The stupidity of trying to "fix" DHCPv6

Jima nanog at
Wed Jun 15 15:22:12 UTC 2011

On 06/14/2011 03:25 PM, Leo Bicknell wrote:
> I urge everyone in this thread to try a simple experiment.  Configure
> an IPv6 segment in your lab.  Make sure there is no IPv4 on it, not
> on the router, and that the IPv4 stack (to the extent possible) is
> disabled on the hosts.  Now try to use one of the hosts to access IPv6
> content.

  Been there, done that, fairly happily -- with both Windows 7 and Linux 
(Fedora 13 or 14, I forget).

> You'll find the box does SLAAC just fine and gets an address.  You'll
> find RA's provide a default gateway and can get your packets out to the
> world.  You'll also find absolutely nothing works, at a bare minimum
> because you have no DNS servers.

  Err, no, that's not universally true.  The version of NetworkManager 
in recent-ish Fedora and Ubuntu (can't attest to other distros) supports 
the RDNSS field in RAs (available in radvd since 1.0, ~2006-11-01).  You 
do need to explicitly disable IPv4 in NM, however, or it'll consider the 
lack of DHCPv4 to be a general network failure.

  RHEL 5 won't work without manually configuring a DNS address; 
everything I've heard indicates that RHEL 6 supports RDNSS, however.

  Windows 7 is a bit of an odd duck; without any defined DNS servers it 
defaults to the following (deprecated) site-local addresses:


  Adding a route/config for those on your actual DNS server(s) allows 
Windows to get working DNS, as well.  (I don't recall if I had to 
explicitly disable IPv4 to get IPv6-only working, though.)

  I will agree that Windows XP is more or less dead in the water in your 
defined scenario (I've heard you can shoehorn IPv6 DNS servers into its 
config, but it's not trivial; I haven't confirmed this); I haven't 
tested Vista but I believe its behavior is probably closer to 7 than XP.

> The IETF is working on one solution, which is to add DNS information to
> the RA's!  So now you'll configure your routers to hand out DNS servers
> to clients, and then everything else (NTP servers, Domain Controllers,
> etc) in DHCP!

  Oh, oops; you did touch upon this.  You might want to let the people 
who've implemented RDNSS in software know that the IETF is working on 
it.  I'm sure that'll be a relief.


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