the project to give birth to Emma. Now instead of baby-sitting me, she
is sitting with a real baby. I’d call that a major promotion!
Tom Critin, who took over as Acquisition Editor, is a career publishing professional.
Tom’s no-nonsense style helps him deal with me and the other authors in the Craig Hunt
Linux Library. Tom deserves special thanks for understanding that the technical quality
and not the production schedule was the most important factor in creating this library.
The Production Editor for this book was Liz Burke—my thanks to Liz for her flexibility
in working around my schedule. Suzanne Goraj was the Editor. I want to thank her for
respecting my writing style while still doing a great job of improving my grammar. Randy
Russell and Eric Gunnett were the Technical Editors. Their suggestions were very helpful
in creating a more accurate book. Randy has a particularly fine eye for technical details.
I would like to thank all of the production people and artists for their hard work: Nila
Nichols, Jennifer Campbell, Nancy Guenther, Nelson Kim, Yariv Rabinovitch, Nanette
Duffy, Nancy Riddiough, Laurie O’Connell, and Andrea Fox.
I’d also like to thank Karen Ruckman of KJR Design in Washington, D.C. Karen is a pro-
fessional photographer and designer. I can attest to the fact that she is one of the best.
Only the best of photographers could make my mug look presentable enough for the
cover of a book.
Twelve-hour days. No vacations. Not even weekends off. When the schedule gets tight and
deadlines loom, I’m not the easiest person to live with. Kathy, thanks for living with me.
Contents at a Glance
Introduction . . . . . . . . . . . . . . . . . . . . . . . xvii
Part 1 How Things Work 1
Chapter 1 Internet Mail Protocols . . . . . . . . . . . . . . . . . . . . . . 3
Chapter 2 Understanding E-Mail Architecture . . . . . . . . . . . . . . . . . .31
Chapter 3 Running Sendmail . . . . . . . . . . . . . . . . . . . . . . .51
Part 2 Essential Configuration 79
Chapter 4 Creating a Basic Sendmail Configuration . . . . . . . . . . . . . . . .81
Chapter 5 Understanding a Vendor’s Configuration . . . . . . . . . . . . . . . 107
Chapter 6 Using Sendmail Databases . . . . . . . . . . . . . . . . . . . 137
Part 3 Advanced Configuration 177
Chapter 7 The sendmail.cf File . . . . . . . . . . . . . . . . . . . . . . 179
Chapter 8 Understanding Rewrite Rules. . . . . . . . . . . . . . . . . . . 219
Chapter 9 Special m4 Configurations. . . . . . . . . . . . . . . . . . . . 247
Part 4 Maintaining a Healthy Server 267
Chapter 10 Testing Sendmail. . . . . . . . . . . . . . . . . . . . . . . 269
Chapter 11 Stopping Spam . . . . . . . . . . . . . . . . . . . . . . . 299
Chapter 12 Sendmail Security . . . . . . . . . . . . . . . . . . . . . . 321
Appendices 359
Appendix A m4 Macro Command Reference . . . . . . . . . . . . . . . . . . 361
Appendix B The sendmail Command . . . . . . . . . . . . . . . . . . . . 395
Appendix C Sendmail Variables, Options, and Flags . . . . . . . . . . . . . . . 411
Index. . . . . . . . . . . . . . . . . . . . . . . . . . 435
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . xvii
Part 1 How Things Work 1
Chapter 1 Internet Mail Protocols. . . . . . . . . . . . . . . 3
The Internet Protocol Suite . . . . . . . . . . . . . 4
A Simple Mail Transport Protocol . . . . . . . . . . . 4
Using SMTP through telnet. . . . . . . . . . . . 6
SMTP Response Codes . . . . . . . . . . . . . 9
Observing SMTP with Verbose Mode . . . . . . . . 11
A Basic Mail Message . . . . . . . . . . . . . . . 13
Message Headers . . . . . . . . . . . . . . . 13
Multipurpose Internet Mail Extensions . . . . . . . . . 15
The Content-Type Header . . . . . . . . . . . . 16
The Content-Transfer-Encoding Header. . . . . . . . 19
Extended SMTP . . . . . . . . . . . . . . . . . 20
Extended Service Keywords . . . . . . . . . . . 21
Mailbox Protocols . . . . . . . . . . . . . . . . 22
Post Office Protocol . . . . . . . . . . . . . . 22
Internet Mail Access Protocol . . . . . . . . . . . 25
