Frank's Technotes

The path of a forwarded packet.

I've just had occasion to figure out (with the help of kgdb) the path a forwarded packet takes through the Linux 2.6 stack. In terms of just called functions, here it is:

netif_receive_skb()	from network driver
ip_rcv()		via packet_type.func
ip_rcv_finish()		via NF_HOOK(PF_INET,
				    NF_IP_PRE_ROUTING,
				    skb, dev, NULL, ip_rcv_finish);
	calls ip_route_input() (which calls ip_route_input_slow())
	to route packet internally
dst_input()		directly
ip_forward()		via skb->dst.input(); set up by
			ip_route_input_slow()
	calls xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb)
		may drop packet if blocked here
	calls xfrm4_route_forward(skb)
		may drop packet if blocked here
ip_forward_finish()	via NF_HOOK(PF_INET, NF_IP_FORWARD,
				    skb, skb->dev,
				    rt->u.dst.dev, ip_forward_finish);
dst_output()
ip_output()		via skb->dst.output(), op cit
ip_finish_output()	if no fragmenting needed
ip_finish_output2()	via NF_HOOK(PF_INET, NF_IP_POST_ROUTING,
				    skb, NULL, dev, ip_finish_output2);
...then device output routine...