I am writing a Kernel Module (will become GPL when ended) to backup VANETS using Cellular Data Network (UMTS)

I'm using Netfilter hooks: The idea seems simple you put an OUTPUT hook and if the dest address is in a list of adrresses to be backuped, you change the dest address
Of course you have to recalculate
IP checksum: ip_send_ceck()
Route: ip_route_me_harder()
TCP/UDP checksums these are build using a fake header with ip addresses and ports. After changing an address you haye to call
ip_nat_cheat_check()

The problem is when my hook is called UDP checksum isn´t in place yet, I don´t know where it is calculated (I know that because I always get the same check value but my datagrams have an increasing order field)

Could any one help me?

I think I have resolved the problem.

It was related with TOE (Tcp offload engine) and you don´t really need to disable it. The only thing you have to do if you're going to mangle a packet is:
if(skb->ip_summed == CHECKSUM_HW)
skb_checksum_help(skb, 0);
Being skb a pointer to the "socket buffer" of the packet. This call will complete the checksum calculation.
Of course after mangling you have to call ip_send_check() , ip_route_me_harder() and ip_nat_cheat_check()

Cheers