Snort is an open source intrusion prevention system offered by Cisco. It is capable of real-time traffic analysis and packet logging on IP networks. It can perform protocol analysis, content searching/matching, and can be used to detect a variety of attacks and probes, such as buffer overflows, stealth port scans, CGI attacks, SMB probes, OS fingerprinting attempts, and much more.
Snort can be used as a packet sniffer like tcpdump, a packet logger (useful for network traffic debugging, etc), network file logging device (capturing files in realtime from network traffic), or as a full blown network intrusion prevention system. The mission for Snort is to deliver the most effective and comprehensive real-time network defense solutions on the planet.
Talos authors the official Snort Subscriber Rule Set.