dupfilter(8) [debian man page]
DUPFILTER(8) Double Precision, Inc. DUPFILTER(8) NAME
dupfilter - Sample Courier mail filter SYNOPSIS
filterctl [[start] | [stop]] [dupfilter] DESCRIPTION
This is an example global mail filter written in C. This is a threaded filter that tries to block junk E-mail by attempting to detect multiple copies of the same message, which are rejected. This filter is presented mostly for educational purposes. It's method for detecting duplicate messages is rather simplistic, and, over time, ways of defeating it will certainly proliferate. dupfilter works by calculating a hash value of the contents of every message it sees. Hash values of recent messages are kept in a circular FIFO queue. When dupfilter starts seeing messages with the same hash value, it will reject them. The hash value is calculated in such a way as to try to detect minor changes to messages' contents that are designed to avoid these kinds of filters, but it's still not a very sophisticated approach, and it can be easily fooled. BUGS
This is not a very smart mail filter. The hash function is very simple and is not expected to be very useful. Don't even think of using dupfilter on a machine that either runs a mailing list, or contains mailboxes that subscribe to mailing lists. dupfilter likes to bounce mailing list traffic. FILES
dupfilter 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/dupfilter-hashsize This file contains a single numerical value that sets the size of dupfilter's circular FIFO queue. /etc/courier/filters/dupfilter-duplevel This file contains a single numerical value that sets how many identical messages dupfilter will tolerate before blocking them. When this many identical messages are seen, dupfilter will start rejecting all messages with the same content. /etc/courier/filters/dupfilter-mode If this file exists and contains the word "all", dupfilter 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/dupfilter-nthreads This file contains a single numerical value that sets the number of threads created (each thread is used to calculate a message's hash value). The default number of threads is 4. SEE ALSO
courierfilter(8)[1]. AUTHOR
Sam Varshavchik Author NOTES
1. courierfilter(8) [set $man.base.url.for.relative.links]/courierfilter.html Courier Mail Server 04/04/2011 DUPFILTER(8)
Check Out this Related Man Page
COURIERPERLFILTER(8) Double Precision, Inc. COURIERPERLFILTER(8) NAME
courierperlfilter - Sample Perl-based mail filter SYNOPSIS
filterctl [[start] | [stop]] [perlfilter] DESCRIPTION
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/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 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 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). CREDITS
A lot of the Perl glue code is based on examples from the perlembed manual page, and other sources. FILES
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. 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. SEE ALSO
courierfilter(8)[1]. AUTHOR
Sam Varshavchik Author NOTES
1. courierfilter(8) [set $man.base.url.for.relative.links]/courierfilter.html Courier Mail Server 04/04/2011 COURIERPERLFILTER(8)