DNS IPv6 Transport Operational Guidelines
draft-ietf-dnsop-3901bis-02

dnsop                                                          Momoka. Y
Internet-Draft                                              WIDE Project
Obsoletes: 3901 (if approved)                                  T. Fiebig
Intended status: Best Current Practice                           MPI-INF
Expires: 9 October 2025                                     7 April 2025

               DNS IPv6 Transport Operational Guidelines
                      draft-ietf-dnsop-3901bis-02

Abstract

   This memo provides guidelines and documents Best Current Practice for
   operating authoritative DNS servers as well as recursive and stub DNS
   resolvers, given that queries and responses are carried in a mixed
   environment of IPv4 and IPv6 networks.  This document expands on RFC
   3901 by recommending that authoritative DNS servers as well as
   recursive DNS resolvers support both IPv4 and IPv6.  It furthermore
   provides guidance on recursive DNS resolver selection for stub DNS
   resolvers.

   This document obsoletes RFC3901. (if approved)

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-dnsop/draft-ietf-dnsop-3901bis.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 9 October 2025.

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Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Name Space Fragmentation  . . . . . . . . . . . . . . . . . .   4
     3.1.  Misconfigurations Causing IP Version Related Name Space
           Fragmentation . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Network Conditions Causing IP Version Related Name Space
           Fragmentation . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Reasons for Intentional IP Version Related Name Space
           Fragmentation . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Policy Based Avoidance of Name Space Fragmentation  . . . . .   7
     4.1.  Guidelines for Authoritative DNS Server Configuration . .   7
     4.2.  Guidelines for DNS Resolvers  . . . . . . . . . . . . . .   8
     4.3.  Guidelines for DNS Stub Resolvers . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   References  . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     Normative References  . . . . . . . . . . . . . . . . . . . . .  10
     Informative References  . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Despite IPv6 being first discussed in the mid-1990s [RFC1883],
   consistent deployment throughout the whole Internet has not yet been
   accomplished [RFC9386].  Hence, today, the Internet is a mixture of
   IPv4, dual-stack (networks connected via both IP versions), and IPv6
   networks.

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   This creates a complex landscape where authoritative DNS servers
   might be accessible only via specific network protocols
   [V6DNSRDY-23].  At the same time, DNS resolvers may only be able to
   access the Internet via either IPv4 or IPv6.  This poses a challenge
   for such resolvers because they may encounter names for which queries
   must be directed to authoritative DNS servers with which they do not
   share an IP version during the name resolution process.

   [RFC3901] was initially written at a time when IPv6 deployment was
   not widespread, focusing primarily on maintaining namespace
   continuity within the IPv4 landscape.  Now, nearly two decades later,
   with IPv6 not only widely deployed but also becoming the de facto
   standard in many areas, this document seeks to expand the scope of
   RFC3901 by recommending IPv6 compatibility for authoritative DNS
   servers, as well as recursive and stub DNS resolvers.

   In this document, we discuss:

   *  IP version related namespace fragmentation and best-practices for
      avoiding it.

   *  Guidelines for configuring authoritative DNS servers for zones.

   *  Guidelines for operating recursive DNS resolvers.

   *  Guidelines for stub DNS resolvers.

   While transitional technologies and dual-stack setups may mitigate
   some of the issues of DNS resolution in a mixed protocol-version
   Internet, making DNS data accessible over both IPv4 and IPv6 is the
   most robust and flexible approach, as it allows resolvers to reach
   the information they need without requiring intermediary translation
   or forwarding services which may introduce additional failure cases.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Terminology

   This document uses DNS terminology as described in [RFC9499].
   Furthermore, the following terms are used with a defined meaning:

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   IPv4 name server:
      A name server providing DNS services reachable via IPv4.  It does
      not imply anything about what DNS data is served, but requires DNS
      queries to be received and answered over IPv4.

   IPv6 name server:
      A name server providing DNS services reachable via IPv6.  It does
      not imply anything about what DNS data is served, but requires DNS
      queries to be received and answered over IPv6.

