snort

Snort is a libpcap-based packet sniffer/logger which can be used as a lightweight network intrusion detection system. It features rules-based logging and can perform content searching/matching in addition to detecting a variety of other attacks and probes, such as buffer overflows, stealth port scans, CGI attacks, SMB probes, and much more. Snort has a real-time alerting capability, with alerts being sent to syslog, a separate “alert” file, or even to a Windows computer via Samba.

Only use this tool against systems you own or are explicitly authorized to test — see the disclaimer.