Independent Submission                                         R. Arends
Internet-Draft                                                     ICANN
Intended status: Best Current Practice                          J. Abley
Expires: April 11, 2021                         Public Interest Registry
                                                        October 08, 2020

                Top-level Domains for Private Internets


   There are no defined private-use namespaces in the Domain Name System
   (DNS).  For a domain name to be considered private-use, it needs to
   be future-proof in that its top-level domain will never be delegated
   from the root zone.  The lack of a private-use namespace has led to
   locally configured namespaces with a top-level domain that is not
   future proof.

   The DNS needs an equivalent of the facilities provided by BCP 5 (RFC
   1918) for private internets, i.e. a range of short, semantic-free
   top-level domains that can be used in private internets without the
   risk of being globally delegated from the root zone.

   The ISO 3166 standard is used for the definition of eligible
   designations for country-code top-level Domains.  This standard is
   maintained by the ISO 3166 Maintenance Agency.  The ISO 3166 standard
   includes a set of user-assigned code elements that can be used by
   those who need to add further names to their local applications.

   Because of the rules set out by ISO in their standard, it is
   extremely unlikely that these user-assigned code elements would ever
   conflict with delegations in the root zone under current practices.
   This document explicitly reserves these code elements to be safely
   used as top-level domains for private DNS resolution.

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

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   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 April 11, 2021.

Copyright Notice

   Copyright (c) 2020 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
   ( 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The ISO 3166-1 alpha-2 and Two-Letter Top-Level Domains . . .   4
   3.  ISO 3166-1 alpha-2 User-assigned Code Elements  . . . . . . .   4
   4.  Examples of Current Uses of the User-assigned Code Elements.    5
   5.  Private-use top-level Domains . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   In private networks where a hostname has no utility in the global
   namespace, it is convenient to have a private-use namespace.  Such
   deployments could theoretically use sub-domains of a domain
   registered for the specific hosting entity, though not all such
   configurations have such a domain available.  When the hostname is
   solely used in a private network, it is not necessary that it
   resolves globally.

   Another situation is where applications use identifiers that are
   similar in appearance to domain names, and may be interpreted by
   software as domain names, but are not intended to use the global DNS
   resolution service.  Using a private-use namespace helps guard
   against conflicts with the global DNS resolution system.

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   Note that a private-use namespace is not a subset of a registered
   special use namespace [IANA-Special].  There is no facility to
   register a specific label using the process defined in [RFC6761].
   The process in RFC 6761 requires that a label has some kind of
   special handling in order to be considered special.  A private-use
   namespace can be considered special on a policy level, but not on a
   technical or protocol level.

   Many protocols outside the DNS have a defined set of elements for
   private use, or an identifier that indicates private use, such as
   "X-headers" MIME types [RFC2045], addresses for private internets
   [BCP-5], the "x-" sub-tag in private-use language tags [BCP-47],
   private-use Autonomous System Numbers [BCP-6], and private-use DNS
   RRTypes and RCODEs [BCP-42].

   There is currently no such facility for the DNS namespace.  A user is
   required to resort to registering a globally unique domain where a
   locally unique domain would suffice, or may configure a domain name
   that is not currently delegated from the root zone.  Additionally,
   there are plenty of examples of device vendors that ship networking
   devices with a default setting for DHCP [RFC2131] option 15 (domain
   name) [RFC2132], containing a top-level domain that is believed to
   not be delegated in the root zone.

   In practice, the lack of a private-use namespace facility has led to
   the deployment of arbitrary, unregistered, semantically meaningful
   top-level domains, such as ".home", ".dhcp", ".lan", ".localdomain",
   ".internal", ".dlink", ".ip" and ".corp" [ITHI].  These examples of
   locally configured strings are derived from traffic to the ICANN
   Managed Root Servers [IMRS] and are part of the most popular observed
   query names [BCP-219].

   While these commonly chosen strings currently do not exist in the
   root zone, there is no guarantee that these strings will not be
   delegated in the root zone in the future.  Therefore, there is no
   guarantee that the local use of these strings (or other strings that
   might be chosen for private use) will be stable, safe, and secure.
   Similarly, there is no guarantee that any of these strings will be
   deemed special-use via an application according to [RFC6761].

