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

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8552.
Author Dave Crocker
Last updated 2021-04-20 (Latest revision 2018-11-16)
Replaces draft-crocker-dns-attrleaf
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Best Current Practice
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Benno Overeinder
Shepherd write-up Show Last changed 2018-07-21
IESG IESG state Became RFC 8552 (Best Current Practice)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Warren "Ace" Kumari
Send notices to "Tim Wicinski" <>, Benno Overeinder <>
IANA IANA review state IANA OK - Actions Needed
IANA action state RFC-Ed-Ack
dnsop                                                         D. Crocker
Internet-Draft                               Brandenburg InternetWorking
Intended status: Standards Track                       November 16, 2018
Expires: May 20, 2019

    DNS Scoped Data Through "Underscore" Naming of Attribute Leaves


   Formally, any DNS resource record 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 an
   _underscore.  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 "DNS Global Underscore
   Scoped Entry Registry" with IANA.  The purpose of the Underscore
   registry is to avoid collisions resulting from the use of the same
   underscore-based name, for different services.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 20, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Underscore Scoping  . . . . . . . . . . . . . . . . . . .   3
     1.2.  Scaling Benefits  . . . . . . . . . . . . . . . . . . . .   4
     1.3.  "Global" Underscored Node Names . . . . . . . . . . . . .   5
     1.4.  Interaction with DNS wildcards  . . . . . . . . . . . . .   5
     1.5.  History . . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  DNS Underscore Scoped Entry Registries Function . . . . . . .   6
   3.  RRset Use Registration Template . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  DNS Underscore Global Scoped Entry Registry . . . . . . .   8
     4.2.  DNS Underscore Global Scoped Entry Registry Definition  .   8
     4.3.  Initial entries . . . . . . . . . . . . . . . . . . . . .   9
     4.4.  _ta . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.5.  _example  . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.6.  Enumservices Registrations Registry . . . . . . . . . . .  11
   5.  Guidance for Expert Review  . . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     7.2.  References - Informative  . . . . . . . . . . . . . . . .  14
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

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

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

1.1.  Underscore Scoping

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

      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

   Because the DNS rules for a "host" (host name) do not allow use of
   the underscore character, this distinguishes the underscored name
   from all legal host names [RFC952].  Effectively, this convention for
   leaf node naming creates a space for the listing of "attributes" --
   in the form of resource record types -- that are associated with the
   parent domain, above the underscored sub-branch.

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

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

   This specification formally defines how underscored labels are used
   as "attribute" enhancements for their parent domain names.  For
   example, 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 underscore-based labels for different
   applications using the same resource record type, this document
   establishes the DNS Underscore Global Scoped Entry IANA Registry.
   Use of such node names, which begin with underscore, are reserved
   when they are the underscored name closest to the DNS root; they are
   considered "global".  Underscore-based names that are farther down
   the hierarchy are handled within the scope of the global underscore

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

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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP14 [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:


   the arrangement is:


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

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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 an _underscore.  A "global"
   underscored node name is the one that is closest to the root of the
   DNS hierarchy, also called the highest-level or top-most.  In the
   presentation convention described in Section 3.1 of [RFC1034] this is
   the right-most name beginning with an underscore.  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 only are interpreted as leaf names, one cannot create
   the equivalent of a wildcard name for prefixed names.  A name such as
   label.* is not a wildcard.

   Conversely, a wildcard such as * 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 underscore-based 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 [RFC2782],
   [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
   underscore-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
   underscore-based names that were reserved without coordination, which
   invites an eventual name-assignment collision.  The remedy is this
   base document, which defines a registry for these names, and attempts
   to register all those already in use, with a companion document

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   [attrleaf-fix] developed to direct changes to the pre-registry
   specifications that used underscore-based (global) node names.

2.  DNS Underscore Scoped Entry Registries Function

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

   o  If a public specification calls for use of an underscore-prefixed
      domain node name, the "global" underscored name -- the underscored
      name that is closest to the DNS root -- MUST be entered into this

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

   Structurally, the registry is defined as a single, flat table of RR
   types, under node names beginning with underscore.  In some cases,
   such as for use of an SRV record, the full scoping name might be
   multi-part, as a sequence of underscored names.  Semantically, that
   sequence represents a hierarchical model and it is theoretically
   reasonable to allow re-use 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 name.
   Therefore they are ignored by this DNS Underscore Global Scoped Entry
   Registry.  This registry is for the definition of highest-level --
   ie, 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 names are registered in the IANA Underscore
   Global table.  From the example, that would mean registering
   "_service3", "_service4", and "_authority" are registered in the IANA
   _service1, _service2, _service3, _service 4, and _authority.

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

   o  Definition and registration of subordinate, underscore node names
      is the responsibility of the specification that creates the global
      registry entry.

