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       dupfilter - Sample Courier mail filter


       filterctl [[start] | [stop]] [dupfilter]


       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.


       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.


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

           This file contains a single numerical value that sets the size of
           dupfilter´s circular FIFO queue.

           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.

           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.

           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.




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