   Dual-stack name server:
      A name server that is both an "IPv4 name server" and also an "IPv6
      name server".

3.  Name Space Fragmentation

   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, based on the referral, only
   accessible over a transport which the resolver cannot use, the
   resolver is unable to continue DNS resolution.

   If this occurs, the DNS has, effectively, fragmented based on the
   recursive DNS resolver's and authoritative DNS server's mismatching
   IP version support.

   In a mixed IP Internet, fragmentation can occur for different
   reasons.  One reason is that DNS zones are consistently configured to
   support only either IPv4 or IPv6.  Another reason is due to
   misconfigurations that make a zone unresolvable by either IPv4 or
   IPv6-only resolvers.  The latter cases are often hard to identify, as
   the impact of misconfigurations for only one IP version (IPv4 or
   IPv6) may be hidden in a dual-stack setting.  In the worst case, a
   specific name may only be resolvable via dual-stack enabled
   resolvers.

3.1.  Misconfigurations Causing IP Version Related Name Space
      Fragmentation

   Even when an administrator assumes that they have enabled support for
   a specific IP version on their authoritative DNS server, various
   misconfigurations may break the DNS delegation chain of a zone for
   that protocol and prevent any of its records from resolving for
   clients only supporting that IP version.  These misconfigurations can
   be kept hidden if most clients can successfully fall back to the
   other IP version.

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   The following name related misconfigurations can cause broken
   delegation for one IP version:

   No A/AAAA records for NS names:
      If all of the NS records for a zone in their parent zone have
      either only A records or only AAAA records, then resolution via
      the other IP version is not possible.

   Missing GLUE:
      If the name from an NS record for a zone is in-domain, i.e., the
      name is within the zone or below, a parent zone must contain both
      IPv4 and IPv6 GLUE records, i.e., a parent must serve the
      corresponding A and AAAA records as ADDITIONAL data when returning
      the NS record(s) as the referral response.

   No A/AAAA record for in-domain NS:
      If an NS record of a child zone, either provided by the parent or
      from the child zone's apex, points to a name in the NS RDATA that
      is in-domain but the name does not contain corresponding A or AAAA
      record(s) in the child zone, name resolution via the concerned IP
      version will fail even if the parent provides GLUE, when the
      recursive DNS resolver revalidates the delegation path
      [I-D.ietf-dnsop-ns-revalidation].

   Zone of sibling domain NSes not resolving:
      If the name from an NS record for a zone is sibling domain, the
      corresponding zone must be resolvable via the IP version in
      question as well It is insufficient if the name pointed to by the
      NS record has an associated A or AAAA record correspondingly.

   Parent zone not resolvable via one IP version:
      For a zone to be resolvable via an IP version the parent zones up
      to the root zone must be resolvable via that IP version as well.
      Any zone not resolvable via the concerned IP version breaks the
      delegation chain for all its children.

   The above misconfigurations are not mutually exclusive.

   Furthermore, any of the misconfigurations above may not only
   materialize via a missing Resource Record (RR) but also via an RR
   providing the IP address of a nameserver that is not configured to
   answer queries via that IP version [V6DNSRDY-23].

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3.2.  Network Conditions Causing IP Version Related Name Space
      Fragmentation

   In addition to explicit misconfigurations in the served DNS zones,
   network conditions may also influence a resolver's ability to resolve
   names in a zone.  The most common issue here are packets requiring
   fragmentation given a reduced path MTU (PMTU) and MTU blackholes,
   i.e., packets being dropped on-path due to exceeding the MTU of the
   link to the next-hop without the sender being notified.  This can
   manifest in the following way:

   DNS-over-UDP packets requiring fragmentation
      When using EDNS0 to communicate support for DNS messages larger
      than 512b [RFC6891], an IP packet carrying a DNS response may
      exceed the PMTU for the path to a resolver.  If an authoritative
      DNS server and does not follow [RFC9715], i.e., honors EDNS0 sizes
      larger than 1232, it will try to fragment the packet according to
      the discovered PMTU.  Such packets mostly occur for DNSKEY
      responses with DNSSEC [RFC4034].

