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Internet Engineering Task Force (IETF)                        D. Crocker
Request for Comments: 8552                   Brandenburg InternetWorking
BCP: 222                                                      March 2019
Category: Best Current Practice
ISSN: 2070-1721


         Scoped Interpretation of DNS Resource Records through
                "Underscored" Naming of Attribute Leaves

Abstract

   Formally, any DNS Resource Record (RR) may occur under any domain
   name.  However, some services use an operational convention for
   defining specific interpretations of an RRset by locating the records
   in a DNS branch under the parent domain to which the RRset actually
   applies.  The top of this subordinate branch is defined by a naming
   convention that uses a reserved node name, which begins with the
   underscore character (e.g., "_name").  The underscored 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
   "Underscored and Globally Scoped DNS Node Names" registry with IANA.
   The purpose of this registry is to avoid collisions resulting from
   the use of the same underscored name for different services.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   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
   BCPs 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
   https://www.rfc-editor.org/info/rfc8552.












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Copyright Notice

   Copyright (c) 2019 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
   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
     1.1.  Underscore-Based Scoping  . . . . . . . . . . . . . . . .   3
     1.2.  Scaling Benefits  . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Global Underscored Node Names . . . . . . . . . . . . . .   4
     1.4.  Interaction with DNS Wildcards  . . . . . . . . . . . . .   5
     1.5.  History . . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  "Underscored and Globally Scoped DNS Node Names" Registry . .   6
   3.  Guidance for Registering RRset Use  . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  "Underscored and Globally Scoped DNS Node Names" Registry   8
     4.2.  Enumservices Registrations Registry . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The core Domain Name System (DNS) technical specifications ([RFC1035]
   and [RFC2181]) 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 resource record



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   types would be added as needed.  Unfortunately, the addition of new
   resource record types has proven extremely challenging, with
   significant adoption and use barriers occurring over the life of the
   DNS.

1.1.  Underscore-Based Scoping

   As an alternative to defining a new RR TYPE, some DNS service
   enhancements call for using an existing resource record type but
   specifying a restricted scope for its occurrence.  Scope is meant as
   a static property, not one dependent on the nature of the query.  It
   is an artifact of the DNS name.  That scope is a leaf node containing
   the specific resource record sets that are formally defined and
   constrained.  Specifically:

      The leaf occurs in a branch having a distinguished naming
      convention: there is a parent domain name to which the scoped data
      applies.  The branch is under this name.  The sub-branch is
      indicated by a sequence of one or more reserved DNS node names; at
      least the first (highest) of these names begins with an underscore
      (e.g., "_name").

   Because the DNS rules for a "host" (host name) do not allow use of
   the underscore character, the underscored name is distinguishable
   from all legal host names [RFC0952].  Effectively, this convention
   for naming leaf nodes 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 its specific purpose.  With underscore-based scoping,
   only the relevant RRsets are returned.

   A simple example is DomainKeys Identified Mail (DKIM) [RFC6376],
   which uses "_domainkey" to define a place to hold a TXT record
   containing signing information for the parent domain.

   This specification formally defines how underscored names are used as
   "attribute" enhancements for their parent domain names.  For example,
   the domain name "_domainkey.example." acts as an attribute of the
   parent domain name "example.".  To avoid collisions resulting from



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   the use of the same underscored names for different applications
   using the same resource record type, this document establishes the
   "Underscored and Globally Scoped DNS Node Names" registry with IANA.
   Use of such node names, which begin with an underscore character, is
   reserved when they are the underscored name closest to the DNS root;
   as in that case, they are considered "global".  Underscored names
   that are farther down the hierarchy are handled within the scope of
   the global underscored node name.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.

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
   specially named branch, which is in turn under the node name that
   would otherwise contain the RRset.  The rules for naming that branch
   define the context for interpreting the RRset.  That is, rather than:

                            domain-name.example
                              /
                             RRset

   the arrangement is:

                        _branch.domain-name.example
                          /
                         RRset

   A direct lookup to the subordinate leaf node produces only the
   desired record types, at no greater cost than a typical DNS lookup.

1.3.  Global Underscored Node Names

   As defined in [RFC1034], the DNS uses names organized in a tree-
   structured or hierarchical fashion.  A domain name might have
   multiple node names that begin with the underscore character (e.g.,
   "_name").  A global underscored node name is the one that is closest
   to the root of the DNS hierarchy, also called the highest level or
   topmost.  In the presentation convention described in Section 3.1 of
   [RFC1034], this is the rightmost name beginning with an underscore.




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   In other presentation environments, it might be positioned
   differently.  To avoid concern for the presentation variations, the
   qualifier "global" is used here.

1.4.  Interaction with DNS Wildcards

   DNS wildcards interact poorly with underscored names in two ways:

   Since wildcards are only interpreted as leaf names, one cannot create
   the equivalent of a wildcard name for prefixed names.  A name such as
   label.*.example.com is not a wildcard.

   Conversely, a wildcard such as *.example.com can match any name
   including an underscored name.  So, a wildcard might match an
   underscored name, returning a record that is the type controlled by
   the underscored name but is not intended to be used in the
   underscored context and does not conform to its rules.

