SLAAC(autoconfig) vs DHCPv6

David W. Hankins David_Hankins at
Tue Aug 19 11:57:20 CDT 2008

On Mon, Aug 18, 2008 at 03:42:29PM -0400, Howard C. Berkowitz wrote:
> If you want to test a resource, be it the end user or an infrastructure
> interface, how do you know how to foo it (foo being some value of ping,
> traceroute, look it up in SNMP/NetFlow, etc)?
> I submit that if you use dynamic assignment of any sort, you really have to
> have DNS dynamic update, so you can use a known name to query the function
> that's indexed by address.  Otherwise, static addresses become rather
> necessary if you want to check a resource. 

That's close.  If you use dynamic assignment via DHCP (v4 or v6),
then you have a handy database of all the IPv4 addresses assigned and
whatever information you want to discern them by (if not by hostname)
that was available to the DHCP server at the time of assignment.
Strictly speaking, Dynamic DNS isn't even necessary, but it could be
reasonably handy (because IPv6 addresses do not pass 'the phone

With technologies like SLAAC, tho, you are right.  You're going to
have to give devices a means to register with the network
independently of their IP address allocation, because it only takes
one client to Router Solicit to configure multiple clients upon the
broadcast Router Advertisement reply.  Unless you start sniffing for
their neighbor discovery probes (part of SLAAC is to ensure the new
address is not already in use), there's no transaction where the
resource(s) are assigned.

There is quite obviously a key distribution problem with that kind of
model, and if you have to manually configure a system to configure
itself dynamically, there is a significantly diminished reward.

At this point in the excercise, you may as well do what the rest of us
in the current SLAAC-only world have done; disable SLAAC and set v6
addresses (and DNS) manually.  Welcome to 1985, the era DHCPv4 saved
us from.

But this leads you back to today's IPv6 operational problem; if you
need registered clients, then you can install any DHCPv6 software you
can find to get it via either its database or Dynamic DNS (quite a
lot of DHCPv6 server software supports Dynamic DNS).  But you still
wont' have any DHCPv6 clients outside of Vista.

This is where the chicken meets the egg on our faces.

Ash bugud-gul durbatuluk agh burzum-ishi krimpatul.
Why settle for the lesser evil?
David W. Hankins	"If you don't do it right the first time,
Software Engineer		     you'll just have to do it again."
Internet Systems Consortium, Inc.		-- Jack T. Hankins
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