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RFC 7553 - The Uniform Resource Identifier (URI) DNS Resource Re

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Internet Engineering Task Force (IETF)                      P. Faltstrom
Request for Comments: 7553                                        Netnod
Category: Informational                                       O. Kolkman
ISSN: 2070-1721                                                     ISOC
                                                               June 2015

       The Uniform Resource Identifier (URI) DNS Resource Record


   This document describes the already registered DNS resource record
   (RR) type, called the Uniform Resource Identifier (URI) RR, that is
   used for publishing mappings from hostnames to URIs.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2015 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
   (http://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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Applicability Statement . . . . . . . . . . . . . . . . . . .   3
   3.  DNS Considerations  . . . . . . . . . . . . . . . . . . . . .   4
   4.  The Format of the URI RR  . . . . . . . . . . . . . . . . . .   4
     4.1.  Owner Name, Class, and Type . . . . . . . . . . . . . . .   4
     4.2.  Priority  . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  Weight  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.4.  Target  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.5.  URI RDATA Wire Format . . . . . . . . . . . . . . . . . .   6
   5.  Usages  . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Example: FTP Server in the example.com Domain . . . . . .   6
     5.2.  Relation to S-NAPTR . . . . . . . . . . . . . . . . . . .   7
     5.3.  Relation to U-NAPTR . . . . . . . . . . . . . . . . . . .   7
     5.4.  Relation to SRV . . . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Registration of the URI Resource Record Type  . . . . . .   7
     6.2.  Registration of Services  . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  The Original RRTYPE Allocation Request . . . . . . .  11
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   This document explains the use of the Domain Name System (DNS) for
   the storage of URIs [RFC3986] and how to resolve hostnames to such
   URIs that can be used by various applications using the URI resource
   record type.  For resolution, the application needs to know both the
   hostname and the protocol that the URI is to be used for.  The
   protocol is registered by IANA.

   Historically, uses of the DNS to map a domain name to a URL have
   relied on the Naming Authority Pointer (NAPTR) RRTYPEs [RFC2915] and
   then on the Dynamic Delegation Discovery System (DDDS) [RFC3401]
   application framework with the DNS as a database as specified in RFC
   3404 [RFC3404].  This has a number of implications such as the fact
   the RRSet returned will contain all URIs "connected" with the owner,
   and not only the ones related to a specific service.

   The URI resource record specified in this document enables the
   querying party to do the equivalent of selecting which of the NAPTR
   records one is interested in and have only those returned.  This is
   possible because data in the service field of the NAPTR record is
   included in the owner part of the URI resource record type.  It is
   also the case that as the URI resource record type includes the
   target URI directly as part of the RDATA, it is very easy to extract
   the correct target URI, instead of applying rewrite rules as in

   Querying for URI resource records is not replacing querying for NAPTR
   resource records (or use of S-NAPTR [RFC3958]).  Instead, the URI
   resource record type provides a complementary mechanism to be used
   when one already knows what service field is interesting.  With it,
   one can directly query for the specific subset of the otherwise
   possibly large RRSet returned when querying for NAPTR resource

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119

2.  Applicability Statement

   In general, it is expected that URI records will be used by clients
   for applications where the relevant protocol to be used is known,
   but, for example, an extra abstraction is needed in order to separate
   a domain name from a point of service (as addressed by the URI).  One
   example of such a situation is when an organization has many domain
   names but only one official web page.

   Applications need to know the specific service to prepend the
   hostname with.  Using repetitive queries for URI records is not a
   replacement for querying for NAPTR records according to the NAPTR
   (DDDS) or S-NAPTR algorithms.  NAPTR records serve the purpose of
   discovering the various services or the URIs (for looking up access
   points for a given service).  These are two very different kinds of

3.  DNS Considerations

   Using prefix labels, such as underscored service tags, for a specific
   owner name may cause a counter-intuitive effect when the owner name
   is a wildcard name.  For example, _s2._s1.*.example.net is not a
   wildcard name and cannot be used to return a synthesized answer for a
   query name of _s2._s1.a.example.net.  See Section 4.5 of RFC 4592
   [RFC4592] for more details.  Besides, underscored service tags used
   for the URI RR (based on the "Service Name and Transport Protocol
   Port Number Registry") may have slightly different semantics than
   service tags used for underscored prefix labels that are used in
   combination with other (yet unspecified) RR types.  This may cause
   subtle management problems when delegation structure that has
   developed within the context of URI RRs is also to be used for other
   RR types.  Because the service labels might be overloaded,
   applications should carefully check that the application-level
   protocol is indeed the protocol they expect.

