Detecting Intruders on Your System Is Fun and Easy
Well, perhaps the title of this chapter is a slightly misleading.
Supposedly, becoming an intruder is fun and easy, too. If you want to
detect intruders, you should know what type of system resources can be
depended on for providing evidence. Should you want to become an
intruder, you ought to know how commercial IDSs look for traces of
Scanners are designed to take a look at your system and to let you
know whether you have configuration problems or holes that can be used
for attacks. If your system was previously set up in a secure fashion,
and an intruder has altered this configuration, a scanner will detect
this change (when you run the scan) and notify you of the problem.
System-level intrusion detection tools differ from scanners in a
couple of ways. If the IDS runs in real time, it can let you know the
instant a compromise has occurred. Also, if the monitor gathers its
data by reading an activity stream on the system, it can detect a
range of features that a single scanner cannot. For example, scanners
will not tell you that someone just entered three bad passwords and
exceeded the failed login threshold.
By the time you finish this chapter, you will understand the
* How to classify attacks according to how they originate and the
threat they pose
* The pros and cons of different data sources that a system monitor
can use for decisions
* What system monitors can and cannot detect
* The tradeoffs you may need to make for monitoring your systems in
* What it takes to really track someone through a network
As you will soon see, you need to consider a number of issues when
trying to build a system-level IDS.
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