[lwip-devel] RFC: ip_input() program flow

[Leon Woestenberg]

Hello,

I am currently implementing interface up/down status in CVS head tree. I 
came across this:

ip_input(p, inp_netif) processes the incoming packet p which was 
received on network interface 'netif'.

In this function, we match p against *ALL* interfaces, i.e. we accept 
host-local forwarding by default.
Why do we do so? Is this standards compliant?

Example: Suppose two interfaces: 192.168.0.1/24 and 10.1.0.1/16
A packet ingressing on the 10.1.0.1/16 netif with destination 
192.168.0.1 will be accepted,
even if routing is disabled.

-- current pseudo code --
drop non-IPv4 packets
drop packets where header is laid out over multiple pbufs (i.e. in 
non-contiguous memory)
drop wrong checksum packets
dest_netif = any local netif IP address that matches destination IP address
if LWIP_DHCP then if dest_netif = NULL then dest_netif := inp_netif
if  dest_netif = NULL {
 if non-broadcast packet then ip_forward(p, inp_netif)
 free(p)
}
reassemble
if p.protocol=udp then udp_input(p, inp_netif);
if p.protocol=tcp then tcp_input(p, inp_netif)
--

-- proposed pseudo code --
drop non-IPv4 packets
drop packets where header is laid out over multiple pbufs (i.e. in 
non-contiguous memory)
drop wrong checksum packets
if netif IP address matches destination IP address && netif.status = up, 
dest_netif := inp_netif
else if forwarding enabled {
 dest_netif = any local netif IP address that matches destination IP 
address && netif.status = up
}
if LWIP_DHCP then if dest_netif = NULL then dest_netif := inp_netif
if  dest_netif = NULL {
 if non-broadcast packet then ip_forward(p, inp_netif)
 free(p)
}
reassemble
if p.protocol=udp then udp_input(p, inp_netif);
if p.protocol=tcp then tcp_input(p, inp_netif)
--

Regards,

Leon.