In Sum . . . . . . . . . . . . . . . . . . . . 29
Chapter 2 Understanding E-Mail Architecture. . . . . . . . . . . 31
The Role of DNS . . . . . . . . . . . . . . . . . 33
Processing MX Records . . . . . . . . . . . . . 35
The Components of Mail Architecture. . . . . . . . . . 36
Formal Definitions. . . . . . . . . . . . . . . 36
Sample Mail Architectures . . . . . . . . . . . . 39
Sendmail’s Roles. . . . . . . . . . . . . . . . . 41
A Message Submission Agent . . . . . . . . . . . 42
A Message Transfer Agent . . . . . . . . . . . . 46
A Client . . . . . . . . . . . . . . . . . . 49
In Sum . . . . . . . . . . . . . . . . . . . . 49
x Contents
Chapter 3 Running Sendmail . . . . . . . . . . . . . . . . 51
Running Sendmail at Start-Up . . . . . . . . . . . . 51
On a BSD-Style Linux System . . . . . . . . . . 53
On a System V–Style Linux System. . . . . . . . . 54
Controlling Sendmail with Signals . . . . . . . . . 59
Installing Sendmail . . . . . . . . . . . . . . . 61
Installing Sendmail with dpkg . . . . . . . . . . 61
Locating RPM Software. . . . . . . . . . . . . 62
Installing Sendmail with RPM . . . . . . . . . . 66
X Tools for Installing Sendmail . . . . . . . . . . 68
Cleaning Up after RPM . . . . . . . . . . . . . 69
Downloading and Compiling Sendmail . . . . . . . . . 71
Known Problems. . . . . . . . . . . . . . . 74
Configuration Compatibility . . . . . . . . . . . 75
In Sum. . . . . . . . . . . . . . . . . . . . 76
Part 2 Essential Configuration 79
Chapter 4 Creating a Basic Sendmail Configuration . . . . . . . . 81
The cf Directory Structure . . . . . . . . . . . . . 82
Little-Used Directories . . . . . . . . . . . . . 83
The domain Directory . . . . . . . . . . . . . 85
The cf Subdirectory . . . . . . . . . . . . . . 85
The ostype Directory . . . . . . . . . . . . . 88
The mailer Directory. . . . . . . . . . . . . . 89
The feature Directory . . . . . . . . . . . . . 90
The m4 Directory. . . . . . . . . . . . . . . 90
The m4 Macro Language. . . . . . . . . . . . . . 91
Controlling m4 Output . . . . . . . . . . . . . 92
The Basic Commands . . . . . . . . . . . . . 94
A Sample Macro Configuration File. . . . . . . . . 97
Building a Simple m4 Configuration File . . . . . . . 99
More m4 Commands . . . . . . . . . . . . . 102
In Sum . . . . . . . . . . . . . . . . . . 105
Contents xi
Chapter 5 Understanding a Vendor’s Configuration. . . . . . . . .107
The Generic Linux Configuration . . . . . . . . . . .108
The Linux OSTYPE File . . . . . . . . . . . . .109
The Generic DOMAIN File . . . . . . . . . . . .112
Adding Support for the .REDIRECT Pseudo-Domain. . . .114
Adding Support for Local Host Aliases . . . . . . . .115
Protecting the root Account from Masquerading. . . . .117
The Essential Mailers . . . . . . . . . . . . . .118
The Red Hat Configuration . . . . . . . . . . . . .122
Modifying the Red Hat Configuration . . . . . . . . . .131
In Sum . . . . . . . . . . . . . . . . . . . .135
Chapter 6 Using Sendmail Databases . . . . . . . . . . . . .137
Adding Database Support. . . . . . . . . . . . . .138
Database Compiler Options. . . . . . . . . . . .138
Configuration Options . . . . . . . . . . . . .142
The Cr, Cw, and Ct Files . . . . . . . . . . . . . .144
The relay-domains File . . . . . . . . . . . . .145
The local-host-names File . . . . . . . . . . . .147
The aliases Database . . . . . . . . . . . . . . .149
Defining Personal Mail Aliases. . . . . . . . . . .153
The User Database . . . . . . . . . . . . . . . .154
The access Database . . . . . . . . . . . . . . .156
The Address Field . . . . . . . . . . . . . . .157
The Action Field . . . . . . . . . . . . . . .159
The virtusertable. . . . . . . . . . . . . . . . .161
Defining a Virtual Domain . . . . . . . . . . . .161
Defining virtusertable Delivery Addresses . . . . . . .163
The mailertable . . . . . . . . . . . . . . . . .166
The genericstable . . . . . . . . . . . . . . . .169
Little-Used Databases . . . . . . . . . . . . . . .171
The makemap Command . . . . . . . . . . . . . .172
In Sum . . . . . . . . . . . . . . . . . . . .174