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Intrusion Detection Systems with Snort: Advanced IDS Techniques Using Snort, Apache, MySQL, PHP, and ACID by Rafeeq Ur Rehman

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4.1. Preprocessors

When a packet is received by Snort, it may not be ready for processing by the main Snort detection engine and application of Snort rules. For example, a packet may be fragmented. Before you can search a string within the packet or determine its exact size, you need to defragment it by assembling all fragments of the data packet. The job of a preprocessor is to make a packet suitable for the detection engine to apply different rules to it. In addition, some preprocessors are used for other tasks such as detection of anomalies and obvious errors in data packets. A detailed description of available preprocessors will show how they work.

During the installation process, you can compile support of different preprocessors into Snort. ...

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