Snort is the most widely deployed open-source intrusion detection system, capable of performing real-time traffic analysis and packet logging on IP networks. Properly configured, Snort acts as an early warning system that alerts administrators to malicious activity before significant damage occurs.
Writing Custom Snort Rules
Snort rules consist of a rule header defining the action, protocol, source, destination, and direction, followed by rule options that specify detection criteria. Understanding rule syntax allows administrators to create custom signatures for threats specific to their environment, such as detecting access to restricted internal resources or identifying proprietary data exfiltration attempts.
Position your Snort sensor on a network tap or mirror port to see all traffic without being in the data path. Running Snort inline as an IPS (Intrusion Prevention System) adds the ability to block malicious traffic, but requires careful tuning to avoid dropping legitimate connections. Start in IDS mode and promote to IPS only after thorough testing.
Update your Snort rulesets regularly using oinkmaster or PulledPork to maintain protection against new threats. Subscribe to the Sourcefire VRT ruleset for the most current signatures. Tune your rules to suppress false positives by creating suppression entries for known benign traffic patterns, as excessive false alerts lead to alert fatigue and missed real incidents.