Specification of the Exim Mail Transfer Agent Exim Maintainers Copyright (c) 2020 University of Cambridge Revision 4.94.2 30 Apr 2021 EM ------------------------------------------------------------------------------- TABLE OF CONTENTS 1. Introduction 1.1. Exim documentation 1.2. FTP site and websites 1.3. Mailing lists 1.4. Bug reports 1.5. Where to find the Exim distribution 1.6. Limitations 1.7. Runtime configuration 1.8. Calling interface 1.9. Terminology 2. Incorporated code 3. How Exim receives and delivers mail 3.1. Overall philosophy 3.2. Policy control 3.3. User filters 3.4. Message identification 3.5. Receiving mail 3.6. Handling an incoming message 3.7. Life of a message 3.8. Processing an address for delivery 3.9. Processing an address for verification 3.10. Running an individual router 3.11. Duplicate addresses 3.12. Router preconditions 3.13. Delivery in detail 3.14. Retry mechanism 3.15. Temporary delivery failure 3.16. Permanent delivery failure 3.17. Failures to deliver bounce messages 4. Building and installing Exim 4.1. Unpacking 4.2. Multiple machine architectures and operating systems 4.3. PCRE library 4.4. DBM libraries 4.5. Pre-building configuration 4.6. Support for iconv() 4.7. Including TLS/SSL encryption support 4.8. Use of tcpwrappers 4.9. Including support for IPv6 4.10. Dynamically loaded lookup module support 4.11. The building process 4.12. Output from "make" 4.13. Overriding build-time options for Exim 4.14. OS-specific header files 4.15. Overriding build-time options for the monitor 4.16. Installing Exim binaries and scripts 4.17. Installing info documentation 4.18. Setting up the spool directory 4.19. Testing 4.20. Replacing another MTA with Exim 4.21. Upgrading Exim 4.22. Stopping the Exim daemon on Solaris 5. The Exim command line 5.1. Setting options by program name 5.2. Trusted and admin users 5.3. Command line options 6. The Exim runtime configuration file 6.1. Using a different configuration file 6.2. Configuration file format 6.3. File inclusions in the configuration file 6.4. Macros in the configuration file 6.5. Macro substitution 6.6. Redefining macros 6.7. Overriding macro values 6.8. Example of macro usage 6.9. Builtin macros 6.10. Conditional skips in the configuration file 6.11. Common option syntax 6.12. Boolean options 6.13. Integer values 6.14. Octal integer values 6.15. Fixed point numbers 6.16. Time intervals 6.17. String values 6.18. Expanded strings 6.19. User and group names 6.20. List construction 6.21. Changing list separators 6.22. Empty items in lists 6.23. Format of driver configurations 7. The default configuration file 7.1. Macros 7.2. Main configuration settings 7.3. ACL configuration 7.4. Router configuration 7.5. Transport configuration 7.6. Default retry rule 7.7. Rewriting configuration 7.8. Authenticators configuration 8. Regular expressions 9. File and database lookups 9.1. Examples of different lookup syntax 9.2. Lookup types 9.3. Single-key lookup types 9.4. Query-style lookup types 9.5. Temporary errors in lookups 9.6. Default values in single-key lookups 9.7. Partial matching in single-key lookups 9.8. Lookup caching 9.9. Quoting lookup data 9.10. More about dnsdb 9.11. Dnsdb lookup modifiers 9.12. Pseudo dnsdb record types 9.13. Multiple dnsdb lookups 9.14. More about LDAP 9.15. Format of LDAP queries 9.16. LDAP quoting 9.17. LDAP connections 9.18. LDAP authentication and control information 9.19. Format of data returned by LDAP 9.20. More about NIS+ 9.21. SQL lookups 9.22. More about MySQL, PostgreSQL, Oracle, InterBase, and Redis 9.23. Specifying the server in the query 9.24. Special MySQL features 9.25. Special PostgreSQL features 9.26. More about SQLite 9.27. More about Redis 10. Domain, host, address, and local part lists 10.1. Expansion of lists 10.2. Negated items in lists 10.3. File names in lists 10.4. An lsearch file is not an out-of-line list 10.5. Results of list checking 10.6. Named lists 10.7. Named lists compared with macros 10.8. Named list caching 10.9. Domain lists 10.10. Host lists 10.11. Special host list patterns 10.12. Host list patterns that match by IP address 10.13. Host list patterns for single-key lookups by host address 10.14. Host list patterns that match by host name 10.15. Behaviour when an IP address or name cannot be found 10.16. Mixing wildcarded host names and addresses in host lists 10.17. Temporary DNS errors when looking up host information 10.18. Host list patterns for single-key lookups by host name 10.19. Host list patterns for query-style lookups 10.20. Address lists 10.21. Case of letters in address lists 10.22. Local part lists 11. String expansions 11.1. Literal text in expanded strings 11.2. Character escape sequences in expanded strings 11.3. Testing string expansions 11.4. Forced expansion failure 11.5. Expansion items 11.6. Expansion operators 11.7. Expansion conditions 11.8. Combining expansion conditions 11.9. Expansion variables 12. Embedded Perl 12.1. Setting up so Perl can be used 12.2. Calling Perl subroutines 12.3. Calling Exim functions from Perl 12.4. Use of standard output and error by Perl 13. Starting the daemon and the use of network interfaces 13.1. Starting a listening daemon 13.2. Special IP listening addresses 13.3. Overriding local_interfaces and daemon_smtp_ports 13.4. Support for the submissions (aka SSMTP or SMTPS) protocol 13.5. IPv6 address scopes 13.6. Disabling IPv6 13.7. Examples of starting a listening daemon 13.8. Recognizing the local host 13.9. Delivering to a remote host 14. Main configuration 14.1. Miscellaneous 14.2. Exim parameters 14.3. Privilege controls 14.4. Logging 14.5. Frozen messages 14.6. Data lookups 14.7. Message ids 14.8. Embedded Perl Startup 14.9. Daemon 14.10. Resource control 14.11. Policy controls 14.12. Callout cache 14.13. TLS 14.14. Local user handling 14.15. All incoming messages (SMTP and non-SMTP) 14.16. Non-SMTP incoming messages 14.17. Incoming SMTP messages 14.18. SMTP extensions 14.19. Processing messages 14.20. System filter 14.21. Routing and delivery 14.22. Bounce and warning messages 14.23. Alphabetical list of main options 15. Generic options for routers 16. The accept router 17. The dnslookup router 17.1. Problems with DNS lookups 17.2. Declining addresses by dnslookup 17.3. Private options for dnslookup 17.4. Effect of qualify_single and search_parents 18. The ipliteral router 19. The iplookup router 20. The manualroute router 20.1. Private options for manualroute 20.2. Routing rules in route_list 20.3. Routing rules in route_data 20.4. Format of the list of hosts 20.5. Format of one host item 20.6. How the list of hosts is used 20.7. How the options are used 20.8. Manualroute examples 21. The queryprogram router 22. The redirect router 22.1. Redirection data 22.2. Forward files and address verification 22.3. Interpreting redirection data 22.4. Items in a non-filter redirection list 22.5. Redirecting to a local mailbox 22.6. Special items in redirection lists 22.7. Duplicate addresses 22.8. Repeated redirection expansion 22.9. Errors in redirection lists 22.10. Private options for the redirect router 23. Environment for running local transports 23.1. Concurrent deliveries 23.2. Uids and gids 23.3. Current and home directories 23.4. Expansion variables derived from the address 24. Generic options for transports 25. Address batching in local transports 26. The appendfile transport 26.1. The file and directory options 26.2. Private options for appendfile 26.3. Operational details for appending 26.4. Operational details for delivery to a new file 26.5. Maildir delivery 26.6. Using tags to record message sizes 26.7. Using a maildirsize file 26.8. Mailstore delivery 26.9. Non-special new file delivery 27. The autoreply transport 27.1. Private options for autoreply 28. The lmtp transport 29. The pipe transport 29.1. Concurrent delivery 29.2. Returned status and data 29.3. How the command is run 29.4. Environment variables 29.5. Private options for pipe 29.6. Using an external local delivery agent 30. The smtp transport 30.1. Multiple messages on a single connection 30.2. Use of the $host and $host_address variables 30.3. Use of $tls_cipher and $tls_peerdn 30.4. Private options for smtp 30.5. How the limits for the number of hosts to try are used 31. Address rewriting 31.1. Explicitly configured address rewriting 31.2. When does rewriting happen? 31.3. Testing the rewriting rules that apply on input 31.4. Rewriting rules 31.5. Rewriting patterns 31.6. Rewriting replacements 31.7. Rewriting flags 31.8. Flags specifying which headers and envelope addresses to rewrite 31.9. The SMTP-time rewriting flag 31.10. Flags controlling the rewriting process 31.11. Rewriting examples 32. Retry configuration 32.1. Changing retry rules 32.2. Format of retry rules 32.3. Choosing which retry rule to use for address errors 32.4. Choosing which retry rule to use for host and message errors 32.5. Retry rules for specific errors 32.6. Retry rules for specified senders 32.7. Retry parameters 32.8. Retry rule examples 32.9. Timeout of retry data 32.10. Long-term failures 32.11. Deliveries that work intermittently 33. SMTP authentication 33.1. Generic options for authenticators 33.2. The AUTH parameter on MAIL commands 33.3. Authentication on an Exim server 33.4. Testing server authentication 33.5. Authentication by an Exim client 34. The plaintext authenticator 34.1. Avoiding cleartext use 34.2. Plaintext server options 34.3. Using plaintext in a server 34.4. The PLAIN authentication mechanism 34.5. The LOGIN authentication mechanism 34.6. Support for different kinds of authentication 34.7. Using plaintext in a client 35. The cram_md5 authenticator 35.1. Using cram_md5 as a server 35.2. Using cram_md5 as a client 36. The cyrus_sasl authenticator 36.1. Using cyrus_sasl as a server 37. The dovecot authenticator 38. The gsasl authenticator 38.1. gsasl auth variables 39. The heimdal_gssapi authenticator 39.1. heimdal_gssapi auth variables 40. The spa authenticator 40.1. Using spa as a server 40.2. Using spa as a client 41. The external authenticator 41.1. External options 41.2. Using external in a server 41.3. Using external in a client 42. The tls authenticator 43. Encrypted SMTP connections using TLS/SSL 43.1. Support for the "submissions" (aka "ssmtp" and "smtps") protocol 43.2. OpenSSL vs GnuTLS 43.3. GnuTLS parameter computation 43.4. Requiring specific ciphers in OpenSSL 43.5. Requiring specific ciphers or other parameters in GnuTLS 43.6. Configuring an Exim server to use TLS 43.7. Requesting and verifying client certificates 43.8. Revoked certificates 43.9. Configuring an Exim client to use TLS 43.10. Use of TLS Server Name Indication 43.11. Multiple messages on the same encrypted TCP/IP connection 43.12. Certificates and all that 43.13. Certificate chains 43.14. Self-signed certificates 43.15. DANE 44. Access control lists 44.1. Testing ACLs 44.2. Specifying when ACLs are used 44.3. The non-SMTP ACLs 44.4. The SMTP connect ACL 44.5. The EHLO/HELO ACL 44.6. The DATA ACLs 44.7. The SMTP DKIM ACL 44.8. The SMTP MIME ACL 44.9. The SMTP PRDR ACL 44.10. The QUIT ACL 44.11. The not-QUIT ACL 44.12. Finding an ACL to use 44.13. ACL return codes 44.14. Unset ACL options 44.15. Data for message ACLs 44.16. Data for non-message ACLs 44.17. Format of an ACL 44.18. ACL verbs 44.19. ACL variables 44.20. Condition and modifier processing 44.21. ACL modifiers 44.22. Use of the control modifier 44.23. Summary of message fixup control 44.24. Adding header lines in ACLs 44.25. Removing header lines in ACLs 44.26. ACL conditions 44.27. Using DNS lists 44.28. Specifying the IP address for a DNS list lookup 44.29. DNS lists keyed on domain names 44.30. Multiple explicit keys for a DNS list 44.31. Data returned by DNS lists 44.32. Variables set from DNS lists 44.33. Additional matching conditions for DNS lists 44.34. Negated DNS matching conditions 44.35. Handling multiple DNS records from a DNS list 44.36. Detailed information from merged DNS lists 44.37. DNS lists and IPv6 44.38. Rate limiting incoming messages 44.39. Ratelimit options for what is being measured 44.40. Ratelimit update modes 44.41. Ratelimit options for handling fast clients 44.42. Limiting the rate of different events 44.43. Using rate limiting 44.44. Address verification 44.45. Callout verification 44.46. Additional parameters for callouts 44.47. Callout caching 44.48. Sender address verification reporting 44.49. Redirection while verifying 44.50. Client SMTP authorization (CSA) 44.51. Bounce address tag validation 44.52. Using an ACL to control relaying 44.53. Checking a relay configuration 45. Content scanning at ACL time 45.1. Scanning for viruses 45.2. Scanning with SpamAssassin and Rspamd 45.3. Calling SpamAssassin from an Exim ACL 45.4. Scanning MIME parts 45.5. Scanning with regular expressions 46. Adding a local scan function to Exim 46.1. Building Exim to use a local scan function 46.2. API for local_scan() 46.3. Configuration options for local_scan() 46.4. Available Exim variables 46.5. Structure of header lines 46.6. Structure of recipient items 46.7. Available Exim functions 46.8. More about Exim's memory handling 47. System-wide message filtering 47.1. Specifying a system filter 47.2. Testing a system filter 47.3. Contents of a system filter 47.4. Additional variable for system filters 47.5. Defer, freeze, and fail commands for system filters 47.6. Adding and removing headers in a system filter 47.7. Setting an errors address in a system filter 47.8. Per-address filtering 48. Message processing 48.1. Submission mode for non-local messages 48.2. Line endings 48.3. Unqualified addresses 48.4. The UUCP From line 48.5. Resent- header lines 48.6. The Auto-Submitted: header line 48.7. The Bcc: header line 48.8. The Date: header line 48.9. The Delivery-date: header line 48.10. The Envelope-to: header line 48.11. The From: header line 48.12. The Message-ID: header line 48.13. The Received: header line 48.14. The References: header line 48.15. The Return-path: header line 48.16. The Sender: header line 48.17. Adding and removing header lines in routers and transports 48.18. Constructed addresses 48.19. Case of local parts 48.20. Dots in local parts 48.21. Rewriting addresses 49. SMTP processing 49.1. Outgoing SMTP and LMTP over TCP/IP 49.2. Errors in outgoing SMTP 49.3. Incoming SMTP messages over TCP/IP 49.4. Unrecognized SMTP commands 49.5. Syntax and protocol errors in SMTP commands 49.6. Use of non-mail SMTP commands 49.7. The VRFY and EXPN commands 49.8. The ETRN command 49.9. Incoming local SMTP 49.10. Outgoing