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Re: IPv6 Link-Local Use Issue for Applications

On Wed, 20 Aug 2003 17:18:02 -0700
"Brian Zill" <bzill@microsoft.com> wrote:

> > that's okay, you can't use ordinary apps with
> > LL addresses (even on ad hoc networks) if the
> > hosts on which those apps run have more than 
> > one active network interface.
> Huh?  I do this all the time.  I daresay a lot of other readers of this
> list do too.  Could you clarify what you mean by this?

To a host with multiple network interfaces, an LL address by itself means
nothing unless the host is willing to try contacting that address on each of
those interfaces.  Even then, the host cannot assume that the address is valid
to other hosts that are participating in the app.  So LL addresses are
unreliable for apps that do referrals.

If you have something like LLMNR, or if you specify an address literal, the
host that does the lookup can know which interface to use.  But this doesn't
solve the problems with referrals.  

Also, in order for LLMNR to work properly (i.e. for it not to break apps) it
must always return results that are consistent with DNS (when given a query
for a DNS name).  It's very tricky to associate LL addresses with DNS names,
even in LLMNR, and to do so in such a way that it won't cause problems for
apps that need consistent results from DNS.  Part of the problem is that the
DNS might more-or-less concurrently appear "up" to one host and "down" to
another host even though both hosts are participating in the same app, so one
host might be querying LLMNR while another host in the same app is querying
DNS for what should be the same information.

Bottom line - it's not safe to associate LL addresses with DNS names, even if
you're using LLMNR as the lookup mechanism.

Now if you want to have "local" names (names that don't look like DNS names)
associated with LL addresses, and you want to use LLMNR to map those names to
addresses, you can do that without confusing apps that make the (reasonable)
assumption that lookups of DNS names are consistent from one host to another
(modulo zone updates that change serial numbers).  But that implies that the
apps are aware of local names, i.e. that local names are treated differently
from DNS names by the apps.  And it still doesn't solve the problem that LL
addresses are not reliable in referrals.

Of course you can get away with using LL addresses with some apps, under
some circumstances, because sometmes LLMNR looks "enough like" DNS to fool the
app.  If you're sufficiently clever about those apps you won't even try to use
them when they wouldn't work, so you won't be annoyed when those apps fail. 
But that doesn't mean that LL addresses are reliable for ad hoc networks for
ordinary users, unless all of those users have hosts with only one active
network interface, and all are connected to the same (perhaps bridged) link.
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