The malware ACL condition lets you connect virus scanner software to Exim. It supports a “generic” interface to scanners called via the shell, and specialized interfaces for “daemon” type virus scanners, which are resident in memory and thus are much faster.

You can set the av_scanner option in first part of the Exim configuration file to specify which scanner to use, together with any additional options that are needed. The basic syntax is as follows:

av_scanner = <scanner-type>:<option1>:<option2>:[...]

If you do not set av_scanner, it defaults to

av_scanner = sophie:/var/run/sophie

If the value of av_scanner starts with dollar character, it is expanded before use.

The following scanner types are supported in this release:

The av_scanner option is expanded each time malware is called. This makes it possible to use different scanners. See further below for an example. The malware condition caches its results, so when you use it multiple times for the same message, the actual scanning process is only carried out once. However, using expandable items in av_scanner disables this caching, in which case each use of the malware condition causes a new scan of the message.

The malware condition takes a right-hand argument that is expanded before use. It can then be one of

You can append /defer_ok to the malware condition to accept messages even if there is a problem with the virus scanner.

When a virus is found, the condition sets up an expansion variable called $malware_name that contains the name of the virus. You can use it in a message modifier that specifies the error returned to the sender, and/or in logging data.

If your virus scanner cannot unpack MIME and TNEF containers itself, you should use the demime condition (see section 40.6) before the malware condition.

Here is a very simple scanning example:

deny message = This message contains malware ($malware_name)
   demime = *
   malware = *

The next example accepts messages when there is a problem with the scanner:

deny message = This message contains malware ($malware_name)
   demime = *
   malware = */defer_ok

The next example shows how to use an ACL variable to scan with both sophie and aveserver. It assumes you have set:

av_scanner = $acl_m0

in the main Exim configuration.

deny message = This message contains malware ($malware_name)
   set acl_m0 = sophie
   malware = *

deny message = This message contains malware ($malware_name)
   set acl_m0 = aveserver
   malware = *

The spam ACL condition calls SpamAssassin’s spamd daemon to get a spam score and a report for the message. You can get SpamAssassin at http://www.spamassassin.org, or, if you have a working Perl installation, you can use CPAN by running:

perl -MCPAN -e 'install Mail::SpamAssassin'

SpamAssassin has its own set of configuration files. Please review its documentation to see how you can tweak it. The default installation should work nicely, however.

After having installed and configured SpamAssassin, start the spamd daemon. By default, it listens on 127.0.0.1, TCP port 783. If you use another host or port for spamd, you must set the spamd_address option in the global part of the Exim configuration as follows (example):

spamd_address = 192.168.99.45 387

You do not need to set this option if you use the default. As of version 2.60, spamd also supports communication over UNIX sockets. If you want to use these, supply spamd_address with an absolute file name instead of a address/port pair:

spamd_address = /var/run/spamd_socket

You can have multiple spamd servers to improve scalability. These can reside on other hardware reachable over the network. To specify multiple spamd servers, put multiple address/port pairs in the spamd_address option, separated with colons:

spamd_address = 192.168.2.10 783 : \
                192.168.2.11 783 : \
                192.168.2.12 783

Up to 32 spamd servers are supported. The servers are queried in a random fashion. When a server fails to respond to the connection attempt, all other servers are tried until one succeeds. If no server responds, the spam condition defers.

Warning: It is not possible to use the UNIX socket connection method with multiple spamd servers.

Here is a simple example of the use of the spam condition in a DATA ACL:

deny message = This message was classified as SPAM
        spam = joe

The right-hand side of the spam condition specifies the username that SpamAssassin should scan for. If you do not want to scan for a particular user, but rather use the SpamAssassin system-wide default profile, you can scan for an unknown user, or simply use “nobody”. However, you must put something on the right-hand side.

The username allows you to use per-domain or per-user antispam profiles. The right-hand side is expanded before being used, so you can put lookups or conditions there. When the right-hand side evaluates to “0” or “false”, no scanning is done and the condition fails immediately.

deny message = This message was classified as SPAM
     condition = ${if < {$message_size}{10K}}
     spam = nobody

The spam condition returns true if the threshold specified in the user’s SpamAssassin profile has been matched or exceeded. If you want to use the spam condition for its side effects (see the variables below), you can make it always return “true” by appending :true to the username.

When the spam condition is run, it sets up the following expansion variables:

The spam condition caches its results. If you call it again with the same user name, it does not scan again, but rather returns the same values as before.

The spam condition returns DEFER if there is any error while running the message through SpamAssassin. If you want to treat DEFER as FAIL (to pass on to the next ACL statement block), append /defer_ok to the right-hand side of the spam condition, like this:

deny message = This message was classified as SPAM
     spam    = joe/defer_ok

This causes messages to be accepted even if there is a problem with spamd.

