Re: a problem with tcp checksum

Hi Fred,

I'm using Mandrake linux 9.2, libnet-1.1.2.1. I'm not sure whether it is the
endianess problem you mentioned. I've done more experiments and found that
not all TCP checksums with odd-size TCP payload are badly computed, while
most of them are. Really strange. And sometimes the first TCP packet with
odd-size payload and the following TCP packets with the same size payload or
even-size payload work well, while once a TCP packet with a different odd
size payload comes up, checksum goes wrong. Did anybody report the similar
problem? Thank you.

Jee


> Hi,
>
> on what architecture and system are you using libnet? I fixed a
> similar bug some times ago that caused that behavior: it was caused by
> an endianess problem (bad casting). And what version of libnet are you
> using ?
>
> Fred Raynal
>
> On Mon, May 31, 2004 at 08:44:09AM +0530, [email protected] wrote:
> >
> >
> >
> >
> > Hi Jeez,
> >
> >    This seems wierd, as i said I havent worked with tcp checksum in
libnet
> > yet, but my guess says that this might be your problem, whenever we
> > calculate checksum for a TCP segment its mandatory that header+text
length
> > is even, if its odd just for checksum calculation purposes the last
byte(on
> > the right) is padded(all zeros). This byte is not transmitted as part of
> > the segment though, maybe libnet doesnt do it itself, this is completely
a
> > guess cos I dont know how checksumming with libnet works.
> >
> > cheers
> > Amit
> >
> >
> >
> >
> >
> > "Jee J.Z." <[email protected]> on 05/31/2004 05:18:02 AM
> >
> > To:    <[email protected]>
> > cc:
> >
> > Subject:    a problem with tcp checksum
> >
> >
> > Hi all,
> >
> > I am going frustrated since a wired problem with tcp checksum seem to
> > always
> > happen. ;-( I really wish to see somebody throw some light on it. The
> > situation is as follows:
> >
> > I am trying to inject TCP packets using libnet_build_tcp_options(),
> > libnet_build_tcp() and libnet_build_ipv4() (checksum automatically
built).
> > When TCP payload size is odd in byte, the tcp checksum is always
incorrect
> > (using ethereal to capture injected packets and check them); while if
TCP
> > payload size is even in byte, everything seems working very well.
> >
> > What I wish to know is do I need to manually change some parameters of
> > libnet_build_tcp_options(), libnet_build_tcp() or libnet_build_ipv4()
when
> > the tcp payload size is odd in byte, in order to let libnet compute tcp
> > checksum correctly? If not, what shall I do to fix the problem? I am
> > looking
> > forward to your help. Thank you very much in advance!
> >
> > Regards,
> > Jee
> >
> >
> >
> >
>