Snort

Snort is a lightweight 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 smbclient.

Snort has three primary uses. 

• It can be used as a straight packet sniffer like tcpdump(1), 

• a packet logger (useful for network traffic debugging, etc), 

or as a full blown network intrusion detection system.

Snort logs packets in either tcpdump(1) binary format or in Snort's decoded ASCII format to logging directories that are named based on the IP address of the "foreign" host 

Snort should work any place where libpcap is installed.

Reference:

Snort home.