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From: Sean McNeil <sean@mcneil.com>
To: Ruslan Ermilov <ru@FreeBSD.org>
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cc: current@freebsd.org
cc: David_Hankins@isc.org
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cc: gallatin@cs.duke.edu
Subject: Re: on amd64 tcp4 cksums are bad (FYI)
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Gentlemen,

> >> >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
looks
> >> >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.

Cheers,
Sean