Re: Tie UDP reply to incoming interface

David Schwartz wrote:

It's not clear that you are fully aware, because you seem to be
confusing the ARP issue (which only occurs with two interfaces in the
same subnet) with the end system model issue issue here (which has
nothing to do with ARP).

Suppose a machine is numbered 192.168.2.10/24 and 192.168.1.10/24 in
two different physical networks. Suppose 192.168.1.1/24 knows this and
gets a packet bound for 192.168.2.10. Since 192.168.1.10 is directly
connected, and "connects" to 192.168.2.10, 192.168.1.1 will ARP for
192.168.1.10 (its next hop on that route) and send packets bound for
192.168.2.10 to 192.168.1.10. The ARP is for the "other" address.

Routers (typically) ARP for the next hop address, not the destination
address.

Of course. Where did I say otherwise? My claim was that an upstream router
will never ARP for the IP address assigned to the ethernet interface that
connects to the other provider.


ISP A ROUTER ISP B ROUTER
| |
| /
\ /
eth1 eth2
linux firewall/router


What I was saying is that eth1 will never get an ARP request for eth2's IP
address, and viceversa. Of course when ISP A router has a packet for some
destination behind the linux firewall, it will arp for eth1's IP address,
and so will ISP B router for eth2, which is what you said.

.

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