On Sat, Mar 24, 2001 at 05:10:04PM +0530, Madhavi Suram wrote:
>
> Hi
>
> I am modifying FreeBSD 4.1 kernel. I am hacking all UDP packets in
> ip_input, changing some headers and finding the udp checksum
> using in_pseudo() and setting the packet header csum_flags to
> CSUM_UDP (I know this is a dirty way of doing it.. but, had to do it
> for efficiency reasons). When I try to see the checksum after the checksum
> calculation is complete in ip_output, I am getting the checksum
> 65535(0xffff). Is this a special checksum(error or something like
> that..)? I have seen INVERT doing something like this in the file
> /sys/alpha/alpha/in_cksum.c.
>
> sum == 0xffff ? 0xffff : ~sum & 0xffff.
>
> After modifying the packet in ip_input(), I call ip_forward() and the
> packet should go and reach the intended destination as usual. But, this
> destination address is the one that is changed in ip_input(). But, the
> destination host is not getting the packet. I think this is a checksum
> problem.
>
> Could anyone please tell me what checksum 0xffff means, or give me some
> pointers to where I can find this information?
>
INVERT was a failed attempt to comply with RFC 768 requirement to transmit
all zero computed checksum as all ones. For some unknown reason, the fix
(sys/alpha/alpha/in_cksum.c, revision 1.5) was not yet MFC'ed.
Cheers,
--
Ruslan Ermilov Oracle Developer/DBA,
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