RFC4501: Domain Name System Uniform Resource Identifiers

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Related keywords:  (DNS) (URI)

Network Working Group                                       S. Josefsson
Request for Comments: 4501                                           SJD
Category: Standards Track                                       May 2006

            Domain Name System Uniform Resource Identifiers

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document defines Uniform Resource Identifiers for Domain Name
   System resources.

Table of Contents

   1.  Introduction and Background  . . . . . . . . . . . . . . . . . 2
   2.  Usage Model  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   3.  DNS URI Registration . . . . . . . . . . . . . . . . . . . . . 3
   4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 7
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 7
   8.  Copying Conditions . . . . . . . . . . . . . . . . . . . . . . 8
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
       9.1.  Normative References . . . . . . . . . . . . . . . . . . 8
       9.2.  Informative References . . . . . . . . . . . . . . . . . 8

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1.  Introduction and Background

   The Domain Name System (DNS) [1] [2] is a widely deployed system
   used, among other things, to translate host names into IP addresses.
   Several protocols use Uniform Resource Identifiers (URIs) to refer to
   data.  By defining a URI scheme for DNS data, the gap between these
   two worlds is bridged.  The DNS URI scheme defined here can be used
   to reference any data stored in the DNS.

   Data browsers may support DNS URIs by forming DNS queries and
   rendering DNS responses using HTML [12], which is similar to what is
   commonly done for FTP [6] resources.  Applications that are
   Multipurpose Internet Mail Extensions (MIME) [7] aware may tag DNS
   data retrieved using this scheme with the text/dns or application/dns
   types as specified in [15].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [3].

2.  Usage Model

   Refer to section 1 of [5] for an in-depth discussion of URI
   classifications.  In particular, the reader is assumed to be familiar
   with the distinction between "name" and "locator".  This section
   describes how the DNS URI scheme is intended to be used and outlines
   future work that may be required to use URIs with the DNS for some

   The URI scheme described in this document focuses on the data stored
   in the DNS.  As such, there is no provision to specify any of the
   fields in the actual DNS protocol.  This is intended so that the URI
   may be used even in situations where the DNS protocol is not used
   directly.  Two examples for this are zone file editors and DNS-
   related configuration files, which may use this URI scheme to
   identify data.  The application would not use the DNS protocol to
   resolve the URIs.

   A limitation of this design is that it does not accommodate all
   protocol parameters within the DNS protocol.  It is expected that,
   for certain applications, a more detailed URI syntax that maps more
   closely to the DNS protocol may be required.  However, such a URI
   definition is not included in this document.  This document specifies
   a URI that is primarily intended to name DNS resources, but it can
   also be used to locate said resources for simple, yet common,

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3.  DNS URI Registration

   This section contains the registration template for the DNS URI
   scheme in accordance with [11].

   URL scheme name: "dns".

   URL scheme syntax: A DNS URI designates a DNS resource record set,
   referenced by domain name, class, type, and, optionally, the
   authority.  The DNS URI follows the generic syntax from RFC 3986 [5]
   and is described using ABNF [4].  Strings are not case sensitive, and
   free insertion of linear-white-space is not permitted.

   dnsurl          = "dns:" [ "//" dnsauthority "/" ]
                     dnsname ["?" dnsquery]

   dnsauthority    = host [ ":" port ]
                                ; See RFC 3986 for the
                                ; definition of "host" and "port".

   dnsname         = *pchar
                                ; See RFC 3986 for the
                                ; definition of "pchar".

                                ; The "dnsname" field may be a
                                ; "relative" or "absolute" name,
                                ; as per RFC 1034, section 3.1.

                                ; Note further that an empty
                                ; "dnsname" value is to be
                                ; interpreted as the root itself.
                                ; See below on relative dnsnames.

   dnsquery        = dnsqueryelement [";" dnsquery]

   dnsqueryelement = ( "CLASS=" dnsclassval ) / ( "TYPE=" dnstypeval )
                                ; Each clause MUST NOT be used more
                                ; than once.

   dnsclassval     = 1*digit / "IN" / "CH" /
                     <Any IANA registered DNS class mnemonic>

   dnstypeval      = 1*digit / "A" / "NS" / "MD" /
                     <Any IANA registered DNS type mnemonic>

   Unless specified in the URI, the authority ("dnsauthority") is
   assumed to be locally known, the class ("dnsclassval") to be the
   Internet class ("IN"), and the type ("dnstypeval") to be the Address

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   type ("A").  These default values match the typical use of DNS: to
   look up addresses for host names.

