Ever since I set up my firewall settings to catch log UDP port opens, I've been getting a lot of UDP 137 port opens when running my Gnutella client (limewire). About every 15 minutes a new one comes in from a different source.

I'm not a TCP expert, 137 is listed as netbios nameserver service in /etc/services. Only two possiblilites come to mind. Either some gnutella client is trying to use this service for some unknown but legitimate purpose, or someone as learned that the gnutella network protocol provides a rich source of IP addresses that can be probed for security holes. The second possibility seems to be the more probable of the two.