Can You Spoof IP Packets?:

Spoofed IP packets are still believed to be a significant problem for the Internet. But are they? The Spoofer Project is attempting to measure the problem. Apparently, 80% of the IP addresses measured no longer support spoofing! Their methodology is simple: have users download a client which attempts to spoof packets to the monitor. Using these packets, they can determine the filter rules. So everyone, download the client and help.