system-config-network & Zeroconf

[John W. Linville]

On Thu, Mar 06, 2008 at 05:58:20PM +0100, Ralf Ertzinger wrote:
> On Thu, 6 Mar 2008 15:54:35 +0100, Lennart Poettering wrote:

> > NAT yes, of course.
> > 
> > proxy arp, no. If you have a route like "route add default dev eth0"
> > this will cause all packets to be adressed to the broadcast MAC
> > address. All other ethernet devices on the same segment should get
> > packets labelled that way because they listen for both their own MAC
> > address and the broadcast MAC address. 
> 
> I'm sorry, but that's just wrong. Setting a net route on a broadcast
> device will cause ARP request for the IP addresses in that network
> to be broadcast on that segment. Nothing more, nothing less. A host has
> to respond to these requests for routing to occur (most likely though
> proxy arp). The only thing being sent with a broadcast MAC are the ARP
> requests, but those are always sent this way.

Ralf is, of course, correct. [1]  In a sense the only purpose of
the routing table is to control which IP address gets ARPed when
sending-out a frame.  Setting the default route to "dev eth0" just
means you ARP for any address.

Just to make sure, I replicated this environment on my local LAN.
Simply setting the default route as "dev eth0" left me with a laptop
that could only reach the local LAN.  Turning on proxy arp at my NAT
router enabled me to communicate with the Internet.  I encourage you
to replicate my experiment. :-)

It's possible that there is some other setting that turns-on the
behavior you describe.  But if there is, I don't know about it.

Thanks,

John

[1] Conveniently, that means I am correct as well. :-)
-- 
John W. Linville
linville at redhat.com