What is special about loopback

What is special about loopback

[DI Roman Fiedler]

Hi All,

There are some statements that PREROUTING modifications are not possible 
on packets transmitted via loopback. It seems that there is some kind of 
"short-circuit" when sending packets from localhost to localhost, I 
guess for speed advantage.

The image 
http://upload.wikimedia.org/wikipedia/de/5/5f/Nfk-traversal.png states 
that a packet would have to enter the kernel with ip_rcv to pass the 
complete netfilter architecture again, but loopback uses netif_rx(skb) 
to feedback sent packets.

Is the assumption correct, that a modified loopback module or some other 
virtual network module could feed back packets in a way that the pass 
the complete filtering arch or would the local routing tables make any 
efforts useless (even when local routing is modified)? Has someone 
already used such a thing for iptables testing? Or would two connected 
tun devices (local tunnel) do the trick?

greetings, Roman