batched SMTP 49.11. Incoming batched SMTP 50. Customizing bounce and warning messages 50.1. Customizing bounce messages 50.2. Customizing warning messages 51. Some common configuration settings 51.1. Sending mail to a smart host 51.2. Using Exim to handle mailing lists 51.3. Syntax errors in mailing lists 51.4. Re-expansion of mailing lists 51.5. Closed mailing lists 51.6. Variable Envelope Return Paths (VERP) 51.7. Virtual domains 51.8. Multiple user mailboxes 51.9. Simplified vacation processing 51.10. Taking copies of mail 51.11. Intermittently connected hosts 51.12. Exim on the upstream server host 51.13. Exim on the intermittently connected client host 52. Using Exim as a non-queueing client 53. Log files 53.1. Where the logs are written 53.2. Logging to local files that are periodically "cycled" 53.3. Datestamped log files 53.4. Logging to syslog 53.5. Log line flags 53.6. Logging message reception 53.7. Logging deliveries 53.8. Discarded deliveries 53.9. Deferred deliveries 53.10. Delivery failures 53.11. Fake deliveries 53.12. Completion 53.13. Summary of Fields in Log Lines 53.14. Other log entries 53.15. Reducing or increasing what is logged 53.16. Message log 54. Exim utilities 54.1. Finding out what Exim processes are doing (exiwhat) 54.2. Selective queue listing (exiqgrep) 54.3. Summarizing the queue (exiqsumm) 54.4. Extracting specific information from the log (exigrep) 54.5. Selecting messages by various criteria (exipick) 54.6. Cycling log files (exicyclog) 54.7. Mail statistics (eximstats) 54.8. Checking access policy (exim_checkaccess) 54.9. Making DBM files (exim_dbmbuild) 54.10. Finding individual retry times (exinext) 54.11. Hints database maintenance 54.12. exim_dumpdb 54.13. exim_tidydb 54.14. exim_fixdb 54.15. Mailbox maintenance (exim_lock) 55. The Exim monitor 55.1. Running the monitor 55.2. The stripcharts 55.3. Main action buttons 55.4. The log display 55.5. The queue display 55.6. The queue menu 56. Security considerations 56.1. Building a more "hardened" Exim 56.2. Root privilege 56.3. Running Exim without privilege 56.4. Delivering to local files 56.5. Running local commands 56.6. Trust in configuration data 56.7. IPv4 source routing 56.8. The VRFY, EXPN, and ETRN commands in SMTP 56.9. Privileged users 56.10. Spool files 56.11. Use of argv[0] 56.12. Use of %f formatting 56.13. Embedded Exim path 56.14. Dynamic module directory 56.15. Use of sprintf() 56.16. Use of debug_printf() and log_write() 56.17. Use of strcat() and strcpy() 57. Format of spool files 57.1. Format of the -H file 57.2. Format of the -D file 58. DKIM, SPF and DMARC 58.1. DKIM (DomainKeys Identified Mail) 58.2. Signing outgoing messages 58.3. Verifying DKIM signatures in incoming mail 58.4. SPF (Sender Policy Framework) 58.5. DMARC 59. Proxies 59.1. Inbound proxies 59.2. Outbound proxies 59.3. Logging 60. Internationalisation 60.1. MTA operations 60.2. MDA operations 61. Events 62. Adding new drivers or lookup types =============================================================================== 1. INTRODUCTION Exim is a mail transfer agent (MTA) for hosts that are running Unix or Unix-like operating systems. It was designed on the assumption that it would be run on hosts that are permanently connected to the Internet. However, it can be used on intermittently connected hosts with suitable configuration adjustments. Configuration files currently exist for the following operating systems: AIX, BSD/OS (aka BSDI), Darwin (Mac OS X), DGUX, Dragonfly, FreeBSD, GNU/Hurd, GNU/ Linux, HI-OSF (Hitachi), HI-UX, HP-UX, IRIX, MIPS RISCOS, NetBSD, OpenBSD, OpenUNIX, QNX, SCO, SCO SVR4.2 (aka UNIX-SV), Solaris (aka SunOS5), SunOS4, Tru64-Unix (formerly Digital UNIX, formerly DEC-OSF1), Ultrix, and UnixWare. Some of these operating systems are no longer current and cannot easily be tested, so the configuration files may no longer work in practice. There are also configuration files for compiling Exim in the Cygwin environment that can be installed on systems running Windows. However, this document does not contain any information about running Exim in the Cygwin environment. The terms and conditions for the use and distribution of Exim are contained in the file NOTICE. Exim is distributed under the terms of the GNU General Public Licence, a copy of which may be found in the file LICENCE. The use, supply, or promotion of Exim for the purpose of sending bulk, unsolicited electronic mail is incompatible with the basic aims of Exim, which revolve around the free provision of a service that enhances the quality of personal communications. The author of Exim regards indiscriminate mass-mailing as an antisocial, irresponsible abuse of the Internet. Exim owes a great deal to Smail 3 and its author, Ron Karr. Without the experience of running and working on the Smail 3 code, I could never have contemplated starting to write a new MTA. Many of the ideas and user interfaces were originally taken from Smail 3, though the actual code of Exim is entirely new, and has developed far beyond the initial concept. Many people, both in Cambridge and around the world, have contributed to the development and the testing of Exim, and to porting it to various operating systems. I am grateful to them all. The distribution now contains a file called ACKNOWLEDGMENTS, in which I have started recording the names of contributors. 1.1 Exim documentation ---------------------- This edition of the Exim specification applies to version 4.94.2 of Exim. Substantive changes from the 4.93 edition are marked in some renditions of this document; this paragraph is so marked if the rendition is capable of showing a change indicator. This document is very much a reference manual; it is not a tutorial. The reader is expected to have some familiarity with the SMTP mail transfer protocol and with general Unix system administration. Although there are some discussions and examples in places, the information is mostly organized in a way that makes it easy to look up, rather than in a natural order for sequential reading. Furthermore, this manual aims to cover every aspect of Exim in detail, including a number of rarely-used, special-purpose features that are unlikely to be of very wide interest. An "easier" discussion of Exim which provides more in-depth explanatory, introductory, and tutorial material can be found in a book entitled The Exim SMTP Mail Server (second edition, 2007), published by UIT Cambridge (https:// www.uit.co.uk/exim-book/). The book also contains a chapter that gives a general introduction to SMTP and Internet mail. Inevitably, however, the book is unlikely to be fully up-to-date with the latest release of Exim. (Note that the earlier book about Exim, published by O'Reilly, covers Exim 3, and many things have changed in Exim 4.) If you are using a Debian distribution of Exim, you will find information about Debian-specific features in the file /usr/share/doc/exim4-base/README.Debian. The command man update-exim.conf is another source of Debian-specific information. As Exim develops, there may be features in newer versions that have not yet made it into this document, which is updated only when the most significant digit of the fractional part of the version number changes. Specifications of new features that are not yet in this manual are placed in the file doc/ NewStuff in the Exim distribution. Some features may be classified as "experimental". These may change incompatibly while they are developing, or even be withdrawn. For this reason, they are not documented in this manual. Information about experimental features can be found in the file doc/experimental.txt. All changes to Exim (whether new features, bug fixes, or other kinds of change) are noted briefly in the file called doc/ChangeLog. This specification itself is available as an ASCII file in doc/spec.txt so that it can easily be searched with a text editor. Other files in the doc directory are: OptionLists.txt list of all options in alphabetical order dbm.discuss.txt discussion about DBM libraries exim.8 a man page of Exim's command line options experimental.txt documentation of experimental features filter.txt specification of the filter language Exim3.upgrade upgrade notes from release 2 to release 3 Exim4.upgrade upgrade notes from release 3 to release 4 openssl.txt installing a current OpenSSL release The main specification and the specification of the filtering language are also available in other formats (HTML, PostScript, PDF, and Texinfo). Section 1.5 below tells you how to get hold of these. 1.2 FTP site and websites ------------------------- The primary site for Exim source distributions is the exim.org FTP site, available over HTTPS, HTTP and FTP. These services, and the exim.org website, are hosted at the University of Cambridge. As well as Exim distribution tar files, the Exim website contains a number of differently formatted versions of the documentation. A recent addition to the online information is the Exim wiki (https://wiki.exim.org), which contains what used to be a separate FAQ, as well as various other examples, tips, and know-how that have been contributed by Exim users. The wiki site should always redirect to the correct place, which is currently provided by GitHub, and is open to editing by anyone with a GitHub account. An Exim Bugzilla exists at https://bugs.exim.org. You can use this to report bugs, and also to add items to the wish list. Please search first to check that you are not duplicating a previous entry. Please do not ask for configuration help in the bug-tracker. 1.3 Mailing lists ----------------- The following Exim mailing lists exist: exim-announce@exim.org Moderated, low volume announcements list exim-users@exim.org General discussion list exim-dev@exim.org Discussion of bugs, enhancements, etc. exim-cvs@exim.org Automated commit messages from the VCS You can subscribe to these lists, change your existing subscriptions, and view or search the archives via the mailing lists link on the Exim home page. If you are using a Debian distribution of Exim, you may wish to subscribe to the Debian-specific mailing list pkg-exim4-users@lists.alioth.debian.org via this web page: https://alioth-lists.debian.net/cgi-bin/mailman/listinfo/pkg-exim4-users Please ask Debian-specific questions on that list and not on the general Exim lists. 1.4 Bug reports --------------- Reports of obvious bugs can be emailed to bugs@exim.org or reported via the Bugzilla (https://bugs.exim.org). However, if you are unsure whether some behaviour is a bug or not, the best thing to do is to post a message to the exim-dev mailing list and have it discussed. 1.5 Where to find the Exim distribution --------------------------------------- The master distribution site for the Exim distribution is https://downloads.exim.org/ The service is available over HTTPS, HTTP and FTP. We encourage people to migrate to HTTPS. The content served at https://downloads.exim.org/ is identical to the content served at https://ftp.exim.org/pub/exim and ftp://ftp.exim.org/pub/exim. If accessing via a hostname containing ftp, then the file references that follow are relative to the exim directories at these sites. If accessing via the hostname downloads then the subdirectories described here are top-level directories. There are now quite a number of independent mirror sites around the world. Those that I know about are listed in the file called Mirrors. Within the top exim directory there are subdirectories called exim3 (for previous Exim 3 distributions), exim4 (for the latest Exim 4 distributions), and Testing for testing versions. In the exim4 subdirectory, the current release can always be found in files called exim-n.nn.tar.xz exim-n.nn.tar.gz exim-n.nn.tar.bz2 where n.nn is the highest such version number in the directory. The three files contain identical data; the only difference is the type of compression. The .xz file is usually the smallest, while the .gz file is the most portable to old systems. The distributions will be PGP signed by an individual key of the Release Coordinator. This key will have a uid containing an email address in the exim.org domain and will have signatures from other people, including other Exim maintainers. We expect that the key will be in the "strong set" of PGP keys. There should be a trust path to that key from the Exim Maintainer's PGP keys, a version of which can be found in the release directory in the file Exim-Maintainers-Keyring.asc. All keys used will be available in public keyserver pools, such as pool.sks-keyservers.net. At the time of the last update, releases were being made by Jeremy Harris and signed with key 0xBCE58C8CE41F32DF. Other recent keys used for signing are those of Heiko Schlittermann, 0x26101B62F69376CE, and of Phil Pennock, 0x4D1E900E14C1CC04. The signatures for the tar bundles are in: exim-n.nn.tar.xz.asc exim-n.nn.tar.gz.asc exim-n.nn.tar.bz2.asc For each released version, the log of changes is made available in a separate file in the directory ChangeLogs so that it is possible to find out what has changed without having to download the entire distribution. The main distribution contains ASCII versions of this specification and other documentation; other formats of the documents are available in separate files inside the exim4 directory of the FTP site: exim-html-n.nn.tar.gz exim-pdf-n.nn.tar.gz exim-postscript-n.nn.tar.gz exim-texinfo-n.nn.tar.gz These tar files contain only the doc directory, not the complete distribution, and are also available in .bz2 and .xz forms. 1.6 Limitations --------------- * Exim is designed for use as an Internet MTA, and therefore handles addresses in RFC 2822 domain format only. It cannot handle UUCP "bang paths", though simple two-component bang paths can be converted by a straightforward rewriting configuration. This restriction does not prevent Exim from being interfaced to UUCP as a transport mechanism, provided that domain addresses are used. * Exim insists that every address it handles has a domain attached. For incoming local messages, domainless addresses are automatically qualified with a configured domain value. Configuration options specify from which remote systems unqualified addresses are acceptable. These are then qualified on arrival. * The only external transport mechanisms that are currently implemented are SMTP and LMTP over a TCP/IP network (including support for IPv6). However, a pipe transport is available, and there are facilities for writing messages to files and pipes, optionally in batched SMTP format; these facilities can be used to send messages to other transport mechanisms such as UUCP, provided they can handle domain-style addresses. Batched SMTP input is also catered for. * Exim is not designed for storing mail for dial-in hosts. When the volumes of such mail are large, it is better to get the messages "delivered" into files (that is, off Exim's queue) and subsequently passed on to the dial-in hosts by other means. * Although Exim does have basic facilities for scanning incoming messages, these are not comprehensive enough to do full virus or spam scanning. Such operations are best carried out using additional specialized software packages. If you compile Exim with the content-scanning extension, straightforward interfaces to a number of common scanners are provided. 