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Date:	Sun, 23 Sep 2007 12:39:36 +0800
From:	Herbert Xu <>
To: (Krzysztof Oledzki)
Subject: Re: incorrect cksum with tcp/udp on lo with 2.6.20/2.6.21/2.6.22

Krzysztof Oledzki <> wrote:
> It seems that after some not very recent changes udp and tcp packes 
> carring data send by a loopback have incorrect cksum:

This correct.  The loopback interfaces has the no checksum flag
set so we only provide a partial checksum on output (i.e., the
pseudoheader without the payload).

We even export this to user-space via a flag.  So you should fix
tcpdump to read this flag and ignore the checksum.

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