Basic Intrusion Detection System with mtree

After the very basic server setup, having some kind of simple Intrusion Detection System (IDS) seems like a good idea. While all the previous steps were designed to prevent someone taking control of your server, if someone has managed to tamper with something, short of being locked out - how would you know? That's where an IDS comes in, that can hopefully tell you if something has changed, and what. 'Intrusion Detection System' sounds very grand and complicated, and you can buy them for a ...