Re: Strange network related data corruption

[Malte Schröder <maltesch@gmx.de>]

On Mon, 8 Oct 2007 14:01:32 +0100
Denys Vlasenko <vda.linux@googlemail.com> wrote:

> On Sunday 07 October 2007 17:47, Malte Schröder wrote:
> > Hello,
> > I am encountering some strange data corruption when transferring
> > data from one of my PCs that I use as a file-server.
> > 
> > on the server:
> > FILE=<large file>; | cut -d" " -f1 | nc -lp5000 -q0; while nc
> > -lp5000 -q0 < $FILE; do : ; done
> 
> $ cat z
> FILE=z; | 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=z ; sha1sum $FILE | cut -d" " -f1 | nc -lp5000 -q0; while nc -lp5000 -q0 < $FILE; do : ;done

> 
> $ sh z
> z: line 1: syntax error near unexpected token `|'
> z: line 1: `FILE=z; | cut -d" " -f1 | nc -lp5000 -q0; while nc -lp5000 -q0 < $FILE; do : ; done'
> 
> > on the client:
> > H=<server>; SUM=$(nc -q0 $H 5000);sleep 1s; while nc -q0 $H 5000 |
> > sha1sum | (grep -v $SUM || echo -n .); do sleep 1s ;done
> > 
> > (output looks somewhat like this:
> > ..............6dd5fb1ce29d270acdfbb02d00921bf75d141773  -
> > ...
> > )
> > 
> > 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.
> 
> 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 ...

> 
> 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
>

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