Introduction

"Intruder Alert! Intruder Alert! Warning, Will Robinson!" When we heard that ominous announcement emanating from a robot as it twisted and turned with arms thrashing and head spinning, we sat galvanized to our televisions waiting for the intruder to reveal itself. Would this be the end of Will Robinson, as we knew him?

All right, this might be a bit dramatic for a prelude to a discussion of intrusion detection, but with most security administrators, when a beeper goes off there is a moment of anxiety. Is this the big one? Did they get in? Do they own my network? Do they own my data?

These and many other questions flood the mind of the well-prepared security administrator. On the other hand, the ill-prepared security administrator, being totally unaware of the intrusion, experiences little anxiety. For him, the anxiety comes later.

Okay, so how can a security-minded administrator protect his network from intrusions? The answer to that question is quite simple, with an intrusion detection system.

What Is Intrusion Detection?

Webster's dictionary defines an intrusion as "the act of thrusting in, or of entering into a place or state without invitation, right, or welcome." When we speak of intrusion detection, we are referring to the act of detecting an unauthorized intrusion by a computer on a network. This unauthorized access, or intrusion, is an attempt to compromise, or otherwise do harm, to other network devices.

An Intrusion Detection System (IDS) is the high-tech equivalent of a burglar...