Protocol-based intrusion detection system
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A protocol-based intrusion detection system (PIDS) is an intrusion detection system which is typically installed on a web server, and is used in the monitoring and analysis of the protocol in use by the computing system. A PIDS will monitor the dynamic behavior and state of the protocol and will typically consist of a system or agent that would typically sit at the front end of a server, monitoring and analyzing the communication between a connected device and the system it is protecting.
A typical use for a PIDS would be at the front end of a web server monitoring the HTTP (or HTTPS) stream. Because it understands the HTTP relative to the web server/system it is trying to protect it can offer greater protection than less in-depth techniques such as filtering by IP address or port number alone, however this greater protection comes at the cost of increased computing on the web server.
Monitoring dynamic behavior
At a basic level a PIDS would look for, and enforce, the correct use of the protocol.
At a more advanced level the PIDS can learn or be taught acceptable constructs of the protocol, and thus better detect anomalous behavior.