      If the requesting resolver is unable to process fragments, or if
      fragments are filtered on-path, resolution will fail over UDP.
      These issues are more prevalent for IPv6, as it no longer allows
      on-path hosts to fragment packets.  Therefore, working Path MTU
      Discovery (PMTUD) is essential for IPv6 DNS-over-UDP packets to be
      fragmented to a size that allows them to traverse all segments on
      a path.

      [RFC9715] provides guidance on preventing this issue by always
      using a maximum EDNS0 size of 1232b.

   DNS-over-TCP packets requiring fragmentation
      If DNS resolution over UDP fails, or if a packet exceeds the
      communicated EDNS0 size, a resolver should fall back to DNS
      resolution over TCP.  However, similar to the case of DNS-over-
      UDP, DNS-over-TCP may encounter MTU blackholes, especially on
      IPv6, if PMTUD does not work, if the MSS honored by the
      authoritative DNS server leads to IP packets exceeding the PMTU.
      In that case, similar to the case of DNS-over-UDP, DNS resolution
      will time out when the recursive DNS resolver did not receive a
      response in time.

      [RFC9715] does not povide explicit guidance on mitigating this
      issue.  However, similar to the guidance in [RFC9715], setting an
      MSS of 1240b for IPv4 and 1220b for IPv6 would similarly mitigate
      this issue.

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3.3.  Reasons for Intentional IP Version Related Name Space
      Fragmentation

   Intentional IP related name space fragmentation occurs if an operator
   consciously decides not to deploy IPv4 or IPv6 for a part of the
   resolution chain.  Most commonly, this is realized by intentionally
   not listing A/AAAA records for NS names.  At the time of writing, the
   share of zones not resolvable via IPv4 is negligible, while a little
   less than 40% of zones are not resolvable via IPv6 [V6DNSRDY-23].
   However, as IPv4 exhaustion progresses, IPv6 adoption will have to
   increase.

4.  Policy Based Avoidance of Name Space Fragmentation

   With the final exhaustion of IPv4 pools in RIRs, e.g., [RIPEV4], and
   the progressing deployment of IPv6, there no longer is a "preferred"
   IP version.  Yet, while we now observe the first zones becoming
   exclusively IPv6 resolvable, we also still see a major portion of
   zones solely relying on IPv4 [V6DNSRDY-23].  Hence, at the moment,
   dual stack connectivity is instrumental to be able to resolve zones
   and avoid name space fragmentation.

   Having zones served only by name servers reachable via one IP version
   would fragment the DNS.  Hence, we need to find a way to avoid this
   fragmentation.

   The recommended approach to maintain name space continuity is to use
   administrative policies, as described in this section.

4.1.  Guidelines for Authoritative DNS Server Configuration

   It is usually recommended that DNS zones contain at least two name
   servers, which are geographically diverse and operate under different
   routing policies [IANANS].  To reduce the chance of DNS name space
   fragmentation, it is RECOMMENDED that at least two name servers for a
   zone are dual stack name servers.  Specifically, this means that the
   following minimal requirements SHOULD be implemented for a zone:

   IPv4 adoption:
      Every DNS zone SHOULD be served by at least one IPv4-reachable
      authoritative DNS server to maintain name space continuity.  The
      delegation configuration (Resolution of the parent, resolution of
      sibling domain names, GLUE) MUST NOT rely on IPv6 connectivity
      being available.  As we acknowledge IPv4 scarcity, operators MAY
      opt not to provide DNS services via IPv4, if they can ensure that
      all clients expected to resolve this zone do support DNS
      resolution via IPv6.

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   IPv6 adoption:
      Every DNS zone SHOULD be served by at least one IPv6-reachable
      authoritative DNS server to maintain name space continuity.  The
      delegation configuration (Resolution of the parent, resolution of
      sibling domain names, GLUE) MUST NOT rely on IPv4 connectivity
      being available.

