Re: Cursed Checksums

[Olivier Galibert]

On Sun, Aug 22, 2004 at 11:03:54PM +0200, Josan Kadett wrote:
> Perhaps there is a way to recompute IP header checksums before they get into
> the interface? As I outlined, I have found a way to manipulate IP source
> address before the packet is flushed to system, but a means of recalculating
> the IP header checksum after that manipulation should be found. Because even
> if I ignore IP header CRC in one system, all other boxes connected to this
> machine has to be patched the same. That is impossible anyway.
> 
> Only if I could find a way to recalculate the checksum in IP headers by
> doing a simple hack to the kernel, everything would be alright. 

Why don't you patch the checksum when you change the IP?  It's just a
not of the sum the 16-bit words so take the old one, not it, add the
two 16-bits differences, re-not it and write it back.

  OG.

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