1.5.  History

   Originally, different uses of underscored node names developed
   largely without coordination.  For TXT records, there is no
   consistent, internal syntax that permits distinguishing among the
   different uses.  In the case of the SRV RR and URI RR, distinguishing
   among different types of use was part of the design (see [RFC2782]
   and [RFC7553]).  The SRV and URI specifications serve as templates,
   defining RRs that might only be used for specific applications when
   there is an additional specification.  The template definition
   included reference to two levels of tables of names from which
   underscored names should be drawn.  The lower-level (local scope) set
   of "_service" names is defined in terms of other IANA tables, namely
   any table with symbolic names.  The upper-level (global scope) SRV
   naming field is "_proto", although its pool of names is not
   explicitly defined.

   The aggregate effect of these independent efforts was a long list of
   underscored names that were reserved without coordination, which
   invites an eventual name-assignment collision.  The remedy is this
   base document and a companion document ([RFC8553]), which define a
   registry for these names and attempt to register all those already in
   use as well as to direct changes to the pre-registry specifications
   that used global underscored node names.









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2.  "Underscored and Globally Scoped DNS Node Names" Registry

   A registry for global DNS node names that begin with an underscore is
   defined here.  The purpose of the "Underscored and Globally Scoped
   DNS Node Names" registry is to avoid collisions resulting from the
   use of the same underscored name for different applications.

      If a public specification calls for use of an underscored node
      name, the global underscored node name -- the underscored name
      that is closest to the DNS root -- MUST be entered into this
      registry.

   An underscored name defines the 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 underscored name.  A given name
   defines a specific, constrained context for one or more RR TYPEs,
   where use of such record types conforms to the defined constraints.

   o  Within a leaf that is underscore scoped, 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 underscored names.  Semantically, that
   sequence represents a hierarchical model, and it is theoretically
   reasonable to allow reuse of a subordinate underscored name in a
   different, global underscored context; that is, a subordinate name is
   meaningful only within the scope of the global underscored node name.
   Therefore, they are ignored by this "Underscored and Globally Scoped
   DNS Node Names" registry.  This registry is for the definition of
   highest-level -- that is, global -- underscored node name used.

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

                  Table 1: Examples of Underscored Names






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   Only global underscored node names are registered in the "Underscored
   and Globally Scoped DNS Node Names" registry.  From the example
   above, that would mean _service1, _service2, _service3, _service 4,
   and _authority would be listed in the IANA registry.

   o  The use of underscored node names is specific to each RR TYPE 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 RR TYPEs.  The sections below describe the way that
      existing underscored names are used with the RR TYPEs that they
      name.

   o  Definition and registration of subordinate underscored node names
      are the responsibility of the specification that creates the
      global underscored node name registry entry.

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

3.  Guidance for Registering RRset Use

   This section provides guidance for specification writers, with a
   basic template they can use, to register new entries in the
   "Underscored and Globally Scoped DNS Node Names" registry.  The text
   can be added to specifications using RR TYPE / _NODE NAME
   combinations that have not already been registered:

      Per RFC 8552, please add the following entry to the "Underscored
      and Globally Scoped DNS Node Names" registry:

   +---------+-------------------+-------------------------------------+
   | RR Type | _NODE NAME        | Reference                           |
   +---------+-------------------+-------------------------------------+
   | {RR     | _{DNS global node | {citation for the document making   |
   | TYPE}   | name}             | the addition.}                      |
   +---------+-------------------+-------------------------------------+

                   Table 2: Template for Entries in the
         "Underscored and Globally Scoped DNS Node Names" Registry







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4.  IANA Considerations

   IANA has established the "Underscored and Globally Scoped DNS Node
   Names" registry.  This section describes the registry, the
   definitions, the initial entries, the use of_ta and _example, and the
   use of [RFC8126] as guidance for expert review.  IANA has also
   updated the "Enumservices Registrations" registry with a pointer to
   this document.

4.1.  "Underscored and Globally Scoped DNS Node Names" Registry

   The "Underscored and Globally Scoped DNS Node Names" registry
   includes any DNS node name that begins with the underscore character
   ("_", ASCII 0x5F) and is the underscored node name closest to the
   root; that is, it defines the highest level of a DNS branch under a
   "parent" domain name.

   o  This registry operates under the IANA rules for "Expert Review"
      registration; see Section 4.1.5.

   o  The contents of each entry in the registry are defined in
      Section 4.1.1.

   o  Each entry in the registry MUST contain values for all of the
      fields specified in Section 4.1.1.

   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.

















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4.1.1.  Contents of an Entry in the "Underscored and Globally Scoped DNS
        Node Names" Registry

   A registry entry contains:

      RR Type:    Lists an RR TYPE that is defined for use within this
                  scope.

      _NODE NAME: Specifies a single, underscored name that defines a
                  reserved name; this name is the global entry name for
                  the scoped resource record types that are associated
                  with that name.  For characters in the name that have
                  an uppercase form and a lowercase form, the character
                  MUST be recorded as lowercase to simplify name
                  comparisons.

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

   Each RR TYPE that is to be used with a _Node Name MUST have a
   separate registry entry.

