DHCP, Dynamic DNS, and IPv6

[Loren M. Lang]

On Thu, 2008-08-28 at 07:51 -0700, David W. Hankins wrote: 
> On Wed, Aug 27, 2008 at 05:26:50PM -0700, Loren M. Lang wrote:
> > I am considering switching/complementing IPv6 auto-configuration which
> > the IPv6 version of DHCP.  I assume it supports Dynamic DNS is the same
> > manner as with IPv4.  Will the two daemons clash when assigning names
> > under the same domain?
> 
> I'm confused as you say 'IPv6 auto configuration' and then 'ISC DHCP
> IPv6 support'.  I'll answer two questions then;

Sorry, I'll try to state what I plan on doing more clearly.  I am using
IPv6 auto-configuration with one router sending out advertisements
including an address prefix.  I have been considering switching to
DHCPv6 in order to do Dynamic DNS updates on client IPv6 addresses which
I cannot do from IPv6 auto-configuration.  RFC2462 states that there are
two flags in the IPv6 Router Advertisements which specify whether
clients should attempt a stateful configuration mechanism, such as
DHCPv6, in addition to what is in the advertisement.  The ManagedFlag,
and OtherConfigFlag from the rfc correspond to the options
AdvManagedFlag and AdvOtherConfigFlag in the radvd.conf configuration
file and should (must?) be set if using DHCPv6.  So the state-less
advertisements from an IPv6 router actually tell clients whether or not
to attempt a stateful protocol such as DHCP.

> 
> SLAAC, as from Router Advertisements, doesn't support DDNS.  It is a
> broadcast query/response protocol, where one reply by the 'server'
> (router) configures many clients.  It has no way of knowing what names
> the clients want, as it will not hear from all of them, nor does it
> know what addresses the names are assigned to, as the clients choose
> those.
> 
> It is possible that someone might write a DDNS updates for SLAAC
> clients, but then you have a key distribution problem or a diminished
> benefit from automation ("manually configuring your clients to
> automatically configure themselves").
> 
> 
> ISC DHCPv6 does support the DHCPv6 FQDN option and Dynamic DNS
> Updates, configuration is precisely the same as with DHCPv4.  As I
> looked at the sources recently, there is one wrinkle (bug); when the
> AAAA's expire, it will remove the last DHCID if there are no more
> AAAA's (assigned by other servers).  It needs to check that there are
> also no more A's.  Repairing this is a SMOP, look to
> "^ddns_remove_fwd" in common/dns.c, and the section of code headed by
> "A RR does not exist".  Simply make this two explicit PREREQs for no
> A and no AAAA (opcode = NXRRSET), rather than one for the prescribed
> "ddns_address_type".
> 
> -- 
> Ash bugud-gul durbatuluk agh burzum-ishi krimpatul.
> Why settle for the lesser evil?	 https://secure.isc.org/store/t-shirt/
-- 
Loren M. Lang
lorenl at north-winds.org
http://www.north-winds.org/


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