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

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