Sentry from the Abacus Project

Description: Port scan detector.

Security Relevance: Sentry detects port scans. That is, when attackers use scanners to probe your system, Sentry will notify you. Sentry offers stealth-scanning detection, too, which is something not widely available. (Stealth scans are where attackers tread lightly, using difficult-to-detect, half-open connections.) Learn more in Chapter 20, "Intrusion Detection."