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Network Intrusion Detection Signatures, Part Two

This is the second in a series of articles on understanding and developing signatures for network intrusion detection systems. In the first installment we looked at signature basics, the functions that signatures serve, header values, signature components, and choosing signatures. In this article we will continue our discussion of IP protocol header values in signatures by closely examining some signature examples. Although it may be relatively easy to develop a signature that matches a particular type of traffic, it will likely cause unexpected false positives and false negatives. Signatures must be carefully developed and tested in order to create a signature set that is highly accurate, yet is also as efficient as possible.

Evaluating the Effectiveness of a Signature

In the previous article, we looked at the characteristics of the packets sent by the synscan tool (as implemented in the Ramen worm) and identified traits that were unusual or suspicious, or that violated standards. We then tried to determine which of these traits might make a good signature. Based on those characteristics, let?s create a signature that will look for all three of the following attributes in each TCP packet:

* Only the SYN and FIN flags set
* IP identification number 39426
* TCP window size of 1028 Network Intrusion Detection Signatures, Part Two

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