Tracing TCP/IP packets from NIC to TCP

Tracing TCP/IP packets from NIC to TCP

[Joshua Stewart]

I've asked questions on this topic in the previous e-mails Subj:"Help me
get up to speed, please" and Subj:"From __cpu_raise_softirq() to
net_rx_action()".  I understand the packet flow from the NIC all the way
to the ip_rcv() function (and a little further).  So, maybe somebody can
fill in another little gap for me.

ip_rcv() call NF_HOOK( PF_INET, NF_IP_PRE_ROUTING, skb, dev, NULL,
ip_rcv_finish) which checks all netfilter hooks that are currently
registered in the PREROUTING hook.  All packets that are accepted by
these hook functions get passed to the ip_rcv_finish() function.  In
ip_rcv_finish, we call ip_route_input() which sets skb->dst.  Then
ip_rv_finish() returns skb->dst->input(skb).

I don't understand enough about the kernel's routing mechanisms to
figure out how these packets (sk_buffs) make their way to the tcp
stack.  Can somebody fill me in on exactly what the skb->dst structure
does and where the rt_hash_table array gets setup.  If I could just
follow this sk_buff until I start hitting code from tcp.c I'd be more
than satisfied.

Thanks,
   Josh