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Re: Multicast and UDP checksums

In article <>,
Frank Kardel  <> wrote:
>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 
>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 ?

Do you have hardware checksumming on, on either side?


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