courierperlfilter - Sample Perl-based mail filter

SamVarshavchik
           Author

       A lot of the Perl glue code is based on examples from the perlembed manual page, and other sources.

       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 perlfilter wrapper can create multiple perlfilter processes, so that multiple
       processes are used to filter incoming mail.

       perlfilter 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 filterctlstartperlfilter to
       activate filtering (if necessary, run courierfilterstart to start the mail filtering subsystem).

   SettingupaPerlscript
       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. The
       filtercontrol() 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 filtercontrol().

       filterdata() 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).

perlfilter uses the following configuration files. Changes to the following files do not take effect
       until the filter has been stopped and restarted.

       /etc/courier/filters/perlfilter-mode
           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)[1] for more information.

       /etc/courier/filters/perlfilter-numprocs
           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 child
           processes.

       /etc/courier/filters/perlfilter
           This file MUST exist and it must contain a single line of text with the filename of the Perl script
           to load.

       /usr/lib/courier/perlfilter-example.pl
           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.

       /usr/lib/courier/perlfilter-ratelimit.pl
           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.

       courierperlfilter - Sample Perl-based mail filter

        1. courierfilter(8)
           http://www.courier-mta.org/courierfilter.html

Courier Mail Server                                01/22/2022                               COURIERPERLFILTER(8)

courierfilter(8)[1].

filterctl [[start] | [stop]] [perlfilter]