In years past, the term intrusion detection had a general meaning: the methods by which an administrator learned about system intrusions, or about attempts to intrude, on a given system. As in most technological areas, intrusion detection has evolved and specialized. The security industry has grown to include a number of disciplines and subspecialties, each with its own cadre of professionals.
Why worry about intrusion detection if you have a good firewall? Just remember: no lock is unpickable. Firewalls have holes so that services can run (web, mail, and so on). Where there's a hole, there's a way. Furthermore, most security experts will tell you that security is not a destination, but a journey. Even if you have outstanding security policies in place, rock-hard firewalls, and a completely trustworthy user and administrator base, you still need to watch your system like a hawk to make sure that everything remains safe.
In this chapter, we explain the basic concepts of intrusion detection and response. We also show you how to use common Linux tools and shell scripts to maintain the security of your system, keep a watchful (and automated) eye on your system, and offer some solutions to common intrusion detection problems.