DHCP, Dynamic DNS, and IPv6

[David W. Hankins]

On Mon, Sep 08, 2008 at 02:12:19PM -0700, Loren M. Lang wrote:
> 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.

That is correct.  The ManagedFlag indicates the client should enter
'stateful' DHCPv6 - the acquisition of IPv6 addresses and other DHCPv6
supplied resources (like FQDN's supplied by DDNS).  The
OtherConfigFlag indicates 'stateless DHCPv6', which is approximately
equivalent to the DHCPv4 DHCPINFORM message; acquiring basic config
parameters with no state engine.  There's also an A-flag which guides
whether or not the client should perform SLAAC on the advertised
prefix (assign its own address automatically).


However;

- There's an RS daemon that forks a DHCPv6 client every time the flags
  toggle from 'off' to 'on'.  So if you have two routers advertising,
  with inconsistent bits set, you'll get a DHCPv6 client spawned on
  every two advertisements.

- The RFCs were never clear on whether or not the client should
  continue with stateless autoconfig addresses when the M bit was set,
  so most (all?) clients seem to get addresses from both sources when
  the A bit is set.  So there doesn't seem to be a configuration that
  lets you limit automatic addressing only to clients that have no
  DHCPv6 support.

- I think there was some permissive language which suggests you could
  run a DHCPv6 client independently of RA flags, so long as the client
  sticks to the appropriate backoff timeouts.

- In addition, I seem to recall manufacturing your own address via
  SLAAC even when the A-bit is zero is also permitted.


So it all comes down to "what do your clients do?"

-- 
Ash bugud-gul durbatuluk agh burzum-ishi krimpatul.
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-- 
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