Understanding IDS for Linux
A port scan to a service like portmap (port 111), which is known to have various exploits, would be alerted by PortSentry:
Dec 9 03:03:17 flamengo portsentry: attackalert: TCP SYN/Normal scan from host: 126.96.36.199/188.8.131.52 to TCP port: 111
Learning how to interpret log files is one of the most important things that an intrusion or security analyst must learn in order to decide what action to take in a given situation. The excerpt from the PortSentry alert above was obtained from the syslog file. This alert states that on December 9 at 03:03, the host called flamengo, which has PortSentry installed, detected an SYN-flag Normal port scan in the TCP port 111 which, in general, runs the service portmap, from host IP 184.108.40.206.
A firewall is a primary security element in a network, but it will not detect attacks on a service that is already opened, such as an attack to your DNS or web server. An IDS by itself will not solve all your problems as a security element, but if you customize it for your needs, it certainly will help alert you to strange behaviors and unauthorized attempts to your host or network. With this information, you should contact the administrator of the network in which the intrusion's IP is located and then inform them of what is going on. Being in contact with the security community is also the best way to keep up to date on new attacks and the signatures to detect them. Be aware—install an IDS!
Pedro Bueno (email@example.com) is a former data engineer from Lucent Technologies and currently is a security engineer at Open Communications Security. He also contributes at Best Linux as a volunteer, and his favorite hobby, besides soccer, is analyzing the alerts generated by Snort.