Re: a problem with tcp checksum

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