courierperlfilter - Sample Perl-based mail filter
[[start] | [stop]] [perlfilter]
This is an example global mail filter that uses an embedded Perl script.
"Embedded" means that the Perl interpreter is loaded once, and the
same Perl code is repeatedly called to accept or reject incoming messages, one
by one. Perl filtering is relatively time consuming (compared to filtering in
C or C++), and excessive delays in mail filters result in incoming mail being
deferred (rejected with a temporary error code). Therefore the
wrapper can create multiple perlfilter
that multiple processes are used to filter incoming mail.
requires Perl 5.10 or higher. The best way to create a Perl
filter is to start with the sample filter,
/usr/lib/courier/perlfilter-example.pl. This filter reject messages that
contain an excessively long Date: header (designed to crash certain
poorly-written mail clients). Use it as a basis for writing your own filter.
You can install your filter in any convenient location, then initialize the
/etc/courier/filters/perlfilter configuration file, as described below. Run
filterctl start perlfilter
to activate filtering (if necessary, run
to start the mail filtering subsystem).
Most of the ugly details of connecting the Perl script to Courier's mail
filtering engine is taken care of by the sample perlfilter-example.pl script.
One big no-no: the script MAY NOT change the current directory. Anything else
goes, for the most part. Loading other modules and classes, pretty much
anything else you can do with Perl, is allowed.
The Perl script, just like any other mail filtering module, receives a pointer
to a data file and one or more control files, each time a message is submitted
to Courier for delivery. The sample script calls the filterdata
function to process the data file. The data file contains the actual message.
() function is called to process each control file.
The control file contains recipient and message metadata. There may be more
than one control file for each message. The example script includes an
implementation of filterdata
() that blocks messages with corrupted
headers. The example script doesn't do anything interesting with
() and filtercontrol
() must return an empty string if no
serious objections are raised for this message. Any other return string is
interpreted as an SMTP-style error code that is used to reject the message.
Care must be taken that any error messages are formatted strictly according to
the format of SMTP error messages (even though the message may not actually
come in via SMTP).
A lot of the Perl glue code is based on examples from the perlembed manual page,
and other sources.
uses the following configuration files. Changes to the
following files do not take effect until the filter has been stopped and
If this file exists and contains the word
"all", perlfilter will create its socket in
/var/lib/courier/allfilters, otherwise the socket will be created in
/var/lib/courier/filters, see courierfilter(8) for more
This file contains a number that sets how many
perlfilter processes are created. The default is 5 processes. There's
always an extra perlfilter process that's used to clean up crashed
This file MUST exist and it must contain a
single line of text with the filename of the Perl script to load.
This is a sample Perl script of the kind that
/etc/courier/filters/perlfilter points to. Use it as an example of writing
your own Perl filters.
This is a complete Perl-based filter than
implements basic rate-limiting features.
Please exercise good judgment in writing Perl-based filters. They should be
reasonably fast, and do not allocate megabytes of memory. They should not be
very promiscuous in creating global Perl variables, and should clean up after
themselves. The current Perl wrapper does not destroy the Perl symbol table
after each call to the filter script. However, do not take that for granted.
This may change in the future.