[avahi] Avahi and new Apple Airports

[Daniel Barkalow]

This weekend I was trying to get my laptop to print to a printer attached 
to an Apple Airport I got this summer. After a bunch of investigation, 
this is what I found:

The Airport is acting as a bridge, so it gets an address from my DHCP 
server for its special functions and otherwise just routes everything 
through. It also assigns itself an IPv6 link-local address and an IPv4 
link-local address.

My laptop gets an address from my DHCP server, and uses that for the 
wireless interface. It assigns itself a link-local IPv4 address for the 
wired interface (with nothing plugged in). It routes the IPv4 link-local 
block to the wired interface.

The airport advertizes all of its addresses, in the order: DHCP-supplied 
IPv4, link-local IPv4, link-local IPv6.

Avahi receives this message, stores the DHCP-supplied address, but 
immediately discards it in favor of the link-local IPv4 address. It then 
supplies the link-local IPv4 address to applications. Any application that 
tries to use the link-local address for the airport fails, because their 
traffic gets routed out the wired interface connected to nothing (and the 
laptop isn't even listening with any link-local address on the wireless 
interface, so it wouldn't be able to get responses even if it sent packets 
the right way).

I hacked around the problem by having avahi discard all link-local IPv4 
addresses, which verified the analysis and got the printer to work for me, 
but it's obviously not the right solution.

I think the right solution is to discard an address on a private or 
link-local address if the interface we heard about it on doesn't have an 
address with that subnet. Sure, the remote device has a couple of 
different sorts of addresses, but we're only interested in ones we can 
actually use, and not ones that are only available to devices with 
addresses of other sorts.

I looked into implementing this for a bit, but there doesn't seem to be a 
function that simultaneously deals with the potential for an interface to 
have multiple addresses and the need to figure out how to extract the 
address from a message.

Of course, it might still be worth having avahi provide multiple addresses 
for devices that advertize them, so programs can try each of them if 
there's a non-obvious reason for one of them to be unusable.

	-Daniel
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