    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>.

    Crocker                   Best Current Practice                [Page 13]

    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>.

    Crocker                   Best Current Practice                [Page 14]

    RFC 8552                      DNS AttrLeaf                    March 2019

    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/

    Crocker                   Best Current Practice                [Page 15]