1.7 Runtime configuration ------------------------- Exim's runtime configuration is held in a single text file that is divided into a number of sections. The entries in this file consist of keywords and values, in the style of Smail 3 configuration files. A default configuration file which is suitable for simple online installations is provided in the distribution, and is described in chapter 7 below. 1.8 Calling interface --------------------- Like many MTAs, Exim has adopted the Sendmail command line interface so that it can be a straight replacement for /usr/lib/sendmail or /usr/sbin/sendmail when sending mail, but you do not need to know anything about Sendmail in order to run Exim. For actions other than sending messages, Sendmail-compatible options also exist, but those that produce output (for example, -bp, which lists the messages in the queue) do so in Exim's own format. There are also some additional options that are compatible with Smail 3, and some further options that are new to Exim. Chapter 5 documents all Exim's command line options. This information is automatically made into the man page that forms part of the Exim distribution. Control of messages in the queue can be done via certain privileged command line options. There is also an optional monitor program called eximon, which displays current information in an X window, and which contains a menu interface to Exim's command line administration options. 1.9 Terminology --------------- The body of a message is the actual data that the sender wants to transmit. It is the last part of a message and is separated from the header (see below) by a blank line. When a message cannot be delivered, it is normally returned to the sender in a delivery failure message or a "non-delivery report" (NDR). The term bounce is commonly used for this action, and the error reports are often called bounce messages. This is a convenient shorthand for "delivery failure error report". Such messages have an empty sender address in the message's envelope (see below) to ensure that they cannot themselves give rise to further bounce messages. The term default appears frequently in this manual. It is used to qualify a value which is used in the absence of any setting in the configuration. It may also qualify an action which is taken unless a configuration setting specifies otherwise. The term defer is used when the delivery of a message to a specific destination cannot immediately take place for some reason (a remote host may be down, or a user's local mailbox may be full). Such deliveries are deferred until a later time. The word domain is sometimes used to mean all but the first component of a host's name. It is not used in that sense here, where it normally refers to the part of an email address following the @ sign. A message in transit has an associated envelope, as well as a header and a body. The envelope contains a sender address (to which bounce messages should be delivered), and any number of recipient addresses. References to the sender or the recipients of a message usually mean the addresses in the envelope. An MTA uses these addresses for delivery, and for returning bounce messages, not the addresses that appear in the header lines. The header of a message is the first part of a message's text, consisting of a number of lines, each of which has a name such as From:, To:, Subject:, etc. Long header lines can be split over several text lines by indenting the continuations. The header is separated from the body by a blank line. The term local part, which is taken from RFC 2822, is used to refer to the part of an email address that precedes the @ sign. The part that follows the @ sign is called the domain or mail domain. The terms local delivery and remote delivery are used to distinguish delivery to a file or a pipe on the local host from delivery by SMTP over TCP/IP to another host. As far as Exim is concerned, all hosts other than the host it is running on are remote. Return path is another name that is used for the sender address in a message's envelope. The term queue is used to refer to the set of messages awaiting delivery because this term is in widespread use in the context of MTAs. However, in Exim's case, the reality is more like a pool than a queue, because there is normally no ordering of waiting messages. The term queue runner is used to describe a process that scans the queue and attempts to deliver those messages whose retry times have come. This term is used by other MTAs and also relates to the command runq, but in Exim the waiting messages are normally processed in an unpredictable order. The term spool directory is used for a directory in which Exim keeps the messages in its queue - that is, those that it is in the process of delivering. This should not be confused with the directory in which local mailboxes are stored, which is called a "spool directory" by some people. In the Exim documentation, "spool" is always used in the first sense. =============================================================================== 2. INCORPORATED CODE A number of pieces of external code are included in the Exim distribution. * Regular expressions are supported in the main Exim program and in the Exim monitor using the freely-distributable PCRE library, copyright (c) University of Cambridge. The source to PCRE is no longer shipped with Exim, so you will need to use the version of PCRE shipped with your system, or obtain and install the full version of the library from ftp:// ftp.csx.cam.ac.uk/pub/software/programming/pcre. * Support for the cdb (Constant DataBase) lookup method is provided by code contributed by Nigel Metheringham of (at the time he contributed it) Planet Online Ltd. The implementation is completely contained within the code of Exim. It does not link against an external cdb library. The code contains the following statements: Copyright (c) 1998 Nigel Metheringham, Planet Online Ltd This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This code implements Dan Bernstein's Constant DataBase (cdb) spec. Information, the spec and sample code for cdb can be obtained from https://cr.yp.to/cdb.html. This implementation borrows some code from Dan Bernstein's implementation (which has no license restrictions applied to it). * Client support for Microsoft's Secure Password Authentication is provided by code contributed by Marc Prud'hommeaux. Server support was contributed by Tom Kistner. This includes code taken from the Samba project, which is released under the Gnu GPL. * Support for calling the Cyrus pwcheck and saslauthd daemons is provided by code taken from the Cyrus-SASL library and adapted by Alexander S. Sabourenkov. The permission notice appears below, in accordance with the conditions expressed therein. Copyright (c) 2001 Carnegie Mellon University. All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. 3. The name "Carnegie Mellon University" must not be used to endorse or promote products derived from this software without prior written permission. For permission or any other legal details, please contact Office of Technology Transfer Carnegie Mellon University 5000 Forbes Avenue Pittsburgh, PA 15213-3890 (412) 268-4387, fax: (412) 268-7395 tech-transfer@andrew.cmu.edu 4. Redistributions of any form whatsoever must retain the following acknowledgment: "This product includes software developed by Computing Services at Carnegie Mellon University (https://www.cmu.edu/computing/." CARNEGIE MELLON UNIVERSITY DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL CARNEGIE MELLON UNIVERSITY BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. * The Exim Monitor program, which is an X-Window application, includes modified versions of the Athena StripChart and TextPop widgets. This code is copyright by DEC and MIT, and their permission notice appears below, in accordance with the conditions expressed therein. Copyright 1987, 1988 by Digital Equipment Corporation, Maynard, Massachusetts, and the Massachusetts Institute of Technology, Cambridge, Massachusetts. All Rights Reserved Permission to use, copy, modify, and distribute this software and its documentation for any purpose and without fee is hereby granted, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation, and that the names of Digital or MIT not be used in advertising or publicity pertaining to distribution of the software without specific, written prior permission. DIGITAL DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL DIGITAL BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. * The DMARC implementation uses the OpenDMARC library which is Copyrighted by The Trusted Domain Project. Portions of Exim source which use OpenDMARC derived code are indicated in the respective source files. The full OpenDMARC license is provided in the LICENSE.opendmarc file contained in the distributed source code. * Many people have contributed code fragments, some large, some small, that were not covered by any specific license requirements. It is assumed that the contributors are happy to see their code incorporated into Exim under the GPL. =============================================================================== 3. HOW EXIM RECEIVES AND DELIVERS MAIL 3.1 Overall philosophy ---------------------- Exim is designed to work efficiently on systems that are permanently connected to the Internet and are handling a general mix of mail. In such circumstances, most messages can be delivered immediately. Consequently, Exim does not maintain independent queues of messages for specific domains or hosts, though it does try to send several messages in a single SMTP connection after a host has been down, and it also maintains per-host retry information. 3.2 Policy control ------------------ Policy controls are now an important feature of MTAs that are connected to the Internet. Perhaps their most important job is to stop MTAs from being abused as "open relays" by misguided individuals who send out vast amounts of unsolicited junk and want to disguise its source. Exim provides flexible facilities for specifying policy controls on incoming mail: * Exim 4 (unlike previous versions of Exim) implements policy controls on incoming mail by means of Access Control Lists (ACLs). Each list is a series of statements that may either grant or deny access. ACLs can be used at several places in the SMTP dialogue while receiving a message from a remote host. However, the most common places are after each RCPT command, and at the very end of the message. The sysadmin can specify conditions for accepting or rejecting individual recipients or the entire message, respectively, at these two points (see chapter 44). Denial of access results in an SMTP error code. * An ACL is also available for locally generated, non-SMTP messages. In this case, the only available actions are to accept or deny the entire message. * When Exim is compiled with the content-scanning extension, facilities are provided in the ACL mechanism for passing the message to external virus and /or spam scanning software. The result of such a scan is passed back to the ACL, which can then use it to decide what to do with the message. * When a message has been received, either from a remote host or from the local host, but before the final acknowledgment has been sent, a locally supplied C function called local_scan() can be run to inspect the message and decide whether to accept it or not (see chapter 46). If the message is accepted, the list of recipients can be modified by the function. * Using the local_scan() mechanism is another way of calling external scanner software. The SA-Exim add-on package works this way. It does not require Exim to be compiled with the content-scanning extension. * After a message has been accepted, a further checking mechanism is available in the form of the system filter (see chapter 47). This runs at the start of every delivery process. 3.3 User filters ---------------- In a conventional Exim configuration, users are able to run private filters by setting up appropriate .forward files in their home directories. See chapter 22 (about the redirect router) for the configuration needed to support this, and the separate document entitled Exim's interfaces to mail filtering for user details. Two different kinds of filtering are available: * Sieve filters are written in the standard filtering language that is defined by RFC 3028. * Exim filters are written in a syntax that is unique to Exim, but which is more powerful than Sieve, which it pre-dates. User filters are run as part of the routing process, described below. 3.4 Message identification -------------------------- Every message handled by Exim is given a message id which is sixteen characters long. It is divided into three parts, separated by hyphens, for example "16VDhn-0001bo-D3". Each part is a sequence of letters and digits, normally encoding numbers in base 62. However, in the Darwin operating system (Mac OS X) and when Exim is compiled to run under Cygwin, base 36 (avoiding the use of lower case letters) is used instead, because the message id is used to construct filenames, and the names of files in those systems are not always case-sensitive. The detail of the contents of the message id have changed as Exim has evolved. Earlier versions relied on the operating system not re-using a process id (pid) within one second. On modern operating systems, this assumption can no longer be made, so the algorithm had to be changed. To retain backward compatibility, the format of the message id was retained, which is why the following rules are somewhat eccentric: * The first six characters of the message id are the time at which the message started to be received, to a granularity of one second. That is, this field contains the number of seconds since the start of the epoch (the normal Unix way of representing the date and time of day). * After the first hyphen, the next six characters are the id of the process that received the message. * There are two different possibilities for the final two characters: 1. If localhost_number is not set, this value is the fractional part of the time of reception, normally in units of 1/2000 of a second, but for systems that must use base 36 instead of base 62 (because of case-insensitive file systems), the units are 1/1000 of a second. 