RFC9476: The .alt Special-Use Top-Level Domain

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Internet Engineering Task Force (IETF)                         W. Kumari
Request for Comments: 9476                                        Google
Category: Standards Track                                     P. Hoffman
ISSN: 2070-1721                                                    ICANN
                                                          September 2023

                 The .alt Special-Use Top-Level Domain


   This document reserves a Top-Level Domain (TLD) label "alt" to be
   used in non-DNS contexts.  It also provides advice and guidance to
   developers creating alternative namespaces.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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   Copyright (c) 2023 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
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   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
     1.2.  Requirements Terminology
   2.  The .alt Namespace
   3.  IANA Considerations
     3.1.  Special-Use Domain Name Registry
     3.2.  Domain Name Reservation Considerations
   4.  Privacy Considerations
   5.  Security Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Authors' Addresses

1.  Introduction

   Many Internet protocols need to name entities.  Names that look like
   DNS names (a series of labels separated with dots) have become
   common, even in systems that are not part of the global DNS
   administered by IANA.  This document reserves the top-level label
   "alt" (short for "alternative") as a special-use domain name
   [RFC6761].  This top-level label can be used as the final (rightmost)
   label to signify that the name is not rooted in the global DNS and
   that it should not be resolved using the DNS protocol.

   Throughout the rest of this document, the top-level "alt" label is
   shown as ".alt" to match the common presentation form of DNS names.

   As detailed in Section 3.1, IANA has added the .alt name to the
   "Special-Use Domain Name" registry.  IANA sets aside names in that
   registry, as described in <https://www.iana.org/domains/reserved>.

   The techniques in this document are primarily intended to address
   some of the issues discussed in [RFC8244], which contains additional
   background on the issues with special-use domain names.

   In this document, ".alt" was chosen for the special-use domain name
   instead of something like "alt.arpa" so that systems that use the
   name do not have to worry that a parent of their name would be
   resolved if the name leaked to the Internet.  Historically, some
   systems that want to use non-DNS names wanted the entire name to be
   not in the DNS, and reserving ".alt" fulfills that use case.

1.1.  Terminology

   This document assumes familiarity with DNS terms; please see
   [RFC8499].  Terminology that is specific to this document is:

   DNS name:  Domain names that are intended to be used with DNS
      resolution, either in the global DNS or in some other context.

   DNS context:  The namespace anchored at the globally unique DNS root
      and administered by IANA.  This is the namespace or context that
      "normal" DNS uses.

   non-DNS context:  Any other (alternative) namespace.

   pseudo-TLD:  A label that appears in a fully qualified domain name in
      the position of a TLD, which is not part of the global DNS.  This
      term is not intended to be pejorative.

   TLD:  See the definition in Section 2 of [RFC8499].

1.2.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.  The .alt Namespace

   This document reserves the .alt label for use as an unmanaged pseudo-
   TLD namespace.  The .alt label can be used in any domain name as a
   pseudo-TLD to signify that this is an alternative (non-DNS) namespace
   and should not be looked up in a DNS context.

   This document uses ".alt" for the pseudo-TLD in the presentation
   format for the DNS, corresponding to a 0x03616c7400 suffix in DNS
   wire format.  The on-the-wire formats for non-DNS protocols might be

   Because names beneath .alt are in an alternative namespace, they have
   no significance in the regular DNS context.  DNS stub and recursive
   resolvers do not need to look them up in the DNS context.

   DNS resolvers that serve the DNS protocol and non-DNS protocols at
   the same time might consider .alt like a DNS entry in the "Transport-
   Independent Locally-Served DNS Zone Registry" that is part of IANA's
   "Locally-Served DNS Zones" registry, except that .alt is always used
   to denote names that are to be resolved by non-DNS protocols.  Note
   that this document does not request adding .alt to these registries
   because .alt, by this specification, is not a DNS name.

   Note that using .alt as a pseudo-TLD does not mandate how the non-DNS
   protocol will handle the name.  To maximize compatibility with
   existing applications, it is suggested, but not required, that non-
   DNS protocols using names that end in .alt follow DNS name syntax.
   If the non-DNS protocol has a wire format like the DNS wire format,
   it might append the null label at the end of the name, but it also
   might not.  This document does not make any suggestion for how non-
   DNS protocols deal with the wire format of their names.

   Groups wishing to create new alternative namespaces may create their
   alternative namespace under a label that names their namespace under
   the .alt pseudo-TLD.  This document defines neither a registry nor a
   governance model for the .alt namespace, as it is not managed by the
   IETF or IANA.  There is no guarantee of unambiguous mappings from
   names to name resolution mechanisms.  Mitigation or resolution of
   collisions that occur under .alt are outside the scope of this
   document and outside the IETF's remit.  Users are advised to consider
   the associated risks when using names under .alt.

   Regardless of the expectations above, names in the .alt pseudo-TLD
   will leak outside the context in which they are valid.  Decades of
   experience show that such names will appear at recursive resolvers
   and will thus also appear at the root servers for the global DNS.

   Sending traffic to the root servers that is known to always elicit an
   NXDOMAIN response, such as queries for names ending in .alt, wastes
   resources on both the resolver and the root server.  Caching
   resolvers performing aggressive use of DNSSEC-validated caches
   (described in [RFC8198]) may mitigate this by synthesizing negative
   answers from cached NSEC records for names under .alt.  Similarly,
   caching resolvers using QNAME minimization (described in [RFC9156])
   will cause less of this traffic to the root servers because the
   negative responses will cover all names under .alt.

