Network Working Group A. Durand
Request for Comments: 3901 SUN Microsystems, Inc.
BCP: 91 J. Ihren
Category: Best Current Practice Autonomica
DNS IPv6 Transport Operational Guidelines
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2004).
This memo provides guidelines and Best Current Practice for operating
DNS in a world where queries and responses are carried in a mixed
environment of IPv4 and IPv6 networks.
1. Introduction to the Problem of Name Space Fragmentation:
following the referral chain
A resolver that tries to look up a name starts out at the root, and
follows referrals until it is referred to a name server that is
authoritative for the name. If somewhere down the chain of referrals
it is referred to a name server that is only accessible over a
transport which the resolver cannot use, the resolver is unable to
finish the task.
When the Internet moves from IPv4 to a mixture of IPv4 and IPv6 it is
only a matter of time until this starts to happen. The complete DNS
hierarchy then starts to fragment into a graph where authoritative
name servers for certain nodes are only accessible over a certain
transport. The concern is that a resolver using only a particular
version of IP and querying information about another node using the
same version of IP can not do it because somewhere in the chain of
servers accessed during the resolution process, one or more of them
will only be accessible with the other version of IP.
With all DNS data only available over IPv4 transport everything is
simple. IPv4 resolvers can use the intended mechanism of following
referrals from the root and down while IPv6 resolvers have to work
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RFC 3901 DNS IPv6 Transport Guidelines September 2004
through a "translator", i.e., they have to use a recursive name
server on a so-called "dual stack" host as a "forwarder" since they
cannot access the DNS data directly.
With all DNS data only available over IPv6 transport everything would
be equally simple, with the exception of IPv4 recursive name servers
having to switch to a forwarding configuration.
However, the second situation will not arise in the foreseeable
future. Instead, the transition will be from IPv4 only to a mixture
of IPv4 and IPv6, with three categories of DNS data depending on
whether the information is available only over IPv4 transport, only
over IPv6 or both.
Having DNS data available on both transports is the best situation.
The major question is how to ensure that it becomes the norm as
quickly as possible. However, while it is obvious that some DNS data
will only be available over v4 transport for a long time it is also
obvious that it is important to avoid fragmenting the name space
available to IPv4 only hosts. For example, during transition it is
not acceptable to break the name space that we presently have
available for IPv4-only hosts.
The phrase "IPv4 name server" indicates a name server available over
IPv4 transport. It does not imply anything about what DNS [1,2] data
is served. Likewise, "IPv6 [4,5,6] name server" indicates a name
server available over IPv6 transport. The phrase "dual-stack name
server" indicates a name server that is actually configured to run
both protocols, IPv4 and IPv6, and not merely a server running on a
system capable of running both but actually configured to run only
3. Policy Based Avoidance of Name Space Fragmentation
Today there are only a few DNS "zones" on the public Internet that
are available over IPv6 transport, and most of them can be regarded
as "experimental". However, as soon as the root and top level
domains are available over IPv6 transport, it is reasonable to expect
that it will become more common to have zones served by IPv6 servers.
Having those zones served only by IPv6-only name server would not be
a good development, since this will fragment the previously
unfragmented IPv4 name space and there are strong reasons to find a
mechanism to avoid it.
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