   A dnsquery element MUST NOT contain more than one occurrence of the
   "CLASS" and "TYPE" fields.  For example, both "dns:
   example?TYPE=A;TYPE=TXT" and "dns:example?TYPE=A;TYPE=A" are invalid.
   However, the fields may occur in any order, so that both "dns:
   example?TYPE=A;CLASS=IN" and "dns:example?CLASS=IN;TYPE=A" are valid.

   The digit representation of types and classes MAY be used when a
   mnemonic for the corresponding value is not well known (e.g., for
   newly introduced types or classes), but it SHOULD NOT be used for the
   types or classes defined in the DNS specification [2].  All
   implementations MUST recognize the mnemonics defined in [2].

   To avoid ambiguity, relative "dnsname" values (i.e., those not ending
   with ".") are assumed to be relative to the root.  For example, "dns:
   host.example" and "dns:host.example." both refer to the same owner
   name; namely, "host.example.".  Further, an empty "dnsname" value is
   considered a degenerative form of a relative name, which refers to
   the root (".").

   To resolve a DNS URI using the DNS protocol [2], a query is created,
   using as input the dnsname, dnsclassval, and dnstypeval from the URI
   string (or the appropriate default values).  If an authority
   ("dnsauthority") is given in the URI string, this indicates the
   server that should receive the DNS query.  Otherwise, the default DNS
   server should receive it.

   Note that DNS URIs could be resolved by other protocols than the DNS
   protocol, or by using the DNS protocol in some other way than as
   described above (e.g., multicast DNS).  DNS URIs do not require the
   use of the DNS protocol, although it is expected to be the typical
   usage.  The previous paragraph only illustrates how DNS URIs are
   resolved using the DNS protocol.

   A client MAY want to check that it understands the dnsclassval and
   dnstypeval before sending a query, so that it will be able to
   understand the response.  However, a typical example of a client that
   would not need to check dnsclassval and dnstypeval would be a proxy
   that would just treat the received answer as opaque data.

   Character encoding considerations: Characters are encoded as per RFC
   3986 [5].  The DNS protocol does not consider character sets; it
   simply transports opaque data.  In particular, the "dnsname" field of
   the DNS URI is to be considered an internationalized domain name
   (IDN) unaware domain name slot, in the terminology of RFC 3940 [14].
   The considerations for "host" and "port" are discussed in [5].

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   Because "." is used as the DNS label separator, an escaping mechanism
   is required to encode a "." that is part of a DNS label.  The
   escaping mechanism is described in section 5.1 of RFC 1035 [2].  For
   example, a DNS label of "exa.mple" can be escaped as "exa\.mple" or
   "exa\046mple".  However, the URI specification disallows the "\"
   character from occurring directly in URIs, so it must be escaped as
   "%5c".  The single DNS label "exa.mple" is thus encoded as "exa%
   5c.mple".  The same mechanism can be used to encode other characters,
   for example, "?" and ";".  Note that "." and "%2e" are equivalent
   within dnsname and are interchangeable.

   This URI specification allows all possible domain names to be
   encoded, provided the encoding rules are observed per [5]).  However,
   certain applications may restrict the set of valid characters.  Care
   should be taken so that invalid characters in these contexts do not
   cause harm.  In particular, host names in the DNS have certain
   restrictions.  It is up to these applications to limit this subset;
   this URI scheme places no restrictions.

   Intended usage: Whenever it is useful for DNS resources to be
   referenced by protocol-independent identifiers.  Often, this occurs
   when the data is more important than the access method.  Since
   software in general has coped without this so far, it is not
   anticipated to be implemented widely, nor migrated to by existing
   systems, but specific solutions (especially security-related) may
   find this appropriate.

   Applications and/or protocols that use this scheme include
   Security-related software, DNS administration tools, and network
   programming packages.

   Interoperability considerations: The data referenced by this URI
   scheme might be transferred by protocols that are not URI aware (such
   as the DNS protocol).  This is not anticipated to have any serious
   interoperability impact.

   Interoperability problems may occur if one entity understands a new
   DNS class/type mnemonic that another entity does not.  This is an
   interoperability problem for DNS software in general, although it is
   not a major practical problem for current DNS deployments, as the DNS
   types and classes are fairly static.  To guarantee interoperability,
   implementations can use integers for all mnemonics not defined in

   Interaction with Binary Labels [10] or other extended label types has
   not been analyzed.  However, binary labels appear to be infrequently
   used in practice.

