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The ALT Special Use Top Level Domain

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9476.
Authors Warren "Ace" Kumari , Paul E. Hoffman
Last updated 2023-04-27 (Latest revision 2023-04-10)
Replaces draft-wkumari-dnsop-alt-tld
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Suzanne Woolf
Shepherd write-up Show Last changed 2023-02-28
IESG IESG state Became RFC 9476 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Robert Wilton
Send notices to
IANA IANA review state IANA OK - Actions Needed
dnsop                                                          W. Kumari
Internet-Draft                                                    Google
Intended status: Standards Track                              P. Hoffman
Expires: 12 October 2023                                           ICANN
                                                           10 April 2023

                  The ALT Special Use Top Level Domain


   This document reserves a TLD label, "alt" to be used in non-DNS
   contexts.  It also provides advice and guidance to developers
   developing alternative namespaces.

   [ This document is being collaborated on in Github at
   <>.  The most
   recent version of the document, open issues, etc should all be
   available here.  The authors (gratefully) accept pull requests. ]

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

   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 12 October 2023.

Copyright Notice

   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
   Provisions Relating to IETF Documents (
   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

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   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.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Terminology  . . . . . . . . . . . . . . . .   3
   2.  The alt Namespace . . . . . . . . . . . . . . . . . . . . . .   3
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Special-Use Domain Name Registry  . . . . . . . . . . . .   5
     3.2.  Domain Name Reservation Considerations  . . . . . . . . .   5
   4.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Changes / Author Notes.  . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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

   In Section 3.1, the IANA is requested to add the .alt name to the
   "Special-Use Domain Name" registry.  IANA sets aside names in that
   registry, as described in

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

   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 "" so that systems that use the
   name do not have to worry that a parent of their name would be

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   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, 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, but 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.

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   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 definition, 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
   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.

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3.  IANA Considerations

3.1.  Special-Use Domain Name Registry

   The IANA is requested to add the .alt name to the "Special-Use Domain
   Name" registry ([RFC6761]), and reference this document.

3.2.  Domain Name Reservation Considerations

   This section exists to meet the requirements of [RFC6761].  The
   questions posed in RFC 6761 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.  Writers 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 differently.

   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 name ending in .alt are not
   DNS names, and queries for those names were leaked into the DNS
   context.  This information can be useful for support or debugging

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   7.  DNS registries/registrars for the global DNS will never register
   names in the .alt pseudo-TLD because .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 value is constructed, that value can act
   similar to a web cookie with all the associated downsides of

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.  Acknowledgements

   We would like to thank Joe Abley, Mark Andrews, Erik Auerswald, Roy
   Arends, Ray Bellis, Vittorio Bertola, Marc Blanchet, John Bond,
   Stephane Bortzmeyer, David Cake, Vint Cerf, David Conrad, Steve
   Crocker, Vladimir Cunat, Brian Dickson, Ralph Droms, Robert Edmonds,
   Patrik Faltstrom, 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 who we forgot to credit.

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

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

7.  References

7.1.  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, <>.

   [RFC8244]  Lemon, T., Droms, R., and W. Kumari, "Special-Use Domain
              Names Problem Statement", RFC 8244, DOI 10.17487/RFC8244,
              October 2017, <>.

7.2.  Informative References

   [RFC8198]  Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
              DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
              July 2017, <>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <>.

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

Appendix A.  Changes / Author Notes.

   [RFC Editor: Please remove this section before publication ]

   From -22 to -23:

   *  Small changes based on inputs from IETF Last Call.

   From -21 to -22:

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   *  Addressed issues from AD review -

   *  Combined some of the acknowledgements into one paragraph.

   From -20 to -21:

   *  During WGLC review, replaced the descriptive text with the
      requirements from RFC 6761 with a list.  This in turn required
      adding in the BCP 14 boilerplate.

   *  During WGLC review, made a few more requested changes

   From -19 to -20:

   *  Expanded the privacy considerations

   *  Clarified benefit of using aggressive NSEC

   *  Clarified that the .alt namespace is unmanaged and thus comes with

   *  Added description of why .alt was chosen instead of

   *  Removed 2119 language because there are no MUSTs or SHOULDs

   From -18 to -19:

   *  Document was discussed at IETF115

   *  Changed the intended status to Standards Track at the request of
      the responsible AD (Rob Wilton)

   *  Clarified that this only deals with some of the problems from RFC

   *  Removed text telling protocol designers that they should
      differentiate their names from other designers

   *  Added a note that .alt names will leak out of the local context

   *  Reminded resolver operators that there are already ways to reduce
      .alt traffic to the root servers

   *  Moved the paragraph related to 6761 to the IANA Considerations

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   *  Strengthened the security considerations

   *  Added references for QNAME minimization and agressive NSEC caching

   From -16 to -18:

   *  Lots of editorial fix-ups

   *  Fixed reference to RFC 8499

   *  Clarified presentation format for .alt

   *  Clarified that IANA will set aside the name when it goes into the
      6761 registry

   *  Removed the loose registry for names under .alt

   *  Added back the required discussion for RFC 6761

   From -15 to -16:

   *  Many simplifications to focus the document on the technical bits
      as much as possible, based on mailing list feedback.

