Exim: The Mail Transfer Agent PDF
Preview Exim: The Mail Transfer Agent
,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 9October 2001 09:13 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
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