đź’€ Monitors a mail server log file and uses a firewall to temporarily block or redirect incoming packets from hosts that display spammer-like behavior. Unmaintained.
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README

= DenySpam

Monitors a mail server log file and uses Packet Filter or another firewall to
temporarily block incoming packets from hosts that display spammer-like
behavior.

Version::   1.0.0
Author::    Ryan Grove (mailto:ryan@wonko.com)
Copyright:: Copyright (c) 2007 Ryan Grove
License::   DenySpam is open source software distributed under the terms of
            the New BSD License.
Website::   http://wonko.com/software/denyspam

=== Dependencies

* Ruby[http://ruby-lang.org/] 1.8.5+
* Some form of mail server. Procmail is supported by default.
* Some form of firewall. Packet Filter is supported by default.

=== How does it work?

A simple and easily customizable regexp-based ruleset assigns a running score
to each host based on the log entries it generates. A host with a negative
score is probably not a spammer, while a host with a positive score probably
is.

To avoid blocking legitimate mail, each host is assigned a low initial
score (the default is -10, but the number can be changed in
<tt>denyspam.conf</tt>). In order to be considered a spammer, a host would
have to display enough bad behavior to accumulate 11 points (for a total
score of 1).

=== What constitutes spammer-like behavior?

Spammers tend to use stupid, badly-written mailers that display easily
recognizable behavior.

For example, spammers often connect to a mail server and attempt to send
messages to huge numbers of nonexistent users in the hope that a few of the
randomly-chosen usernames actually exist. Legitimate mailers may occasionally
attempt to send mail to a nonexistent user, but almost never more than one
at a time.

DenySpam would assign the connecting host one point for each attempt to send
mail to a nonexistent user. A spammer would quickly accumulate enough points
to reach a positive score, causing DenySpam to tell PF to block packets from
that host for a certain amount of time (ten minutes per positive point by
default).

=== Isn't it possible that DenySpam could block a legitimate host?

It's possible, but not likely. DenySpam's default configuration is
conservative, yet effective.

Since all hosts start with negative points by default, it's unlikely that
a legitimate host would accumulate a positive score. In addition, DenySpam
rewards good behavior (like successfully delivering an email without
generating any errors) with negative points. This way, legitimate hosts will
quickly accumulate very low scores, making it even less likely that a false
positive will occur.

Nevertheless, if a legitimate host is actually quirky enough to get itself
blocked, the mail will most likely only be delayed. Almost all legitimate
mail servers will continue trying to send a message for up to four days
before giving up. Since DenySpam only blocks hosts for 10 minutes per point
by default, it's extremely unlikely that a legitimate mailer will dig itself
a hole so deep that DenySpam will block it for four entire days.

In other words, with DenySpam's default settings, you probably won't lose
any legitimate mail unless the sender behaves so badly that it accumulates a
score of 576. A score that high would be a pretty big achievement even for
a spammer!