Changing IP destination in NF_IP_PRE_ROUTING hook

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Hi,

I am not very familiar with Linux kernel internals. I am trying to
write a simple netfilter hook but facing some issues and did not find
any clear answer in available docs on packet filter or netfilter
hacking.

I am writing an experimental kernel module that will register a
NF_IP_PRE_ROUTING hook. This hook will change the destination address
of the IP header. After that I'll update the checksum of the IP header
by calling ip_nat_cheat_check as it is done in manip_pkt. Do I need to
update the TCP checksum as well (if it is a TCP packet) since the
pseudo header of the packet has changed? If I need to do update the
tcp checksum as well then why isn't it being done  mainp_pkt after
changing the source or destination IP address?

I also want to do similar modification for IPv6 packets. For IPv4 I
have seen that skb_ip_make_writable is being called before modifying
the IPv4 headers. I did not find anything similar for IPv6. What is
the logic I should follow for IPv6?

Regards,
Partha
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