Re: 2 nics in the same machine...
From: Helge Hafting (helgehaf_at_aitel.hist.no)
Date: 03/20/04
- Previous message: William Lee Irwin III: "Re: can device drivers return non-ram via vm_ops->nopage?"
- In reply to: Willy Tarreau: "Re: 2 nics in the same machine..."
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Date: Sat, 20 Mar 2004 19:42:52 +0100 To: Willy Tarreau <willy@w.ods.org>
On Fri, Mar 19, 2004 at 11:36:56PM +0100, Willy Tarreau wrote:
> Hi,
>
> On Fri, Mar 19, 2004 at 10:16:36AM +0100, Helge Hafting wrote:
>
> > 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.
>
> Nearly right. He will need to enter static ARP entries for this to
> work because his host will try to resolve the gateway's address first,
> so nothing except ARP will go out.
>
I didn't think of that. Of course, capturing an ARP broadcast is probably
good enough for testing cables. One might want better than that
for testing the NIC driver though.
> DNAT out + SNAT in may be OK. I've used this setup in the past, but didn't
> not go on because of performance problems. Now, Julian Anastasov has written
> a wonderful patch named "send-to-self" which does the trick automagically.
> You can get it on his site ( http://www.ssi.bg/~ja/ IIRC ).
>
> > 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.
>
> Right. Except that in some very weird cases, the higher MTU on loopback may
> affect the app's behaviour (less packets, or bigger reads at once, etc...).
ifconfig lo mtu 1500 :-)
Helge Hafting
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- Previous message: William Lee Irwin III: "Re: can device drivers return non-ram via vm_ops->nopage?"
- In reply to: Willy Tarreau: "Re: 2 nics in the same machine..."
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