Use one of the most powerful (and free) network intrusion detection systems available to help you keep an eye on your network.
Monitoring your logs can take you only so far in detecting intrusions. If the logs are being generated by a service that has been compromised, welcome to the security admin’s worst nightmare: you can no longer trust your logs. This is where NIDS come into play. They can alert you to intrusion attempts, or even intrusions in progress.
The undisputed champion of open source NIDS is Snort (http://www.snort.org). Some of the features that make Snort so powerful are its signature-based rule engine and its easy extensibility through plug-ins and preprocessors. These features allow you to extend Snort in whichever direction you need. Consequently, you don’t have to depend on anyone else to provide you with rules when a new exploit comes to your attention: with a basic knowledge of TCP/IP, you can write your own rules quickly and easily. This is probably Snort’s most important feature, since new attacks are invented and reported all the time. Additionally, Snort has a very flexible reporting mechanism that allows you to send alerts to a syslogd, flat files, or even a database.
To compile and install a plain-vanilla version
of Snort, download the latest version
and unpack it. Run the configure script and then
./configure && make
Then become root and run:
Note that all the headers and libraries for libpcap (http://www.tcpdump.org ...