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rfc:rfc3901

Network Working Group A. Durand Request for Comments: 3901 SUN Microsystems, Inc. BCP: 91 J. Ihren Category: Best Current Practice Autonomica

                                                        September 2004
             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 Notice

 Copyright (C) The Internet Society (2004).

Abstract

 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

Durand & Ihren Best Current Practice [Page 1] 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.

2. Terminology

 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
 one.

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.

Durand & Ihren Best Current Practice [Page 2] RFC 3901 DNS IPv6 Transport Guidelines September 2004

 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:
  1. every recursive name server SHOULD be either IPv4-only or dual

stack,

       This rules out IPv6-only recursive servers.  However, one might
       design configurations where a chain of IPv6-only name server
       forward queries to a set of dual stack recursive name server
       actually performing those recursive queries.
  1. every DNS zone SHOULD be served by at least one IPv4-reachable

authoritative name server.

       This rules out DNS zones served only by IPv6-only authoritative
       name servers.
 Note: 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

 The guidelines described in this memo introduce no new security
 considerations into the DNS protocol or associated operational
 scenarios.

6. Acknowledgment

 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.

Durand & Ihren Best Current Practice [Page 3] RFC 3901 DNS IPv6 Transport Guidelines September 2004

7. Normative References

 [1]  Mockapetris, P., "Domain names - concepts and facilities", STD
      13, RFC 1034, November 1987.
 [2]  Mockapetris, P., "Domain names - implementation and
      specification", STD 13, RFC 1035, November 1987.
 [3]  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.
 [4]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
      Specification", RFC 2460, December 1998.
 [5]  Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
      Addressing Architecture", RFC 3513, April 2003.
 [6]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, "DNS
      Extensions to Support IP Version 6", RFC 3596, October 2003.

8. Authors' Addresses

 Alain Durand
 SUN Microsystems, Inc
 17 Network circle UMPK17-202
 Menlo Park, CA, 94025
 USA
 EMail: Alain.Durand@sun.com
 Johan Ihren
 Autonomica
 Bellmansgatan 30
 SE-118 47 Stockholm
 Sweden
 EMail: johani@autonomica.se

Durand & Ihren Best Current Practice [Page 4] RFC 3901 DNS IPv6 Transport Guidelines September 2004

9. Full Copyright Statement

 Copyright (C) The Internet Society (2004).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/S HE
 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
 INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
 IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the IETF's procedures with respect to rights in IETF Documents can
 be found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at ietf-
 ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Durand & Ihren Best Current Practice [Page 5]

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