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Date: Sun, 11 Jan 2004 15:35:33 +0000
From: Bruce M Simpson <bms@spc.org>
To: YACINE GHANJAOUI <001VA36979@stud.alakhawayn.ma>
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Subject: Re: freebsd-hackers Digest, Vol 42, Issue 6
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On Fri, Jan 09, 2004 at 03:57:07PM +0000, YACINE GHANJAOUI wrote:
> when trying to intercept UDP packet after changing the protocol number
> from 17 to a user one (99) in the ip_input.c file. when trying to
> regenrate the packet after inserting some bit errors an error message
> appears in the reciever telling that The udp checksum is incorrect even if
> i just change the ip Header.
> What do you think the problem is?

You didn't read how UDP checksums were calculated, did you? Here is one free
clue: Look at the definition of in_pseudo() and where it is used.

Use the source. look!

BMS