1.2 Testing a new filter file

Filter files, especially the more complicated ones, should always be tested, as it is easy to make mistakes. Exim provides a facility for preliminary testing of a filter file before installing it. This tests the syntax of the file and its basic operation, and can also be used with ordinary `.forward' Exim Overview

`The Exim Mail Transport Agent`

Table of Contents Exim Overview Background Availability Limitations Main features Performance Interface Method of operation Mail filtering Directors Routers Transports Exim logs Exim databases SMTP batching Retries Header rewriting Host verification SMTP port reservation Control of relaying Sender verification Sender lock out Receiver verification The `percent hack' Security The Exim Monitor Exim Overview Date: 10 July 1997 Exim is a mail transport agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet. It is freely available under the terms of the GNU General Public Licence. In style it is similar to Smail 3, but its facilities are more extensive, and in particular it has some defences against mail bombs and unsolicited junk mail, in the form of options for refusing messages from particular hosts, networks, or senders. Exim is in production use on a number of sites that move tens of thousands of messages per day. This document contains an overview description of the way Exim works, with a certain amount of omission and simplification to keep it fairly short. Please address any enquiries about Exim to Philip Hazel: Email: <ph10@cus.cam.ac.uk> Phone: +44 1223 334714 Fax: +44 1223 334679 University of Cambridge Computer Laboratory Pembroke Street Cambridge CB2 3QG United Kingdom This document is copyright (c) University of Cambridge 1997, but copying permission is granted to all. ------------------------------------------------------------------------- "If I have seen further it is by standing on the shoulders of giants." (Isaac Newton) Background 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 mailer. Many of the ideas and configuration interfaces are taken from Smail 3, though the actual code of Exim is entirely new. My intention was to write a mailer that had more functionality than Smail 3, but which retained the simple lightweight approach, as this seemed to me to be all that was needed for systems directly connected to the Internet, where most messages are delivered almost immediately. Availability The current distribution of Exim is available from $st{ftp://ftp.cus.cam.ac.uk/pub/software/programs/exim/exim-$si{n.nn}.tar.gz} where n.nn is the version number. The distribution contains an ASCII copy of the documentation; other formats are available from $st{ftp://ftp.cus.cam.ac.uk/pub/software/programs/exim/exim-postscript-$si{n.nn}.tar.gz} $st{ftp://ftp.cus.cam.ac.uk/pub/software/programs/exim/exim-texinfo-$si{n.nn}.tar.gz} The following operating systems are currently supported: AIX, BSDI, DGUX, FreeBSD, HI-OSF (Hitachi), HP-UX, IRIX, Linux, MIPS RISCOS, NetBSD, OpenBSD, DEC OSF1 (aka Digital UNIX), SCO, SCO SVR4.2 (aka UNIX-SV), SunOS4, SunOS5, Ultrix, and Unixware. Limitations For the benefit of those reading this overview to see whether Exim is of interest to them, its limitations are listed first. Exim is written in ANSI C. This should not be much of a limitation these days. However, to help with systems that lack a true ANSI C library, Exim avoids making any use of the value returned by the `sprintf()' function, which is one of the main incompatibilities. It has its own version of `strerror()' for use with SunOS4 and any other system that lacks this function, and a macro can be defined to turn `memmove()' into `bcopy()' if necessary. Exim uses file names that are longer than 14 characters. Exim is intended for use as an Internet mailer, and therefore handles addresses in RFC 822 domain format only. It cannot handle `bang paths', though simple two-component bang paths can be converted by a straightforward rewriting configuration. 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. The only external transport currently implemented is an SMTP transport over a TCP/IP network (using sockets), suitable for machines on the Internet. However, a pipe transport is available, and there are facilities for writing messages to files in `batched SMTP' format; both of these can be used to send messages to some other transport mechanism such as UUCP. Batched SMTP input is also catered for. Main features Exim follows the same general approach of decentralized control that Smail 3 does. There is no central process doing overall management of mail delivery. However, unlike Smail, the independent delivery processes share data in the form of `hints', which makes delivery more efficient in some cases. The hints are kept in a number of DBM files. If any of these files are lost, the only effect is to change the pattern of delivery attempts and