4.1.2.  Initial Node Names

   The initial entries in the registry are as follows:

          +------------+-----------------------+---------------+
          | RR Type    | _NODE NAME            | Reference     |
          +------------+-----------------------+---------------+
          | *          | _example              | Section 4.1.4 |
          | NULL       | _ta-* {Section 4.1.3} | [RFC8145]     |
          | OPENPGPKEY | _openpgpkey           | [RFC7929]     |
          | SMIMEA     | _smimecert            | [RFC8162]     |
          | SRV        | _dccp                 | [RFC2782]     |
          | SRV        | _http                 | [RFC4386]     |
          | SRV        | _ipv6                 | [RFC5026]     |
          | SRV        | _ldap                 | [RFC4386]     |
          | SRV        | _ocsp                 | [RFC4386]     |
          | SRV        | _sctp                 | [RFC2782]     |
          | SRV        | _sip                  | [RFC5509]     |
          | SRV        | _tcp                  | [RFC2782]     |
          | SRV        | _udp                  | [RFC2782]     |
          | SRV        | _xmpp                 | [RFC3921]     |
          | TLSA       | _dane                 | [RFC7671]     |
          | TLSA       | _sctp                 | [RFC6698]     |
          | TLSA       | _tcp                  | [RFC6698]     |



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          | TLSA       | _udp                  | [RFC6698]     |
          | TXT        | _acme-challenge       | [RFC8555]     |
          | TXT        | _dmarc                | [RFC7489]     |
          | TXT        | _domainkey            | [RFC6376]     |
          | TXT        | _mta-sts              | [RFC8461]     |
          | TXT        | _spf                  | [RFC7208]     |
          | TXT        | _sztp                 | [ZEROTOUCH]   |
          | TXT        | _tcp                  | [RFC6763]     |
          | TXT        | _udp                  | [RFC6763]     |
          | TXT        | _vouch                | [RFC5518]     |
          | URI        | _acct                 | [RFC6118]     |
          | URI        | _dccp                 | [RFC7566]     |
          | URI        | _email                | [RFC6118]     |
          | URI        | _ems                  | [RFC6118]     |
          | URI        | _fax                  | [RFC6118]     |
          | URI        | _ft                   | [RFC6118]     |
          | URI        | _h323                 | [RFC6118]     |
          | URI        | _iax                  | [RFC6118]     |
          | URI        | _ical-access          | [RFC6118]     |
          | URI        | _ical-sched           | [RFC6118]     |
          | URI        | _ifax                 | [RFC6118]     |
          | URI        | _im                   | [RFC6118]     |
          | URI        | _mms                  | [RFC6118]     |
          | URI        | _pres                 | [RFC6118]     |
          | URI        | _pstn                 | [RFC6118]     |
          | URI        | _sctp                 | [RFC6118]     |
          | URI        | _sip                  | [RFC6118]     |
          | URI        | _sms                  | [RFC6118]     |
          | URI        | _tcp                  | [RFC6118]     |
          | URI        | _udp                  | [RFC6118]     |
          | URI        | _unifmsg              | [RFC6118]     |
          | URI        | _vcard                | [RFC6118]     |
          | URI        | _videomsg             | [RFC6118]     |
          | URI        | _voice                | [RFC6118]     |
          | URI        | _voicemsg             | [RFC6118]     |
          | URI        | _vpim                 | [RFC6118]     |
          | URI        | _web                  | [RFC6118]     |
          | URI        | _xmpp                 | [RFC6118]     |
          +------------+-----------------------+---------------+

                     Table 3: Initial Contents of the
         "Underscored and Globally Scoped DNS Node Names" Registry

4.1.3.  _ta

   Under the NULL RR Type, the entry "_ta-*" denotes all node names
   beginning with the string "_ta-*".  It does NOT refer to a DNS
   wildcard specification.



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4.1.4.  _example

   The node name "_example" is reserved across all RRsets.

4.1.5.  Guidance for Expert Review

   This section provides guidance for expert review of registration
   requests in the "Underscored and Globally Scoped DNS Node Names"
   registry.

      This review is solely to determine adequacy of a requested entry
      in this registry, and it 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 underscored 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.

4.2.  Enumservices Registrations Registry

   The following note has been added to the "Enumservice Registrations"
   registry:

      When adding an entry to this registry, strong consideration should
      be given to also adding an entry to the "Underscored and Globally
      Scoped DNS Node Names" registry.

5.  Security Considerations

   This memo raises no security issues.








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6.  References

6.1.  Normative References

   [RFC0952]  Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
              host table specification", RFC 952, DOI 10.17487/RFC0952,
              October 1985, <https://www.rfc-editor.org/info/rfc952>.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
              <https://www.rfc-editor.org/info/rfc2181>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/info/rfc2782>.

   [RFC3921]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence",
              RFC 3921, DOI 10.17487/RFC3921, October 2004,
              <https://www.rfc-editor.org/info/rfc3921>.

   [RFC4386]  Boeyen, S. and P. Hallam-Baker, "Internet X.509 Public Key
              Infrastructure Repository Locator Service", RFC 4386,
              DOI 10.17487/RFC4386, February 2006,
              <https://www.rfc-editor.org/info/rfc4386>.

   [RFC5026]  Giaretta, G., Ed., Kempf, J., and V. Devarapalli, Ed.,
              "Mobile IPv6 Bootstrapping in Split Scenario", RFC 5026,
              DOI 10.17487/RFC5026, October 2007,
              <https://www.rfc-editor.org/info/rfc5026>.







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   [RFC5509]  Loreto, S., "Internet Assigned Numbers Authority (IANA)
              Registration of Instant Messaging and Presence DNS SRV RRs
              for the Session Initiation Protocol (SIP)", RFC 5509,
              DOI 10.17487/RFC5509, April 2009,
              <https://www.rfc-editor.org/info/rfc5509>.

