Snort is an open source network intrusion detection system (NIDS) created by Martin Roesch. Snort is a packet sniffer that monitors network traffic in real time, scrutinizing each packet closely to detect a dangerous payload or suspicious anomalies.
Snort is based on libpcap (for library packet capture), a tool that is widely used in TCP/IP traffic sniffers and analyzers. Through protocol analysis and content searching and matching, Snort detects attack methods, including denial of service, buffer overflow, CGI attacks, stealth port scans, and SMB probes. When suspicious behavior is detected, Snort sends a real-time alert to syslog, a separate 'alerts' file, or to a pop-up window.
NSS Group, a European network security testing organization, tested Snort along with intrusion detection system (IDS) products from 15 major vendors including Cisco, Computer Associates, and Symantec. According to NSS, Snort, which was the sole open source freeware product tested, clearly out-performed the proprietary products.