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Re: Using NBAR to block Nimda
From: Daniel Senie <dts () senie com>
Date: Wed, 19 Sep 2001 23:29:10 -0400

At 10:25 PM 9/19/01, Randy Benn wrote:

The basics of using NBAR as an IDS can be found here:

The page above is specifically for Code Red, but the same technique can be
used for blocking many different exploits.  Just modify the class map as you
like to block Nimda or anything else.

I'm presently running using the policy map config example, and having some real problems. While the traffic is no longer getting to the servers, the servers wind up with massive quantities of open TCP sessions. These take long enough to die that Apache winds up maxing out on processes. Two possible alternative approaches that I'd like to explore:

1. Some mechanism that builds on the present stuff, but sends a TCP RST off to the web server to get the TCP session terminated.

2. Alternative approach: use the timed access lists to place a temporary filter rule into the input filter for any IP address which matches on URL. This would protect the servers better, in that it'd block the TCP connections (after the first one) from a server entirely. This wasn't an issue really for CodeRed, but is a major issue for nimda, since it opens many connections.

If anyone has insight on how to implement either of these, I'd like to hear about it.

Daniel Senie                                        dts () senie com
Amaranth Networks Inc.                    http://www.amaranth.com

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