    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>.

    Crocker                   Best Current Practice                [Page 13]

    RFC 8553                    DNS AttrLeaf Fix                  March 2019

       [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>.

    Crocker                   Best Current Practice                [Page 14]

    RFC 8553                    DNS AttrLeaf Fix                  March 2019

       [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/

    Crocker                   Best Current Practice                [Page 15]