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Re: arp on same wire

Quoting J.D. Bronson (jbronson_(_at_)_wixb_(_dot_)_com):
> At 01:15 PM 05/14/2004, you wrote:
> >> Is this going to cause ARP messages (like in freebsd) and if so, is 
> >there a
> >> way to shut them up?
> >
> >P A R E N T A L   A D V I S O R Y
> >
> >Using IP on broadcast type networks causes ARP messages.
> >
> >P A R E N T A L   A D V I S O R Y
> >
> >Seriously now, do you mean to ask if using both NICs on the same
> >ethernet segment will cause ARP problems, or something like that?
> >
> >Please elaborate.
> I mean those dreaded arp messages from one NIC to the OTHER.
> (not normal ones!!!)
> "expected a reply from NIC 1 but came in on NIC 2...."

Really roughly:
- Mr Dest IP is generated in a packet or, say, a router has a packet
  with that Dest IP.
_ The machine that has it, checks its routing table.
  - If a route for that IP is found, it's passed to the machine
    that has that route (usually "default")
  - Lather, rinse, repeat until it's one your network.
- Eventually it gets to a machine on the same physical network as your host.
- There's no route for this host.
- Then the ARP table is looked at (or the MAC add is requested -
  tcpdump will show the "who-has" packets).
- The packet is put on the wire with your host's MAC address.
  Your host picks it up.

Oh! You're running a BROKEN ethernet.  You're host has 2 devices
on the same network.  Bad for routing, easy to make broadcast
storms, quick way to get collisions.  There are hacks around it.
But hacks for generally to work around brokeness that you've invoked.

You can route the exhaust from your car into the living room.
It will kill you.
Of course you can wear an oxygen mask and do ok.  It's a hack.

Perhaps setting up your network RIGHT is less uncomfortable than
wearing an oxygen mask.  Because you will be working around the fact
that it breaks the specs for an ethernet.

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