Network Working Group                                         D. Crocker
Internet-Draft                               Brandenburg InternetWorking
Intended status: Best Current                               July 8, 2007
Expires: January 9, 2008

                DNS Scoped Data Through Attribute Leaves

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   Historically, any DNS RR may occur for any domain name.  Recent
   additions have defined DNS leaf nodes that contain a reserved node
   name, beginning with an underscore.  The underscore construct is used
   to define a semantic scope for the associated, parent domain name,
   within which the use of some RRs is constrained.  Hence the
   underscore construct defines a basic paradigm modification to the
   DNS.  This note explores the nature of this DNS usage and defines the

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   procedures for registering "underscore names" with IANA.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Disclaimer  . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.2.  Procedural Model  . . . . . . . . . . . . . . . . . . . . . 4
     1.3.  Discussion Venue  . . . . . . . . . . . . . . . . . . . . . 4
   2.  Scaling Benefits and TXT and SRV Resource Records . . . . . . . 4
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     5.1.  References -- Normative . . . . . . . . . . . . . . . . . . 7
     5.2.  References -- Informative . . . . . . . . . . . . . . . . . 7
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9

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1.  Introduction

   Historically, any DNS RR may occur for any domain name.  The DNS
   technical specifications assign no semantics to domain names and no
   constraints upon which resource records may be associated with a
   particular name.  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 semantics.
   This freedom in the basic technology has permitted a wide range of
   administrative and semantic policies to be used -- in parallel --
   with the DNS.  In the DNS, data semantics have been limited to the
   specifications of particular resource records, on the expectation
   that new ones would be added as needed.  Although there remains the
   view that this method of enhancement is preferred, alternative
   approaches have been explored and gained widespread deployment.

   Recent additions have defined reserved DNS node names, beginning with
   an underscore.  The underscore construct is used to define a scope
   for the occurrence of particular resource records, notably particular
   uses of those RRs.  Hence the underscore construct defines a basic
   paradigm modification to the DNS.  Within the scope of a defined
   underscore leaf, the uses of specific resource records can be
   formally defined and constrained.  An established example is the SRV
   record [RFC2782] which generalizes concepts long-used for email
   routing in the MX record.[RFC0974][RFC2821] The use of special DNS
   names has significant benefits and detriments.  Some of these are
   explored in [I-D.iab-dns-choices].

   [Comment]:   The terms "resolution context" and "scoping rules" have
      been suggested, in place of "semantic scope".  In order to avoid
      concern for matters of semantics, this specification uses the term
      "scoping rules", to create a focus on the mechanics being defined,
      rather than nuances of interpretation for the mechanism.

   One use that has perhaps not been noticed is that the underscore
   construct substantially changes possible concerns for scaling
   effects.  For example, different uses for the same RR, such as the
   free-form TXT record, become manageable when those are defined to be
   within different, scoped leaf nodes.

   This note discusses this enhancement, provides an explicit definition
   of it, and establishes an IANA registry for the reserved names
   beginning with underscore.

1.1.  Disclaimer

   This document does not seek to recommend or debate the merits of
   using sub-domain names that begin with underscore.  The practise

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   already exists, for multiple services.  The sole goal for this
   document is to specify a registry for the underscore-based names that
   get used.

1.2.  Procedural Model

   NOTE:   This procedure is modeled after that specified in [RFC2489].

   "The author of a new DHCP option will follow these steps to obtain
   approval for the option and publication of the specification of the
   option as an RFC:

   1.  The author devises the new option.

   2.  The author documents the new option as an Internet Draft,
       choosing a node name that has not yet been registered.

   3.  The author submits the Internet Draft for publication as an RFC,
       either as an independent submission or as an IETF-approved

   4.  The specification of the new option is reviewed for publication
       by the appropriate bodies.

   5.  At the time of publication as an RFC, IANA formally lists the
       node name."

1.3.  Discussion Venue

   Discussion about this draft is directed to the [1]mailing list of the IETF DNSOP Working
   Group [2].

2.  Scaling Benefits and TXT and SRV Resource Records

   Some resource records are have a generic form, with additional rules
   of use, internal syntax, or naming node naming conventions to
   distinguish among particular types.  The TXT and SRV records are the
   notable concern for this.  Some of these approaches scale poorly,
   particularly when the same RR can be present in the same node, but
   with different uses.  An approach with good scaling properties uses
   underscore-based names can be used to define sections with particular
   uses for particular RRs.

   In the case of TXT records, use for different scoping rules has
   developed organically and largely without coordination.  Underscore-
   based names therefore provide an administrative way of separating TXT

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   records that might have different uses, but otherwise would have no
   syntactic markers for distinguishing among them.