retries. Here is a summary of Exim's main features. More details are given in the sections which follow. Many configuration options can be given as expansion strings, and as these can include file lookups, much of Exim's operation can be made table-driven if desired. For example, it is possible to do local delivery on a machine on which the users do not have accounts. Regular expressions are available i Exim 4.00 Specification chapter 11 Previous Next Contents (Exim 4.00 Specification) 11. String expansions Many strings that are given in Exim's run time configuration are expanded before use. Some of them are expanded every time they are used; others are expanded only once. When a string is being expanded it is copied verbatim from left to right except when a dollar or backslash character is encountered. A dollar specifies the start of a portion of the string which is interpreted and replaced as described below in section 11.4 onwards. 11.1. Literal text in expanded strings An uninterpreted dollar can be included in an expanded string by putting a backslash in front of it. A backslash can be used to prevent any special character being treated specially in an expansion, including itself. If the string appears in quotes in the configuration file, two backslashes are required because the quotes themselves cause interpretation of backslashes when the string is read in. An entire portion of the string can specified as non-expandable by placing it between two occurrences of \N. This is particularly useful for protecting regular expressions, which often contain backslashes and dollar signs. For example: deny senders = \N^\d{8}[a-z]@some\.site\.example$\N On encountering the first \N, the expander copies subsequent characters without interpretation until it reaches the next \N or the end of the string. 11.2. Character escape sequences in expanded strings A backslash followed by one of the letters ``n'', ``r'', or ``t'' in an expanded string is recognized as an escape sequence for the character newline, carriage return, or tab, respectively. A backslash followed by up to three octal digits is recognized as an octal encoding for a single character, and a backslash followed by ``x'' and up to two hexadecimal digits is a hexadecimal encoding. These escape sequences are also recognized in quoted strings when they are read in. Their interpretation in expansions as well is useful for unquoted strings, and for other cases such as looked-up strings that are then expanded. 11.3. Testing string expansions Many expansions can be tested by calling Exim with the -be option. This takes the command arguments, or lines from the standard input if there are no arguments, runs them through the string expansion code, and writes the results to the standard output. Variables based on configuration values are set up, but since no message is being processed, variables such as $local_part have no value. Nevertheless the -be option can be useful for checking out file and database lookups, and the use of expansion operators such as substr and hash. Exim gives up its root privilege when it is called with the -be option, and instead runs under the uid and gid it was called with, to prevent users from using -be for reading files to which they normally do not have access. 11.4. Expansion items The following items are recognized in expanded strings. White space may be used between sub-items that are keywords or substrings enclosed in braces inside an outer set of braces, to improve readability. Within braces, however, white space is significant. $<variable name> or ${ The Exim FAQ The Exim FAQ This is the FAQ for the Exim Mail Transfer Agent. Many thanks to the many people who provided the original information. This file would be amazingly cluttered if I tried to list them all. Suggestions for corrections, improvements, and additions are always welcome. This version of the FAQ applies to Exim 4.00 and later releases. It has been extensively revised, and material that was relevant only to earlier releases has been removed. As this caused some whole sections to disappear, I've taken the opportunity to re-arrange the sections and renumber everything except the configuration samples. References of the form Cnnn, Fnnn, and Lnnn are to the sample configuration, filter, and local_scan() files. These are hyperlinked from the HTML version of this FAQ. They can also be found in the separately distributed directory called config.samples. The primary location is ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/exim4/config.samples.tar.gz ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/exim4/config.samples.tar.bz2 There are brief descriptions of these files at the end of this document. Philip Hazel <ph10@cus.cam.ac.uk> Last update: 16-July-2002 Index A Keyword-in-context index to the questions is available. Contents The FAQ is divided into the following sections: 0. General