Re: 2 nics in the same machine...

[Helge Hafting <helgehaf@aitel.hist.no>]

RANDAZZO@ddc-web.com wrote:
> I have connected two nics in the same machine with a null modem cable...
> 
> I have configured nic 1 to be 156.15.16.1 and nic 2 to be 156.15.16.2
> 
> I have adjusted the routes so that nic 1 routes to nic 2 and vice versa...
> 
> how come when I ping 156.15.16.1 it does not go out nic 2...
> 
> am I missing something?

Kernel sees that it have a ping packet destined for 156.15.16.1
Kernel notices it has several NICs available, so it chooses
the one closest to the destination.  NIC1 _is_ 156.15.16.1 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