   *  Removed unused references.

   *  Removed the RFC 2119 language because it is no longer used in the

   *  Added a non-normative IANA registry.

   *  Added Paul Hoffman as second author to help get the draft moving
      in the DNSOP WG again.

   From -14 to -15:

   *  [Pinky]: Gee, Brain.  What are we going to do tonight?

   *  [The Brain]: The same thing we do every 6 months, Pinky.  Post a
      new version of this document, with only the version number

   From -13 to -14:

   *  Andrew asked to be removed as co-author, due to potential
      perception of CoI.

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   *  Erik Auerswald provided Github issues and comments re: references
      and grammar.

   From -12 to -13:

   *  Just bumping versions to prevent expiration.

   From -08 to -12:

   *  Just bumping versions to prevent expiration.

   *  Updated references (aggressive-nsec is now RFC 8198, draft-ietf-
      dnsop-sutld-ps is now 8244).

   From -07 to -08:

   *  Made it clear that this is only for non-DNS.

   *  As per Interim consensus, removed the "add this to local zones"

   *  Added a Privacy Considerations section

   *  Grammar fix -- "alternative" is more correct than "alternate",

   From -06 to -07:

   *  Rolled up the GItHub releases in to a full release.

   From -07.2 to -07.3 (GitHub point release):

      Removed 'sandbox' at Stephane's suggestion -

      Suggested (in 4.1 bullet 3) that DNS libraries ignore these -- Bob
      Harold -

      Added some pointers to the SUTLD document.

   From -07.1 to -07.2 (Github point release):

   *  Reverted the <TBD> string (at request of chairs).

   *  Added an editors note explaining the above.

   *  Removed some more background, editorializing, etc.

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   From -06 to -07.1 (

   *  Replaced ALT with <TBD> at the suggestions of George.

   From -05 to -06:

   *  Removed a large amount of background - we now have the (adopted)
      tldr document for that.

   *  Made it clear that pseudo-TLD is not intended to be pejorative.

   *  Tried to make it cleat that this is something people can choose to
      use - or not.

   From -04 to -05:

   *  Version bump - we are waiting in the queue for progress on SUN,
      bumping this to keep it alive.

   From -03 to -04:

   *  3 changes - the day, the month and the year (a bump to keep

   From -02 to -03:

   *  Incorporate suggestions from Stephane and Paul Hoffman.

   From -01 to -02:

   *  Merged a bunch of changes from Paul Hoffman.  Thanks for sending a
      git pull.

   From -00 to 01:

   *  Removed the "delegated to new style AS112 servers" text -this was
      legacy from the omnicient AS112 days.  (Joe Abley)

   *  Removed the "Advice to implemntors" section.  This used to
      recommend that people used a subdomain of a domain in the DNS.  It
      was pointed out that this breaks things badly if the domain

   *  Added text about why we don't want to adminster a registry for

   From Individual-06 to DNSOP-00

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   *  Nothing changed, simply renamed draft-wkumari-dnsop-alt-tld to

   From -05 to -06

   *  Incorporated comments from a number of people, including a number
      of suggestion heard at the IETF meeting in Dallas, and the DNSOP
      Interim meeting in May, 2015.

   *  Removed the "Let's have an (optional) IANA registry for people to
      (opportinistically) register their string, if they want that
      option" stuff.  It was, um, optional....

   From -04 to -05

   *  Went through and made sure that I'd captured the feedback

   *  Comments from Ed Lewis.

   *  Filled in the "Domain Name Reservation Considerations" section of

   *  Removed examples from .Onion.

   From -03 to -04

   *  Incorporated some comments from Paul Hoffman

   From -02 to -03

   *  After discussions with chairs, made this much more generic (not
      purely non-DNS), and some cleanup.

   From -01 to -02

   *  Removed some fluffy wording, tightened up the language some.

   From -00 to -01.

   *  Fixed the abstract.

   *  Recommended that folk root their non-DNS namespace under a DNS
      namespace that they control (Joe Abley)

Authors' Addresses

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   Warren Kumari
   1600 Amphitheatre Parkway
   Mountain View, CA,  94043
   United States of America

   Paul Hoffman

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