   There are many uses for private-use names.  It is not feasible to
   assign a semantically meaningful, relatively short top-level label to
   each individual private-use of a namespace in multiple languages.
   Similar to "X-headers" MIME types, and analogous in concept to
   address allocation for private internets, this document defines a
   range of abstract, two-letter labels that are aligned with the user-
   assigned two-letter code elements in the ISO 3166-1 alpha-2
   [ISO3166-1] standard.

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2.  The ISO 3166-1 alpha-2 and Two-Letter Top-Level Domains

   IANA's practice of governing the delegation of ASCII two-letter
   domain names in the DNS [STD13] root zone is to align it with
   assignment of two-letter (known as "alpha-2") code elements in the
   ISO 3166-1 standard [ISO3166-1].  The ISO 3166-1 standard contains
   many categories of code elements, with the "officially assigned" and
   some "exceptionally reserved" code elements being used in the DNS to
   represent entities as country-code top-level domains (ccTLDs)
   [RFC1591].  The interrelationship is documented in "ICANN and the
   ISO, A Common Interest in ISO Standard 3166" [ICANNISO].

   In addition to the assigned, available, and reserved code elements,
   there are code elements designated as "user-assigned".  The intent of
   user-assigned code elements is to provide the user with a code
   element when no other code element satisfies the intended use.

3.  ISO 3166-1 alpha-2 User-assigned Code Elements

   The ISO 3166-1 standard states in section 5.2:

      "In addition, exactly 42 alpha-2 code elements are not used in the
      ISO 3166, AA, QM to QZ, XA to XZ, ZZ."

   And explains in clause 8.1 "Special Provisions":

      "Users sometimes need to extend or alter the use of country-code
      elements for special purposes.  The following provisions give
      guidance for meeting such needs within the framework of this part
      of ISO 3166. "

   And finally, clause 8.1.3 "User assigned code element":

      "If users need code elements to represent country names not
      included in this part of ISO 3166, the series of letters AA, QM to
      QZ, XA to XZ, and ZZ, and the series AAA to AAZ, QMA to QZZ, XAA
      to XZZ, and ZZA to ZZZ respectively and the series of numbers 900
      to 999 are available.  NOTE Users are advised that the above
      series of codes are not universals, those code elements are not
      compatible between different entities."

   As shown above, the ISO 3166-1 user-assigned alpha-2 code elements
   are defined to be AA, QM to QZ, XA to XZ, and ZZ.  The ranges ("to")
   are alphabetic and contain only characters in the US-ASCII definition

   It is unlikely that the user-assigned range will change.

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4.  Examples of Current Uses of the User-assigned Code Elements.

   Using code elements to represent an entity other than a country name
   may appear to deviate from the intended use of the ISO 3166-1
   standard.  However, many organizations, including the IETF and the
   ISO, have used the user-assigned range to represent entities other
   than country names.  The following list is not exhaustive but
   illustrates the wide variety of current uses of codes within the ISO
   3166-1 user-assigned alpha-2 range.

   o  The International Standard Recording Code (ISRC) [ISO3901] uses
      code element "ZZ" from the User-assigned range for direct
      registrants independent of any country.

   o  The ISO Currency Codes standard [ISO4217] uses code elements "XA"
      to "XZ" from the user-assigned range for transactions and precious

   o  International Securities Identification Numbers [ISO6166] uses the
      following code elements from the user-assigned range:

      QS: internally used by Euroclear France

      QT: internally used in Switzerland

      QW: internally used in WM Datenservice Germany for historical data

      XA: CUSIP Global Services substitute agencies

      XB: NSD Russia substitute agencies

      XC: WM Datenservice Germany substitute agencies

      XD: SIX Telekurs substitute agencies

      XF: internally assigned, non-unique numbers

      XS: Euroclear and Clearstream international securities

   o  The International Civil Aviation Organization [ICAO] Machine
      Readable Travel Documents standard uses code element "ZZ" from the
      user-assigned range for UN travel documents.

   o  The World Intellectual Property Organization [WIPO] Standard 3
      uses the following code elements from the user-assigned range:

      QZ: Community Plant Variety Office (European Union) (CPVO).

