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       courierperlfilter - Sample Perl-based mail filter


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

       perlfilter requires Perl 5.004 or higher. The best way to create a Perl
       filter is to start with the sample filter,
       /usr/lib/courier/ 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 courierfilter start to start
       the mail filtering subsystem).

   Setting up a Perl script
       Most of the ugly details of connecting the Perl script to Courier´s
       mail filtering engine is taken care of by the sample 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).


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


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

           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.

           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.

           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.

       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.




        1. courierfilter(8)
           [set $man.base.url.for.relative.links]/courierfilter.html