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"The purpose of an intrusion detection system (or IDS) is to detect unauthorized access or misuse of a computer system. Intrusion detection systems are kind of like burglar alarms for computers. They sound alarms and sometimes even take corrective action when an intruder or abuser is detected. Many different intrusion detection systems have been developed but the detection schemes generally fall into one of two categories, anomaly detection or misuse detection. Anomaly detectors look for behavior that deviates from normal system use. Misuse detectors look for behavior that matches a known attack scenario. A great deal of time and effort has been invested in intrusion detection, and this list provides links to many sites that discuss some of these efforts"(http://www.cerias.purdue.edu/about/history/coast_resources/intrusion_detection/)
There is a sub-category of intrusion detection systems called network intrusion detection systems (NIDS). These systems are looking for suspicious activity and monitor the packets. Network intrusion detection systems can monitor many computers at a time over a network, while other intrusion detection systems may monitor only one.
Who wants to breaking into your system?
One common misconception of software hackers is that it is usually people outside your network who break into your systems and cause mayhem. The reality, especially for corporate workers, is that insiders can and usually do cause the majority of security breaches. Insiders often impersonate people with more privileges then themselves to gain access to sensitive information.
How do intruders break into your system?
The simplest and easiest way to break in is to let someone have physical access to a system. Despite the best of efforts, it is often impossible to stop someone once they have physical access to a machine. Also, if someone has an account on a system already, at a low permission level, another way to break in is to use tricks of the trade to be granted higher-level privileges through holes in your system. Finally, there are a lot of ways to gain access to systems even if one is working remotely. Remote intrusion techniques have become harder and more complex to fight.
How does one stop intrusions?
There are several Freeware/shareware Intrusion Detection Systems as well as commercial intrusion detection systems.
Open Source Intrusion Detection Systems
Below are a few of the open source intrusion detection systems:
AIDE (http://sourceforge.net/projects/aide) Self-described as "AIDE (Advanced Intrusion Detection Environment) is a free replacement for Tripwire. It does the same things as the semi-free Tripwire and more. There are other free replacements available so why build a new one? All the other replacements do not achieve the level of Tripwire. And I wanted a program that would exceed the limitations of Tripwire."
File System Saint (http://sourceforge.net/projects/fss) - Self-described as, "File System Saint is a lightweight host-based intrusion detection system with primary focus on speed and ease of use."
Snort (www.snort.org) Self-described as "Snort® is an open source network intrusion prevention and detection system utilizing a rule-driven language, which combines the benefits of signature, protocol and anomaly based inspection methods. With millions of downloads to date, Snort is the most widely deployed intrusion detection and prevention technology worldwide and has become the de facto standard for the industry."
Commercial Intrusion Detection Systems
If you are looking for Commercial Intrusion Detection Systems, here are a few of these as well:
Tripwire
http://www.tripwire.com
Touch Technology Inc (POLYCENTER Security Intrusion Detector)
Http://www.ttinet.com
Internet Security Systems (Real Secure Server Sensor)
http://www.iss.net
eEye Digital Security (SecureIIS Web Server Protection)
http://www.eeye.com