Exim: The Mail Transfer Agent

632 Pages·2001·5.69 MB·English

by  Philip Hazel

,Title.10724 Page 1 Tuesday, October 9, 2001 9:25 AM
Exim
The Mail Transfer Agent
,Title.10724 Page 2 Tuesday, October 9, 2001 9:25 AM
,Title.10724 Page 3 Tuesday, October 9, 2001 9:25 AM
Exim
The Mail Transfer Agent
Philip Hazel
Beijing• Cambridge• Farnham• Köln• Paris• Sebastopol• Taipei• Tokyo
,Copyright.10561 Page 1 Tuesday, October 9, 2001 9:25 AM
Exim: The Mail Transfer Agent
by Philip Hazel
Copyright © 2001 O’Reilly & Associates, Inc. All rights reserved.
Printed in the United States of America.
Published by O’Reilly & Associates, Inc., 101 Morris Street, Sebastopol, CA 95472.
Editor: Andy Oram
Production Editor: Mary Brady
Cover Designer: Ellie Volckhausen
Printing History:
June 2001: First Edition.
Nutshell Handbook, the Nutshell Handbook logo, and the O’Reilly logo are registered
trademarks of O’Reilly & Associates, Inc. Many of the designations used by manufacturers
andsellerstodistinguishtheirproductsareclaimedastrademarks. Wherethosedesignations
appear in this book, and O’Reilly & Associates, Inc. was aware of a trademark claim, the
designationshavebeenprintedincapsorinitialcaps.Theassociationbetweentheimageof
an aye-aye and Exim is a trademark of O’Reilly & Associates, Inc.
Whileeveryprecautionhasbeentakeninthepreparationofthisbook,thepublisherassumes
no responsibility for errors or omissions, or for damages resulting from the use of the
information contained herein.
Library of Congress Cataloging-in-Publication Data
Hazel, Philip
Exim: the mail transfer agent/by Philip Hazel p.cm.
ISBN 0-596-00098-7
1. Exim (Computer program) 2. Email--Computer programs I. Title
TK5105.73 .H39 2001
004.692--dc21 2001036079
[DS]
Ta ble of Contents
Preface .................................................................................................................. xiii
1. Introduction .................................................................................................. 1
2. How Inter net Mail Works ......................................................................... 5
Dif ferent Types of MTA .................................................................................... 10
Inter net Message Standards ............................................................................. 11
RFC 822 Message Format ................................................................................. 11
The Message ‘‘On the Wir e’’ ............................................................................ 13
Summary of the SMTP Protocol ....................................................................... 15
Forgery .............................................................................................................. 18
Authentication and Encryption ........................................................................ 18
Routing a Message ............................................................................................ 18
Checking Incoming Mail .................................................................................. 19
Overview of the DNS ....................................................................................... 21
DNS Records Used for Mail Routing ............................................................... 24
Related DNS Records ....................................................................................... 25
Common DNS Errors ........................................................................................ 27
Role of the Postmaster ..................................................................................... 29
3. Exim Over view .......................................................................................... 30
Exim Philosophy ............................................................................................... 30
Exim’s Queue ................................................................................................... 31
Receiving and Delivering Messages ................................................................ 31
Exim Processes ................................................................................................. 32
v
9October 2001 09:13
vi Table of Contents
Coordination Between Processes .................................................................... 32
How Exim Is Configured ................................................................................. 33
How Exim Delivers Messages .......................................................................... 35
Local and Remote Addresses ........................................................................... 37
Pr ocessing an Address ..................................................................................... 38
A Simple Example ............................................................................................ 40
Complications While Directing and Routing ................................................... 46
Complications During Delivery ....................................................................... 48
Complications After Delivery ........................................................................... 49
Use of Transports by Directors and Routers ................................................... 49
4. Exim Operations Over view ................................................................... 52
How Exim Identifies Messages ........................................................................ 52
Watching Exim at Work .................................................................................... 53
The Runtime Configuration File ...................................................................... 54
The Default Qualification Domain .................................................................. 61
Handling Frozen Bounce Messages ................................................................ 62
Reducing Activity at High Load ....................................................................... 62
Limiting Message Sizes ..................................................................................... 65
Parallel Remote Delivery .................................................................................. 65
Contr olling the Number of Delivery Processes .............................................. 66
Large Message Queues ..................................................................................... 66
Large Installations ............................................................................................. 67
5. Extending the Deliver y Configuration ............................................. 71
Multiple Local Domains ................................................................................... 71
Virtual Domains ................................................................................................ 74
Mailing Lists ...................................................................................................... 78
Using an External Local Delivery Agent ......................................................... 85
Multiple User Addresses ................................................................................... 87
Mixed Local/Remote Domains ......................................................................... 88
Delivering to UUCP .......................................................................................... 90
Ignoring the Local Part in Local Deliveries ..................................................... 91
Handling Local Parts in a Case-Sensitive Manner ........................................... 93
Scanning Messages for Viruses ........................................................................ 94
Modifying Message Bodies .............................................................................. 99
9October 2001 09:13
Ta ble of Contents vii
6. Options Common to Director s and Routers ................................ 101
Conditional Running of Routers and Directors ............................................. 102
