v4 given v6

[Richard Draves]

> > Given a 6bone node's v6 address, what's the best way to find a v4
> address
> > for the node (if it has one)?
> 
> I would look up the PTR record in ip6.int, then look up the A record
> for that name.  However, there are some obstacles:
> 
	[Richard Draves]  Yes, in practice the DNS doesn't do the job, at
least today.

	It's too bad I can't just send a Neighbor Solicitation to the v6
address and ask the node what it's link-level address is... the node would
receive the solicitation on its tunnel interface and the v4 address would be
the link-level address for the tunnel interface. But it won't work because
NS validation will fail: the packet will have a hop count != 255. In fact I
can see implementations disabling all ND on tunnel pseudo-interfaces used
for configured and automatic tunnels, since those forms of tunneling don't
use ND.

	Maybe we can define a limited form of NS/NA for use with tunnel
pseudo-interfaces? Or would that be too confusing?

	Or perhaps more generally, for all interfaces allow a NS with hop
count != 255 to query the link-level address, but if the hop count != 255,
don't update the Neighbor Cache in any way because the source isn't actually
a neighbor.

	Rich