   [RFC5518]  Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
              Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009,
              <https://www.rfc-editor.org/info/rfc5518>.

   [RFC6118]  Hoeneisen, B. and A. Mayrhofer, "Update of Legacy IANA
              Registrations of Enumservices", RFC 6118,
              DOI 10.17487/RFC6118, March 2011,
              <https://www.rfc-editor.org/info/rfc6118>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <https://www.rfc-editor.org/info/rfc6376>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              DOI 10.17487/RFC7208, April 2014,
              <https://www.rfc-editor.org/info/rfc7208>.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
              <https://www.rfc-editor.org/info/rfc7489>.






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RFC 8552                      DNS AttrLeaf                    March 2019


   [RFC7553]  Faltstrom, P. and O. Kolkman, "The Uniform Resource
              Identifier (URI) DNS Resource Record", RFC 7553,
              DOI 10.17487/RFC7553, June 2015,
              <https://www.rfc-editor.org/info/rfc7553>.

   [RFC7566]  Goix, L. and K. Li, "Enumservice Registration for 'acct'
              URI", RFC 7566, DOI 10.17487/RFC7566, June 2015,
              <https://www.rfc-editor.org/info/rfc7566>.

   [RFC7671]  Dukhovni, V. and W. Hardaker, "The DNS-Based
              Authentication of Named Entities (DANE) Protocol: Updates
              and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
              October 2015, <https://www.rfc-editor.org/info/rfc7671>.

   [RFC7929]  Wouters, P., "DNS-Based Authentication of Named Entities
              (DANE) Bindings for OpenPGP", RFC 7929,
              DOI 10.17487/RFC7929, August 2016,
              <https://www.rfc-editor.org/info/rfc7929>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8145]  Wessels, D., Kumari, W., and P. Hoffman, "Signaling Trust
              Anchor Knowledge in DNS Security Extensions (DNSSEC)",
              RFC 8145, DOI 10.17487/RFC8145, April 2017,
              <https://www.rfc-editor.org/info/rfc8145>.

   [RFC8162]  Hoffman, P. and J. Schlyter, "Using Secure DNS to
              Associate Certificates with Domain Names for S/MIME",
              RFC 8162, DOI 10.17487/RFC8162, May 2017,
              <https://www.rfc-editor.org/info/rfc8162>.

   [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>.

   [RFC8461]  Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
              and J. Jones, "SMTP MTA Strict Transport Security (MTA-
              STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,
              <https://www.rfc-editor.org/info/rfc8461>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.




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6.2.  Informative References

   [RFC8553]  Crocker, D., "DNS Attrleaf Changes: Fixing Specifications
              That Use Underscored Node Names", RFC 8553,
              DOI 10.17487/RFC8553, March 2019,
              <https://www.rfc-editor.org/info/rfc8553>.

   [ZEROTOUCH]
              Watsen, K., Abrahamsson, M., and I. Farrer, "Secure Zero
              Touch Provisioning (SZTP)", Work in Progress, draft-ietf-
              netconf-zerotouch-29, January 2019.

Acknowledgements

   Thanks go to Bill Fenner, Dick Franks, Tony Hansen, Martin Hoffmann,
   Paul Hoffman, Peter Koch, Olaf Kolkman, Murray Kucherawy, John
   Levine, Benno Overeinder, and Andrew Sullivan for diligent review of
   the (much) earlier draft versions.  For the later enhancements,
   thanks to Stephane Bortzmeyer, Alissa Cooper, Bob Harold, Joel
   Jaeggli, Benjamin Kaduk, Mirja Kuehlewind, Warren Kumari, John
   Levine, Benno Overeinder, Eric Rescorla, Adam Roach, Petr Spacek,
   Ondrej Sury, 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
   United States of America

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













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=========================================================================





Internet Engineering Task Force (IETF)                        D. Crocker
Request for Comments: 8553                   Brandenburg InternetWorking
BCP: 222                                                      March 2019
Updates: 2782, 3263, 3529, 3620, 3832,
         3887, 3958, 4120, 4227, 4386,
         4387, 4976, 5026, 5328, 5389,
         5415, 5518, 5555, 5617, 5679,
         5766, 5780, 5804, 5864, 5928,
         6120, 6186, 6376, 6733, 6763,
         7208, 7489, 8145
Category: Best Current Practice
ISSN: 2070-1721


                         DNS AttrLeaf Changes:
         Fixing Specifications That Use Underscored Node Names

Abstract

   Using an underscore for a prefix creates a space for constrained
   interoperation of resource records.  Original uses of an underscore
   character as a domain node name prefix were specified without the
   benefit of an IANA registry.  This produced an entirely uncoordinated
   set of name-creation activities, all drawing from the same namespace.
   A registry for these names has now been defined by RFC 8552.
   However, the existing specifications that use underscored naming need
   to be modified in order to be in line with the new registry.  This
   document specifies those changes.  The changes preserve existing
   software and operational practice, while adapting the specifications
   for those practices to the newer underscore registry model.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   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
   BCPs 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
   https://www.rfc-editor.org/info/rfc8553.