2. If localhost_number is set, it is multiplied by 200 (100) and added to the fractional part of the time, which in this case is in units of 1/ 200 (1/100) of a second. After a message has been received, Exim waits for the clock to tick at the appropriate resolution before proceeding, so that if another message is received by the same process, or by another process with the same (re-used) pid, it is guaranteed that the time will be different. In most cases, the clock will already have ticked while the message was being received. 3.5 Receiving mail ------------------ The only way Exim can receive mail from another host is using SMTP over TCP/IP, in which case the sender and recipient addresses are transferred using SMTP commands. However, from a locally running process (such as a user's MUA), there are several possibilities: * If the process runs Exim with the -bm option, the message is read non-interactively (usually via a pipe), with the recipients taken from the command line, or from the body of the message if -t is also used. * If the process runs Exim with the -bS option, the message is also read non-interactively, but in this case the recipients are listed at the start of the message in a series of SMTP RCPT commands, terminated by a DATA command. This is called "batch SMTP" format, but it isn't really SMTP. The SMTP commands are just another way of passing envelope addresses in a non-interactive submission. * If the process runs Exim with the -bs option, the message is read interactively, using the SMTP protocol. A two-way pipe is normally used for passing data between the local process and the Exim process. This is "real" SMTP and is handled in the same way as SMTP over TCP/IP. For example, the ACLs for SMTP commands are used for this form of submission. * A local process may also make a TCP/IP call to the host's loopback address (127.0.0.1) or any other of its IP addresses. When receiving messages, Exim does not treat the loopback address specially. It treats all such connections in the same way as connections from other hosts. In the three cases that do not involve TCP/IP, the sender address is constructed from the login name of the user that called Exim and a default qualification domain (which can be set by the qualify_domain configuration option). For local or batch SMTP, a sender address that is passed using the SMTP MAIL command is ignored. However, the system administrator may allow certain users ("trusted users") to specify a different sender addresses unconditionally, or all users to specify certain forms of different sender address. The -f option or the SMTP MAIL command is used to specify these different addresses. See section 5.2 for details of trusted users, and the untrusted_set_sender option for a way of allowing untrusted users to change sender addresses. Messages received by either of the non-interactive mechanisms are subject to checking by the non-SMTP ACL if one is defined. Messages received using SMTP (either over TCP/IP or interacting with a local process) can be checked by a number of ACLs that operate at different times during the SMTP session. Either individual recipients or the entire message can be rejected if local policy requirements are not met. The local_scan() function (see chapter 46) is run for all incoming messages. Exim can be configured not to start a delivery process when a message is received; this can be unconditional, or depend on the number of incoming SMTP connections or the system load. In these situations, new messages wait on the queue until a queue runner process picks them up. However, in standard configurations under normal conditions, delivery is started as soon as a message is received. 3.6 Handling an incoming message -------------------------------- When Exim accepts a message, it writes two files in its spool directory. The first contains the envelope information, the current status of the message, and the header lines, and the second contains the body of the message. The names of the two spool files consist of the message id, followed by "-H" for the file containing the envelope and header, and "-D" for the data file. By default, all these message files are held in a single directory called input inside the general Exim spool directory. Some operating systems do not perform very well if the number of files in a directory gets large; to improve performance in such cases, the split_spool_directory option can be used. This causes Exim to split up the input files into 62 sub-directories whose names are single letters or digits. When this is done, the queue is processed one sub-directory at a time instead of all at once, which can improve overall performance even when there are not enough files in each directory to affect file system performance. The envelope information consists of the address of the message's sender and the addresses of the recipients. This information is entirely separate from any addresses contained in the header lines. The status of the message includes a list of recipients who have already received the message. The format of the first spool file is described in chapter 57. Address rewriting that is specified in the rewrite section of the configuration (see chapter 31) is done once and for all on incoming addresses, both in the header lines and the envelope, at the time the message is accepted. If during the course of delivery additional addresses are generated (for example, via aliasing), these new addresses are rewritten as soon as they are generated. At the time a message is actually delivered (transported) further rewriting can take place; because this is a transport option, it can be different for different forms of delivery. It is also possible to specify the addition or removal of certain header lines at the time the message is delivered (see chapters 15 and 24). 3.7 Life of a message --------------------- A message remains in the spool directory until it is completely delivered to its recipients or to an error address, or until it is deleted by an administrator or by the user who originally created it. In cases when delivery cannot proceed - for example when a message can neither be delivered to its recipients nor returned to its sender, the message is marked "frozen" on the spool, and no more deliveries are attempted. An administrator can "thaw" such messages when the problem has been corrected, and can also freeze individual messages by hand if necessary. In addition, an administrator can force a delivery error, causing a bounce message to be sent. There are options called ignore_bounce_errors_after and timeout_frozen_after, which discard frozen messages after a certain time. The first applies only to frozen bounces, the second to all frozen messages. While Exim is working on a message, it writes information about each delivery attempt to its main log file. This includes successful, unsuccessful, and delayed deliveries for each recipient (see chapter 53). The log lines are also written to a separate message log file for each message. These logs are solely for the benefit of the administrator and are normally deleted along with the spool files when processing of a message is complete. The use of individual message logs can be disabled by setting no_message_logs; this might give an improvement in performance on very busy systems. All the information Exim itself needs to set up a delivery is kept in the first spool file, along with the header lines. When a successful delivery occurs, the address is immediately written at the end of a journal file, whose name is the message id followed by "-J". At the end of a delivery run, if there are some addresses left to be tried again later, the first spool file (the "-H" file) is updated to indicate which these are, and the journal file is then deleted. Updating the spool file is done by writing a new file and renaming it, to minimize the possibility of data loss. Should the system or Exim crash after a successful delivery but before the spool file has been updated, the journal is left lying around. The next time Exim attempts to deliver the message, it reads the journal file and updates the spool file before proceeding. This minimizes the chances of double deliveries caused by crashes. 3.8 Processing an address for delivery -------------------------------------- The main delivery processing elements of Exim are called routers and transports , and collectively these are known as drivers. Code for a number of them is provided in the source distribution, and compile-time options specify which ones are included in the binary. Runtime options specify which ones are actually used for delivering messages. Each driver that is specified in the runtime configuration is an instance of that particular driver type. Multiple instances are allowed; for example, you can set up several different smtp transports, each with different option values that might specify different ports or different timeouts. Each instance has its own identifying name. In what follows we will normally use the instance name when discussing one particular instance (that is, one specific configuration of the driver), and the generic driver name when discussing the driver's features in general. A router is a driver that operates on an address, either determining how its delivery should happen, by assigning it to a specific transport, or converting the address into one or more new addresses (for example, via an alias file). A router may also explicitly choose to fail an address, causing it to be bounced. A transport is a driver that transmits a copy of the message from Exim's spool to some destination. There are two kinds of transport: for a local transport, the destination is a file or a pipe on the local host, whereas for a remote transport the destination is some other host. A message is passed to a specific transport as a result of successful routing. If a message has several recipients, it may be passed to a number of different transports. An address is processed by passing it to each configured router instance in turn, subject to certain preconditions, until a router accepts the address or specifies that it should be bounced. We will describe this process in more detail shortly. First, as a simple example, we consider how each recipient address in a message is processed in a small configuration of three routers. To make this a more concrete example, it is described in terms of some actual routers, but remember, this is only an example. You can configure Exim's routers in many different ways, and there may be any number of routers in a configuration. The first router that is specified in a configuration is often one that handles addresses in domains that are not recognized specifically by the local host. Typically these are addresses for arbitrary domains on the Internet. A precondition is set up which looks for the special domains known to the host (for example, its own domain name), and the router is run for addresses that do not match. Typically, this is a router that looks up domains in the DNS in order to find the hosts to which this address routes. If it succeeds, the address is assigned to a suitable SMTP transport; if it does not succeed, the router is configured to fail the address. The second router is reached only when the domain is recognized as one that "belongs" to the local host. This router does redirection - also known as aliasing and forwarding. When it generates one or more new addresses from the original, each of them is routed independently from the start. Otherwise, the router may cause an address to fail, or it may simply decline to handle the address, in which case the address is passed to the next router. The final router in many configurations is one that checks to see if the address belongs to a local mailbox. The precondition may involve a check to see if the local part is the name of a login account, or it may look up the local part in a file or a database. If its preconditions are not met, or if the router declines, we have reached the end of the routers. When this happens, the address is bounced. 