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Re: Multicast and UDP checksums

On 08/21/12 08:58, Christos Zoulas wrote:
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?

On and off.
    started with on on both sides - bad
    then off on receiver - bad
    then off on sender and receiver - bad
so, no improvement.

broadcasts make it through with no problems (sane checksums).

tcpdump does not complain when looking at the packets (multicast and broadcast) and I believe tcpdump makes these checks also.

I have yet to look very close at packet data and checksums (earliest next WE) .
I seem to remember, that multicast reception was working some time ago.


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