I want to find out about worms and viruses.

• The Worm FAQ ~ An excellent FAQ on "worms and worm containment".

How do I change my MAC address?

• Changing Your MAC Address ~ Much easier to do on Linux than Windows, unsurprisingly ...

FAQ: Specific Tools

Scan ports with nmap.

• The Art of Port Scanning ~ This article, by Fyodor, the creator of nmap, is old but still contains some essential information about port scanning.
• Nmap network security scanner man page ~ The most up-to-date version of the nmap man page.
• Using Nmap to Find Random Websites ~ Not just Web sites, but FTP, email, or anything else. Quite simple, really.

Detect intruders with snort.

• The Snort FAQ ~ Written by the Snort team.

View traffic on a network with tcpdump.

• Tutorial: tcpdump ~ A simple overview of tcpdump & how to use it.