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Copyright Notice

   Copyright (c) 2019 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
   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.  Underscored RRset Use in Specifications . . . . . . . . . . .   3
     2.1.  TXT RRset . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  SRV RRset . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  URI RRset . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Underscored Template Specifications . . . . . . . . . . . . .   7
     3.1.  SRV Specification Changes . . . . . . . . . . . . . . . .   7
     3.2.  URI Specification Changes . . . . . . . . . . . . . . . .   8
     3.3.  DNSSEC Signaling Specification Changes  . . . . . . . . .  10
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Original uses of an underscore character as a domain node name
   [RFC1035] prefix, which creates a space for constrained
   interpretation of resource records, were specified without the
   benefit of an IANA registry [IANA-reg].  This produced an entirely
   uncoordinated set of name-creation activities, all drawing from the
   same namespace.  A registry has now been defined (see Section 4 of
   [RFC8552]); the RFC that defined it discusses the background for the
   use of underscored domain names [RFC8552].







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   The basic model for underscored name registration, as specified in
   [RFC8552], is to have each registry entry be unique in terms of the
   combination of a resource record type and a "global" (highest-level)
   underscored node name; that is, the node name beginning with an
   underscore that is the closest to the DNS root.

   The specifications describing the existing uses of underscored naming
   do not reflect the existence of this integrated registry.  For the
   new reader or the new editor of one of those documents, there is
   currently nothing signaling that the underscored name(s) defined in
   the document are now processed through an IANA registry.  This
   document remedies that, by marking such a published document with an
   update that indicates the nature of the change.

   Further, the documents that define the SRV [RFC2782] and URI
   [RFC7553] DNS resource records provide a meta-template for
   underscored name assignments, partially based on separate registries
   [RFC6335].  For the portion that selects the global (highest-level)
   underscored node name, this perpetuates uncoordinated assignment
   activities by separate technical specifications, out of the same
   namespace.  This document remedies that by providing detail for
   revisions to the SRV and URI specifications to bring their use in
   line with the single, integrated "Underscored and Globally Scoped DNS
   Node Names" registry.

   The result of these changes preserves existing software and
   operations practices while adapting the technical specifications to
   the newer underscore registry model.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.  Underscored RRset Use in Specifications

   The use of underscored node names is specific to each RR TYPE 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 RR
   TYPEs.  The sections below describe the way that existing underscored
   names are used with the RR TYPEs that they name.







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2.1.  TXT RRset



      NOTE -  Documents falling into this category include: [RFC5518],
         [RFC5617], [RFC6120], [RFC6376], [RFC6763], [RFC7208], and
         [RFC7489].

   This section provides a generic approach for changes to existing
   specifications that define straightforward use of underscored node
   names when scoping the use of a TXT RRset.  The approach provides the
   information needed for adapting such specifications to the use of the
   IANA "Underscored and Globally Scoped DNS Node Names" registry
   [RFC8552].  Hence, the approach is meant both as an update to these
   existing specifications and as guidance for changes when those
   documents are revised.

   For any document that specifies the use of a TXT RRset under one or
   more underscored names, the global node name is expected to be
   registered in the IANA "Underscored and Globally Scoped DNS Node
   Names" registry [RFC8552].  An effort has been made to locate
   existing documents that do this, to register the global underscored
   node names, and to list them in the initial set of names added to the
   registry.

   If a public specification defines use of a TXT RRset and calls for
   the use of an underscored node name, here is a template of suggested
   text for registering the global underscored node name -- the one
   closest to the root -- that can be used through the IANA
   Considerations section of the specification:

      "Per [RFC8552], please add the following entry to the "Underscored
      and Globally Scoped DNS Node Names" registry:"

   +--------+----------------+-----------------------------------------+
   | RR     | _NODE NAME     | Reference                               |
   | Type   |                |                                         |
   +--------+----------------+-----------------------------------------+
   | TXT    | _{DNS node     | {citation for the document making the   |
   |        | name}          | addition}                               |
   +--------+----------------+-----------------------------------------+

        Table 1: Entry for the "Underscored and Globally Scoped DNS
                    Node Names" Registry for TXT RR Use







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2.2.  SRV RRset

      NOTE -  Documents falling into this category include:

         [RFC3263], [RFC3529], [RFC3620], [RFC3832], [RFC3887],
         [RFC3958], [RFC4120], [RFC4227], [RFC4386], [RFC4387],
         [RFC4976], [RFC5026], [RFC5328], [RFC5389], [RFC5415],
         [RFC5555], [RFC5679], [RFC5766], [RFC5780], [RFC5804],
         [RFC5864], [RFC5928], and [RFC6186].

   Specification of the SRV resource record [RFC2782] provides a
   template for use of underscored node names.  The global node name is
   characterized as referencing the 'protocol' that is associated with
   SRV RRset usage.

   This section provides a generic approach for changes to existing
   specifications that define the use of an SRV RRset.  The approach
   provides the information needed for adapting such specifications to
   the use of the IANA "Underscored and Globally Scoped DNS Node Names"
   registry [RFC8552].  Hence, the approach is meant both as an update
   to these existing specifications and as guidance for changes when
   those documents are revised.

   For any document that specifies the use of an SRV RRset, the global
   ('protocol') underscored node name is expected to be registered in
   the IANA "Underscored and Globally Scoped DNS Node Names" registry
   [RFC8552].  An effort has been made to locate existing documents that
   do this, to register the global underscored node names, and to list
   them in the initial set of names added to the registry.