debugging 14. Dial-up and ISDN 1. Building and installing 15. UUCP 2. Routing in general 16. Modifying message bodies 3. Routing to remote hosts 17. Encryption (TLS/SSL) 4. Routing for local delivery 20. Millennium 5. Filtering 50. Miscellaneous 6. Delivery 92. Freebsd 7. Policy controls 93. HP-UX 8. Rewriting addresses 94. BSDI 9. Headers 95. IRIX 10. Performance 96. Linux 11. Majordomo 97. Sun systems 12. Fetchmail 98. Configuration cookbook 13. Perl 99. List of sample configurations List of questions 0. GENERAL DEBUGGING Q0001: Exim is crashing. What is wrong? Q0002: Exim is not working. What is wrong? How can I check what it is doing? Q0003: What does the error Child process of address_pipe transport returned 69 from command xxx mean? Q0004: My virtual domain setup isn't working. How can I debug it? Q0005: Why is Exim not rejecting incoming messages addressed to non-existent users at SMTP time? Q0006: I've put an entry for *.my.domain in a DBM lookup file, but it isn't getting recognized. Q0007: I've put the entry *@domain.com in a lookup database, but it isn't working. The expansion I'm using is: Q0008: If I run ./exim -d -bt user@domain all seems well, but when I send a message from my User Agent, it does not arrive at its destination. Q0009: What does no immediate delivery: too many messages received in one SMTP connection mean? Q0010: Exim puts for <address> in the Received: headers of some, but not all, messages. Is this a bug? Q0011: Instead of exim_dbmbuild, I'm using a homegrown program to build DBM (or cdb) files, but Exim doesn't seem to be able to use them. Q0012: Exim is unable to route to any remote domains. It doesn't seen to be able to access the DNS. Q0013: What does the error message transport system_aliases: cannot find transport driver "redirect" in line 92 mean? Q0014: Exim is timing out after receiving and responding to the DATA command from one particular host, and yet the client host also claims to be timing out. This seems to affect only certain messages. Q0015: What does the message Socket bind() to port 25 for address (any) failed: address already in use mean? Q0016: I've set verify = header_syntax in my ACL, but this causes Exim to complain about header lines like To: Work: Jim <jims@email>, Home: Bob <bobs@email> which look all right to me. Is this a bug? Q0017: Whenever Exim tries to deliver a specific message to a particular server, it fails, giving the error Remote end closed connection after data or Broken pipe or a timeout. What's going on? Q0018: Why do messages not get delivered down the same connection when I do something like: exim -v -R @aol.com? For other domains, I do this and I see the appropriate waiting for passed connections to get used messages. Q0019: There seems to be a problem in the string expansion code: it doesn't recognize references to headers such as ${h_to}. Q0020: Why do connections to my machine's SMTP port take a long time to respond with the banner, when connections to other ports respond instantly? It sometimes takes 30 seconds to respond. Q0021: What does failed to create child process to send failure message mean? This is a busy mail server with smtp_accept_max set to 500, but this problem started to occur at about 300 incoming connections. Q0022: What does No transport set by system filter in a log line mean? Q0023: Why is Exim refusing to relay, saying failed to find host name from IP address when I have the sender's IP address in an ACL condition? My configuration contains this ACL statement: Q0024: When I run exim -bd -q10m I get PANIC LOG: exec of exim -q failed. Q0025: I can't seem to get a pipe command to run when I include a ${if expansion in it. This fails: Q0026: I'm trying to get Exim to connect an alias to a pipe, but it always gives error code 69, with the comment (could mean service or program unavailable). Q0027: What does the error Spool file is locked mean? Q0028: Exim is reporting IP addresses as 0.0.0.0 or 255.255.255.255 instead of their correct values. What's going on? Q0029: I can't seem to figure out why PAM support doesn't work correctly. Q0030: I'm trying to use a query-style lookup for hosts that are allowed to relay, but it is giving really weird errors. Q0031: Exim is rejecting connections from hosts that have more than one IP address, for no apparent reason. Q0032: Exim is failing to find the MySQL library, even though is it present within LD_LIBRARY_PATH. I'm getting this error: Q0033: What does the error lookup of host "xx.xx.xx" failed in yyy router mean? Q0034: Exim works fine on one host, but when I copied the binary to another identical host, it stopped working (it could not resolve DNS names). Q0035: I set a hosts condition in an ACL to do a lookup in a file of IP addresses, but it doesn't work. Q0036: Why do I get the error Permission denied: creating lock file hitching post when Exim tries to do a local delivery?