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Date: Tue, 16 Oct 2007 20:54:24 +0200
From: Kris Kennaway <kris@FreeBSD.org>
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To: Jelte Jansen <jelte@NLnetLabs.nl>
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Cc: freebsd-current@freebsd.org
Subject: Re: re(4) checksum offload problem
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Jelte Jansen wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> 
> Hi,
> 
> i'm seeing bad IP/UDP checksums on packets on a realtek gigabit network
> card. Turning offloading off (ifconfig -txcsum) seems to make the
> problem disappear.
> 
> Attached are pcap files showing the bad checksums; the IP header is set
> to 0x0000 and then TCP/UDP headers are wrong too. The machine in
> question is on .99. These were taken on the host itself but i also see
> them on the other machine.

Are you sure?  The host itself does not see the correct checksums on the 
packets it sends, because they are not calculated until the packet is in 
the driver.  This is even documented in the tcpdump manpage :)

Kris