Samhain is an integrity checker and host intrusion detection system that can be used on single hosts as well as large, UNIX-based networks. It supports central monitoring as well as powerful (and new) stealth features to run undetected in memory, using steganography.
Main features
Complete integrity check
uses cryptographic checksums of files to detect modifications, can find rogue SUID executables anywhere on a disk, and Centralized monitoring native support for logging to a central server via encrypted and authenticated connections Tamper resistance database and configuration files can be signed log file entries and e-mail reports are signed support for stealth operation