Re: Changing IP destination in NF_IP_PRE_ROUTING hook

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On Friday 2008-09-12 07:28, Partha Majumdar wrote:
>
>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?

What you want to achieve sounds pretty much like the RAWNAT
extension:
http://dev.medozas.de/gitweb.cgi?p=xtables-addons;a=summary

>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?

It is always skb_make_writable().
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