Procmail and Filter Information
Submitted by Peggy Yetter
Already existing on some campus mail servers is mail filtering software
which can be configured to catch some spam mail. This software was not
written explicitly to manage spam and how effective this kind of mail
filtering would be will depend greatly on the filtering rules available
in
the software and the expertise of the user.
Two such programs are:
1. Filter - http://www.bolthole.com/filter/
2. Procmail - http://www.procmail.org/
Both Filter and Procmail use rulesets that examine an incoming message
and
have a set of actions that will be called if the message matches any of
the
rules. Actions include moving the message to a file other than the default
mailbox, deleting the message, copying the message, forwarding the message,
or executing another
command.
Filter, once part of the Elm mail client package, is now being developed
separately. This package is easy to use since it uses English commands but
the ruleset is quite small and Filter only examines certain fields within
a
message header. Filter cannot act on the content of the body of the
message. This is quite a limitation since spammers have learned how to
constantly change the
mail headers to defeat the filtering rules that can be applied by
Filter.
Procmail is a more powerful piece of software. It can examine the entire
message and can be installed on a system with system-wide rules that would
be applied to all mail coming into the server. The disadvantage to Procmail
is that the user must learn the syntax for the Procmail rulesets. The
Procmail recipes are not written in English like Filter and the syntax can
be quite complicated. The potential to make a mistake in creating a recipe
is high and mail can be easily lost.
Either mail filtering program can simulate whitelists and/or blacklists.
With either of these mail filtering programs the user must be
the barometer on what constitutes spam, constantly maintaining the ruleset
as spam evolves. There is no heuristic mechanisms built into mail filtering
software that can help the user.
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