   Subtle management issues may also arise when the delegations from
   service to sub-service labels involve several parties and different

4.  The Format of the URI RR

   This is the presentation format of the URI RR:

      Owner name TTL Class URI Priority Weight Target

   The URI RR does not cause any kind of Additional Section processing.

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

   The type number for the URI record is 256.

   The URI resource record is class independent.

   The URI RR has no special Time-to-Live (TTL) requirements.

4.2.  Priority

   This field holds the priority of the target URI in this RR.  Its
   range is 0-65535.  A client MUST attempt to contact the URI with the
   lowest-numbered priority it can reach; URIs with the same priority
   SHOULD be selected according to probabilities defined by the weight

4.3.  Weight

   This field holds the server selection mechanism.  The weight field
   specifies a relative weight for entries with the same priority.
   Larger weights SHOULD be given a proportionately higher probability
   of being selected.  The range of this number is 0-65535.

4.4.  Target

   This field holds the URI of the target, enclosed in double-quote
   characters ('"'), where the URI is as specified in RFC 3986
   [RFC3986].  Resolution of the URI is according to the definitions for
   the Scheme of the URI.

   Since the URI will not be encoded as a <character-string> (see
   Section 3.3 of RFC 1035 [RFC1035]), there is no 255-character size

   The Target MUST NOT be an empty URI ("").

4.5.  URI RDATA Wire Format

   The RDATA for a URI RR consists of a 2-octet Priority field, a
   2-octet Weight field, and a variable-length Target field.

   Priority and Weight are unsigned integers in network byte order.

   The remaining data in the RDATA contains the Target field.  The
   Target field contains the URI as a sequence of octets (without the
   enclosing double-quote characters used in the presentation format).

   The length of the Target field MUST be greater than zero.

                        1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |          Priority             |          Weight               |
   /                                                               /
   /                             Target                            /
   /                                                               /

5.  Usages

5.1.  Example: FTP Server in the example.com Domain

   An organization has the domain names example.com and example.net, and
   their FTP archive is at ftp://ftp1.example.com/public.  Given the
   service name "ftp" and transport protocol "tcp" (from the IANA
   "Service Name and Transport Protocol Port Number Registry"), the
   following URI resource records could be made available in the
   respective zones (example.com and example.net):

   $ORIGIN example.com.
   _ftp._tcp    IN URI 10 1 "ftp://ftp1.example.com/public"

   $ORIGIN example.net.
   _ftp._tcp    IN URI 10 1 "ftp://ftp1.example.com/public"

5.2.  Relation to S-NAPTR

   The URI resource record type is not a replacement for the S-NAPTR.
   It is instead an extension and the second step of the S-NAPTR
   resolution can resolve a URI resource record instead of using SRV
   records and yet another algorithm for how to use SRV records for the
   specific protocol.

   $ORIGIN example.com.
   ;;                  order pref flags
              IN NAPTR 100   10   "D"   "EM:ProtA"  ( ; service
                       ""                             ; regexp
                       _http._tcp.example.com. )      ; replacement

   _http._tcp IN URI   10 1 "http://www.example.com/path"

5.3.  Relation to U-NAPTR

   The URI resource record type, together with S-NAPTR, can be viewed as
   a replacement for U-NAPTR [RFC4848].  The URI resource record type is
   only interesting when one know a base domain name, a protocol, and a
   service so that one can compose the record to look up.  NAPTR records
   of any kind are used to look up what services exist for a certain
   domain, which is one step before the URI resource record is used.

