mod_evasive is an evasive maneuvers module for Apache to provide evasive 
action in the event of an HTTP DoS or DDoS attack or brute force attack.
It is also designed to be a detection and network management tool, and can be
easily configured to talk to ipchains, firewalls, routers, and etcetera.
mod_evasive presently reports abuses via email and syslog facilities.

Detection is performed by creating an internal dynamic hash table of IP
Addresses and URIs, and denying any single IP address from any of the
following:

    * Requesting the same page more than a few times per second
    * Making more than 50 concurrent requests on the same child per second
    * Making any requests while temporarily blacklisted (on a blocking list) 

This method has worked well in both single-server script attacks as well as
distributed attacks, but just like other evasive tools, is only as useful to
the point of bandwidth and processor consumption (e.g. the amount of bandwidth
and processor required to receive/process/respond to invalid requests), which
is why it's a good idea to integrate this with your firewalls and routers for
maximum protection.

This module instantiates for each listener individually, and therefore has a
built-in cleanup mechanism and scaling capabilities. Because of this per-child
design, legitimate requests are never compromised (even from proxies and NAT
addresses) but only scripted attacks. Even a user repeatedly clicking on
'reload' should not be affected unless they do it maliciously. mod_dosevasive
is fully tweakable through the Apache configuration file, easy to incorporate
into your web server, and easy to use. 

WWW: http://www.zdziarski.com/projects/mod_evasive/