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   Contact: simon@josefsson.org

   Author/Change Controller: simon@josefsson.org

4.  Examples

   A DNS URI is of the following general form.  This is intended to
   illustrate, not define, the scheme:


   The following illustrates a URI for a resource with the absolute name
   "www.example.org.", the Internet (IN) class, and the Address (A)


   Since the default class is IN and the default type is A, the same
   resource can be identified by a shorter URI using a relative name:


   The following illustrates a URI for a resource with the name
   "simon.example.org" for the CERT type in the Internet (IN) class:


   The following illustrates a URI for a resource with the name
   "ftp.example.org", in the Internet (IN) class and the address (A)
   type, but from the DNS authority instead of the default


   The following illustrates various escaping techniques.  The owner
   name would be "world wide web.example\.domain.org", where "\."
   denotes the character "." as part of a label, and "." denotes the
   label separator:


   The following illustrates a strange but valid DNS resource:


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5.  Acknowledgements

   Thanks to Stuart Cheshire, Donald Eastlake, Pasi Eronen, Bill Fenner,
   Ted Hardie, Russ Housley, Peter Koch, Andrew Main, Larry Masinter,
   Michael Mealling, Steve Mattson, Marcos Sanz, Jason Sloderbeck, Paul
   Vixie, Sam Weiler, and Bert Wijnen for comments and suggestions.  The
   author acknowledges RSA Laboratories for supporting the work that led
   to this document.

6.  Security Considerations

   If a DNS URI references domains in the Internet DNS environment, both
   the URI itself and the information referenced by the URI is public
   information.  If a DNS URI is used within an "internal" DNS
   environment, both the DNS URI and the data referenced should be
   handled using the same considerations that apply to DNS data in the
   "internal" environment.

   If information referenced by DNS URIs are used to make security
   decisions (such data includes, but is not limited to, certificates
   stored in the DNS [9]), implementations may need to employ security
   techniques such as Secure DNS [16], CMS [13], or OpenPGP [8], to
   protect the data during transport.  How to implement this will depend
   on the usage scenario, and it is not up to this URI scheme to define
   how the data referenced by DNS URIs should be protected.

   If applications accept unknown dnsqueryelement values in a URI (e.g.,
   URI "dns:www.example.org?secret=value") without knowing what the
   "secret=value" dnsqueryelement means, a covert channel used to "leak"
   information may be enabled.  The implications of covert channels
   should be understood by applications that accept unknown
   dnsqueryelement values.

   Slight variations, such as the difference between upper and lower
   case in the dnsname field, can be used as a covert channel to leak

7.  IANA Considerations

   The IANA has registered the DNS URI scheme, using the template in
   section 3, in accordance with RFC 2717 [11].

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8.  Copying Conditions

   Copyright (c) 2000, 2001, 2002, 2003, 2004, 2005, 2006 Simon

   Regarding this entire document or any portion of it, the author makes
   no guarantees and is not responsible for any damage resulting from
   its use.  The author grants irrevocable permission to anyone to use,
   modify, and distribute it in any way that does not diminish the
   rights of anyone else to use, modify, and distribute it, provided
   that redistributed derivative works do not contain misleading author
   or version information.  Derivative works need not be licensed under
   similar terms.

9.  References

9.1.  Normative References

   [1]  Mockapetris, P., "Domain names - concepts and facilities", STD
        13, RFC 1034, November 1987.

   [2]  Mockapetris, P., "Domain names - implementation and
        specification", STD 13, RFC 1035, November 1987.

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

   [4]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 4234, October 2005.

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

9.2.  Informative References

   [6]  Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC
        959, October 1985.

   [7]  Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
        Mail Extensions (MIME) Part Four: Registration Procedures", BCP
        13, RFC 2048, November 1996.

   [8]  Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, "OpenPGP
        Message Format", RFC 2440, November 1998.

   [9]  Eastlake 3rd, D. and O. Gudmundsson, "Storing Certificates in
        the Domain Name System (DNS)", RFC 2538, March 1999.

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   [10] Crawford, M., "Binary Labels in the Domain Name System", RFC
        2673, August 1999.

   [11] Petke, R. and I. King, "Registration Procedures for URL Scheme
        Names", BCP 35, RFC 2717, November 1999.

   [12] Connolly, D. and L. Masinter, "The 'text/html' Media Type", RFC
        2854, June 2000.

   [13] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852,
        July 2004.

   [14] Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing
        Domain Names in Applications (IDNA)", RFC 3490, March 2003.

   [15] Josefsson, S., "Domain Name System Media Types", RFC 4027, April

   [16] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
        "DNS Security Introduction and Requirements", RFC 4033, March

Author's Address

   Simon Josefsson

   EMail: simon@josefsson.org

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RFC 4501                        DNS URI                         May 2006

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