SNORT

Snort is a network intrusion detector created in 1998 by Martin Roesch.

Often used as a probe, it also has an active mode that allows it to block any suspicious traffic when installed on routing equipment. In other words, it is a Network Intrusion Detection System (NIDS) that performs real-time traffic analyses on a given network segment.

Although the engine is distributed under the GPL, it is of no use without a rule base, which does entail a paid subscription with the vendor. Nonetheless, the rules are available free of charge after a period of 30 days.