How do you do rDNS for IPv6 ?

Sun Dec 5 22:54:43 UTC 2010

On Dec 5, 2010, at 2:13 PM, John Levine wrote:

> I've been pondering IPv6 setups, and I don't understand how IPv6 rDNS
> is supposed to work.  It's clear enough how you look up any particular
> address, but it's not at all clear to me what you put into an rDNS
> zone and how you put it there.
> 
Pretty much the same thing you put into an IPv4 zone... PTR records.

For example:

owen.delong.com.	IN	AAAA 2620:0:930::200:2
2.0.0.0.0.0.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.3.9.0.0.0.0.0.0.2.6.2.ip6.arpa.	IN	PTR	owen.delong.com.

> In IPv4 land, it is standard to assign matching forward and reverse
> DNS for every live IP, and a fair number of services treat requests
> from hosts without rDNS with added scepticism. For consumer networks,
> it's often something like 12-34-56-78.adsl.incompetent.net, with the
> numbers being the IP address forward or backwards.
> 
Ah, so you're not talking about assigning to live hosts, your talking about
the unfortunate habit of assigning to every possible host. Yeah, that trick
doesn't work in IPv6.

> So if every customer gets a /64, what do you do?  You can use a
> wildcard to give the same rDNS to all 2^64 addresses, but you can't do
> matching forward DNS, since a DNS response with 2^64 AAAA records
> would be, ah, a little unwieldy.
> 
First, customers should be getting more than a /64. A /64 should be a single
subnet and customers should, ideally, be getting a /48 for each end site.

In general, for the most part, the services that treat missing rDNS with additional
skepticism also treat rDNS entries like 12-34-56-78.adsl.incompetent.net with
that same or greater skepticism, so, I wouldn't worry too much about it.

For hosts where it does matter, you've got to create an AAAA record somehow
(just like you needed to create an A record somehow), so, you should be
able to use that same process to generate the AAAA and PTR records.

> When hosts self-configure their low 64 bits, do you install a suitable
> PTR and AAAA into your DNS?  If so, how?  Do you use DHCPv6 and have it
> install the DNS?  Do you do something else?
> 
If you care, you probably need to use DHCPv6 for this and it should be able
to build both the AAAA and PTR records.

Owen