on amd64 tcp4 cksums are bad (FYI)

Sean McNeil sean at mcneil.com
Fri Aug 20 20:20:14 PDT 2004


> >> >You're almost certainly using a driver which offloads transmit
> >> >checksums.  (both fxp and em do) Since BPF sniffs the packet
before it
> >> >leaves the host, the checksum has not yet been calculated, so it
> >> >bad.

I reported this a long time ago.  Hardware transmit checksums are broken
somewhere.  Most likely UDP and perhaps ipfw/nat have something to do
with it.  I've just checked and the problem still exists.  In order for
something like NFS to work, you have to turn off txcsum.  Other people
see this same issue and it has led me to conclude it isn't nic related.


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