I used to work as a CISSP-certified security analyst for a largish company. I ran the enterprise firewall and tested software for functionality and problems (e.g. race conditions and other types of vulnerabilities.) I also wrote a nice little set of scripts to look for users who violated company computer policies. (If I did it all over again from scratch, I'd probably use php for the layout and perl for the parsing instead of doing it all in perl.) I used linux at work back then and currently dabble with NetBSD, but my security background makes me a major fan of openbsd. If you are interested in highly audited code that uses the standard unix security model, this is your OS.

I've been out of that field for several years, but it was interesting. I got to meet both security luminaries and people who were just plain clueless. Naturally, I also attended a few computer conventions, subscribed to bugtraq and hung out at securityfocus . Over all it was a fun chapter in life.

watchtower is a perl/cgi parser for squid logs. It can give breakdowns of internet usage by IP, rank domains by percentage of traffic, summarize log files, flag suspicious traffic based on that show up in keywords in URLs, etc. It has been several years since I maintained this code, so use at your own risk.