3.9 Processing an address for verification ------------------------------------------ As well as being used to decide how to deliver to an address, Exim's routers are also used for address verification. Verification can be requested as one of the checks to be performed in an ACL for incoming messages, on both sender and recipient addresses, and it can be tested using the -bv and -bvs command line options. When an address is being verified, the routers are run in "verify mode". This does not affect the way the routers work, but it is a state that can be detected. By this means, a router can be skipped or made to behave differently when verifying. A common example is a configuration in which the first router sends all messages to a message-scanning program unless they have been previously scanned. Thus, the first router accepts all addresses without any checking, making it useless for verifying. Normally, the no_verify option would be set for such a router, causing it to be skipped in verify mode. 3.10 Running an individual router --------------------------------- As explained in the example above, a number of preconditions are checked before running a router. If any are not met, the router is skipped, and the address is passed to the next router. When all the preconditions on a router are met, the router is run. What happens next depends on the outcome, which is one of the following: * accept: The router accepts the address, and either assigns it to a transport or generates one or more "child" addresses. Processing the original address ceases unless the unseen option is set on the router. This option can be used to set up multiple deliveries with different routing (for example, for keeping archive copies of messages). When unseen is set, the address is passed to the next router. Normally, however, an accept return marks the end of routing. Any child addresses generated by the router are processed independently, starting with the first router by default. It is possible to change this by setting the redirect_router option to specify which router to start at for child addresses. Unlike pass_router (see below) the router specified by redirect_router may be anywhere in the router configuration. * pass: The router recognizes the address, but cannot handle it itself. It requests that the address be passed to another router. By default, the address is passed to the next router, but this can be changed by setting the pass_router option. However, (unlike redirect_router) the named router must be below the current router (to avoid loops). * decline: The router declines to accept the address because it does not recognize it at all. By default, the address is passed to the next router, but this can be prevented by setting the no_more option. When no_more is set, all the remaining routers are skipped. In effect, no_more converts decline into fail. * fail: The router determines that the address should fail, and queues it for the generation of a bounce message. There is no further processing of the original address unless unseen is set on the router. * defer: The router cannot handle the address at the present time. (A database may be offline, or a DNS lookup may have timed out.) No further processing of the address happens in this delivery attempt. It is tried again next time the message is considered for delivery. * error: There is some error in the router (for example, a syntax error in its configuration). The action is as for defer. If an address reaches the end of the routers without having been accepted by any of them, it is bounced as unrouteable. The default error message in this situation is "unrouteable address", but you can set your own message by making use of the cannot_route_message option. This can be set for any router; the value from the last router that "saw" the address is used. Sometimes while routing you want to fail a delivery when some conditions are met but others are not, instead of passing the address on for further routing. You can do this by having a second router that explicitly fails the delivery when the relevant conditions are met. The redirect router has a "fail" facility for this purpose. 3.11 Duplicate addresses ------------------------ Once routing is complete, Exim scans the addresses that are assigned to local and remote transports and discards any duplicates that it finds. During this check, local parts are treated case-sensitively. This happens only when actually delivering a message; when testing routers with -bt, all the routed addresses are shown. 3.12 Router preconditions ------------------------- The preconditions that are tested for each router are listed below, in the order in which they are tested. The individual configuration options are described in more detail in chapter 15. * The local_part_prefix and local_part_suffix options can specify that the local parts handled by the router may or must have certain prefixes and/or suffixes. If a mandatory affix (prefix or suffix) is not present, the router is skipped. These conditions are tested first. When an affix is present, it is removed from the local part before further processing, including the evaluation of any other conditions. * Routers can be designated for use only when not verifying an address, that is, only when routing it for delivery (or testing its delivery routing). If the verify option is set false, the router is skipped when Exim is verifying an address. Setting the verify option actually sets two options, verify_sender and verify_recipient, which independently control the use of the router for sender and recipient verification. You can set these options directly if you want a router to be used for only one type of verification. Note that cutthrough delivery is classed as a recipient verification for this purpose. * If the address_test option is set false, the router is skipped when Exim is run with the -bt option to test an address routing. This can be helpful when the first router sends all new messages to a scanner of some sort; it makes it possible to use -bt to test subsequent delivery routing without having to simulate the effect of the scanner. * Routers can be designated for use only when verifying an address, as opposed to routing it for delivery. The verify_only option controls this. Again, cutthrough delivery counts as a verification. * Individual routers can be explicitly skipped when running the routers to check an address given in the SMTP EXPN command (see the expn option). * If the domains option is set, the domain of the address must be in the set of domains that it defines. * If the local_parts option is set, the local part of the address must be in the set of local parts that it defines. If local_part_prefix or local_part_suffix is in use, the prefix or suffix is removed from the local part before this check. If you want to do precondition tests on local parts that include affixes, you can do so by using a condition option (see below) that uses the variables $local_part, $local_part_prefix, $local_part_prefix_v, $local_part_suffix and $local_part_suffix_v as necessary. * If the check_local_user option is set, the local part must be the name of an account on the local host. If this check succeeds, the uid and gid of the local user are placed in $local_user_uid and $local_user_gid and the user's home directory is placed in $home; these values can be used in the remaining preconditions. * If the router_home_directory option is set, it is expanded at this point, because it overrides the value of $home. If this expansion were left till later, the value of $home as set by check_local_user would be used in subsequent tests. Having two different values of $home in the same router could lead to confusion. * If the senders option is set, the envelope sender address must be in the set of addresses that it defines. * If the require_files option is set, the existence or non-existence of specified files is tested. * If the condition option is set, it is evaluated and tested. This option uses an expanded string to allow you to set up your own custom preconditions. Expanded strings are described in chapter 11. Note that require_files comes near the end of the list, so you cannot use it to check for the existence of a file in which to lookup up a domain, local part, or sender. However, as these options are all expanded, you can use the exists expansion condition to make such tests within each condition. The require_files option is intended for checking files that the router may be going to use internally, or which are needed by a specific transport (for example, .procmailrc). 3.13 Delivery in detail ----------------------- When a message is to be delivered, the sequence of events is as follows: * If a system-wide filter file is specified, the message is passed to it. The filter may add recipients to the message, replace the recipients, discard the message, cause a new message to be generated, or cause the message delivery to fail. The format of the system filter file is the same as for Exim user filter files, described in the separate document entitled Exim's interfaces to mail filtering. (Note: Sieve cannot be used for system filter files.) Some additional features are available in system filters - see chapter 47 for details. Note that a message is passed to the system filter only once per delivery attempt, however many recipients it has. However, if there are several delivery attempts because one or more addresses could not be immediately delivered, the system filter is run each time. The filter condition first_delivery can be used to detect the first run of the system filter. * Each recipient address is offered to each configured router, in turn, subject to its preconditions, until one is able to handle it. If no router can handle the address, that is, if they all decline, the address is failed. Because routers can be targeted at particular domains, several locally handled domains can be processed entirely independently of each other. * A router that accepts an address may assign it to a local or a remote transport. However, the transport is not run at this time. Instead, the address is placed on a list for the particular transport, which will be run later. Alternatively, the router may generate one or more new addresses (typically from alias, forward, or filter files). New addresses are fed back into this process from the top, but in order to avoid loops, a router ignores any address which has an identically-named ancestor that was processed by itself. * When all the routing has been done, addresses that have been successfully handled are passed to their assigned transports. When local transports are doing real local deliveries, they handle only one address at a time, but if a local transport is being used as a pseudo-remote transport (for example, to collect batched SMTP messages for transmission by some other means) multiple addresses can be handled. Remote transports can always handle more than one address at a time, but can be configured not to do so, or to restrict multiple addresses to the same domain. * Each local delivery to a file or a pipe runs in a separate process under a non-privileged uid, and these deliveries are run one at a time. Remote deliveries also run in separate processes, normally under a uid that is private to Exim ("the Exim user"), but in this case, several remote deliveries can be run in parallel. The maximum number of simultaneous remote deliveries for any one message is set by the remote_max_parallel option. The order in which deliveries are done is not defined, except that all local deliveries happen before any remote deliveries. * When it encounters a local delivery during a queue run, Exim checks its retry database to see if there has been a previous temporary delivery failure for the address before running the local transport. If there was a previous failure, Exim does not attempt a new delivery until the retry time for the address is reached. However, this happens only for delivery attempts that are part of a queue run. Local deliveries are always attempted when delivery immediately follows message reception, even if retry times are set for them. This makes for better behaviour if one particular message is causing problems (for example, causing quota overflow, or provoking an error in a filter file). * Remote transports do their own retry handling, since an address may be deliverable to one of a number of hosts, each of which may have a different retry time. If there have been previous temporary failures and no host has reached its retry time, no delivery is attempted, whether in a queue run or not. See chapter 32 for details of retry strategies. * If there were any permanent errors, a bounce message is returned to an appropriate address (the sender in the common case), with details of the error for each failing address. Exim can be configured to send copies of bounce messages to other addresses. * If one or more addresses suffered a temporary failure, the message is left on the queue, to be tried again later. Delivery of these addresses is said to be deferred. * When all the recipient addresses have either been delivered or bounced, handling of the message is complete. The spool files and message log are deleted, though the message log can optionally be preserved if required. 3.14 Retry mechanism -------------------- Exim's mechanism for retrying messages that fail to get delivered at the first attempt is the queue runner process. You must either run an Exim daemon that uses the -q option with a time interval to start queue runners at regular intervals or use some other means (such as cron) to start them. If you do not arrange for queue runners to be run, messages that fail temporarily at the first attempt will remain in your queue forever. A queue runner process works its way through the queue, one message at a time, trying each delivery that has passed its retry time. You can run several queue runners at once. Exim uses a set of configured rules to determine when next to retry the failing address (see chapter 32). These rules also specify when Exim should give up trying to deliver to the address, at which point it generates a bounce message. If no retry rules are set for a particular host, address, and error combination, no retries are attempted, and temporary errors are treated as permanent. 3.15 Temporary delivery failure ------------------------------- There are many reasons why a message may not be immediately deliverable to a particular address. Failure to connect to a remote machine (because it, or the connection to it, is down) is one of the most common. Temporary failures may be detected during routing as well as during the transport stage of delivery. Local deliveries may be delayed if NFS files are unavailable, or if a mailbox is on a file system where the user is over quota. Exim can be configured to impose its own quotas on local mailboxes; where system quotas are set they will also apply. If a host is unreachable for a period of time, a number of messages may be waiting for it by the time it recovers, and sending them in a single SMTP connection is clearly beneficial. Whenever a delivery to a remote host is deferred, Exim makes a note in its hints database, and whenever a successful SMTP delivery has happened, it looks to see if any other messages are waiting for the same host. If any are found, they are sent over the same SMTP connection, subject to a configuration limit as to the maximum number in any one connection. 