Nessus is an open-source UNIX-based vulnerability scanner available for free download from http://www.nessus.org. The solution uses the classic client/server model—a server-based scan engine (similar to Network Associates' Distributed CyberCop Scanner 2.0) to probe network clients and a client that collects and monitors scan data. You can export the scan results to a few basic formats, including ASCII and HTML. The client and server run on UNIX platforms such as Linux, Sun Microsystems' Solaris, and FreeBSD. Nessus also provides clients for Win32 and Java platforms.

Nessus is a viable competitor to commercial products. It provides open, extensible source code; a robust scanning engine; and external, modular plugins that form the vulnerability-scan database. A community of developers maintains and updates the library of available plugins at the Nessus site. If you have inhouse programming expertise, you can write a custom vulnerability plugin.

To install Nessus, download the most current distribution libraries from the Web site and compile them in proper order. Next, create a Nessus account, configure and start the Nessus daemons, and configure the client. Then, use Nessus's fairly intuitive GUI to configure scan options, and you're ready to start scanning.

The choice of Nessus over a commercial scanner depends on how comfortable you are with an open-source product. Certainly, the price is right, but you'll need some level of expertise on one of the Nessus-supported UNIX platforms. You'll also need to alter your support expectations. A user community—rather than a vendor—supports and develops Nessus. So, you'll get no maintenance contracts or support guarantees. In a nutshell, Nessus is an attractive solution for organizations that have appropriate expectations and expertise.