dnsop                                                         D. Crocker
Internet-Draft                               Brandenburg InternetWorking
Intended status: Best Current Practice                    March 25, 2018
Expires: September 26, 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 records
   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

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 26, 2018.

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   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
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   (https://trustee.ietf.org/license-info) in effect on the date of

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  _Underscore Scoping . . . . . . . . . . . . . . . . . . .   2
     1.2.  Scaling Benefits for TXT, SRV, and URI Resource Records .   3
   2.  DNS Underscore Scoped Entry Registries Function . . . . . . .   4
     2.1.  DNS Underscore Global Scoped Entry Registry Definition  .   5
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  DNS Underscore Global ScopedEntry Registry  . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     5.2.  References -- Informative . . . . . . . . . . . . . . . .   7
     5.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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 records (RRs) are permitted to be associated with
   particular names.[RFC1035] 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 records, on the expectation that
   new ones would be added as needed.  Unfortunately, the addition of
   new resource records has proved extremely challenging, over the life
   of the DNS, with significant adoption and use barriers.

1.1.  _Underscore Scoping

   As an alternative to defining new RRs, some DNS service enhancements
   call for using an existing resource record, but specify a restricted
   scope for its occurrence.  That scope is a leaf node, within which
   the uses of specific resource records 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

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   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
   [RFC1035].  Effectively, this convention for leaf node naming creates
   a space for the listing of 'attributes' -- in the form of resource
   records -- that are associated with the parent domain, above the
   underscore sub-branch.

   The scoping feature is particularly useful when generalized resource
   records 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 "RR"s 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 the records do not adequately self-
   identify.  With underscore-based scoping, only the relevant "RR"s are

   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, 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.

1.2.  Scaling Benefits for TXT, SRV, and URI Resource Records

   Some resource records are generic and support a variety of uses.
   Each additional use defines its own rules and, possibly, its own
   internal syntax and node-naming conventions to distinguish among
   particular types.  The "TXT", "SRV", and "URI" records are notable
   examples.  Their use can scale poorly, particularly when the same
   "RR" can be present in the same leaf node, but with different uses.

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   An increasingly-popular approach, with excellent scaling properties,
   place the RR under a node with an underscore-based name, at a defined
   place in the DNS tree, so as to constrain the use of particular "RR"s
   farther down the branch with that name.  This means that a direct
   lookup produces only the desired records, 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 RRs;
   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 'global' (right-most) node name that uses an _underscore
      prefix MUST be entered into this registry.

   The names define scope of use for specific resource records, 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 records, in which use of such records MUST conform to the
      defined constraints.  Within this scope, other resource records
      that are not specified MAY be used.

   The purpose of the Underscore Global Registry is to avoid collisions
   resulting from the use of the same _underscore-based name, for
   different applications.

   Structurally, the registry is defined as a single, flat table of
   names that begin 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 first (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.

      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

2.1.  DNS Underscore Global Scoped Entry Registry Definition

   A registry entry contains:

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

   RR(s):    Lists the RRs that are defined for use within this scope.

   References  Lists specification that define the records and their use
             under this Name.  The organization producing the
             specification retains control over the registry entry for
             the _Node Name.

3.  IANA Considerations

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

      DNS Underscore Global Scoped Entry Registry

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

3.1.  DNS Underscore Global ScopedEntry Registry

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

   o  This registry is to operate under the IANA rules for "First Come
      First Served" registration.

   o  The contents of each entry in the Global registry are defined in
      Section 2.1.

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

   Initial entries in the registry are:

                    | _NODE NAME  | RR  | REFERENCE  |
                    | _tcp        | SRV | [RFC2782]  |
                    | _udp        | SRV | [RFC2782]  |
                    | _sctp       | SRV | [RFC2782]  |
                    | _dccp       | SRV | [RFC2782]  |
                    | _domainkey  | TXT | [RFC6376]  |
                    | _spf        | TXT | [RFC7208]  |
                    | _dmarc      | TXT | [RFC7489]  |
                    | _vouch      | TXT | [RFC5518]  |

           Table 1: Underscore Global Registry (initial entries)

4.  Security Considerations

   This memo raises no security issues.

5.  References

5.1.  Normative References

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

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5.2.  References -- Informative

   [IANA]     M. Cotton, B. Leiba, and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", I-D
              draft-leiba-cotton-iana-5226bis-11, 2017.

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

   [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.

   [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.

5.3.  URIs

   [1] mailto:dnsop@ietf.org

Appendix A.  Acknowledgements

   Thanks go to Bill Fenner, Tony Hansen, 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, John Levine, Joel Jaeggli, Petr Špaček, Ondřej
   Surř, Tim Wicinski, and Paul Wouters.

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   Special thanks to Ray Bellis for more than 12 years of 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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