Changing a Driver’s Successful Outcome ..................................................... 107
Adding Data for Use by Transports ............................................................... 108
Debugging Directors and Routers ................................................................. 113
Summary of Director/Router Generic Options ............................................. 114
7. The Director s ............................................................................................ 118
Conditional Running of Directors .................................................................. 119
Optimizing Single-Level Aliasing ................................................................... 120
Adding Data for Use by Transports ............................................................... 121
The aliasfile and forwardfile Directors .......................................................... 121
The aliasfile Director ...................................................................................... 133
The forwardfile Director ................................................................................ 138
The localuser Director .................................................................................... 146
The smartuser Director .................................................................................. 147
8. The Routers ............................................................................................... 150
Timeouts While Routing ................................................................................ 150
Domains That Route to the Local Host ......................................................... 151
The lookuphost Router .................................................................................. 154
The domainlist Router .................................................................................... 158
The ipliteral Router ........................................................................................ 169
The queryprogram Router ............................................................................. 169
9. The Transpor ts ......................................................................................... 173
Options Common to All Transports .............................................................. 174
The smtp Transport ........................................................................................ 184
Envir onment for Local Transports ................................................................. 194
Options Common to the appendfile and pipe Transports ........................... 196
The appendfile Transport .............................................................................. 203
The pipe Transport ........................................................................................ 222
The lmtp Transport ........................................................................................ 231
The autoreply Transport ................................................................................ 232
10. Message Filter ing .................................................................................... 238
Examples of Filter Commands ....................................................................... 239
Filtering Compared with an External Delivery Agent .................................. 241
Setting Up a User Filter .................................................................................. 242
Setting Up a System Filter .............................................................................. 242
Testing Filter Files .......................................................................................... 244
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viii Table of Contents
For mat of Filter Files ...................................................................................... 246
Significant Actions .......................................................................................... 248
Filter Commands ............................................................................................ 249
The add Command ........................................................................................ 249
Delivery Commands ....................................................................................... 250
Mail Commands .............................................................................................. 253
Logging Commands ....................................................................................... 256
The testprint Command ................................................................................. 256
The finish Command ...................................................................................... 257
Obeying Filter Commands Conditionally ...................................................... 257
Additional Features for System Filters ........................................................... 262
11. Shared Data and Exim Processes ..................................................... 265
Message Files .................................................................................................. 266
Locking Message Files .................................................................................... 268
Hints Files ....................................................................................................... 269
Log Files .......................................................................................................... 271
User and Group IDs for Exim Processes ...................................................... 271
Pr ocess Relationships ..................................................................................... 272
The Daemon Process ..................................................................................... 273
Reception Processes ....................................................................................... 277
Queue Runner Processes ............................................................................... 279
Delivery Processes ......................................................................................... 281
Summary of Message Handling Process Types ............................................ 283
Other Types of Process .................................................................................. 283
12. Deliver y Er ror s and Retrying ............................................................ 284
Retrying After Errors ....................................................................................... 284
Remote Delivery Errors .................................................................................. 285
Local Delivery Errors ...................................................................................... 288
Routing and Directing Errors ......................................................................... 289
Retry Rules ...................................................................................................... 289
Computing Retry Times ................................................................................. 292
Using Retry Times .......................................................................................... 293
Retry Rule Examples ...................................................................................... 294
Timeout of Retry Data .................................................................................... 295
Long-Ter m Failur es ........................................................................................ 295
Ultimate Address Timeout ............................................................................. 297
Inter mittently Connected Hosts ..................................................................... 297
9October 2001 09:13
Ta ble of Contents ix
13. Message Reception and Polic y Controls ......................................... 302