   Consistency:
      Both IPv4 and IPv6 transports should serve identical DNS data to
      ensure a consistent resolution experience across different network
      types.

   Avoiding IP Fragmentation:
      IP fragmentation has been reported to be fragile [RFC8900].
      Furthermore, IPv6 transition technologies can introduce unexpected
      MTU breaks, e.g., when NAT64 is used [RFC7269].  Therefore, IP
      fragmentation SHOULD be avoided by following guidance on maximum
      DNS payload sizes [RFC9715] and providing TCP fall-back options
      [RFC7766].  Furthermore, similar to the guidance in [RFC9715], it
      is RECOMMENDED that authoritative DNS servers sets an MSS of 1240b
      for IPv4 and 1220b for IPv6 in TCP sessions carrying DNS
      responses.

   Note: zone validation processes SHOULD ensure that there is at least
   one IPv4 address record and one IPv6 address record available for the
   name servers of any child delegation within the zone.

4.2.  Guidelines for DNS Resolvers

   Every recursive DNS resolver SHOULD be dual stack.

   While the zones that IPv6-only recursive DNS resolvers can resolve
   are growing, they do not yet cover all zones.  Hence, a recursive DNS
   resolver MAY be IPv6-only, if it uses a transition mechanism that
   allows it to also query IPv4-only authoritative DNS server or uses a
   configuration where it forwards queries failing IPv6-only DNS
   resolution to a recursive DNS resolver that is able to perform DNS
   resolution over IPv4.

   If a recursive DNS resolver runs in a network that uses XLAT
   [RFC6877], and the recursive DNS resolver is aware of the used PREF64
   [RFC6146], it SHOULD synthesize mapped IPv6 addresses for remote
   authoritative DNS servers directly for DNS resolution, instead of
   relying on the socket translation layer of the operating system.
   Additionally, the PREF64 in use MAY also be statically configured for
   the DNS resolver.

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   Similarly, a recursive DNS resolver MAY be IPv4-only, if it uses a
   configuration where such resolvers forward queries failing IPv4-only
   DNS resolution to a recursive DNS resolver that is able to perform
   DNS resolution over IPv6.

   Finally, when responding to recursive queries sent by stub DNS
   resolvers, a DNS resolver SHOULD follow the above guidance for
   communication between authoritative DNS servers and recursive DNS
   resolvers analogously.

4.3.  Guidelines for DNS Stub Resolvers

   In general, DNS Stub Resolvers SHOULD follow the same guidance as
   outlined for recursive DNS resolvers when they are deployed to a
   dual-stack network not using IPv4-IPv6 transition techniques.
   Contrary to authoritative DNS servers and recursive DNS resolvers,
   stub DNS resolvers are more likely to find themselves in either an
   IPv6 mostly or IPv4 only environment, as they are usually run on end-
   hosts / clients.

   For IPv4 only environments, a stub DNS resolver has to rely on the
   provided recursive DNS server following guidance in this document.
   However, in an IPv6 mostly scenario, the environment might appear
   similar to a dual-stack scenario, and the host running the stub DNS
   resolver may receive multiple IPv4 and IPv6 addresses for possible
   DNS resolvers to use via different protocols (DHCPv4, DHCPv6, SLAAC).

   Hence, when a host running a stub DNS resolver receives addresses for
   IPv4 and IPv6 recursive DNS resolver to use, it SHOULD prioritize
   reachable IPv6 recursive DNS resolvers.  If the host is aware of a
   PREF64 being used, it SHOULD NOT use IPv4 recursive DNS resolvers for
   which it also received the corresponding mapped address in the
   PREF64, but instead SHOULD only use the supplied IPv6 address.
   Additionally, if the host receives multiple IPv6 reachable recursive
   DNS resolvers and is aware of a PREF64 being in use, it SHOULD
   prioritize recursive DNS resolvers outside the PREF64.

5.  Security Considerations

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

6.  IANA Considerations

   This document requests IANA to update its technical requirements for
   authoritative DNS servers to require both IPv4 and IPv6 addresses for
   each authoritative server [IANANS].

