ntpd on machine B never sees the multicast packets, because they get dropped due to 'bad checksum' in the udp4_realinput() path. We have seen multicast checksum issues, but haven't reported it because it seemed likely we introduced it due to our changes (in a tree based on netbsd-6 with extensive changes). A crazy theory without any justification, but it's the first thing I'd go looking at: udp on loopback disables checksumming, but there's an edge case where received multicast appears to be on lo0 when it isn't.
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