linux-kernel - Re: Networking messed up, bad checksum, incorrect length

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Date:	Fri, 27 Oct 2006 13:37:07 +1000
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	lkml@...ipnet.com (Carlos Velasco)
Cc:	linux-kernel@...r.kernel.org, linux-net@...r.kernel.org
Subject: Re: Networking messed up, bad checksum, incorrect length

Carlos Velasco <lkml@...ipnet.com> wrote:
> 
> 03:12:22.484863 00:13:21:cc:54:a6 > 00:0d:bc:a0:e2:82, ethertype IPv4
> (0x0800), length 12546: 192.168.128.182.59061 > 193.147.150.12.25: .
> 68786:81266(12480) ack 228 win 40 <nop,nop,timestamp 9644391 425655748>
> 
> LENGTH: 12546 !!

> But still worse, I wrote the full sniffer traces to a file for further
> analysis with Ethereal, and there I see that not only these long packets
> are above MTU, all they have bad TCP checksums.

These packets look like normal TSO packets.  Linux will send a
packet containing more data than fits in a packet to the NIC.  The
NIC will then segment the packet for us.

In order to see what really goes out, you'll need to run a packet
dump beyond the NIC.

If that is not possible, you can try disabling TSO with ethtool -K.

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