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Acknowledgments

   Valuable input for this draft was provided by: Bob Harold, Andreas
   Schulze, Tommy Jensen, Nick Buraglio, Jen Linkova, Tim Chown, Brian E
   Carpenter

   Thank you for reading this draft.

References

Normative References

   [I-D.ietf-dnsop-ns-revalidation]
              Huque, S., Vixie, P. A., and W. Toorop, "Delegation
              Revalidation by DNS Resolvers", Work in Progress,
              Internet-Draft, draft-ietf-dnsop-ns-revalidation-09, 27
              February 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-dnsop-ns-revalidation-09>.

   [RFC1883]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 1883, DOI 10.17487/RFC1883,
              December 1995, <https://www.rfc-editor.org/info/rfc1883>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3901]  Durand, A. and J. Ihren, "DNS IPv6 Transport Operational
              Guidelines", BCP 91, RFC 3901, DOI 10.17487/RFC3901,
              September 2004, <https://www.rfc-editor.org/info/rfc3901>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <https://www.rfc-editor.org/info/rfc4034>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, April 2013,
              <https://www.rfc-editor.org/info/rfc6877>.

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   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <https://www.rfc-editor.org/info/rfc6891>.

   [RFC7766]  Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
              <https://www.rfc-editor.org/info/rfc7766>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC9715]  Fujiwara, K. and P. Vixie, "IP Fragmentation Avoidance in
              DNS over UDP", RFC 9715, DOI 10.17487/RFC9715, January
              2025, <https://www.rfc-editor.org/info/rfc9715>.

Informative References

   [I-D.ietf-v6ops-ipv6-only-resolver]
              Yamamoto, M. and Y. Toyota, "IPv6-only Capable Resolvers
              Utilising NAT64", Work in Progress, Internet-Draft, draft-
              ietf-v6ops-ipv6-only-resolver-00, 23 October 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-v6ops-
              ipv6-only-resolver-00>.

   [IANANS]   IANA, "Technical requirements for authoritative name
              servers",
              <https://www.iana.org/help/nameserver-requirements>.

   [RFC7269]  Chen, G., Cao, Z., Xie, C., and D. Binet, "NAT64
              Deployment Options and Experience", RFC 7269,
              DOI 10.17487/RFC7269, June 2014,
              <https://www.rfc-editor.org/info/rfc7269>.

   [RFC8900]  Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
              and F. Gont, "IP Fragmentation Considered Fragile",
              BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
              <https://www.rfc-editor.org/info/rfc8900>.

   [RFC9386]  Fioccola, G., Volpato, P., Palet Martinez, J., Mishra, G.,
              and C. Xie, "IPv6 Deployment Status", RFC 9386,
              DOI 10.17487/RFC9386, April 2023,
              <https://www.rfc-editor.org/info/rfc9386>.

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   [RFC9499]  Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
              RFC 9499, DOI 10.17487/RFC9499, March 2024,
              <https://www.rfc-editor.org/info/rfc9499>.

   [RIPEV4]   RIPE NCC, "The RIPE NCC has run out of IPv4 Addresses",
              November 2019, <https://www.ripe.net/publications/news/
              about-ripe-ncc-and-ripe/the-ripe-ncc-has-run-out-of-
              ipv4-addresses>.

   [V6DNSRDY-23]
              Streibelt, F., Sattler, P., Lichtblau, F., Hernandez-
              Gañán, C., Gasser, O., and T. Fiebig, "How Ready is DNS
              for an IPv6-Only World?", March 2023,
              <https://link.springer.com/
              chapter/10.1007/978-3-031-28486-1_22>.

Authors' Addresses

   Momoka Yamamoto
   WIDE Project
   Email: momoka.my6@gmail.com

   Additional contact information:

      山本 桃歌
      WIDE Project

   Tobias Fiebig
   Max-Planck-Institut fuer Informatik
   Campus E14
   66123 Saarbruecken
   Germany
   Phone: +49 681 9325 3527
   Email: tfiebig@mpi-inf.mpg.de

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