3.16 Permanent delivery failure ------------------------------- When a message cannot be delivered to some or all of its intended recipients, a bounce message is generated. Temporary delivery failures turn into permanent errors when their timeout expires. All the addresses that fail in a given delivery attempt are listed in a single message. If the original message has many recipients, it is possible for some addresses to fail in one delivery attempt and others to fail subsequently, giving rise to more than one bounce message. The wording of bounce messages can be customized by the administrator. See chapter 50 for details. Bounce messages contain an X-Failed-Recipients: header line that lists the failed addresses, for the benefit of programs that try to analyse such messages automatically. A bounce message is normally sent to the sender of the original message, as obtained from the message's envelope. For incoming SMTP messages, this is the address given in the MAIL command. However, when an address is expanded via a forward or alias file, an alternative address can be specified for delivery failures of the generated addresses. For a mailing list expansion (see section 51.2) it is common to direct bounce messages to the manager of the list. 3.17 Failures to deliver bounce messages ---------------------------------------- If a bounce message (either locally generated or received from a remote host) itself suffers a permanent delivery failure, the message is left in the queue, but it is frozen, awaiting the attention of an administrator. There are options that can be used to make Exim discard such failed messages, or to keep them for only a short time (see timeout_frozen_after and ignore_bounce_errors_after). =============================================================================== 4. BUILDING AND INSTALLING EXIM 4.1 Unpacking ------------- Exim is distributed as a gzipped or bzipped tar file which, when unpacked, creates a directory with the name of the current release (for example, exim-4.94.2) into which the following files are placed: ACKNOWLEDGMENTS contains some acknowledgments CHANGES contains a reference to where changes are documented LICENCE the GNU General Public Licence Makefile top-level make file NOTICE conditions for the use of Exim README list of files, directories and simple build instructions Other files whose names begin with README may also be present. The following subdirectories are created: Local an empty directory for local configuration files OS OS-specific files doc documentation files exim_monitor source files for the Exim monitor scripts scripts used in the build process src remaining source files util independent utilities The main utility programs are contained in the src directory and are built with the Exim binary. The util directory contains a few optional scripts that may be useful to some sites. 4.2 Multiple machine architectures and operating systems -------------------------------------------------------- The building process for Exim is arranged to make it easy to build binaries for a number of different architectures and operating systems from the same set of source files. Compilation does not take place in the src directory. Instead, a build directory is created for each architecture and operating system. Symbolic links to the sources are installed in this directory, which is where the actual building takes place. In most cases, Exim can discover the machine architecture and operating system for itself, but the defaults can be overridden if necessary. A C99-capable compiler will be required for the build. 4.3 PCRE library ---------------- Exim no longer has an embedded PCRE library as the vast majority of modern systems include PCRE as a system library, although you may need to install the PCRE package or the PCRE development package for your operating system. If your system has a normal PCRE installation the Exim build process will need no further configuration. If the library or the headers are in an unusual location you will need to either set the PCRE_LIBS and INCLUDE directives appropriately, or set PCRE_CONFIG=yes to use the installed pcre-config command. If your operating system has no PCRE support then you will need to obtain and build the current PCRE from ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/. More information on PCRE is available at https://www.pcre.org/. 4.4 DBM libraries ----------------- Even if you do not use any DBM files in your configuration, Exim still needs a DBM library in order to operate, because it uses indexed files for its hints databases. Unfortunately, there are a number of DBM libraries in existence, and different operating systems often have different ones installed. If you are using Solaris, IRIX, one of the modern BSD systems, or a modern Linux distribution, the DBM configuration should happen automatically, and you may be able to ignore this section. Otherwise, you may have to learn more than you would like about DBM libraries from what follows. Licensed versions of Unix normally contain a library of DBM functions operating via the ndbm interface, and this is what Exim expects by default. Free versions of Unix seem to vary in what they contain as standard. In particular, some early versions of Linux have no default DBM library, and different distributors have chosen to bundle different libraries with their packaged versions. However, the more recent releases seem to have standardized on the Berkeley DB library. Different DBM libraries have different conventions for naming the files they use. When a program opens a file called dbmfile, there are several possibilities: 1. A traditional ndbm implementation, such as that supplied as part of Solaris, operates on two files called dbmfile.dir and dbmfile.pag. 2. The GNU library, gdbm, operates on a single file. If used via its ndbm compatibility interface it makes two different hard links to it with names dbmfile.dir and dbmfile.pag, but if used via its native interface, the filename is used unmodified. 3. The Berkeley DB package, if called via its ndbm compatibility interface, operates on a single file called dbmfile.db, but otherwise looks to the programmer exactly the same as the traditional ndbm implementation. 4. If the Berkeley package is used in its native mode, it operates on a single file called dbmfile; the programmer's interface is somewhat different to the traditional ndbm interface. 5. To complicate things further, there are several very different versions of the Berkeley DB package. Version 1.85 was stable for a very long time, releases 2.x and 3.x were current for a while, but the latest versions when Exim last revamped support were numbered 4.x. Maintenance of some of the earlier releases has ceased. All versions of Berkeley DB could be obtained from http://www.sleepycat.com/, which is now a redirect to their new owner's page with far newer versions listed. It is probably wise to plan to move your storage configurations away from Berkeley DB format, as today there are smaller and simpler alternatives more suited to Exim's usage model. 6. Yet another DBM library, called tdb, is available from https:// sourceforge.net/projects/tdb/files/. It has its own interface, and also operates on a single file. Exim and its utilities can be compiled to use any of these interfaces. In order to use any version of the Berkeley DB package in native mode, you must set USE_DB in an appropriate configuration file (typically Local/Makefile). For example: USE_DB=yes Similarly, for gdbm you set USE_GDBM, and for tdb you set USE_TDB. An error is diagnosed if you set more than one of these. At the lowest level, the build-time configuration sets none of these options, thereby assuming an interface of type (1). However, some operating system configuration files (for example, those for the BSD operating systems and Linux) assume type (4) by setting USE_DB as their default, and the configuration files for Cygwin set USE_GDBM. Anything you set in Local/Makefile , however, overrides these system defaults. As well as setting USE_DB, USE_GDBM, or USE_TDB, it may also be necessary to set DBMLIB, to cause inclusion of the appropriate library, as in one of these lines: DBMLIB = -ldb DBMLIB = -ltdb Settings like that will work if the DBM library is installed in the standard place. Sometimes it is not, and the library's header file may also not be in the default path. You may need to set INCLUDE to specify where the header file is, and to specify the path to the library more fully in DBMLIB, as in this example: INCLUDE=-I/usr/local/include/db-4.1 DBMLIB=/usr/local/lib/db-4.1/libdb.a There is further detailed discussion about the various DBM libraries in the file doc/dbm.discuss.txt in the Exim distribution. 4.5 Pre-building configuration ------------------------------ Before building Exim, a local configuration file that specifies options independent of any operating system has to be created with the name Local/ Makefile. A template for this file is supplied as the file src/EDITME, and it contains full descriptions of all the option settings therein. These descriptions are therefore not repeated here. If you are building Exim for the first time, the simplest thing to do is to copy src/EDITME to Local/Makefile, then read it and edit it appropriately. There are three settings that you must supply, because Exim will not build without them. They are the location of the runtime configuration file (CONFIGURE_FILE), the directory in which Exim binaries will be installed (BIN_DIRECTORY), and the identity of the Exim user (EXIM_USER and maybe EXIM_GROUP as well). The value of CONFIGURE_FILE can in fact be a colon-separated list of filenames; Exim uses the first of them that exists. There are a few other parameters that can be specified either at build time or at runtime, to enable the same binary to be used on a number of different machines. However, if the locations of Exim's spool directory and log file directory (if not within the spool directory) are fixed, it is recommended that you specify them in Local/Makefile instead of at runtime, so that errors detected early in Exim's execution (such as a malformed configuration file) can be logged. Exim's interfaces for calling virus and spam scanning software directly from access control lists are not compiled by default. If you want to include these facilities, you need to set WITH_CONTENT_SCAN=yes in your Local/Makefile. For details of the facilities themselves, see chapter 45. If you are going to build the Exim monitor, a similar configuration process is required. The file exim_monitor/EDITME must be edited appropriately for your installation and saved under the name Local/eximon.conf. If you are happy with the default settings described in exim_monitor/EDITME, Local/eximon.conf can be empty, but it must exist. This is all the configuration that is needed in straightforward cases for known operating systems. However, the building process is set up so that it is easy to override options that are set by default or by operating-system-specific configuration files, for example, to change the C compiler, which defaults to gcc. See section 4.13 below for details of how to do this. 4.6 Support for iconv() ----------------------- The contents of header lines in messages may be encoded according to the rules described RFC 2047. This makes it possible to transmit characters that are not in the ASCII character set, and to label them as being in a particular character set. When Exim is inspecting header lines by means of the $h_ mechanism, it decodes them, and translates them into a specified character set (default is set at build time). The translation is possible only if the operating system supports the iconv() function. However, some of the operating systems that supply iconv() do not support very many conversions. The GNU libiconv library (available from https://www.gnu.org/ software/libiconv/) can be installed on such systems to remedy this deficiency, as well as on systems that do not supply iconv() at all. After installing libiconv, you should add HAVE_ICONV=yes to your Local/Makefile and rebuild Exim. 4.7 Including TLS/SSL encryption support ---------------------------------------- Exim is usually built to support encrypted SMTP connections, using the STARTTLS command as per RFC 2487. It can also support clients that expect to start a TLS session immediately on connection to a non-standard port (see the tls_on_connect_ports runtime option and the -tls-on-connect command line option). If you want to build Exim with TLS support, you must first install either the OpenSSL or GnuTLS library. There is no cryptographic code in Exim itself for implementing SSL. If you do not want TLS support you should set DISABLE_TLS=yes in Local/Makefile. If OpenSSL is installed, you should set USE_OPENSL=yes TLS_LIBS=-lssl -lcrypto in Local/Makefile. You may also need to specify the locations of the OpenSSL library and include files. For example: USE_OPENSSL=yes TLS_LIBS=-L/usr/local/openssl/lib -lssl -lcrypto TLS_INCLUDE=-I/usr/local/openssl/include/ If you have pkg-config available, then instead you can just use: USE_OPENSSL=yes USE_OPENSSL_PC=openssl If GnuTLS is installed, you should set USE_GNUTLS=yes TLS_LIBS=-lgnutls -ltasn1 -lgcrypt in Local/Makefile, and again you may need to specify the locations of the library and include files. For example: USE_GNUTLS=yes TLS_LIBS=-L/usr/gnu/lib -lgnutls -ltasn1 -lgcrypt TLS_INCLUDE=-I/usr/gnu/include If you have pkg-config available, then instead you can just use: USE_GNUTLS=yes USE_GNUTLS_PC=gnutls You do not need to set TLS_INCLUDE if the relevant directory is already specified in INCLUDE. Details of how to configure Exim to make use of TLS are given in chapter 43. 4.8 Use of tcpwrappers ---------------------- Exim can be linked with the tcpwrappers library in order to check incoming SMTP calls using the tcpwrappers control files. This may be a convenient alternative to Exim's own checking facilities for installations that are already making use of tcpwrappers for other purposes. To do this, you should set USE_TCP_WRAPPERS in Local/Makefile, arrange for the file tcpd.h to be available at compile time, and also ensure that the library libwrap.a is available at link time, typically by including -lwrap in EXTRALIBS_EXIM. For example, if tcpwrappers is installed in /usr/local, you might have USE_TCP_WRAPPERS=yes CFLAGS=-O -I/usr/local/include EXTRALIBS_EXIM=-L/usr/local/lib -lwrap in Local/Makefile. The daemon name to use in the tcpwrappers control files is "exim". For example, the line exim : LOCAL 192.168.1. .friendly.domain.example in your /etc/hosts.allow file allows connections from the local host, from the subnet 192.168.1.0/24, and from all hosts in friendly.domain.example. All other connections are denied. The daemon name used by tcpwrappers can be changed at build time by setting TCP_WRAPPERS_DAEMON_NAME in Local/Makefile, or by setting tcp_wrappers_daemon_name in the configure file. Consult the tcpwrappers documentation for further details. 4.9 Including support for IPv6 ------------------------------ Exim contains code for use on systems that have IPv6 support. Setting "HAVE_IPV6=YES" in Local/Makefile causes the IPv6 code to be included; it may also be necessary to set IPV6_INCLUDE and IPV6_LIBS on systems where the IPv6 support is not fully integrated into the normal include and library files. Two different types of DNS record for handling IPv6 addresses have been defined. AAAA records (analogous to A records for IPv4) are in use, and are currently seen as the mainstream. Another record type called A6 was proposed as better than AAAA because it had more flexibility. However, it was felt to be over-complex, and its status was reduced to "experimental". Exim used to have a compile option for including A6 record support but this has now been withdrawn. 