Someone told me IP{V6,}_ADD_MEMBERSHIP sockopts were deprecated and we should use MCAST_JOIN_GROUP instead. That fails for 32-bit userspace on 64-bit kernels, though. Fixed by upstream commit dae50295. Test case at http://david.woodhou.se/MulticastTest.c should print '5 bytes received' but will just hang. Note that on RHEL4 you seem to have to have a global IPv6 address in order for the test to work; I think that might be a separate bug.