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      XN: Nordic Patent Institute (NPI).

      XU: International Union for the Protection of New Varieties of
      Plants (UPOV).

      XV: Visegrad Patent Institute (VPI)

      XX: recommended to refer to unknown states, other entities or

   o  The United Nations Code for Trade and Transport Locations
      [UNLOCODE] uses the code element "XZ" from the user-assigned range
      for international waters in accordance with ISO 3166-1 clause

      "3.2.5 In cases where no ISO 3166 country-code element is
      available, e.g. installations in international waters or
      international cooperation zones, the code element "XZ", available
      for user assignment in accordance with clause 8.1.3 of ISO
      3166-1/1997, will be used."

   o  The World Bank Country API [WORLDBANK] uses the following code
      elements from the User-assigned range:

      XC: Euro area

      XD: High income

      XE: Heavily indebted poor countries (HIPC)

      XF: International Bank for Reconstruction and Development

      XH: Blend

      XI: International Development Association

      XJ: Latin America and Caribbean (excluding high income)

      XL: Least developed countries: UN classification

      XM: Low income

      XN: Lower middle income

      XO: Low & middle income

      XP: Middle income

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      XQ: Middle East & North Africa (excluding high income)

      XT: Upper middle income

      XU: North America

      XX: Not classified

      XY: Not Classified

   o  The Interpol Implementation data format for the interchange of
      fingerprint, facial & scar-mark-tattoo information [INTERPOL] uses
      code element "ZZ" from the user-assigned range as follows:
      Destination Agency Identifier "ZZ/ALL" is reserved for
      transactions which shall be distributed by INTERPOL AFIS to all
      INTERPOL member states."

   o  The Certificate Authority Browser Forum Baseline Requirements for
      the Issuance and Management of Publicly-Trusted Certificates
      [CABForum] states that if a country is not represented by an
      official ISO 3166-1 alpha-2 country-code, the CA may specify the
      user-assigned code element "XX" to indicate that an official code
      element has not been assigned.

   o  The UNICODE Common Locale Data Repository (CLDR) [UNICODE] version
      37 uses the following code elements from the user-assigned range:

      QO: Outlying Oceania; countries in Oceania that do not have a

      XA: Pseudo-Accents; special code indicating derived testing locale
      with English + added accents and lengthened.

      XB: Pseudo-Bidi; special code indicating derived testing locale
      with forced RTL English.

      ZZ: Unknown Region; used in APIs or as a replacement for invalid

   o  The IETF Best Current Practice 47 [BCP-47] contains a section and
      examples dedicated to private-use subtags, using code elements
      from the user-assigned range:

      "For example, the region subtags 'AA', 'ZZ', and those in the
      ranges 'QM'-'QZ' and 'XA'-'XZ' (derived from the ISO 3166-1
      alpha-2 private use codes) can be used to form a language tag.  A
      tag such as "zh-Hans-XQ" conveys a great deal of public,
      interchangeable information about the language material"

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   o  The IETF Proposed Standard "Internationalized Domain Names for
      Applications" [RFC5890] uses the XN-- prefix.  The method that was
      used to decide on the prefix was explained in an email from the
      IANA to the IETF IDN Working Group list [XNIDN]:

      "The following steps will be used to select the two-character

      The code will be selected from among a subset of the entries on
      the ISO 3166-1, clause 8.1.3 User-assigned alpha-2 code elements:
      AA, QM to QZ, XA to XZ, and ZZ.  The selection is limited to these
      codes because of the following:

      The use of ISO 3166-1 User-assigned elements removes the
      possibility that the code will duplicate a present or future ccTLD

5.  Private-use top-level Domains

   The user-assigned classification of these code elements in the ISO
   3166-1 alpha-2 standard allows for the assumption that these code
   elements will not risk delegation as country-code top-level Domains
   through future assignments to represent a country or territory.  To
   quote [XNIDN]:

      "The use of ISO 3166-1 User-assigned elements removes the
      possibility that the code will duplicate a present or future ccTLD

