Multicast and UDP checksums

[Frank Kardel <kardel%netbsd.org@localhost>]

Hi,

while debugging ntp multicast issues I found a strange behaviour on 6.99.5.

ntpd in machine A (amd64, wm interface) sends multicast packets to 
224.0.1.1.

tcpdump on machine B (amd64, wm interface) sees both packets types
fine (no complaints about the checksum).

ntpd on machine B never sees the multicast packets, because they get 
dropped due to 'bad checksum' in the udp4_realinput() path.

ntpd on machine B sees the plain broadcast packets, because the checksum
is fine.

Any ideas why the ntp multicast packets get dropped, but tcpdump
finds nothing wrong about them ?

Frank