GNU General Public License (GPL)
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Snort is an open source network intrusion detection system, capable of performing 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 uses a flexible rules language to describe traffic that it should collect or pass, as well as a detection engine that utilizes a modular plugin architecture. Snort has a real-time alerting capability as well, incorporating alerting mechanisms for syslog, a user specified file, a UNIX socket, or WinPopup messages to Windows clients using Samba's smb client.
Snort has three primary uses. It can be used as a straight packet sniffer like tcp dump(1), a packet logger (useful for network traffic debugging, etc), or as a full blown network intrusion detection system.
There are three main modes in which Snort can be configured: sniffer, packet logger, and network intrusion detection system. Sniffer mode simply reads the packets off of the network and displays them for you in a continuous stream on the console. Packet logger mode logs the packets to the disk. Network intrusion detection mode is the most complex and configurable configuration, allowing Snort to analyze network traffic for matches against a user defined rule set and perform several actions based upon what it sees.