   In the case of the SRV RR this method of distinguishing among uses
   was part of the design.  [RFC2782] In reality, the SRV specification
   defines an RR that may only be used for specific applications when
   there is an additional specification.  So the SRV specification is
   best thought of as a template for future specifications.  The
   template definition includes reference to tables of names from which
   underscore-names should be drawn.  So, the set of <service> names is
   defined in terms of other IANA tables, namely any table with symbolic
   names.  The other SRV naming field is <proto>, although its pool of
   names is not explicitly defined.

3.  IANA Considerations

   IANA is requested to establish a set of DNS Underscore Name
   Registries, for DNS node names that begin with the underscore
   character and have been specified in any published RFC.

   The "DNS Underscore SCOPE Registry" creates the top-level of a
   potentially multi-field sequence of underscore names.  Additional
   registries are defined by the specification that creates a particular
   underscore name, if it provides for subordinate underscore

   The purpose of these tables is to define portions of the DNS for
   which there is a scope of use, with specific meanings for specific
   resource records, when they occur under the domain name having the
   underscore name(s).  They do not constrain the usage of other
   resource records that are not specified.  The purpose of the
   registries is to avoid collisions resulting from the use of the same
   underscore name, for different applications.

   A request to register an entry in a DNS Underscore Name Registry MUST

      Name:   Specifies a textual name for this scoped portion of the
         DNS.  The name will usually be taken from the specification
         cited in the "Defined" column and is intended for use in
         discussions about the entry.

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      Label:   Specifies the underscore name that is being reserved.
         The name may be specified directly or by citing a table of
         names, with the implication that a name from the table will be
         prefaced with an underscore.  Referencing a table of names
         incorporates those names into the table, so as to create a set
         of additional entries.

      Subordinate:   Refers to a registry of underscore names that
         defines the next level of domain name field, below the current
         one.  If this cell contains a reference, the RR cell must be

      RR(s):   Specifies the Resource Records that are explicitly
         defined for the scope of this registration.  The specification
         is either by directly listing the RR(s) or by citing a table of
         RRs.  This cell in the table is to be empty, if the Subordinate
         cell contains a reference.

      Defined:   Specifies the particular use for specific RR(s),
         defined for use within the scope of the registered underscore

   | NAME | LABEL | SUBORDINATE            | RR(s) | DEFINED           |
   | SRV  | _tcp  | DNS SRV Underscore     |       | [RFC3263]         |
   | TCP  |       | Name Registry          |       |                   |
   |      |       | (Table 2)              |       |                   |
   | SRV  | _udp  | DNS Underscore SRV     |       | [RFC3263]         |
   | UDP  |       | Name Registry          |       |                   |
   |      |       | (Table 2)              |       |                   |
   | SPF  | _spf  |                        | TXT   | [RFC4408]         |
   | URI  | _e2u  | ENUM (Table 3)         |       | (new enumuri      |
   |      |       |                        |       | draft pending.    |
   |      |       |                        |       | /d)               |

     Table 1: DNS Underscore SCOPE Name Registry (with initial values)

                 Initial entries in the registry comprise:

             | NAME | LABEL | SUBORDINATE | RR(s) | DEFINED |

      Table 2: DNS Underscore SRV Name Registry (with initial values)

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   | NAME  | LABEL | SUBORDINATE | RR(s)                   | DEFINED   |
   | ENUM  | _e2u  |             | IANA Service Table      | (new      |
   | (or   |       |             | enum-services; or RFC   | enumuri   |
   | E2U?) |       |             | 3968, Section 6.5,      | draft     |
   |       |       |             | <experimental-service>  | pending.  |
   |       |       |             |                         | /d)       |

     Table 3: DNS Underscore ENUM Name Registry (with initial values)

4.  Security Considerations

   This memo raises no security issues.

5.  References

5.1.  References -- Normative

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

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [RFC4408]  Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
              for Authorizing Use of Domains in E-Mail, Version 1",
              RFC 4408, April 2006.

5.2.  References -- Informative

              Faltstrom, P., "Design Choices When Expanding DNS",
              draft-iab-dns-choices-04 (work in progress), October 2006.

   [RFC0974]  Partridge, C., "Mail routing and the domain system",
              RFC 974, January 1986.

   [RFC2489]  Droms, R., "Procedure for Defining New DHCP Options",
              BCP 29, RFC 2489, January 1999.

   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

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   [1]  <>

   [2]  <>

Appendix A.  Acknowledgements

   Thanks go to Bill Fenner, Tony Hansen, Peter Koch, Olaf Kolkman, and
   Andrew Sullivan for diligent review.

Author's Address

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

   Phone: +1.408.246.8253

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