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

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