The OSSEC free intrusion detection and host-based intrusion prevention system detects and fixes security problems in real time at the operating system level with functions such as log analysis, file integrity checks, Windows registry monitoring, and rootkit detection. It can be deployed virtually anywhere and supports the Linux, Windows, and macOS platforms.

Security Onion offers a comprehensive security suite for intrusion detection that involves surprisingly little work.

Does OPNids combine the Suricata IDS with machine learning to detect attack threats automatically, as advertised?