Thanks all for the suggestions. That C source code, (the setting of theIP_ADD_MEMBERSHIP socket option in particular), led me to this rather useful document: http://tldp.org/HOWTO/Multicast-HOWTO-7.html ...which describes how it is the kernel's task to send IGMP out for sockets set as above, and not the application's problem, which explains why there aren't many apps around that send IGMP. And using 'tcpdump -n igmp', on the client end, I can see that it's doing what it should be doing :) Thanks.