Understanding Linux Network Internals

Chapter 20. Internet Protocol Version 4 (IPv4): Forwarding and Local Delivery

At the end of the ip_rcv_finish function, if the destination address is different from the local interface, the kernel has to forward packets to the appropriate host. On the other hand, if the destination address is local, the kernel has to prepare the packet for use by higher layers. As discussed in the section "The ip_rcv_finish Function" in Chapter 19, the correct choice is taken from the skb buffer through a call to dst_input. Let’s see now how the two tasks (forwarding and local delivery) are accomplished.

Forwarding

As with many networking activities described in the previous chapter, forwarding is split into two functions: ip_forward and ip_forward_finish. The second is called at the end of the first, if Netfilter allows it. Both functions are defined in net/ipv4/ip_forward.c.

By this time, thanks to the call to ip_route_input in ip_rcv_finish described in Chapter 19, the sk_buff buffer contains all the information needed to forward the packet. Forwarding consists of the following steps:

1. Process the IP options. This may involve recording the local IP address and a timestamp if options in the IP header require them.

2. Make sure that the packet can be forwarded, based on the IP header fields.

3. Decrement the Time To Live (TTL) field of the IP header and discard the packet if the TTL becomes 0.

4. Handle fragmentation if needed, based on the MTU associated with the route.

5. Send the packet out to the outgoing device. ...