4.10 Dynamically loaded lookup module support --------------------------------------------- On some platforms, Exim supports not compiling all lookup types directly into the main binary, instead putting some into external modules which can be loaded on demand. This permits packagers to build Exim with support for lookups with extensive library dependencies without requiring all users to install all of those dependencies. Most, but not all, lookup types can be built this way. Set "LOOKUP_MODULE_DIR" to the directory into which the modules will be installed; Exim will only load modules from that directory, as a security measure. You will need to set "CFLAGS_DYNAMIC" if not already defined for your OS; see OS/Makefile-Linux for an example. Some other requirements for adjusting "EXTRALIBS" may also be necessary, see src/EDITME for details. Then, for each module to be loaded dynamically, define the relevant "LOOKUP_"< lookup_type> flags to have the value "2" instead of "yes". For example, this will build in lsearch but load sqlite and mysql support on demand: LOOKUP_LSEARCH=yes LOOKUP_SQLITE=2 LOOKUP_MYSQL=2 4.11 The building process ------------------------- Once Local/Makefile (and Local/eximon.conf, if required) have been created, run make at the top level. It determines the architecture and operating system types, and creates a build directory if one does not exist. For example, on a Sun system running Solaris 8, the directory build-SunOS5-5.8-sparc is created. Symbolic links to relevant source files are installed in the build directory. If this is the first time make has been run, it calls a script that builds a make file inside the build directory, using the configuration files from the Local directory. The new make file is then passed to another instance of make. This does the real work, building a number of utility scripts, and then compiling and linking the binaries for the Exim monitor (if configured), a number of utility programs, and finally Exim itself. The command "make makefile" can be used to force a rebuild of the make file in the build directory, should this ever be necessary. If you have problems building Exim, check for any comments there may be in the README file concerning your operating system, and also take a look at the FAQ, where some common problems are covered. 4.12 Output from "make" ----------------------- The output produced by the make process for compile lines is often very unreadable, because these lines can be very long. For this reason, the normal output is suppressed by default, and instead output similar to that which appears when compiling the 2.6 Linux kernel is generated: just a short line for each module that is being compiled or linked. However, it is still possible to get the full output, by calling make like this: FULLECHO='' make -e The value of FULLECHO defaults to "@", the flag character that suppresses command reflection in make. When you ask for the full output, it is given in addition to the short output. 4.13 Overriding build-time options for Exim ------------------------------------------- The main make file that is created at the beginning of the building process consists of the concatenation of a number of files which set configuration values, followed by a fixed set of make instructions. If a value is set more than once, the last setting overrides any previous ones. This provides a convenient way of overriding defaults. The files that are concatenated are, in order: OS/Makefile-Default OS/Makefile- Local/Makefile Local/Makefile- Local/Makefile- Local/Makefile-- OS/Makefile-Base where is the operating system type and is the architecture type. Local/Makefile is required to exist, and the building process fails if it is absent. The other three Local files are optional, and are often not needed. The values used for and are obtained from scripts called scripts/os-type and scripts/arch-type respectively. If either of the environment variables EXIM_OSTYPE or EXIM_ARCHTYPE is set, their values are used, thereby providing a means of forcing particular settings. Otherwise, the scripts try to get values from the uname command. If this fails, the shell variables OSTYPE and ARCHTYPE are inspected. A number of ad hoc transformations are then applied, to produce the standard names that Exim expects. You can run these scripts directly from the shell in order to find out what values are being used on your system. OS/Makefile-Default contains comments about the variables that are set therein. Some (but not all) are mentioned below. If there is something that needs changing, review the contents of this file and the contents of the make file for your operating system (OS/Makefile-) to see what the default values are. If you need to change any of the values that are set in OS/Makefile-Default or in OS/Makefile-, or to add any new definitions, you do not need to change the original files. Instead, you should make the changes by putting the new values in an appropriate Local file. For example, when building Exim in many releases of the Tru64-Unix (formerly Digital UNIX, formerly DEC-OSF1) operating system, it is necessary to specify that the C compiler is called cc rather than gcc. Also, the compiler must be called with the option -std1, to make it recognize some of the features of Standard C that Exim uses. (Most other compilers recognize Standard C by default.) To do this, you should create a file called Local/Makefile-OSF1 containing the lines CC=cc CFLAGS=-std1 If you are compiling for just one operating system, it may be easier to put these lines directly into Local/Makefile. Keeping all your local configuration settings separate from the distributed files makes it easy to transfer them to new versions of Exim simply by copying the contents of the Local directory. Exim contains support for doing LDAP, NIS, NIS+, and other kinds of file lookup, but not all systems have these components installed, so the default is not to include the relevant code in the binary. All the different kinds of file and database lookup that Exim supports are implemented as separate code modules which are included only if the relevant compile-time options are set. In the case of LDAP, NIS, and NIS+, the settings for Local/Makefile are: LOOKUP_LDAP=yes LOOKUP_NIS=yes LOOKUP_NISPLUS=yes and similar settings apply to the other lookup types. They are all listed in src/EDITME. In many cases the relevant include files and interface libraries need to be installed before compiling Exim. However, there are some optional lookup types (such as cdb) for which the code is entirely contained within Exim, and no external include files or libraries are required. When a lookup type is not included in the binary, attempts to configure Exim to use it cause runtime configuration errors. Many systems now use a tool called pkg-config to encapsulate information about how to compile against a library; Exim has some initial support for being able to use pkg-config for lookups and authenticators. For any given makefile variable which starts "LOOKUP_" or "AUTH_", you can add a new variable with the "_PC" suffix in the name and assign as the value the name of the package to be queried. The results of querying via the pkg-config command will be added to the appropriate Makefile variables with "+=" directives, so your version of make will need to support that syntax. For instance: LOOKUP_SQLITE=yes LOOKUP_SQLITE_PC=sqlite3 AUTH_GSASL=yes AUTH_GSASL_PC=libgsasl AUTH_HEIMDAL_GSSAPI=yes AUTH_HEIMDAL_GSSAPI_PC=heimdal-gssapi Exim can be linked with an embedded Perl interpreter, allowing Perl subroutines to be called during string expansion. To enable this facility, EXIM_PERL=perl.o must be defined in Local/Makefile. Details of this facility are given in chapter 12. The location of the X11 libraries is something that varies a lot between operating systems, and there may be different versions of X11 to cope with. Exim itself makes no use of X11, but if you are compiling the Exim monitor, the X11 libraries must be available. The following three variables are set in OS/ Makefile-Default: X11=/usr/X11R6 XINCLUDE=-I$(X11)/include XLFLAGS=-L$(X11)/lib These are overridden in some of the operating-system configuration files. For example, in OS/Makefile-SunOS5 there is X11=/usr/openwin XINCLUDE=-I$(X11)/include XLFLAGS=-L$(X11)/lib -R$(X11)/lib If you need to override the default setting for your operating system, place a definition of all three of these variables into your Local/Makefile- file. If you need to add any extra libraries to the link steps, these can be put in a variable called EXTRALIBS, which appears in all the link commands, but by default is not defined. In contrast, EXTRALIBS_EXIM is used only on the command for linking the main Exim binary, and not for any associated utilities. There is also DBMLIB, which appears in the link commands for binaries that use DBM functions (see also section 4.4). Finally, there is EXTRALIBS_EXIMON, which appears only in the link step for the Exim monitor binary, and which can be used, for example, to include additional X11 libraries. The make file copes with rebuilding Exim correctly if any of the configuration files are edited. However, if an optional configuration file is deleted, it is necessary to touch the associated non-optional file (that is, Local/Makefile or Local/eximon.conf) before rebuilding. 4.14 OS-specific header files ----------------------------- The OS directory contains a number of files with names of the form os.h- . These are system-specific C header files that should not normally need to be changed. There is a list of macro settings that are recognized in the file OS/os.configuring, which should be consulted if you are porting Exim to a new operating system. 4.15 Overriding build-time options for the monitor -------------------------------------------------- A similar process is used for overriding things when building the Exim monitor, where the files that are involved are OS/eximon.conf-Default OS/eximon.conf- Local/eximon.conf Local/eximon.conf- Local/eximon.conf- Local/eximon.conf-- As with Exim itself, the final three files need not exist, and in this case the OS/eximon.conf- file is also optional. The default values in OS/ eximon.conf-Default can be overridden dynamically by setting environment variables of the same name, preceded by EXIMON_. For example, setting EXIMON_LOG_DEPTH in the environment overrides the value of LOG_DEPTH at runtime. 4.16 Installing Exim binaries and scripts ----------------------------------------- The command "make install" runs the exim_install script with no arguments. The script copies binaries and utility scripts into the directory whose name is specified by the BIN_DIRECTORY setting in Local/Makefile. The install script copies files only if they are newer than the files they are going to replace. The Exim binary is required to be owned by root and have the setuid bit set, for normal configurations. Therefore, you must run "make install" as root so that it can set up the Exim binary in this way. However, in some special situations (for example, if a host is doing no local deliveries) it may be possible to run Exim without making the binary setuid root (see chapter 56 for details). Exim's runtime configuration file is named by the CONFIGURE_FILE setting in Local/Makefile. If this names a single file, and the file does not exist, the default configuration file src/configure.default is copied there by the installation script. If a runtime configuration file already exists, it is left alone. If CONFIGURE_FILE is a colon-separated list, naming several alternative files, no default is installed. One change is made to the default configuration file when it is installed: the default configuration contains a router that references a system aliases file. The path to this file is set to the value specified by SYSTEM_ALIASES_FILE in Local/Makefile (/etc/aliases by default). If the system aliases file does not exist, the installation script creates it, and outputs a comment to the user. The created file contains no aliases, but it does contain comments about the aliases a site should normally have. Mail aliases have traditionally been kept in /etc/aliases. However, some operating systems are now using /etc/mail/ aliases. You should check if yours is one of these, and change Exim's configuration if necessary. The default configuration uses the local host's name as the only local domain, and is set up to do local deliveries into the shared directory /var/mail, running as the local user. System aliases and .forward files in users' home directories are supported, but no NIS or NIS+ support is configured. Domains other than the name of the local host are routed using the DNS, with delivery over SMTP. It is possible to install Exim for special purposes (such as building a binary distribution) in a private part of the file system. You can do this by a command such as make DESTDIR=/some/directory/ install This has the effect of pre-pending the specified directory to all the file paths, except the name of the system aliases file that appears in the default configuration. (If a default alias file is created, its name is modified.) For backwards compatibility, ROOT is used if DESTDIR is not set, but this usage is deprecated. Running make install does not copy the Exim 4 conversion script convert4r4. You will probably run this only once if you are upgrading from Exim 3. None of the documentation files in the doc directory are copied, except for the info files when you have set INFO_DIRECTORY, as described in section 4.17 below. For the utility programs, old versions are renamed by adding the suffix .O to their names. The Exim binary itself, however, is handled differently. It is installed under a name that includes the version number and the compile number, for example, exim-4.94.2-1. The script then arranges for a symbolic link called exim to point to the binary. If you are updating a previous version of Exim, the script takes care to ensure that the name exim is never absent from the directory (as seen by other processes). If you want to see what the make install will do before running it for real, you can pass the -n option to the installation script by this command: make INSTALL_ARG=-n install The contents of the variable INSTALL_ARG are passed to the installation script. You do not need to be root to run this test. Alternatively, you can run the installation script directly, but this must be from within the build directory. For example, from the top-level Exim directory you could use this command: (cd build-SunOS5-5.5.1-sparc; ../scripts/exim_install -n) There are two other options that can be supplied to the installation script. * -no_chown bypasses the call to change the owner of the installed binary to root, and the call to make it a setuid binary. * -no_symlink bypasses the setting up of the symbolic link exim to the installed binary. INSTALL_ARG can be used to pass these options to the script. For example: make INSTALL_ARG=-no_symlink install The installation script can also be given arguments specifying which files are to be copied. For example, to install just the Exim binary, and nothing else, without creating the symbolic link, you could use: make INSTALL_ARG='-no_symlink exim' install 4.17 Installing info documentation ---------------------------------- Not all systems use the GNU info system for documentation, and for this reason, the Texinfo source of Exim's documentation is not included in the main distribution. Instead it is available separately from the FTP site (see section 1.5). If you have defined INFO_DIRECTORY in Local/Makefile and the Texinfo source of the documentation is found in the source tree, running "make install" automatically builds the info files and installs them. 4.18 Setting up the spool directory ----------------------------------- When it starts up, Exim tries to create its spool directory if it does not exist. The Exim uid and gid are used for the owner and group of the spool directory. Sub-directories are automatically created in the spool directory as necessary. 