Re: Recalculate checksums in netfilter queue

[Justin Yaple <yaplej@gmail.com>]

> If you solve this problem, I'll be too grateful, because I'm looking
> for a solution since the last year in order to do NAT in userspace.
> Good luck.


If I get it figured out I will let you know.  Im kind of supprised
that there isnt a function for this in libnetfilter_queue already.  It
seems like it would be a pretty common requirement to recalculate the
tcp, and ip checksums.  I have spent two days trying to get something
to work without much luck.

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;