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Network Working Group                                          A. DurandRequest for Comments: 3901                        SUN Microsystems, Inc.BCP: 91                                                         J. IhrenCategory: Best Current Practice                               Autonomica                                                          September 2004DNS IPv6 Transport Operational GuidelinesStatus 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 workDurand & 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:      - 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.      - 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 20047.  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",BCP9,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.seDurand & Ihren           Best Current Practice                  [Page 4]

RFC 3901             DNS IPv6 Transport Guidelines        September 20049.  Full Copyright Statement   Copyright (C) The Internet Society (2004).   This document is subject to the rights, licenses and restrictions   contained inBCP 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 inBCP 78 andBCP 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 athttp://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]