5.4.  Relation to SRV

   The URI resource record type can be viewed as a replacement for the
   SRV record.  This is because it, like the SRV record, can only be
   looked up if one knows the base domain, the protocol, and the
   service.  It has a similar functionality and uses the same registry
   for service names, but instead of returning a hostname and port
   number, the URI record returns a full URI.  As such, it can be viewed
   as a more powerful resource record than SRV.

6.  IANA Considerations

6.1.  Registration of the URI Resource Record Type

   After an expert review in February 2011 (see Appendix A), IANA
   allocated RRTYPE 256 for the URI resource record type in the registry
   named "Resource Record (RR) TYPEs" (as defined in [BCP42], which was
   RFC 6195 at the time but has since been replaced by RFC 6895) located
   at <http://www.iana.org/assignments/dns-parameters>.

   IANA has updated the reference for this registration to refer to this

6.2.  Registration of Services

   No new registry is needed for the registration of services as the
   Service Name, Transport Protocol Port Numbers, Enumservices and the
   DNS SRV Service Type registries are also used for the URI resource
   record type.

7.  Security Considerations

   Using the URI resource record together with security mechanisms that
   rely on verification of authentication of hostnames, like TLS, makes
   it important to choose the correct domain name when doing the
   comparison and ensure that the change in the hostname to be used is
   secured by DNSSEC so that it can be trusted in a similar way as a
   redirect in HTTP using TLS.

   If, for example, the URI resource record is not signed with the help
   of DNSSEC and then validated successfully, trusting the non-signed
   URI will effectively lead to a downgrade attack.

   The basic mechanism for successful use of URI works as follows:

   1.   Announce that example.com is hosted at example.org (with some
        URL) in DNS.

   2.   Secure the URI resource record with DNSSEC.  This is best done
        by doing validation in the application doing the lookup, but it
        could also be done in the local recursive resolver or in the
        trusted recursive resolver also doing validation.  All are
        according to the local trust policy.

   3.   Verify the TLS (for example) certificate for the connection to
        example.org matches, i.e., use the hostname in the URI and not
        the hostname used originally when looking up the URI resource

   4.   If needed, do application-layer authentication, etc., over the
        then encrypted connection.

   It is also possible that the URI in the resource record type has
   errors in it.  Applications using the URI resource record type for
   resolution should behave similarly as if the user typed (or copied
   and pasted) the URI.  At least it must be clear to the user that the
   error is not due to any error from his side.

   One SHOULD NOT include userinfo (see "User Information",
   Section 3.2.1 of RFC 3986 [RFC3986]) in a URI that is used in a URI
   resource record as DNS data must be viewed as publicly available

8.  References

8.1.  Normative References

   [BCP42]    Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, April 2013,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://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,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [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, <http://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,

8.2.  Informative References

   [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,

   [RFC2915]  Mealling, M. and R. Daniel, "The Naming Authority Pointer
              (NAPTR) DNS Resource Record", RFC 2915,
              DOI 10.17487/RFC2915, September 2000,

   [RFC3401]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part One: The Comprehensive DDDS", RFC 3401,
              DOI 10.17487/RFC3401, October 2002,

   [RFC3403]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Three: The Domain Name System (DNS) Database",
              RFC 3403, DOI 10.17487/RFC3403, October 2002,

   [RFC3404]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Four: The Uniform Resource Identifiers (URI)",
              RFC 3404, DOI 10.17487/RFC3404, October 2002,

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
              2003, <http://www.rfc-editor.org/info/rfc3597>.

   [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, <http://www.rfc-editor.org/info/rfc3958>.

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, DOI 10.17487/RFC4592, July 2006,

   [RFC4848]  Daigle, L., "Domain-Based Application Service Location
              Using URIs and the Dynamic Delegation Discovery Service
              (DDDS)", RFC 4848, DOI 10.17487/RFC4848, April 2007,

   [RFC5507]  IAB, Faltstrom, P., Ed., Austein, R., Ed., and P. Koch,
              Ed., "Design Choices When Expanding the DNS", RFC 5507,
              DOI 10.17487/RFC5507, April 2009,

Appendix A.  The Original RRTYPE Allocation Request

   On February 22, 2011 IANA assigned RRTYPE 256 for the URI resource
   record based on a request that followed the procedure documented in
   [BCP42] (which was RFC 6195 at the time but has since been replaced
   by RFC 6895).  The DNS RRTYPE PARAMETER ALLOCATION form as submitted
   to IANA at that time is replicated below for reference.