   Using these code elements as top-level domains for the purpose of
   private-use TLDs is in line with the intended use of these code
   elements and follows the many examples of other standards and
   protocols.  Furthermore, they are short and free of any semantic

   This document does not recommend any specific ISO 3166-1 alpha-2
   user-assigned code as a private use, but instead proposes that any of
   them can be used by a network or application for private use.  That
   is, there is no attempt to choose just one of the ISO 3166-1 Alpha-2
   user-assigned codes for use as private-use TLDs, just as other
   organizations use multiple user-assigned codes for many internal

   Note that there may be software that treats labels beginning with XN
   differently due to the use of the XN- prefix in internationalized
   domain names [RFC5890].

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6.  IANA Considerations

   This document makes the observation that the policy of assigning
   ccTLD labels is to align with the ISO-3166-1 alpha-2 standard
   [RFC1591], which includes user-assigned code elements that will never
   be assigned to a territory [ISO3166-1].  This is then consistent with
   existing policies that those user-assigned codes will never be
   delegated from the DNS root zone and, for that reason, will never
   give rise to collisions with any future new TLD.

7.  Security Considerations

   Use of private-use identifiers of any sort almost always results in
   unexpected collisions in practice.  This has repeatedly been shown
   for private-use addresses, private-use identifiers (such as "x-
   headers") and private-use names in the DNS.  These unexpected
   collisions can easily have security ramifications that are well
   beyond what the user understands.

8.  Acknowledgements

   This document is based on an earlier draft by Ed Lewis.  David
   Conrad, Paul Hoffman, Sion Lloyd, Alain Durand, Jaap Akkerhuis, Kal
   Feher, Andrew Sullivan, Joe Abley, Petr Spacek, Patrick Mevzek and
   Kim Davies have played a role.

9.  Informative References

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

   [BCP-42]   Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <>.

   [BCP-47]   Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <>.

   [BCP-5]    Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,

   [BCP-6]    Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", BCP 6, RFC 6996, DOI 10.17487/RFC6996, July
              2013, <>.

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              "CA/Browser Forum Baseline Requirements for the Issuance
              and Management of Publicly-Trusted Certificates, version
              1.6.9", March 2020, <

              "Special-Use Domain Names", n.d.,

              "ICANN and the International Organization for
              Standardization (ISO)", n.d.,

   [ICAO]     "International Civil Aviation Organization, Machine
              Readable Travel Documents, Part 3; Specifications Common
              to all MRTDs", n.d., <

   [IMRS]     "ICANN Managed Root Server", n.d.,

              "Interpol Implementation data format for the interchange
              of fingerprint, facial & smt information", n.d.,

              "ISO 3166-1:2013 Codes for the representation of names of
              countries and their subdivisions - Part 1: Country codes",
              2013, <>.

   [ISO3901]  "Information and documentation -- International Standard
              Recording Code (ISRC)", n.d.,

   [ISO4217]  "ISO 4217; Codes for the representation of currencies and
              funds", n.d.,

   [ISO6166]  "Securities and related financial instruments --
              International securities identification numbering system
              (ISIN)", n.d., <>.

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   [ITHI]     "ICANN's Identifier Technology Health Indicator; Queries
              to frequently leaked strings", n.d.,

   [RFC1591]  Postel, J., "Domain Name System Structure and Delegation",
              RFC 1591, DOI 10.17487/RFC1591, March 1994,

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,

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

   [STD13]    Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [STD80]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,

   [UNICODE]  "CLDRv37 - Unicode Common Locale Data Repository version
              37", April 2020,

              "United Nations Code for Trade and Transport Locations;
              UN/LOCODE Manual", n.d.,

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   [WIPO]     "World Intellectual Property Organization; Recommended
              standard on two-letter codes for the representation of
              states, other entities and intergovernmental
              organizations.", n.d.,

              "Worldbank API V2 Country API", n.d..

   [XNIDN]    "Results of IANA Selection of IDNA Prefix", February 2003,

Authors' Addresses

   Roy Arends


   Joe Abley
   Public Interest Registry
   470 Moore Street
   London, true  N6C 2C2


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