From mboxrd@z Thu Jan  1 00:00:00 1970
From: Morgon.J.Kanter@Dartmouth.edu
Subject: Re: Recalculate checksums in netfilter queue
Date: Thu, 22 Apr 2010 08:43:13 -0400
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Cc: netfilter-devel@vger.kernel.org
To: Justin Yaple <yaplej@gmail.com>
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  My understanding is that the one complement of each 16-bit word is
> summed, and then the result of that is bitwise complemented.
>
> So for each word we would do something like this.
> for each word{
>      sum += ~word;
> }
>
> then you would take the bitwise complement of that total.
> sum = ~sum;

The RFC for IP contains C code that you can literally copy and paste  
for this task.

-- Morgon