   If a public specification defines use of an SRV RRset and calls for
   the use of an underscored node name, here is a template of suggested
   text for registering the global underscored node name -- the one
   closest to the root -- that can be used through the IANA
   Considerations section of the specification:
















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      "Per [RFC8552], please add the following entry to the "Underscored
      and Globally Scoped DNS Node Names" registry:

   +--------+----------------------+-----------------------------------+
   | RR     | _NODE NAME           | Reference                         |
   | Type   |                      |                                   |
   +--------+----------------------+-----------------------------------+
   | SRV    | _{DNS 'protocol'     | {citation for the document making |
   |        | node name}           | the addition}                     |
   +--------+----------------------+-----------------------------------+

     Table 2: Entry for the "Underscored and Globally Scoped DNS Node
                      Names" Registry for SRV RR Use

2.3.  URI RRset

   Specification of the URI resource record [RFC7553] provides a
   template for use of underscored node names.  The global node name is
   characterized as naming the 'protocol' that is associated with URI RR
   usage or by reversing an Enumservice sequence [RFC6117].

   This section provides a generic approach for changes to existing
   specifications that define use of a URI RRset.  The approach provides
   the information needed for adapting such specifications to the use of
   the IANA "Underscored and Globally Scoped DNS Node Names" registry
   [RFC8552].  Hence, the approach is meant both as an update to these
   existing specifications and as guidance for changes when those
   documents are revised.

   For any document that specifies the use of a URI RRset, the global
   ('protocol' or highest-level Enumservice) underscored node name is
   expected to be registered in the IANA "Underscored and Globally
   Scoped DNS Node Names" registry [RFC8552].  An effort has been made
   to locate existing documents that do this, to register the global
   underscored node names, and to list them in the initial set of names
   added to the registry.

   If a public specification defines use of a URI RRset and calls for
   the use of an underscored node name, here is a template of suggested
   text for registering the global underscored node name -- the one
   closest to the root -- that can be used through the IANA
   Considerations section of the specification:









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      "Per [RFC8552], please add the following entry to the "Underscored
      and Globally Scoped DNS Node Names" registry:

   +-------+----------------------------+------------------------------+
   | RR    | _NODE NAME                 | Reference                    |
   | Type  |                            |                              |
   +-------+----------------------------+------------------------------+
   | URI   | _{DNS 'protocol' or        | {citation for the document   |
   |       | Enumservice node name}     | making the addition}         |
   +-------+----------------------------+------------------------------+

     Table 3: Entry for the "Underscored and Globally Scoped DNS Node
                      Names" Registry for URI RR Use

3.  Underscored Template Specifications

3.1.  SRV Specification Changes

   The specification for a domain name, under which an SRV resource
   record [RFC2782] appears, provides a template for use of underscored
   node names.  The global underscored node name is characterized as
   indicating the 'protocol' that is associated with SRV RR usage.

   The text of [RFC2782] is changed as described below.  In addition,
   note that a normative reference to RFC 8552 is added to the
   References section of RFC 2782.

      OLD:

   The format of the SRV RR

    Here is the format of the SRV RR, whose DNS type code is 33:
          _Service._Proto.Name TTL Class SRV Priority Weight Port Target
    ...
    Proto
         The symbolic name of the desired protocol, with an underscore
         (_) prepended to prevent collisions with DNS labels that occur
         in nature.  _TCP and _UDP are at present the most useful values
         for this field, though any name defined by Assigned Numbers or
         locally may be used (as for Service).  The Proto is case
         insensitive.










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      NEW:

         The format of the SRV RR

         Here is the format of the SRV RR, whose DNS type code is 33:

            "_Service._Proto.Name TTL Class SRV Priority Weight Port
            Target"

            _..._

         Proto

            The symbolic name of the desired protocol with an underscore
            (e.g., "_name") prepended to prevent collisions with DNS
            labels that occur in nature. _TCP and _UDP are at present
            the most useful values for this field.  The Proto is case
            insensitive.

            The SRV RRset 'protocol' (global) underscored node name
            SHOULD be registered in the IANA "Underscored and Globally
            Scoped DNS Node Names" registry [RFC8552].

3.2.  URI Specification Changes

   Specification for the domain name (under which a URI resource record
   [RFC7553] occurs) is similar to that for the SRV resource record
   [RFC2782], although the text refers only to 'service' name, rather
   than distinguishing 'service' from 'protocol'.  Further, the URI RR
   specification permits alternative underscored naming schemes:

      One matches what is used for SRV, with the global underscored node
      name called 'protocol'.

      The other is based on a reversing of an Enumservice [RFC6117]
      sequence.

   Text of [RFC7553] is changed as described below.  In addition, a
   normative reference to RFC 8552 is added to the References section of
   RFC 7553.











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      OLD:

   4.1.  Owner Name, Class, and Type

   The URI owner name is subject to special conventions.

   Just like the SRV RR [RFC2782], the URI RR has service information
   encoded in its owner name.  In order to encode the service for a
   specific owner name, one uses service parameters.  Valid service
   parameters are those registered by IANA in the "Service Name and
   Transport Protocol Port Number Registry" [RFC6335] or as "Enumservice
   ---
   Registrations [RFC6117].  The Enumservice Registration parameters are
   reversed (i.e., subtype(s) before type), prepended with an underscore
   (_), and prepended to the owner name in separate labels.  The
   underscore is prepended to the service parameters to avoid collisions
   with DNS labels that occur in nature, and the order is reversed to
   make it possible to do delegations, if needed, to different zones
   (and therefore providers of DNS).