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

3.  RRset Use Registration Template

   This section provides a basic template that can be used to register
   new entries in the IANA DNS Underscore Global Scoped Entry Registry,
   if the global underscored name above the RRTYPE is not already
   registered.  The text can be added to specifications using
   RRTYPE/_Node-name combinations that have not already been registered:

      Per {RFC Attrleaf} please add the following entry to the DNS
      Underscore Global Scoped Entry Registry:

   | RR Type  | _NODE NAME        | REFERENCE                          |
   | {RRTYPE} | _{DNS global node | {citation for the document making  |
   |          | name}             | the addition.}                     |

            Table 2: Underscore Global Registry Entry Template

   Note to RFC Editor:   Please replace the above "{RFC Attrleaf}" text
      with a reference to this document's RFC number. /d

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4.  IANA Considerations

   Per [RFC8126] IANA is requested to establish the:

                DNS Underscore Global Scoped Entry Registry

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

4.1.  DNS Underscore Global Scoped Entry Registry

   The DNS Global Underscore Scoped Entry Registry is any DNS node name
   that begin 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 is to operate under the IANA rules for "Expert
      Review" registration; see Section 5.

   o  The contents of each entry in the Global registry are defined in
      Section 4.2.

   o  Each entry in the registry MUST contain values for all of the
      fields specified in Section 4.2.

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

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

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

4.2.  DNS Underscore Global Scoped Entry Registry Definition

   A registry entry contains:

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

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

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                with that name; for characters in the name that have an
                upper-case form and a lower-case form, the character
                MUST be recorded as lower-case, to simplify name

      References:    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.

4.3.  Initial entries

   Initial entries in the registry are:

            | RR Type    | _NODE NAME          | REFERENCE   |
            | *          | _example            | Section 4.5 |
            | NULL       | _ta-* {Section 4.4} | [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]   |
            | TLSA       | _udp                | [RFC6698]   |
            | TXT        | _acme-challenge     | [ACME]      |
            | TXT        | _dmarc              | [RFC7489]   |
            | TXT        | _domainkey          | [RFC6376]   |
            | TXT        | _mta-sts            | [MTA-STS]   |
            | TXT        | _spf                | [RFC7208]   |
            | TXT        | _tcp                | [RFC6763]   |
            | TXT        | _udp                | [RFC6763]   |
            | TXT        | _vouch              | [RFC5518]   |
            | URI        | _acct               | [RFC6118]   |
            | URI        | _dccp               | [RFC7566]   |

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            | 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: Underscore Global Registry (initial entries)

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

4.4.  _ta

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

4.5.  _example

   The node name "_example" is reserved across all RRsets

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4.6.  Enumservices Registrations Registry

   Please add a note to the Enumservice Registrations registry with the
   following -- or similar -- language:

      "When adding an entry to this registry, strong consideration
      should be given to also adding an entry to the 'DNS Underscore
      Global Scoped Entry Registry'."

5.  Guidance for Expert Review

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

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

   The review is for the purposes of ensuring that:

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

   o  The combination of the 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.

6.  Security Considerations

   This memo raises no security issues.

7.  References

7.1.  Normative References

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

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

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

   [RFC1034]  Mockapetris, P., "Domain Names - Concepts and Facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain Names - Implementation and
              Specification", STD 13, RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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

   [RFC3921]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence",
              RFC 3921, DOI 10.17487/RFC3921, October 2004,

   [RFC4386]  Boeyen, S. and P. Hallam-Baker, "Internet X.509 Public Key
              Infrastructure Repository Locator Service", RFC 4386,
              DOI 10.17487/RFC4386, February 2006,

   [RFC5026]  Giaretta, G., Ed., Kempf, J., and V. Devarapalli, Ed.,
              "Mobile IPv6 Bootstrapping in Split Scenario", RFC 5026,
              DOI 10.17487/RFC5026, October 2007,

   [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,

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

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   [RFC6118]  Hoeneisen, B. and A. Mayrhofer, "Update of Legacy IANA
              Registrations of Enumservices", RFC 6118,
              DOI 10.17487/RFC6118, March 2011,

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

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

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

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,

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

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

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

   [RFC7566]  Goix, L. and K. Li, "Enumservice Registration for 'acct'
              URI", RFC 7566, DOI 10.17487/RFC7566, June 2015,

   [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, <>.

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

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

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   [RFC8145]  Wessels, D., Kumari, W., and P. Hoffman, "Signaling Trust
              Anchor Knowledge in DNS Security Extensions (DNSSEC)",
              RFC 8145, April 2017.

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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

7.2.  References - Informative

              Crocker, D., "Changes to Rationalize Underscore DNS Node
              Names", I-D draft-crocker-attrleaf-simplification-00,

7.3.  URIs


Appendix A.  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 drafts.  For the later enhancements, thanks to:
   Stephane Bortzmeyer, Alissa Cooper, Bob Harold, Benjamin Kaduk, Mirja
   Kuehlewind, Warren Kumari, John Levine, Joel Jaeggli, Benno
   Overeinder, Eric Rescorla, Adam Roach, Petr &#352;pa&#269;ek,
   Ond&#345;ej 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.

   NOTE TO RFC EDITOR:    The listed names Petr &#352;pa&#269;ek,
      Ond&#345;ej Sury -- Petr &#352;pa&#269;ek, Ond&#345;ej Sury --
      render properly in xml-to-html conversion but the production
      xml2rfc engine does not render it properly to text.  Making the
      xml version match the documented form for txt will mean that the
      names are not properly rendered for output formats that can
      support extended character sets. /d

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Author's Address

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

   Phone: +1.408.246.8253

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