[lug] physical/logical network interfaces

[rm at fabula.de]

On Tue, Mar 22, 2005 at 11:13:49AM -0700, Hugh Brown wrote:
> 
> 
> On Tue, 22 Mar 2005, Nick Golder wrote:
> 
> > On 2005-03-22 10:31 -0700, Hugh Brown wrote:
> > > I always love responding to my own posts.  So there really are 5 ports,
> > > the onboard NIC, and two Intel pro 100 dual port cards.
> > >
> > > So the question morphs.  The four ports in the dual port cards have no
> > > cables in them.  Only the onboard NIC is connected.  So now my conundrum
> > > is this:
> > >
> > > Why would the kernel allow me to configure eth1 as an interface (and
> > > receive traffic for that interface) when there's no cable connected to it?
> > >
> > > Is it an artifact of all of the cards using the same driver (e100)?
> > >
> > > This strikes me as obscurely broken.
> > >
> >
> > The kernel sees 5 NICs and enumerates them all.  The question is why
> > shouldn't the kernel allow you configure a NIC that doesn't have a cable
> > connected.  The kernel thinks that if you didn't want to be
> > able to configure them, you probably wouldn't have them installed.
> >
> > It would strike me as "obscurely broken" if I couldn't configure a NIC
> > that isn't pysically connected to a network.
> 
> Agreed.  Here's my thought process on what's "broken"
> 
> I have a disconnected nic named eth1.  I configure it.  Since there's no
> cabled connected, I would expect that packets destined for it would fall
> on the floor.
> 
> Instead, what I'm seeing is eth0 is picking up all packets destined to
> eth1 and the IP assigned to eth1 is associated with the MAC address for
> eth0.

No, the kernel is trying to be nice to you :-)
What tool did you use to configure/set up your interfaces? 'ifconfig' or 'ip'?
If you happen to have the 'ip' program installed, what does 'ip link list' show?


> So what I think is "broken" is that eth0 has pre-empted eth1, not that I
> was able to configure eth1 in the first place.


I know, i've fallen into this trap myself (when i tried to have hosts connected
by two cables with seperate networks on each cable. When i shut down one interface
(assuming that i'd still have the other one to connect to the box i had 
to learn that that one interface was handling _all_ traffic ... and since our
providers switch cached the arp entries itg wasn't too easy to get the system
connected again).
BTW, there _is_ some /proc fiddling to prevent all this IIRC ....

 HTH and cheers Ralf Mattes
> 
> Hugh
> 
> 
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