dnsop                                                         D. Crocker
Internet-Draft                               Brandenburg InternetWorking
Intended status: Best Current Practice                      May 22, 2018
Expires: November 23, 2018

    DNS Scoped Data Through '_Underscore' Naming of Attribute Leaves


   Formally, any DNS resource record may occur for any domain name.
   However some services have defined an operational convention, which
   applies to DNS leaf nodes that are under a DNS branch having one or
   more reserved node names, each beginning with an underscore.  The
   underscore naming construct defines a semantic scope for DNS record
   types that are associated with the parent domain, above the
   underscored branch.  This specification explores the nature of this
   DNS usage and defines the "DNS Global Underscore Scoped Entry
   Registry" with IANA.  The purpose of the Underscore registry is to
   avoid collisions resulting from the use of the same underscore-based
   name, for different services.

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
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   This Internet-Draft will expire on November 23, 2018.

Copyright Notice

   Copyright (c) 2018 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

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  _Underscore Scoping . . . . . . . . . . . . . . . . . . .   2
     1.2.  Scaling Benefits  . . . . . . . . . . . . . . . . . . . .   4
   2.  DNS Underscore Scoped Entry Registries Function . . . . . . .   4
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  DNS Underscore Global Scoped Entry Registry . . . . . . .   6
     3.2.  DNS Underscore Global Scoped Entry Registry Definition  .   6
     3.3.  Initial entries . . . . . . . . . . . . . . . . . . . . .   7
   4.  Guidance for Expert Review  . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The core Domain Name System (DNS) technical specifications assign no
   semantics to domain names or their parts, and no constraints upon
   which resource record (RR) types are permitted to be stored under
   particular names [RFC1035], [RFC2181].  Over time, some leaf node
   names, such as "www" and "ftp" have come to imply support for
   particular services, but this is a matter of operational convention,
   rather than defined protocol semantics.  This freedom in the basic
   technology has permitted a wide range of administrative and semantic
   policies to be used -- in parallel.  DNS data semantics have been
   limited to the specification of particular resource record types, on
   the expectation that new ones would be added as needed.
   Unfortunately, the addition of new resource record types has proven
   extremely challenging, over the life of the DNS, with significant
   adoption and use barriers.

1.1.  _Underscore Scoping

   As an alternative to defining a new RR type, some DNS service
   enhancements call for using an existing resource record type, but
   specify a restricted scope for its occurrence.  Scope is meant as a
   static property, not one dependent on the nature of the query.  It is

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   an artifact of the DNS name.  That scope is a leaf node, within which
   the uses of specific resource record sets can be formally defined and
   constrained.  The leaf occurs in a branch having a distinguished
   naming convention: At the top of the branch -- beneath the parent
   domain name to which the scope applies -- one or more reserved DNS
   node names begin with an underscore ("_").  Because the DNS rules for
   a "host" (host name) are not allowed to use the underscore character,
   this distinguishes the underscore name from all legal host names
   [RFC952].  Effectively, this convention for leaf node naming creates
   a space for the listing of 'attributes' -- in the form of resource
   record types -- that are associated with the parent domain, above the
   underscored sub-branch.

   The scoping feature is particularly useful when generalized resource
   record types are used -- notably "TXT", "SRV", and "URI" [RFC1035],
   [RFC2782], [RFC6335], [RFC7553].  It provides efficient separation of
   one use of them from others.  Absent this separation, an
   undifferentiated mass of these "RRsets" is returned to the DNS
   client, which then must parse through the internals of the records in
   the hope of finding ones that are relevant.  Worse, in some cases the
   results are ambiguous because a record type might not adequately
   self-identify.  With underscore-based scoping, only the relevant
   "RRsets"s are returned.

   A simple example is DKIM [RFC6376] , which uses "_domainkeys" for
   defining a place to hold a "TXT" record containing signing
   information for the parent domain.

   This specification formally defines how underscore labels are used as
   "attribute" enhancements for their parent domain names.  For example,
   domain name "_domainkey.example." acts as attribute of parent domain
   name "example."  To avoid collisions resulting from the use of the
   same underscore-based labels for different applications using the
   same resource record type, this document establishes DNS Underscore
   Global Scoped Entry IANA Registry for the highest-level reserved
   names that begin with _underscore; _underscore-based names that are
   farther down the hierarchy are handled within the scope of the
   highest-level _underscore name.

   Discussion Venue:    Discussion about this draft should be directed
      to the dnsop@ietf.org [1] mailing list.

      NOTE TO RFC EDITOR:    Please remove "Discussion Venue" paragraph
         prior to publication.

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1.2.  Scaling Benefits

   Some resource record types are used in a fashion that can create
   scaling problems, if an entire RRset associated with a domain name is
   aggregated in the leaf node for that name.  An increasingly-popular
   approach, with excellent scaling properties, places the RRset under a
   node having an underscore-based name, at a defined place in the DNS
   tree under the 'parent' name.  This constrains the use of particular
   "RR" types associated with that parent name.  A direct lookup to the
   subordinate leaf node produces only the desired record types, at no
   greater cost than a typical DNS lookup.

