ip_forward, routing

h725 · 11-23-2008, 10:31 AM

Hi, this is the scenario:

LAPTOP: eth0 10.0.0.1/24

PC has 2 interfaces: eth0 10.0.0.2/24 and eth1 192.168.1.1/24

Here the connections:

laptop(eth0) <--------> (eth0)PC(eth1)

PC has ip_forward = 0.

on "laptop" I type:

# route add -net 192.168.1.0/24 eth0

Question is: why I'm able to ping 192.168.1.1 if the ip_forwarding between the 2 interfaces is off?

Thank you.

bmarx · 11-23-2008, 02:17 PM

The PC is not forwarding packets if the packets are destined for the localhost, even if on a different interface.

rayfordj · 11-23-2008, 02:19 PM

because an IP really isnt "bound" to a specific interface. The kernel owns it and knows which interface to route the traffic.

Since the kernel "owns" both 10.0.0.2 and 192.168.1.1 it is able to properly respond on the 10. interface to your ping to 192.168.1.1 since it does not require any forwarding to another network. It never leaves the PC system.

h725 · 11-24-2008, 07:10 AM

Thank you!