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Domain Name System Uniform Resource Identifiers

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 4501.
Author Simon Josefsson
Last updated 2020-01-21 (Latest revision 2005-08-05)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Stream WG state (None)
Document shepherd (None)
IESG IESG state Became RFC 4501 (Proposed Standard)
Action Holders
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Ted Hardie
Send notices to <>
Network Working Group                                       S. Josefsson
Internet-Draft                                            August 5, 2005
Expires: February 6, 2006

            Domain Name System Uniform Resource Identifiers

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on February 6, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).


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

   See <> for more information.

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Table of Contents

   1.  Introduction and Background  . . . . . . . . . . . . . . . . .  3
   2.  Usage Model  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  DNS URI Registration . . . . . . . . . . . . . . . . . . . . .  5
   4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   8.  Copying conditions . . . . . . . . . . . . . . . . . . . . . . 10
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.1   Normative References . . . . . . . . . . . . . . . . . . . 10
     9.2   Informative References . . . . . . . . . . . . . . . . . . 10
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 11
   A.  Revision Changes . . . . . . . . . . . . . . . . . . . . . . . 11
     A.1   Changes since -06  . . . . . . . . . . . . . . . . . . . . 11
     A.2   Changes since -07  . . . . . . . . . . . . . . . . . . . . 12
     A.3   Changes since -08  . . . . . . . . . . . . . . . . . . . . 12
     A.4   Changes since -09  . . . . . . . . . . . . . . . . . . . . 12
     A.5   Changes since -10  . . . . . . . . . . . . . . . . . . . . 12
     A.6   Changes since -11  . . . . . . . . . . . . . . . . . . . . 12
     A.7   Changes since -12  . . . . . . . . . . . . . . . . . . . . 13
       Intellectual Property and Copyright Statements . . . . . . . . 14

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1.  Introduction and Background

   The Domain Name System (DNS) [1] [2] is a widely deployed system used
   to, among other things, translate host names into IP addresses.
   Several protocols are using Uniform Resource Identifier (URI) to
   refer to data.  By defining a URI scheme for DNS data, the gap
   between these two worlds are 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 render
   DNS responses using HTML [14], similar to what is commonly done for
   FTP [6] resources.  Applications that are Multipurpose Internet Mail
   Extension (MIME) [7] aware may tag DNS data retrieve using this
   scheme with the text/dns or application/dns types as specified in

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

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2.  Usage Model

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

   The URI scheme described in this document focus 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 intentional, so that the URI may
   be used even in situations where the DNS protocol is not used
   directly.  Two examples for this is 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 do not accommodate all
   protocol parameters within the DNS protocol.  It is expected that for
   certain applications, a more detailed URI syntax that map more
   closely to the DNS protocol may be required.  However, such an URI
   definition is not included in this document.  This document specify a
   URI that is primarily intended to name DNS resources, but it can also
   be used to locate said resources for simple (but common)

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3.  DNS URI Registration

   The section contain the registration template for the DNS URI scheme
   in accordance with [13].

   URL scheme name: "dns".

   URL scheme syntax: A DNS URI designate 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 dnsname's.

   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 occurance 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 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 to be a degenerative form of a relative name, which refer
   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 indicate 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 illustrate 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: The characters are encoded as per
   RFC 3986 [5].  The DNS protocol do 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 [16].  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.  For
   example, a DNS label of "exa.mple" can be escaped as "exa\.mple" or
   "exa\046mple".  However, the URI specification disallow the "\"
   character from occuring 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 interchangable.

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

   Intended usage: Whenever DNS resources are useful to reference by
   protocol independent identifiers, often 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 which use this scheme: Security related
   software.  DNS administration tools.  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 though.

   Interoperability problems may occur if one entity understands a new
   DNS class/type mnemonic and another entity do not understand it.
   This is an interoperability problem for DNS software in general,
   although it is not a major practical problem 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 [12], or other extended label types,
   has not been analyzed.  However, they appear to be infrequently used
   in practice.