   For example, suppose we are looking for the URI for a service with
   ENUM Service Parameter "A:B:C" for host example.com.  Then we would
   query for (QNAME,QTYPE)=("_C._B._A.example.com","URI").

   As another example, suppose we are looking for the URI for a service
   with Service Name "A" and Transport Protocol "B" for host
   example.com.  Then we would query for
   (QNAME,QTYPE)=("_A._B.example.com","URI").

      NEW:

         4.1.  Owner Name, Class, and Type

         The URI owner name is subject to special conventions.

         As for the SRV RRset [RFC2782], the URI RRset global (highest-
         level) underscored node name SHOULD be registered in the IANA
         "Underscored and Globally Scoped DNS Node Names" registry
         [RFC8552].

         Just like the SRV RRset, the URI RRset has service information
         encoded in its owner name.  In order to encode the service for
         a specific owner name, one uses service parameters.  Valid
         service parameters are:

         +  Those registered by IANA in the "Service Name and Transport
            Protocol Port Number Registry" [RFC6335].  The underscore is
            prepended to the service parameters to avoid collisions with



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            DNS labels that occur in nature, and the order is reversed
            to make it possible to do delegations, if needed, to
            different zones (and therefore providers of DNS).

         +  Those listed in "Enumservice Registrations" [RFC6117].  The
            Enumservice Registration parameters are reversed (i.e.,
            subtype(s) before type), prepended with an underscore (e.g.,
            "_name"), and prepended to the owner name in separate
            labels.  The highest-level (global) underscored Enumservice
            name becomes the global name per RFC 8552 to register.

         For example, suppose we are looking for the URI for a service
         with ENUM Service Parameter "A:B:C" for host example.com.  Then
         we would query for
         (QNAME,QTYPE)=("_C._B._A.example.com","URI").

         As another example, suppose we are looking for the URI for a
         service with Service Name "A" and Transport Protocol "B" for
         host example.com.  Then we would query for
         (QNAME,QTYPE)=("_A._B.example.com","URI").

3.3.  DNSSEC Signaling Specification Changes

   "Signaling Trust Anchor Knowledge in DNS Security Extensions
   (DNSSEC)" [RFC8145] defines a use of DNS node names that effectively
   consumes all names beginning with the string "_ta-" when using the
   NULL RR in the query.

   Text of Section 5.1, "Query Format", of RFC 8145 is changed as
   described below.  In addition, a normative reference to RFC 8552 is
   added to the References section of RFC 8145.

      OLD:

   For example, a validating DNS resolver ...
                              QNAME=_ta-4444.

      NEW:

         For example, a validating DNS resolver ...  "QNAME=_ta-4444".

         Under the NULL RR, an entry is registered in the IANA
         "Underscored and Globally Scoped DNS Node Names" registry
         [RFC8552] for all node names beginning with "_ta-".







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4.  IANA Considerations

   Although this document makes reference to IANA registries, it
   introduces no new IANA registries or procedures.

5.  Security Considerations

   This memo raises no security issues.

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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC6117]  Hoeneisen, B., Mayrhofer, A., and J. Livingood, "IANA
              Registration of Enumservices: Guide, Template, and IANA
              Considerations", RFC 6117, DOI 10.17487/RFC6117, March
              2011, <https://www.rfc-editor.org/info/rfc6117>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

   [RFC7553]  Faltstrom, P. and O. Kolkman, "The Uniform Resource
              Identifier (URI) DNS Resource Record", RFC 7553,
              DOI 10.17487/RFC7553, June 2015,
              <https://www.rfc-editor.org/info/rfc7553>.

   [RFC8145]  Wessels, D., Kumari, W., and P. Hoffman, "Signaling Trust
              Anchor Knowledge in DNS Security Extensions (DNSSEC)",
              RFC 8145, DOI 10.17487/RFC8145, April 2017,
              <https://www.rfc-editor.org/info/rfc8145>.

   [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>.

   [RFC8552]  Crocker, D., "Scoped Interpretation of DNS Resource
              Records through "Underscored" Naming of Attribute Leaves",
              RFC 8552, DOI 10.17487/RFC8552, March 2019,
              <https://www.rfc-editor.org/info/rfc8552>.



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6.2.  Informative References

   [IANA-reg]
              IANA, "Protocol Registries",
              <https://www.iana.org/protocols>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/info/rfc2782>.

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              DOI 10.17487/RFC3263, June 2002,
              <https://www.rfc-editor.org/info/rfc3263>.

   [RFC3529]  Harold, W., "Using Extensible Markup Language-Remote
              Procedure Calling (XML-RPC) in Blocks Extensible Exchange
              Protocol (BEEP)", RFC 3529, DOI 10.17487/RFC3529, April
              2003, <https://www.rfc-editor.org/info/rfc3529>.

   [RFC3620]  New, D., "The TUNNEL Profile", RFC 3620,
              DOI 10.17487/RFC3620, October 2003,
              <https://www.rfc-editor.org/info/rfc3620>.

   [RFC3832]  Zhao, W., Schulzrinne, H., Guttman, E., Bisdikian, C., and
              W. Jerome, "Remote Service Discovery in the Service
              Location Protocol (SLP) via DNS SRV", RFC 3832,
              DOI 10.17487/RFC3832, July 2004,
              <https://www.rfc-editor.org/info/rfc3832>.

   [RFC3887]  Hansen, T., "Message Tracking Query Protocol", RFC 3887,
              DOI 10.17487/RFC3887, September 2004,
              <https://www.rfc-editor.org/info/rfc3887>.