   Note: Although "ownername" should be "owner name", "ownername" has
   been preserved below because it was part of the original request form
   submitted to IANA.

   A.   Submission Date:

        May 23, 2009

   B.   Submission Type:

        [X] New RRTYPE
        [ ] Modification to existing RRTYPE

   C.   Contact Information for submitter:

        Name: Patrik Faltstrom
        Email Address: paf@cisco.com
        International telephone number: +46-8-6859131
        Other contact handles:
        (Note: This information will be publicly posted.)

   D.   Motivation for the new RRTYPE application?

        There is no easy way to get from a domain name to a URI.  Some
        mechanisms exists via use of the NAPTR [RFC3403] resource
        record.  That implies quite complicated rules that are
        simplified via the S-NAPTR [RFC3958] specification.  But, the
        ability to directly look up a URI still exists.  This
        specification uses a prefix based naming mechanism originated in
        the definition of the SRV [RFC2782] resource record, and the
        RDATA is a URI, encoded as one text field.

        See also above (Section 1).

   E.   Description of the proposed RR type.

        The format of the URI resource record is as follows:

        Ownername TTL Class URI Priority Weight Target

        The URI RR has service information encoded in its ownername.  In
        order to encode the service for a specific ownername one uses
        service parameters.  Valid service parameters used are either
        Enumservice Registrations registered by IANA, or prefixes used
        for the SRV resource record.

        The wire format of the RDATA is as follows:

                           1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |          Priority             |          Weight               |
      /                                                               /
      /                             Target                            /
      /                                                               /

   F.   What existing RRTYPE or RRTYPEs come closest to filling that
        need and why are they unsatisfactory?

        The RRTYPE that come closest is the NAPTR resource record.  It
        is for example used in the DDDS and S-NAPTR algorithms.  The
        main problem with the NAPTR is that selection of what record (or
        records) one is interested in is based on data stored in the
        RDATA portion of the NAPTR resource record.  This, as explained
        in RFC 5507 [RFC5507], is not optimal for DNS lookups.  Further,
        most applications using NAPTR resource records uses regular
        expression based rewrite rules for creation of the URI, and that
        has shown be complicated to implement.

        The second closest RRTYPE is the SRV record that given a
        prefixed based naming just like is suggested for the URI
        resource record, one get back a port number and domain name.
        This can also be used for creation of a URI, but, only URIs
        without path components.

   G.   What mnemonic is requested for the new RRTYPE (optional)?


   H.   Does the requested RRTYPE make use of any existing IANA Registry
        or require the creation of a new IANA sub-registry in DNS

        Yes, partially.

        One of the mechanisms to select a service is to use the
        Enumservice Registry managed by IANA.  Another is to use
        services and protocols used for SRV records.

   I.   Does the proposal require/expect any changes in DNS servers/
        resolvers that prevent the new type from being processed as an
        unknown RRTYPE (see [RFC3597])?


   J.   Comments:



   Ideas on how to split the two different kinds of queries, "What
   services exists for this domain name" and "What is the URI for this
   service", came from Scott Bradner and Lawrence Conroy.  Other people
   that have contributed to this document include Richard Barnes, Leslie
   Daigle, Victor Dukhovni, Olafur Gudmundsson, Philip Hallam-Baker, Ted
   Hardie, Sam Hartman, Evan Hunt, John Klensin, Peter Koch, Eliot Lear,
   Andy Newton, Mark Nottingham, Penn Pfautz, Jinmei Tatuya, Willem
   Toorop, and Nico Williams.

   Cisco is acknowledged as Mr. Faltstrom's employer at the time this
   document was developed.

   The NLnet Labs is acknowledged as Mr. Kolkman's employer at the time
   this document was developed.

Authors' Addresses

   Patrik Faltstrom

   EMail: paf@netnod.se

   Olaf Kolkman
   Internet Society

   EMail: kolkman@isoc.org


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