The decision of whether to implement an intrusion-detection system (IDS) is a complicated one. Unfortunately, IDS has a well-deserved reputation for requiring a lot of "care and feeding" and commercial systems can be very expensive. However, there is an enterprise-grade open source IDS called Snort that may tip the scales over to a "can't lose" position.

Snort is free and runs on any modern operating system and any old hardware you have. The real investment in Snort is your time and effort, but I guarantee in less than a week you will learn things about your network you never knew. At the end of three months you will have fixed a number of problems on your network that you didn't even know you had. For example, you may find a misconfigured management station that's using the SNMP community string 'public' for everything. We all know that's a bad thing, right?

In addition to learning about your network, you'll learn about the interesting world of malicious code. With Snort, rules are powerful, flexible and relatively easy to write, so new rules to detect the latest malware are often written by the Snort community within hours of an outbreak. Add one to your local or experimental rules file, restart Snort, and you're well on your way to detecting, containing and eliminating any infestation that makes it past your other layers of security.

In addition to getting new Snort rules from Snort.org and the community, you can easily write them yourself. Perhaps you have an obscure application or protocol you need to keep tabs on or want to implement a policy-based IDS. This self-service feature is a very powerful concept that works in certain environments where you define all the traffic that is known and allowed, and then alerts on anything else.

One of the best and most overlooked things about Snort rules is that they are open source as well. (See also: Is Snort better than proprietary IDS?) Ironically both ISS RealSecure and Symantec ManHunt have modules that allow you to use Snort rules. But, the real beauty is that you can actually read a Snort rule, which is something you can't do with most commercial vendors. The best you get with them is a few paragraphs that someone wrote about the rule, which is not the same. Access to the actual rules allows you to make a very informed decision as to the exact criteria that caused an alert and the relevance to your environment.

Regardless of your needs or your reason for implementing Snort, you will probably want to read the FAQ at http://www.snort.org/docs/FAQ.txt and join one of the Snort mailing lists (especially Snort users) at http://www.snort.org/lists.html.

About the author
JP Vossen, CISSP, is the integration manager for Counterpane Internet Security and a technical editor for Information Security magazine. Vossen is involved with various open source projects including Snort and has previously worked as an information security consultant and systems engineer.