   The definition of a underscore global registry, provided in this
   specification, primarily attends to the top-most names used for
   scoping an RR type; that is the _underscore "global" names.

2.  DNS Underscore Scoped Entry Registries Function

   A global registry for DNS nodes names that begin with an _underscore
   is defined here.  The purpose of the Underscore Global Registry is to
   avoid collisions resulting from the use of the same _underscore-based
   name, for different applications.

   o  If a public specification calls for use of an _underscore-prefixed
      domain node name, the 'global' (right-most) _underscored name MUST
      be entered into this registry.

   The _underscore names define scope of use for specific resource
   record types, which are associated with the domain name that is the
   "parent" to the branch defined by the _underscore naming.  A given
   name defines a specific, constrained context for one or more RR
   types, where use of such record types conforms to the defined

   o  Within an _underscore scoped leaf, other RRsets that are not
      specified as part of the scope MAY be used.

   Structurally, the registry is defined as a single, flat table of RR
   types, under node names beginning with _underscore.  In some cases,
   such as for use of an "SRV" record, the full scoping name might be
   multi-part, as a sequence of underscore names.  Semantically, that
   sequence represents a hierarchical model and it is theoretically
   reasonable to allow re-use of a subordinate underscore name in
   different underscore context; that is, a subordinate name is
   meaningful only within the scope of the right-most (top-level)
   underscore name.  Therefore they are ignored by this DNS Underscore
   Global Scoped Entry Registry.  This registry is for the definition of
   highest-level -- ie, global -- underscore node name used.

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                      |                       NAME |
                      |                  _service1 |
                      |         ._protoB._service2 |
                      |          _protoB._service3 |
                      |          _protoC._service3 |
                      |    _useX._protoD._service4 |
                      | _protoE._region._authority |

                        Example of Underscore Names

   Only the right-most _underscore names are registered in the IANA
   Underscore Global table.

      The use of underscored node names is specific to each RRTYPE that
      is being scoped.  Each name defines a place, but does not define
      the rules for what appears underneath that place, either as
      additional underscored naming or as a leaf node with resource
      records.  Details for those rules are provided by specifications
      for individual RRTYPEs.  The sections below describe the way that
      existing underscore labels are used with the RRTYPEs that they

      Definition and registration of the subordinate underscore node
      names is the responsibility of the specification that creates the
      highest-level (right-most) global registry entry.

      That is, if a scheme using a global underscore node name also has
      one or more subordinate levels of underscore node naming, the
      namespaces from which names for those lower levels is chosen is
      controlled by the parent underscore node name.  Each globally-
      registered underscore name owns a distinct, subordinate name

3.  IANA Considerations

   Per [RFC8126], IANA is requested to establish the:

      DNS Underscore Global Scoped Entry Registry

   This section describes actions requested of IANA.  The guidance in
   [IANA] is used.

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3.1.  DNS Underscore Global Scoped Entry Registry

   The DNS Global Underscore Scoped Entry Registry is for DNS node names
   that begin with the underscore character (_) and are the right-most
   occurrence of any underscored names in a domain name sequence having
   that form; that is they are the "top" of a DNS branch, under a
   "parent" domain name.

   o  This registry is to operate under the IANA rules for "Expert
      Review" registration; see Section 4.

   o  The contents of each entry in the Global registry are defined in
      Section 3.2.

   o  Each entry in the registry MUST contain values for all of the
      fields specified in Section 3.2.

   o  Within the registry, the combination of RR Type and _Node Name
      MUST be unique.

   o  The table is to be maintained with entries sorted by the first
      column (RR Type) and within that the second column (_Node Name).

   o  The required Reference for an entry MUST have a stable resolution
      to the organization controlling that registry entry

3.2.  DNS Underscore Global Scoped Entry Registry Definition

   A registry entry contains:

   RR Type:    Lists an RR type that is defined for use within this

   _Node Name:    Specifies a single _underscore name that defines a
             reserved name; this name is the "global" entry name for the
             scoped resource record types that are associated with that

   References:    Lists specification that defines a record type and its
             use under this Name.  The organization producing the
             specification retains control over the registry entry for
             the _Node Name.

   Each RR type that is to be used MUST have a separate registry entry.

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3.3.  Initial entries

   Initial entries in the registry are:

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               | RR Type    | _NODE NAME      | REFERENCE  |
               | OPENPGPKEY | _openpgpkey     | [RFC7929]  |
               | SMIMEA     | _smimecert      | [RFC8162]  |
               | SRV        | _dccp           | [RFC2782]  |
               | SRV        | _sctp           | [RFC2782]  |
               | SRV        | _tcp            | [RFC2782]  |
               | SRV        | _udp            | [RFC2782]  |
               | TLSA       | _sctp           | [RFC6698]  |
               | TLSA       | _tcp            | [RFC6698]  |
               | TLSA       | _udp            | [RFC6698]  |
               | TXT        | _mta-sts        | [MTA-STS]  |
               | TXT        | _acme-challenge | [ACME]     |
               | TXT        | _dmarc          | [RFC7489]  |
               | TXT        | _domainkey      | [RFC6376]  |
               | TXT        | _spf            | [RFC7208]  |
               | TXT        | _vouch          | [RFC5518]  |
               | URI        | _iax            | [RFC7553]  |
               | URI        | _acct           | [RFC7553]  |
               | URI        | _dccp           | [RFC7553]  |
               | URI        | _email          | [RFC7553]  |
               | URI        | _ems            | [RFC7553]  |
               | URI        | _fax            | [RFC7553]  |
               | URI        | _ft             | [RFC7553]  |
               | URI        | _h323           | [RFC7553]  |
               | URI        | _ical-sched     | [RFC7553]  |
               | URI        | _ical-access    | [RFC7553]  |
               | URI        | _ifax           | [RFC7553]  |
               | URI        | _im             | [RFC7553]  |
               | URI        | _mms            | [RFC7553]  |
               | URI        | _pres           | [RFC7553]  |
               | URI        | _pstn           | [RFC7553]  |
               | URI        | _sctp           | [RFC7553]  |
               | URI        | _sip            | [RFC7553]  |
               | URI        | _sms            | [RFC7553]  |
               | URI        | _tcp            | [RFC7553]  |
               | URI        | _udp            | [RFC7553]  |
               | URI        | _unifmsg        | [RFC7553]  |
               | URI        | _vcard          | [RFC7553]  |
               | URI        | _videomsg       | [RFC7553]  |
               | URI        | _voice          | [RFC7553]  |
               | URI        | _voicemsg       | [RFC7553]  |
               | URI        | _vpim           | [RFC7553]  |
               | URI        | _xmp            | [RFC7553]  |

           Table 1: Underscore Global Registry (initial entries)

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4.  Guidance for Expert Review

   This section provides guidance for expert review of registration
   requests in the of DNS Underscore Global Scoped Entry Registry.

      This review is solely to determine adequacy of a requested entry
      in this Registry, and does not include review of other aspects of
      the document specifying that entry.  For example such a document
      might also contain a definition of the resource record type that
      is referenced by the requested entry.  Any required review of that
      definition is separate from the expert review required here.

   The review is for the purposes of ensuring that:

   o  The details for creating the registry entry are sufficiently
      clear, precise and complete

   o  The combination of the _underscore name, under which the listed
      resource record type is used, and the resource record type, is
      unique in the table

   For the purposes of this Expert Review, other matters of the
   specification's technical quality, adequacy or the like are outside
   of scope.

5.  Security Considerations

   This memo raises no security issues.

6.  References

6.1.  Normative References

   [ACME]     Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", I-D draft-ietf-acme-acme-11, March 2018.

   [IANA]     M. Cotton, B. Leiba, and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", RFC 8126,
              June 2017.

   [MTA-STS]  Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
              and J. Jones, "SMTP MTA Strict Transport Security (MTA-
              STS)", I-D draft-ietf-uta-mta-sts.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

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   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC5518]  Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
              Reference", RFC 5518, April 2009.

   [RFC6335]  Cotton, M., Eggert, L., Tpuch, J., Westerlund, M., and S.
              Cheshire, "nternet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", RFC 6335, Aug

   [RFC6376]  Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
              Identified Mail (DKIM) Signatures", RFC 6376, Sept 2011.

   [RFC6698]  Hoffman, J. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August .

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in E-Mail, Version 1",
              RFC 7208, April 2014.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, March 2015.

   [RFC7553]  Falstrom, P. and O. Kolkman, "The Uniform Resource
              Identifier (URI) DNS Resource Record", RFC 7553,
              ISSN 2070-1721, June 2015.

   [RFC7929]  Wouters, P., , RFC 7929, August 2016.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", RFC 8126,
              June 2017.

   [RFC8162]  Hoffman, P. and J. Schlyter, "Using Secure DNS to
              Associate Certificates with Domain Names for S/MIME",
              RFC 8162, May 2017.

   [RFC952]   Harrenstien, K., Stahl, M., and E. Feinler, "DOD Internet
              Host Table Specification", RFC 952, October 1985.

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6.2.  URIs

   [1] mailto:dnsop@ietf.org

Appendix A.  Acknowledgements

   Thanks go to Bill Fenner, Tony Hansen, Martin Hoffmann, Peter Koch,
   Olaf Kolkman, and Andrew Sullivan for diligent review of the (much)
   earlier drafts.  For the later enhancements, thanks to: Stephane
   Bortzmeyer, Bob Harold, Warren Kumari, John Levine, Joel Jaeggli,
   Petr Špaček, Ondřej Surř, Paul Vixie, Tim
   Wicinski, and Paul Wouters.

   Special thanks to Ray Bellis for his persistent encouragement to
   continue this effort, as well as the suggestion for an essential
   simplification to the registration model.

Author's Address

   Dave Crocker
   Brandenburg InternetWorking
   675 Spruce Dr.
   Sunnyvale, CA  94086

   Phone: +1.408.246.8253
   Email: dcrocker@bbiw.net
   URI:   http://bbiw.net/

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