   [RFC3958]  Daigle, L. and A. Newton, "Domain-Based Application
              Service Location Using SRV RRs and the Dynamic Delegation
              Discovery Service (DDDS)", RFC 3958, DOI 10.17487/RFC3958,
              January 2005, <https://www.rfc-editor.org/info/rfc3958>.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,
              <https://www.rfc-editor.org/info/rfc4120>.



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   [RFC4227]  O'Tuathail, E. and M. Rose, "Using the Simple Object
              Access Protocol (SOAP) in Blocks Extensible Exchange
              Protocol (BEEP)", RFC 4227, DOI 10.17487/RFC4227, January
              2006, <https://www.rfc-editor.org/info/rfc4227>.

   [RFC4386]  Boeyen, S. and P. Hallam-Baker, "Internet X.509 Public Key
              Infrastructure Repository Locator Service", RFC 4386,
              DOI 10.17487/RFC4386, February 2006,
              <https://www.rfc-editor.org/info/rfc4386>.

   [RFC4387]  Gutmann, P., Ed., "Internet X.509 Public Key
              Infrastructure Operational Protocols: Certificate Store
              Access via HTTP", RFC 4387, DOI 10.17487/RFC4387, February
              2006, <https://www.rfc-editor.org/info/rfc4387>.

   [RFC4976]  Jennings, C., Mahy, R., and A. Roach, "Relay Extensions
              for the Message Sessions Relay Protocol (MSRP)", RFC 4976,
              DOI 10.17487/RFC4976, September 2007,
              <https://www.rfc-editor.org/info/rfc4976>.

   [RFC5026]  Giaretta, G., Ed., Kempf, J., and V. Devarapalli, Ed.,
              "Mobile IPv6 Bootstrapping in Split Scenario", RFC 5026,
              DOI 10.17487/RFC5026, October 2007,
              <https://www.rfc-editor.org/info/rfc5026>.

   [RFC5328]  Adolf, A. and P. MacAvock, "A Uniform Resource Name (URN)
              Namespace for the Digital Video Broadcasting Project
              (DVB)", RFC 5328, DOI 10.17487/RFC5328, September 2008,
              <https://www.rfc-editor.org/info/rfc5328>.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
              <https://www.rfc-editor.org/info/rfc5389>.

   [RFC5415]  Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
              Ed., "Control And Provisioning of Wireless Access Points
              (CAPWAP) Protocol Specification", RFC 5415,
              DOI 10.17487/RFC5415, March 2009,
              <https://www.rfc-editor.org/info/rfc5415>.

   [RFC5518]  Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
              Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009,
              <https://www.rfc-editor.org/info/rfc5518>.

   [RFC5555]  Soliman, H., Ed., "Mobile IPv6 Support for Dual Stack
              Hosts and Routers", RFC 5555, DOI 10.17487/RFC5555, June
              2009, <https://www.rfc-editor.org/info/rfc5555>.



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   [RFC5617]  Allman, E., Fenton, J., Delany, M., and J. Levine,
              "DomainKeys Identified Mail (DKIM) Author Domain Signing
              Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617, August
              2009, <https://www.rfc-editor.org/info/rfc5617>.

   [RFC5679]  Bajko, G., "Locating IEEE 802.21 Mobility Services Using
              DNS", RFC 5679, DOI 10.17487/RFC5679, December 2009,
              <https://www.rfc-editor.org/info/rfc5679>.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766,
              DOI 10.17487/RFC5766, April 2010,
              <https://www.rfc-editor.org/info/rfc5766>.

   [RFC5780]  MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
              Using Session Traversal Utilities for NAT (STUN)",
              RFC 5780, DOI 10.17487/RFC5780, May 2010,
              <https://www.rfc-editor.org/info/rfc5780>.

   [RFC5804]  Melnikov, A., Ed. and T. Martin, "A Protocol for Remotely
              Managing Sieve Scripts", RFC 5804, DOI 10.17487/RFC5804,
              July 2010, <https://www.rfc-editor.org/info/rfc5804>.

   [RFC5864]  Allbery, R., "DNS SRV Resource Records for AFS", RFC 5864,
              DOI 10.17487/RFC5864, April 2010,
              <https://www.rfc-editor.org/info/rfc5864>.

   [RFC5928]  Petit-Huguenin, M., "Traversal Using Relays around NAT
              (TURN) Resolution Mechanism", RFC 5928,
              DOI 10.17487/RFC5928, August 2010,
              <https://www.rfc-editor.org/info/rfc5928>.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <https://www.rfc-editor.org/info/rfc6120>.

   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186,
              DOI 10.17487/RFC6186, March 2011,
              <https://www.rfc-editor.org/info/rfc6186>.

   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <https://www.rfc-editor.org/info/rfc6376>.





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   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              DOI 10.17487/RFC7208, April 2014,
              <https://www.rfc-editor.org/info/rfc7208>.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
              <https://www.rfc-editor.org/info/rfc7489>.

Acknowledgements

   Thanks go to Bill Fenner, Dick Franks, Tony Hansen, Peter Koch, Olaf
   Kolkman, and Andrew Sullivan for diligent review of the (much)
   earlier draft versions.  For the later enhancements, thanks to Tim
   Wicinski, John Levine, Bob Harold, Joel Jaeggli, Ondrej Sury, 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
   United States of America

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














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