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Date: Tue, 9 Oct 2007 13:25:53 +0200
From: Malte =?UTF-8?B?U2NocsO2ZGVy?= <maltesch@gmx.de>
To: Denys Vlasenko <vda.linux@googlemail.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Strange network related data corruption
Message-ID: <20071009132553.439f7d8b@cp771133>
In-Reply-To: <200710081401.32375.vda.linux@googlemail.com>
References: <20071007184741.3b1951c1@highlander.home.lan>
	<200710081401.32375.vda.linux@googlemail.com>
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On Mon, 8 Oct 2007 14:01:32 +0100
Denys Vlasenko <vda.linux@googlemail.com> wrote:

> On Sunday 07 October 2007 17:47, Malte Schr=C3=B6der wrote:
> > Hello,
> > I am encountering some strange data corruption when transferring
> > data from one of my PCs that I use as a file-server.
> >=20
> > on the server:
> > FILE=3D<large file>; | cut -d" " -f1 | nc -lp5000 -q0; while nc
> > -lp5000 -q0 < $FILE; do : ; done
>=20
> $ cat z
> FILE=3Dz; | cut -d" " -f1 | nc -lp5000 -q0; while nc -lp5000 -q0 < $FILE;=
 do : ; done

Sorry, there is a copy'n'paste error in my post, correct line would be:
FILE=3Dz ; sha1sum $FILE | cut -d" " -f1 | nc -lp5000 -q0; while nc -lp5000=
 -q0 < $FILE; do : ;done

>=20
> $ sh z
> z: line 1: syntax error near unexpected token `|'
> z: line 1: `FILE=3Dz; | cut -d" " -f1 | nc -lp5000 -q0; while nc -lp5000 =
-q0 < $FILE; do : ; done'
>=20
> > on the client:
> > H=3D<server>; SUM=3D$(nc -q0 $H 5000);sleep 1s; while nc -q0 $H 5000 |
> > sha1sum | (grep -v $SUM || echo -n .); do sleep 1s ;done
> >=20
> > (output looks somewhat like this:
> > ..............6dd5fb1ce29d270acdfbb02d00921bf75d141773  -
> > ...
> > )
> >=20
> > I would expect the sha1sum to be the same in every pass (assuming the
> > source file does not change). But every few passes (with no apparent
> > pattern) there is a different sum returned. I first noticed this when
> > transferring large files (backups) with with SMB and NFS(v3 and v4) but
> > to rule that out I tried netcat in the way noted above.
>=20
> Does it happen over loopback?

I just tried a few times and yes, it also happens on loopback, but
much less frequently. Now I am really confused ...

>=20
> tcpdump / tcpflow may help seeing whether there is some corruption
> (TCP checksumming should have catched that, but worth looking into).
> Basically, you wait for "wrong" checksum to appear, then
> you stop script and look into tcpdump/tcpflow logs.
> --
> vda
>=20

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