Re: Networking Q concerning /etc/network/interfaces
On Wed 15 Feb 2012 at 10:07:52 -0500, Harry Putnam wrote:
> In the confused thread I butchered, I missed this little snipped from
> Brian. I very much appreciate this explanation. I had it wrong.
You're being too hard on yourself. I get things wrong all the time and
it may be only months later I realise it.
> I thought that without the machine being set up as a router. That is,
> with /proc/sys/net/ipv4/ip_forward set to 0, in that case the eth1 adaptor
> would NOT ask eth0 anything, and would not communicate anything about
> eth0 back to host b.
The eth1 interface is obliged by ARP to either discard the request or to
communicate back what it has been able to find out.
> I had no experience with that scenario, but that was my first thought
> on it.
>
> I'm still a little unclear just how eth1 queries eth0. The part in
> Brians explanation that says:
> "[ ...] eth1 is aware 192.168.1.54 isn't its address but, being
> conscientious, asks about on the machine it lives on. eth0 says:
> 'hey that's me!'. eth1 then tells host b: 'I've found what you are
> looking for, 192.168.1.54's traffic can be sent to 192.168.1.42 first."
Did I say that? It's a demonstration of what can happen when you become
overenthusiastic! But, in the absence of a networking guru, the essence
is there.
> Just how is that asking and reporting part performed? Does eth1 then
> handle all traffic to and from eth0?
eth1 doesn't actually query eth0 directly. I was being dramatic. :)
The ARP request goes to the kernel, which knows about everything on the
machine. It instigates the reply that the IP asked for is known there.
How traffic to eth0 is dealt with depends on how the machine is set up
but it must first go through eth1 in the situation you describe - no
network cable attached to it.
An analogy: Mr Harry Putman walks into Hotel Debian. He approaches the
reception desk and asks whether Mr Important Person (IP to his friends)
is staying there and whether he may speak with him. The concierge says
he is but all communication must go through her and, for a small
consideration, she will set up the channels.
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