From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <shemminger@linux-foundation.org>
Subject: Re: UDP checksum broken since 2.6.18?
Date: Tue, 22 May 2007 16:12:17 -0700
Message-ID: <20070522161217.20875c39@freepuppy>
References: <465364EA.3020100@openpma.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: QUOTED-PRINTABLE
Cc: netdev@vger.kernel.org,
	Philipp =?UTF-8?B?R8O8aHJpbmc=?= <pg@futureware.at>
To: "Thomas B. =?UTF-8?B?UsO8Y2tlciI=?= <dm8tbr@openpma.org>"@smtp.osdl.org
Return-path: <netdev-owner@vger.kernel.org>
Received: from smtp.osdl.org ([207.189.120.12]:48412 "EHLO smtp.osdl.org"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1760767AbXEVXOq convert rfc822-to-8bit (ORCPT
	<rfc822;netdev@vger.kernel.org>); Tue, 22 May 2007 19:14:46 -0400
In-Reply-To: <465364EA.3020100@openpma.org>
Sender: netdev-owner@vger.kernel.org
List-Id: netdev.vger.kernel.org

On Tue, 22 May 2007 21:47:22 +0000
"Thomas B. R=C3=BCcker" <dm8tbr@openpma.org> wrote:

> hi,
>=20
> a friend of mine recently contacted me about what he at first thought
> were IPv6 issues with some java software.
>=20
> As it turns out it probably is a general IP issue with the Linux kern=
el:
>=20
> He wrote this piece of c which sends an UDP packet to 127.28.50.50 -
> http://www2.futureware.at/~philipp/udp-problem.c
> Packets generated by this code were captured by tcpdump and wireshark=
=2E
> When feeding the dump into wireshark it says:
> "Checksum: 0x62fd [incorrect, should be 0xe4f3]" for the udp packet.
>=20
> We've tested this on several kernel versions.
> Wireshark reports checksum broken:
> Linux version 2.6.18-4-vserver-686 (Debian 2.6.18.dfsg.1-12)
> (waldi@debian.org) (gcc version 4.1.2 20061115 (prerelease) (Debian
> 4.1.1-21)) #1 SMP Mon Mar 26 19:55:22 UTC 2007
> Linux version 2.6.19-dm8tbr-1 (root@oberon) (gcc version 4.1.2 200610=
28
> (prerelease) (Debian 4.1.1-19)) #3 SMP PREEMPT Sun Dec 3 18:31:00 CET
> 2006 - (that's vanilla)
> Linux version 2.6.21.1-dm8tbr-1 (root@oberon) (gcc version 4.1.2
> 20061115 (prerelease) (Debian 4.1.1-21)) #3 SMP Fri May 18 09:04:55 C=
EST
> 2007 - (that's vanilla + dscape patch)
>=20
> Wireshark reports checksum ok:
> Linux version 2.6.16.13-4-default (geeko@buildhost) (gcc version 4.1.=
0
> (SUSE Linux)) #1 Wed May 3 04:53:23 UTC 2006
> Linux version 2.6.17-11-386 (root@terranova) (gcc version 4.1.2 20060=
928
> (prerelease) (Ubuntu 4.1.1-13ubuntu5)) #2 Tue Mar 13 23:30:30 UTC 200=
7
> (Ubuntu 2.6.17-11.37-386)
>=20
> So my guess is something between 2.6.17 and 2.6.18 broke.
>=20
> Second option is: The way you are supposed to send UDP packets change=
d
> in 2.6.18 and sun javavm and that piece of c are broken for the same =
reason.
>=20
> Third option: everything is perfectly ok, the UDP checksum is compute=
d
> in a different way since 2.6.18 - due to some reason I don't know - a=
nd
> Wireshark is broken.
>=20
> We'd be grateful for some enlightment.
>=20
> Cheers
>=20
> Thomas
>

The packet passed to packet capture programs may not have a valid check=
sum
if you have checksum offload configured on the device.  What kind of ha=
rdware
do you have on sender and receiver?  Try disabling checksum offload wit=
h
ethtool.

If you are getting bad UDP checksums then the counters in 'netstat -s'
will be increasing.