From mboxrd@z Thu Jan  1 00:00:00 1970
From: Justin Yaple <yaplej@gmail.com>
Subject: Recalculate checksums in netfilter queue
Date: Tue, 20 Apr 2010 18:02:42 -0700
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Hello,

I am having some trouble figuring out how to re-calculate the tcp/ip
checksums after pulling a packet into user space with
libnetfilter_queue.  In a netfilter hook I was able to use this:
tcph->check = 0;
tcph->check = tcp_v4_check(tcph, tcplen,
	iph->saddr,
	iph->daddr,
	csum_partial((char *)tcph, tcplen, 0));
	iph->check = 0;
	ip_send_check(iph); //ip checksum

In user space these functions tcp_v4_check(), csum_partial() and
ip_send_check() do not exists.  Are there equivalent functions in user
space so I can recalculate the ip/tcp checksums before passing the new
packet back to the kernel with nfq_set_verdict()?

Thanks.