There are open source IDS products out there. If you need a Host IDS, or HIDS, then there is Daniel Cid's excellent OSSEC product. If you need a Network IDS, or NIDS, then the obvious choice is Snort.
While it was recently announced that Checkpoint would acquire Snort's owners, Sourcefire, it appears the application will remain open source and continue to be developed under a GPL license.
Dig Deeper on Linux servers
Related Q&A from James Turnbull
A user wants to implement OSSEC on a Windows server because he has no server side Linux operating system. Continue Reading
Solaris 10 Trusted Extensions and SELinux are best suited to different system requirements and administrator skill sets. Our security expert explains... Continue Reading
Configuring spam filters Spamassassin and dspam together in the email server Postfix is easy with the resources listed by our security expert. Continue Reading