Internet Engineering Task Force                         A.Durand
INTERNET-DRAFT                             SUN Microsystems,inc.
November, 24, 2003                                      J. Ihren
Expires May 25, 2004                                  Autonomica

               DNS IPv6 transport operational guidelines

Status of this Memo

   This memo provides information to the Internet community. It does not
   specify an Internet standard of any kind. This memo is in full
   conformance with all provisions of Section 10 of RFC2026

   Internet-Drafts are draft documents valid for a maximum of six months
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Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   This memo provides guidelines and Best Current Practice to operate
   DNS in a world where queries and responses are carried in a mixed
   environment of IPv4 and IPv6 networks.


   This document is the result of many conversations that happened in
   the DNS community at IETF and elsewhere since 2001. During that
   period of time, a number of Internet drafts have been published to
   clarify various aspects of the issues at stake. This document focuses
   on the conclusion of those discussions.

   The authors would like to acknowledge the role of Pekka Savola in his
   thorough review of the document.

1. Terminology

   The phrase "IPv4 name server" indicates a name server available over
   IPv4 transport. It does not imply anything about what DNS data is
   served. Likewise, "IPv6 name server" indicates a name server
   available over IPv6 transport. The phrase "dual-stack DNS server"
   indicates a DNS 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 one.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [2119].

2. Introduction to the Problem of Name Space Fragmentation:
     following the referral chain

   The caching resolver that tries to look up a name starts out at the
   root, and follows referrals until it is referred to a nameserver that
   is authoritative for the name.  If somewhere down the chain of
   referrals it is referred to a nameserver that is only accessible over
   an unavailable type of transport, a traditional nameserver 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
   nameservers for certain nodes are only accessible over a certain
   transport. What is feared is that a node using only a particular
   version of IP, 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
   through a "translator", i.e. they have to use a second 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 old legacy IPv4 name servers
   having to switch to a forwarding configuration.

   However, the second situation will not arise in a foreseeable time.
   Instead, it is expected that the transition will be from IPv4 only to
   a mixture of IPv4 and IPv6, with DNS data of theoretically three
   categories depending on whether it 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 as quickly as possible
   becomes the norm. 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. I.e. during transition it is not
   acceptable to break the name space that we presently have available
   for IPv4-only hosts.

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.

   The RECOMMENDED approach to maintain name space continuity is to use
   administrative policies, as described in the next section.

4. DNS IPv6 Transport RECOMMENDED Guidelines

   In order to preserve name space continuity, the following administrative
   policies are RECOMMENDED:
      - every recursive DNS server SHOULD be either IPv4-only or dual
      - every single DNS zone SHOULD be served by at least one IPv4
      reachable DNS server.

   This rules out IPv6-only DNS servers performing full recursion and
   DNS zones served only by IPv6-only DNS servers.  However, one could
   very well design a configuration where a chain of IPv6 only DNS
   servers forward queries to a set of dual stack DNS servers actually
   performing those recursive queries.  This approach could be revisited
   if/when translation techniques between IPv4 and IPv6 were to be
   widely deployed.

   In order to help enforcing the second point, the optional operational
   zone validation processes SHOULD ensure that there is at least one
   IPv4 address record available for the name servers of any child
   delegations within the zone.

5. Security Considerations

   Being a critical piece of the Internet infrastructure, the DNS is a
   potential value target and thus should be protected.  Great care
   should be taken not to weaken the security of DNS while introducing
   IPv6 operation.

   Keeping the DNS name space from fragmenting is a critical thing for
   the availability and the operation of the Internet; this memo
   addresses this issue by clear and simple operational guidelines.

   The RECOMMENDED guidelines are compatible with the operation of
   DNSSEC and do not introduce any new security issues.

6. Author Addresses

   Alain Durand
   SUN Microsystems, Inc
   17 Network circle UMPK17-202
   Menlo Park, CA, 94025

   Johan Ihren
   Bellmansgatan 30
   SE-118 47 Stockholm, Sweden

7. Normative References

   [2119]  Bradner, S., "Key Words for Use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.

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