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6. Netfilter Hooks for Tunnel Writers

Authors of tunnel (or encapsulation) drivers should follow two simple rules for the 2.4 kernel (as do the drivers inside the kernel, like net/ipv4/ipip.c):

The canonical way to do the first is to insert code like the following before you wrap or unwrap the packet:

        /* Tell the netfilter framework that this packet is not the
          same as the one before! */
        skb->nfct = NULL;
        skb->nf_debug = 0;

Usually, all you need to do for the second, is to find where the newly encapsulated packet goes into "ip_send()", and replace it with something like:

        /* Send "new" packet from local host */
        NF_HOOK(PF_INET, NF_IP_LOCAL_OUT, skb, NULL, rt->, ip_send);

Following these rules means that the person setting up the packet filtering rules on the tunnel box will see something like the following sequence for a packet being tunnelled:

  1. FORWARD hook: normal packet (from eth0 -> tunl0)
  2. LOCAL_OUT hook: encapsulated packet (to eth1).

And for the reply packet:

  1. LOCAL_IN hook: encapsulated reply packet (from eth1)
  2. FORWARD hook: reply packet (from eth1 -> eth0).

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