A host-based intrusion detection system (HIDS) is a system that monitors a host on which it is installed to detect an intrusion and/or misuse, and responds by logging the activity and notifying the system administrators.
Defend against a live attack
Install HIDS right after installing the system.
Tripwire creates a database of information related to your system, then compares that to what it finds when ran regularly, which it should, in order to get some real use out of it.
Rkhunter is a Unix-based tool that scans for rootkits, backdoors and possible local exploits.
Samhain provides file integrity checking, log file monitoring/analysis, rootkit detection, port monitoring, detection of rogue SUID executables, and hidden processes. Like having tripwire and rkhunter rolled into one. <=