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Date: Fri, 20 Aug 2004 22:54:13 +0300
From: Ruslan Ermilov <ru@freebsd.org>
To: Andrew Gallatin <gallatin@cs.duke.edu>
Message-ID: <20040820195413.GD57723@ip.net.ua>
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cc: "David W. Hankins" <David_Hankins@isc.org>
cc: current@freebsd.org
Subject: Re: on amd64 tcp4 cksums are bad (FYI)
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On Fri, Aug 20, 2004 at 03:41:26PM -0400, Andrew Gallatin wrote:
>=20
> David W. Hankins writes:
>  >=20
>  > This is as observed via tcpdump on [client], which is what is producing
>  > the bad checksums.  Obviously it doesn't cause a problem since no one
>  > listens to TCP checksums, but it's interesting.  I only noticed it
>  > because I was tcpdump'ing for completely unrelated reasons, and it cau=
ght
>  > my eye.
>=20
> <...>
>=20
>  > Client machine is amd64, running 64-bit mode 5-current fresh as of
>  > yesterday.  Network interface is e1000, so fxp.  Server is also freebsd
>=20
> e1000 is actually em. =20
>=20
> 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.
>=20
> See this thread:
> http://lists.FreeBSD.org/pipermail/freebsd-net/2004-August/004703.html
>=20
This should be added to FAQ.


Cheers,
--=20
Ruslan Ermilov
ru@FreeBSD.org
FreeBSD committer

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