netdev - Re: incorrect cksum with tcp/udp on lo with 2.6.20/2.6.21/2.6.22

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Message-Id: <E1IZJFo-0006ta-00@gondolin.me.apana.org.au>
Date:	Sun, 23 Sep 2007 12:39:36 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	olel@....pl (Krzysztof Oledzki)
Cc:	netdev@...r.kernel.org
Subject: Re: incorrect cksum with tcp/udp on lo with 2.6.20/2.6.21/2.6.22

Krzysztof Oledzki <olel@....pl> 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.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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