From the O'Reilly LinuxDevCenter article, Essential Security Tools for Linux:
there are open source, Linux-based solutions that can give you all of the benefits of a commercial product (along with the ability to extend the software) at a fraction of the price.
Two packages that make network diagnostics and troubleshooting easier are Ethereal and Netwatch.
Download your copies today!
- The original Ethereal is now supplanted by Wireshark, which you should use instead.