   Currently deployed projects and protocols that are using pseudo-TLDs
   are recommended to move under the .alt pseudo-TLD, but this is not a
   requirement.  Rather, the .alt pseudo-TLD is being reserved so that
   current and future projects of a similar nature have a designated
   place to create alternative resolution namespaces that will not
   conflict with the regular DNS context.

3.  IANA Considerations

3.1.  Special-Use Domain Name Registry

   The IANA has added the .alt name to the "Special-Use Domain Name"
   registry [RFC6761] with a reference to this RFC.

3.2.  Domain Name Reservation Considerations

   This section exists to meet the requirements of [RFC6761].  The
   questions posed in [RFC6761] were largely written assuming a DNS
   resolution system, and so some of the questions are not especially
   relevant or well suited.

   1.  Users might or might not recognize that names in the .alt pseudo-
       TLD as special.

   2.  Application software that uses alternative namespaces in the .alt
       pseudo-TLD are expected to have their own processing rules for
       their own names, probably in specialized resolver APIs,
       libraries, and/or application software.  Application software
       that is not specifically designed to use names in the .alt
       pseudo-TLD are not expected to make their software recognize
       these names as special.

   3.  Developers of name resolution APIs and libraries that are
       specifically designed to implement resolution of an alternative
       name resolution system are expected to recognize names in the
       .alt pseudo-TLD as special and thus perform resolution of those
       names.  The exact mechanism used by the name resolution APIs and
       libraries will obviously depend on the particular alternative
       resolution system.  Regular DNS resolution APIs and libraries are
       not expected to recognize or treat names in the .alt pseudo-TLD

   4.  Caching DNS servers SHOULD NOT recognize names in the .alt
       pseudo-TLD as special and SHOULD NOT perform any special handling
       with them.

   5.  Authoritative DNS servers SHOULD NOT recognize names in the .alt
       pseudo-TLD as special and SHOULD NOT perform any special handling
       with them.

   6.  DNS server operators will treat names in the .alt pseudo-TLD as
       they would names in any other TLD not in the global DNS.  DNS
       server operators may be aware that queries for names ending in
       .alt are not DNS names and that queries for those names were
       leaked into the DNS context.  This information can be useful for
       support or debugging purposes.

   7.  It is not possible for DNS registries/registrars to register DNS
       names in the .alt pseudo-TLD as the .alt will not exist in the
       global DNS root.

4.  Privacy Considerations

   This document reserves .alt to be used to indicate that a name is not
   a DNS name.  Unfortunately, these queries will undoubtedly leak into
   the global DNS.  This is a general problem with alternative
   namespaces and not confined to names ending in .alt.

   For example, a value such as "example.alt" could easily cause a
   privacy issue for any names in that namespace that are leaked to the
   Internet.  In addition, if a name ending in .alt is sufficiently
   unique, long-lasting, and frequently leaks into the global DNS, then
   regardless of how the name is constructed, it can act similar to a
   web cookie with all the associated downsides of identification or re-

5.  Security Considerations

   Because names in the .alt pseudo-TLD are explicitly outside of the
   DNS context, it is impossible to rely on any DNS-related security
   considerations.  Care must be taken when mapping the pseudo-TLD into
   its corresponding non-DNS name resolution system in order to get
   whatever security is offered by that system.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,

   [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>.

   [RFC8244]  Lemon, T., Droms, R., and W. Kumari, "Special-Use Domain
              Names Problem Statement", RFC 8244, DOI 10.17487/RFC8244,
              October 2017, <https://www.rfc-editor.org/info/rfc8244>.

6.2.  Informative References

   [RFC8198]  Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
              DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
              July 2017, <https://www.rfc-editor.org/info/rfc8198>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

   [RFC9156]  Bortzmeyer, S., Dolmans, R., and P. Hoffman, "DNS Query
              Name Minimisation to Improve Privacy", RFC 9156,
              DOI 10.17487/RFC9156, November 2021,


   We would like to thank Joe Abley, Mark Andrews, Erik Auerswald, Roy
   Arends, Ray Bellis, Vittorio Bertola, Marc Blanchet, John Bond,
   Stéphane Bortzmeyer, David Cake, Vint Cerf, David Conrad, Steve
   Crocker, Vladimir Cunat, Brian Dickson, Ralph Droms, Robert Edmonds,
   Patrik Fältström, Bernd Fix, Christian Grothoff, Olafur Gudmundsson,
   Ted Hardie, Bob Harold, Wes Hardaker, Geoff Huston, Joel Jaeggli,
   John C Klensin, Eliot Lear, Barry Leiba, Ted Lemon, Edward Lewis,
   John Levine, George Michaelson, Ed Pascoe, Libor Peltan, Jim Reid,
   Martin Schanzenbach, Ben Schwartz, Arturo Servin, Peter Thomassen,
   Paul Vixie, Duane Wessels, Paul Wouters, and Suzanne Woolf for

   This document was many years in the making, and we would like to
   sincerely apologize for anyone whom we forgot to credit.

   We would also like to thank Rob Wilton for serving as Responsible AD
   for this document.

   In addition, Andrew Sullivan was an author from adoption (2015)
   through version 14 (2021).

Authors' Addresses

   Warren Kumari
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States of America
   Email: warren@kumari.net

   Paul Hoffman
   Email: paul.hoffman@icann.org