Message Sources ............................................................................................. 303
Message Size Control ..................................................................................... 303
Messages from Local Processes ..................................................................... 304
Unqualified Addresses from Remote Hosts .................................................. 307
Checking a Remote Host ............................................................................... 308
Checking Remote Sender Addresses ............................................................. 314
Checking Recipient Addresses ....................................................................... 322
Checking Header Line Syntax ....................................................................... 326
Relay Control .................................................................................................. 326
Customizing Prohibition Messages ................................................................ 332
Incoming Message Processing ....................................................................... 333
14. Rewr iting Addresses .............................................................................. 339
Automatic Rewriting ....................................................................................... 339
Configur ed Rewriting ..................................................................................... 340
Rewriting Rules ............................................................................................... 343
Rewriting Patterns .......................................................................................... 345
Rewriting Flags ............................................................................................... 347
A Further Rewriting Example ........................................................................ 351
Testing Rewriting Rules .................................................................................. 354
15. Authentication, Encryption, and Other SMTP Processing ...... 355
SMTP Authentication ...................................................................................... 355
Encrypted SMTP Connections ....................................................................... 367
SMTP over TCP/IP .......................................................................................... 372
Local SMTP ..................................................................................................... 376
Batched SMTP ................................................................................................ 377
16. File and Database Lookups ................................................................ 378
Single-Key Lookup Types .............................................................................. 379
Query-Style Lookup Types ............................................................................ 382
Quoting Lookup Data .................................................................................... 382
NIS+ ................................................................................................................ 383
LDAP ............................................................................................................... 384
MySQL and PostgreSQL ................................................................................. 386
DNS Lookups .................................................................................................. 388
Implicit Keys in Query-Style Lookups .......................................................... 388
Temporary Errors in Lookups ........................................................................ 389
Default Values in Single-Key Lookups .......................................................... 389
9October 2001 09:13
x Table of Contents
Partial Matching in Single-Key Lookups ....................................................... 390
Lookup Caching ............................................................................................. 391
17. String Expansion .................................................................................... 392
Variable Substitution ....................................................................................... 394
Header Insertion ............................................................................................. 394
Operations on Substrings ............................................................................... 395
Character Translation ..................................................................................... 398
Text Substitution ............................................................................................. 399
Conditional Expansion ................................................................................... 399
Lookups in Expansion Strings ....................................................................... 406
Extracting Fields from Substrings .................................................................. 410
IP Address Masking ........................................................................................ 412
Quoting ........................................................................................................... 413
Reexpansion ................................................................................................... 416
Running Embedded Perl ................................................................................ 417
Testing String Expansions .............................................................................. 418
18. Domain, Host, and Address Lists ..................................................... 420
Negative Items in Lists ................................................................................... 421
List Items in Files ............................................................................................ 422
Lookup Items in Lists ..................................................................................... 423
Domain Lists ................................................................................................... 423
Host Lists ......................................................................................................... 426
Addr ess Lists ................................................................................................... 432
19. Miscellany .................................................................................................. 435
Security Issues ................................................................................................ 435
Privileged Users .............................................................................................. 442
RFC Conformance .......................................................................................... 444
Timestamps ..................................................................................................... 449
Checking Spool Space ................................................................................... 450
Contr ol of DNS Lookups ................................................................................ 451
Bounce Message Handling ............................................................................ 451
Miscellaneous Controls .................................................................................. 456
20. Command-Line Interface to Exim ................................................... 458
Input Mode Control ........................................................................................ 459
Additional Message Data ............................................................................... 462
Immediate Delivery Control .......................................................................... 464
Err or Routing .................................................................................................. 465
9October 2001 09:13