Here is a longer, commented example of the use of the spam condition:

# put headers in all messages (no matter if spam or not)
warn  message = X-Spam-Score: $spam_score ($spam_bar)
      spam = nobody:true
warn  message = X-Spam-Report: $spam_report
      spam = nobody:true

# add second subject line with *SPAM* marker when message
# is over threshold
warn  message = Subject: *SPAM* $h_Subject:
      spam = nobody

# reject spam at high scores (> 12)
deny   message = This message scored $spam_score spam points.
       spam = nobody:true
       condition = ${if >{$spam_score_int}{120}{1}{0}}

At the start of a MIME ACL, a number of variables are set from the header information for the relevant MIME part. These are described below. The contents of the MIME part are not by default decoded into a disk file except for MIME parts whose content-type is “message/rfc822”. If you want to decode a MIME part into a disk file, you can use the decode modifier. The general syntax is:

decode = [/<path>/]<filename>

The right hand side is expanded before use. After expansion, the value can be:

You can easily decode a file with its original, proposed filename using

decode = $mime_filename

However, you should keep in mind that $mime_filename might contain anything. If you place files outside of the default path, they are not automatically unlinked.

For RFC822 attachments (these are messages attached to messages, with a content-type of “message/rfc822”), the ACL is called again in the same manner as for the primary message, only that the $mime_is_rfc822 expansion variable is set (see below). Attached messages are always decoded to disk before being checked, and the files are unlinked once the check is done.

The MIME ACL supports the regex and mime_regex conditions. These can be used to match regular expressions against raw and decoded MIME parts, respectively. They are described in section 40.5.

The following list describes all expansion variables that are available in the MIME ACL:

$mime_filename:: This is perhaps the most important of the MIME variables. It contains a proposed filename for an attachment, if one was found in either the Content-Type: or Content-Disposition: headers. The filename will be RFC2047 decoded, but no additional sanity checks are done. If no filename was found, this variable contains the empty string.

You can specify your own custom regular expression matches on the full body of the message, or on individual MIME parts.

The regex condition takes one or more regular expressions as arguments and matches them against the full message (when called in the DATA ACL) or a raw MIME part (when called in the MIME ACL). The regex condition matches linewise, with a maximum line length of 32K characters. That means you cannot have multiline matches with the regex condition.

The mime_regex condition can be called only in the MIME ACL. It matches up to 32K of decoded content (the whole content at once, not linewise). If the part has not been decoded with the decode modifier earlier in the ACL, it is decoded automatically when mime_regex is executed (using default path and filename values). If the decoded data is larger than 32K, only the first 32K characters are checked.

The regular expressions are passed as a colon-separated list. To include a literal colon, you must double it. Since the whole right-hand side string is expanded before being used, you must also escape dollar signs and backslashes with more backslashes, or use the \N facility to disable expansion. Here is a simple example that contains two regular expressions:

deny message = contains blacklisted regex ($regex_match_string)
       regex = [Mm]ortgage : URGENT BUSINESS PROPOSAL

The conditions returns true if any one of the regular expressions matches. The $regex_match_string expansion variable is then set up and contains the matching regular expression.

Warning: With large messages, these conditions can be fairly CPU-intensive.

The demime ACL condition provides MIME unpacking, sanity checking and file extension blocking. It is usable only in the DATA and non-SMTP ACLs. The demime condition uses a simpler interface to MIME decoding than the MIME ACL functionality, but provides no additional facilities. Please note that this condition is deprecated and kept only for backward compatibility. You must set the WITH_OLD_DEMIME option in Local/Makefile at build time to be able to use the demime condition.

The demime condition unpacks MIME containers in the message. It detects errors in MIME containers and can match file extensions found in the message against a list. Using this facility produces files containing the unpacked MIME parts of the message in the temporary scan directory. If you do antivirus scanning, it is recommened that you use the demime condition before the antivirus (malware) condition.

On the right-hand side of the demime condition you can pass a colon-separated list of file extensions that it should match against. For example:

deny message = Found blacklisted file attachment
     demime  = vbs:com:bat:pif:prf:lnk

If one of the file extensions is found, the condition is true, otherwise it is false. If there is a temporary error while demimeing (for example, “disk full”), the condition defers, and the message is temporarily rejected (unless the condition is on a warn verb).

The right-hand side is expanded before being treated as a list, so you can have conditions and lookups there. If it expands to an empty string, “false”, or zero (“0”), no demimeing is done and the condition is false.

The demime condition set the following variables:

$found_extension:: When the demime condition is true, this variable contains the file extension it found.

Both $demime_errorlevel and $demime_reason are set by the first call of the demime condition, and are not changed on subsequent calls.

If you do not want to check for file extensions, but rather use the demime condition for unpacking or error checking purposes, pass “*” as the right-hand side value. Here is a more elaborate example of how to use this facility:

# Reject messages with serious MIME container errors
deny  message = Found MIME error ($demime_reason).
      demime = *
      condition = ${if >{$demime_errorlevel}{2}{1}{0}}

# Reject known virus spreading file extensions.
# Accepting these is pretty much braindead.
deny  message = contains $found_extension file (blacklisted).
      demime  = com:vbs:bat:pif:scr

# Freeze .exe and .doc files. Postmaster can
# examine them and eventually thaw them.
deny  log_message = Another $found_extension file.
      demime = exe:doc
      control = freeze