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SubjectRe: 2 nics in the same machine... wrote:
> I have connected two nics in the same machine with a null modem cable...
> I have configured nic 1 to be and nic 2 to be
> I have adjusted the routes so that nic 1 routes to nic 2 and vice versa...
> how come when I ping it does not go out nic 2...
> am I missing something?

Kernel sees that it have a ping packet destined for
Kernel notices it has several NICs available, so it chooses
the one closest to the destination. NIC1 _is_ so it
_is_ the destination.
Kernel figures that the packet is already here, and never sends it
out on the wire.

A crossed cable between two NICS in the same machine is
a testing setup, right?

If you want to test NICs (or cables & hubs) do this:

1. Run a packet sniffer on the "listening" NIC. Run it in
promiscuous mode so it'll even sniff packets not meant for it.

2. Set a default route to the "sending" NIC. Or at least a route
to some network that isn't on your machine.

3. Ping the remote network. You will not get an answer, but:
The packet will be sent through the "sending" NIC,
and sniffed by the "listening" NIC. So you'll verify that
NICs and cable works. Optionally make a script that reverses
the roles of the two NICs if you want to test both ways.

If, on the other hand, you're testing apps/protocols, don't worry that
the traffic don't hit the wire. A test utilizing internal loopback
is just as good. If you really need traffic to go out on a wire, then
you likely need two or more machines for a realistic test too.

Helge Hafting

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