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    SubjectRe: 2 nics in the same machine...
    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

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