4.19 Testing ------------ Having installed Exim, you can check that the runtime configuration file is syntactically valid by running the following command, which assumes that the Exim binary directory is within your PATH environment variable: exim -bV If there are any errors in the configuration file, Exim outputs error messages. Otherwise it outputs the version number and build date, the DBM library that is being used, and information about which drivers and other optional code modules are included in the binary. Some simple routing tests can be done by using the address testing option. For example, exim -bt should verify that it recognizes a local mailbox, and exim -bt a remote one. Then try getting it to deliver mail, both locally and remotely. This can be done by passing messages directly to Exim, without going through a user agent. For example: exim -v postmaster@your.domain.example From: user@your.domain.example To: postmaster@your.domain.example Subject: Testing Exim This is a test message. ^D The -v option causes Exim to output some verification of what it is doing. In this case you should see copies of three log lines, one for the message's arrival, one for its delivery, and one containing "Completed". If you encounter problems, look at Exim's log files (mainlog and paniclog) to see if there is any relevant information there. Another source of information is running Exim with debugging turned on, by specifying the -d option. If a message is stuck on Exim's spool, you can force a delivery with debugging turned on by a command of the form exim -d -M You must be root or an "admin user" in order to do this. The -d option produces rather a lot of output, but you can cut this down to specific areas. For example, if you use -d-all+route only the debugging information relevant to routing is included. (See the -d option in chapter 5 for more details.) One specific problem that has shown up on some sites is the inability to do local deliveries into a shared mailbox directory, because it does not have the "sticky bit" set on it. By default, Exim tries to create a lock file before writing to a mailbox file, and if it cannot create the lock file, the delivery is deferred. You can get round this either by setting the "sticky bit" on the directory, or by setting a specific group for local deliveries and allowing that group to create files in the directory (see the comments above the local_delivery transport in the default configuration file). Another approach is to configure Exim not to use lock files, but just to rely on fcntl() locking instead. However, you should do this only if all user agents also use fcntl() locking. For further discussion of locking issues, see chapter 26. One thing that cannot be tested on a system that is already running an MTA is the receipt of incoming SMTP mail on the standard SMTP port. However, the -oX option can be used to run an Exim daemon that listens on some other port, or inetd can be used to do this. The -bh option and the exim_checkaccess utility can be used to check out policy controls on incoming SMTP mail. Testing a new version on a system that is already running Exim can most easily be done by building a binary with a different CONFIGURE_FILE setting. From within the runtime configuration, all other file and directory names that Exim uses can be altered, in order to keep it entirely clear of the production version. 4.20 Replacing another MTA with Exim ------------------------------------ Building and installing Exim for the first time does not of itself put it in general use. The name by which the system's MTA is called by mail user agents is either /usr/sbin/sendmail, or /usr/lib/sendmail (depending on the operating system), and it is necessary to make this name point to the exim binary in order to get the user agents to pass messages to Exim. This is normally done by renaming any existing file and making /usr/sbin/sendmail or /usr/lib/sendmail a symbolic link to the exim binary. It is a good idea to remove any setuid privilege and executable status from the old MTA. It is then necessary to stop and restart the mailer daemon, if one is running. Some operating systems have introduced alternative ways of switching MTAs. For example, if you are running FreeBSD, you need to edit the file /etc/mail/ mailer.conf instead of setting up a symbolic link as just described. A typical example of the contents of this file for running Exim is as follows: sendmail /usr/exim/bin/exim send-mail /usr/exim/bin/exim mailq /usr/exim/bin/exim -bp newaliases /usr/bin/true Once you have set up the symbolic link, or edited /etc/mail/mailer.conf, your Exim installation is "live". Check it by sending a message from your favourite user agent. You should consider what to tell your users about the change of MTA. Exim may have different capabilities to what was previously running, and there are various operational differences such as the text of messages produced by command line options and in bounce messages. If you allow your users to make use of Exim's filtering capabilities, you should make the document entitled Exim's interface to mail filtering available to them. 4.21 Upgrading Exim ------------------- If you are already running Exim on your host, building and installing a new version automatically makes it available to MUAs, or any other programs that call the MTA directly. However, if you are running an Exim daemon, you do need to send it a HUP signal, to make it re-execute itself, and thereby pick up the new binary. You do not need to stop processing mail in order to install a new version of Exim. The install script does not modify an existing runtime configuration file. 4.22 Stopping the Exim daemon on Solaris ---------------------------------------- The standard command for stopping the mailer daemon on Solaris is /etc/init.d/sendmail stop If /usr/lib/sendmail has been turned into a symbolic link, this script fails to stop Exim because it uses the command ps -e and greps the output for the text "sendmail"; this is not present because the actual program name (that is, "exim") is given by the ps command with these options. A solution is to replace the line that finds the process id with something like pid=`cat /var/spool/exim/exim-daemon.pid` to obtain the daemon's pid directly from the file that Exim saves it in. Note, however, that stopping the daemon does not "stop Exim". Messages can still be received from local processes, and if automatic delivery is configured (the normal case), deliveries will still occur. =============================================================================== 5. THE EXIM COMMAND LINE Exim's command line takes the standard Unix form of a sequence of options, each starting with a hyphen character, followed by a number of arguments. The options are compatible with the main options of Sendmail, and there are also some additional options, some of which are compatible with Smail 3. Certain combinations of options do not make sense, and provoke an error if used. The form of the arguments depends on which options are set. 5.1 Setting options by program name ----------------------------------- If Exim is called under the name mailq, it behaves as if the option -bp were present before any other options. The -bp option requests a listing of the contents of the mail queue on the standard output. This feature is for compatibility with some systems that contain a command of that name in one of the standard libraries, symbolically linked to /usr/sbin/sendmail or /usr/lib/ sendmail. If Exim is called under the name rsmtp it behaves as if the option -bS were present before any other options, for compatibility with Smail. The -bS option is used for reading in a number of messages in batched SMTP format. If Exim is called under the name rmail it behaves as if the -i and -oee options were present before any other options, for compatibility with Smail. The name rmail is used as an interface by some UUCP systems. If Exim is called under the name runq it behaves as if the option -q were present before any other options, for compatibility with Smail. The -q option causes a single queue runner process to be started. If Exim is called under the name newaliases it behaves as if the option -bi were present before any other options, for compatibility with Sendmail. This option is used for rebuilding Sendmail's alias file. Exim does not have the concept of a single alias file, but can be configured to run a given command if called with the -bi option. 5.2 Trusted and admin users --------------------------- Some Exim options are available only to trusted users and others are available only to admin users. In the description below, the phrases "Exim user" and "Exim group" mean the user and group defined by EXIM_USER and EXIM_GROUP in Local/Makefile or set by the exim_user and exim_group options. These do not necessarily have to use the name "exim". * The trusted users are root, the Exim user, any user listed in the trusted_users configuration option, and any user whose current group or any supplementary group is one of those listed in the trusted_groups configuration option. Note that the Exim group is not automatically trusted. Trusted users are always permitted to use the -f option or a leading "From " line to specify the envelope sender of a message that is passed to Exim through the local interface (see the -bm and -f options below). See the untrusted_set_sender option for a way of permitting non-trusted users to set envelope senders. For a trusted user, there is never any check on the contents of the From: header line, and a Sender: line is never added. Furthermore, any existing Sender: line in incoming local (non-TCP/IP) messages is not removed. Trusted users may also specify a host name, host address, interface address, protocol name, ident value, and authentication data when submitting a message locally. Thus, they are able to insert messages into Exim's queue locally that have the characteristics of messages received from a remote host. Untrusted users may in some circumstances use -f, but can never set the other values that are available to trusted users. * The admin users are root, the Exim user, and any user that is a member of the Exim group or of any group listed in the admin_groups configuration option. The current group does not have to be one of these groups. Admin users are permitted to list the queue, and to carry out certain operations on messages, for example, to force delivery failures. It is also necessary to be an admin user in order to see the full information provided by the Exim monitor, and full debugging output. By default, the use of the -M, -q, -R, and -S options to cause Exim to attempt delivery of messages on its queue is restricted to admin users. However, this restriction can be relaxed by setting the prod_requires_admin option false (that is, specifying no_prod_requires_admin). Similarly, the use of the -bp option to list all the messages in the queue is restricted to admin users unless queue_list_requires_admin is set false. Warning: If you configure your system so that admin users are able to edit Exim's configuration file, you are giving those users an easy way of getting root. There is further discussion of this issue at the start of chapter 6. 5.3 Command line options ------------------------ Exim's command line options are described in alphabetical order below. If none of the options that specifies a specific action (such as starting the daemon or a queue runner, or testing an address, or receiving a message in a specific format, or listing the queue) are present, and there is at least one argument on the command line, -bm (accept a local message on the standard input, with the arguments specifying the recipients) is assumed. Otherwise, Exim outputs a brief message about itself and exits. -- This is a pseudo-option whose only purpose is to terminate the options and therefore to cause subsequent command line items to be treated as arguments rather than options, even if they begin with hyphens. --help This option causes Exim to output a few sentences stating what it is. The same output is generated if the Exim binary is called with no options and no arguments. --version This option is an alias for -bV and causes version information to be displayed. -Ac, -Am These options are used by Sendmail for selecting configuration files and are ignored by Exim. -B This is a Sendmail option for selecting 7 or 8 bit processing. Exim is 8-bit clean; it ignores this option. -bd This option runs Exim as a daemon, awaiting incoming SMTP connections. Usually the -bd option is combined with the -q