Re: Networking Q concerning /etc/network/interfaces

[Harry Putnam <reader@newsguy.com>]

Brian <ad44@cityscape.co.uk> writes:

>> So, it seems there is no way around thinking both addresses are on a
>> single nic since there is only one ethernet wire attached to
>> localhost.
>
> As above; erroneous. Find out about MAC addresses and ARP. They are
> basic to communication on an ethernet network and will help you see a
> way round your thinking.
>
> When pinging, host b sends an ARP broadcast to all machines asking: 'any
> of you out there know about the IP address 192.168.1.54?' 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.'
>
> Two addresses on two interfaces. ifconfig says as much too.

In the confused thread I butchered, I missed this little snipped from
Brian. I very much appreciate this explanation.  I had it wrong.

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.

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."

Just how is that asking and reporting part performed?  Does eth1 then
handle all traffic to and from eth0?