Macs don't need antivirus? You may want to rethink that. Just because it's easier to write exploits for MS platforms doesn't mean they don't exist for other platforms. Computer security is a process not a state. Whenever one feels safe, one is being cocky and therefore vulnerable.
Back to the topic:
If you're confident enough to get into some fairly advanced networking stuff, I suggest you download and install
Ethereal (2 Mac versions are vailable from the download page -- a quick glance indicates that the
Fink Project binary is probably easier to install than the
Darwin Ports) and take a trace of the interactions between your computer and AlfaBB. The trace will show exactly what goes on at the Ethernet level (on top of which is TCP/IP and HTTP). I can't help much with setting up Ethereal on a Mac, but I'll analyze the trace for you if you send me the file via e-mail.