I am aware of that. Sometimes those socket options have the same name, sometimes not. When looking this up I got the impression that it was added to Linux copying from BSD, but it appears to be not the case. In the Apple variant of BSD the same option exists but it is named differently. Ok... well, it is possible to calculate the checksum of course but it is a bit tricky as it is not only a checksum of the actual packet but also of a "pseudo header" that is temporarily added in front of it... But again, it should not be required. The 0000 checksum indicates "no checksum" and it is valid. I think Maiko has a different issue, maybe the problem with multicast sockets? (try -r option to force raw mode) Rob