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   Author/Change Controller:

4.  Examples

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


   The following illustrate a URI for a resource with the absolute name
   "", the Internet (IN) class and the Address (A) type:;tYpE=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 illustrate a URI for a resource with the name
   "", for the CERT type, in the Internet (IN) class:

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


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


   The following illustrate a strange, but valid, DNS resource:


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5.  Acknowledgments

   Thanks to Stuart Cheshire, Donald Eastlake, Pasi Eronen, Bill Fenner,
   Ted Hardie, Russ Housley, Peter Koch, Andrew Main, Larry Masinter,
   Michael Mealling, Steve Mattson, Paul Vixie, Sam Weiler, and Bert
   Wijnen for comments and suggestions.  The author acknowledges the 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 is referenced should be
   handled using the same considerations that apply to DNS data in the

   If information referenced by DNS URIs are used to make security
   decisions (examples of such data include, but is not limited to,
   certificates stored in the DNS [10]), implementations may need to
   employ security techniques such as Secure DNS [9], or even CMS [15]
   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 (e.g., accepts
   the URI "" 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 accepts unknown
   dnsqueryelement values.

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

7.  IANA Considerations

   The IANA is asked to register the DNS URI scheme, using the template
   in section 3, in accordance with RFC 2717 [13].

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8.  Copying conditions

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

   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 2234, November 1997.

   [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, D., "Domain Name System Security Extensions",
         RFC 2535, March 1999.

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

   [11]  Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams,
         "X.509 Internet Public Key Infrastructure Online Certificate
         Status Protocol - OCSP", RFC 2560, June 1999.

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

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

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

   [15]  Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369,
         August 2002.

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

   [17]  Yergeau, F., "UTF-8, a transformation format of ISO 10646",
         STD 63, RFC 3629, November 2003.

   [18]  Josefsson, S., "Domain Name System Media Types", RFC 4027,
         April 2005.

Author's Address

   Simon Josefsson


Appendix A.  Revision Changes

   Note to RFC editor: Remove this appendix before publication.

A.1  Changes since -06

   The MIME registration templates for text/dns and application/dns was
   removed, and will be defined in separate documents.

   Improved discussion related to which mnemonics that must be
   supported.  The interoperability problem that provoked the
   clarification is also mentioned.

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   Security consideration improvements.

A.2  Changes since -07

   Author/Change Controller changed to author of this document, not
   IESG.  Terminology section collapsed into introduction.  The second
   paragraph of the introduction rewritten and gives explicit examples.
   Intended usage and applications fields fixed.  Moved this revision
   tracking information to an appendix.  Mention IDN in charset section.
   All previous thanks to suggestions by Larry Masinter.

A.3  Changes since -08

   Modifications derived from Last-Call comments: Made more clear that
   DNS URIs does not imply use of the DNS protocol, but the issue is not
   stressed because of the apparent inflamatory state of affairs.  Added
   informative references to HTML and FTP.  Clarified that dnsname can
   be empty.  Clarified that first dnsqueryelement "win" in case of
   ambiguity.  Clarified security consideration with respect to unknown
   dnsqueryelements.  Use "authority" instead of "server".  Say "IANA
   registered" instead of "standard".  Interoperability note about
   binary DNS labels.  Typos.

A.4  Changes since -09

   Use legal texts from RFC 3667.  Update UTF-8 reference to RFC 3629.
   Simplified introduction.  Discuss relative and absolute dnsname's.
   Clarify that empty dnsname correspond to the root.  Change so that
   dns:foo?TYPE=A;TYPE=TXT is invalid, instead of meaning TYPE=A. The
   underspecified extension mechanism was dropped; now only TYPE= and
   CLASS= are permitted.  Remove background discussion of why the
   dnsname field is made a IDN unaware domain name slot.  Use standard
   DNS escaping (i.e, "\." for ".") instead of broken approach that
   violated the URI specification.  Improve examples.  Add security

A.5  Changes since -10

   Add section "Usage Model".  Move acknowledgements, as per rfc2223bis.
   Add permissive copying condition.  Updates to align with RFC 3986.

A.6  Changes since -11

   Fix typos.  IESG feedback: Move RFC2119 reference to normative
   section.  Replace OCSP example with X.509 CRL Distribution Point
   extension.  Fix ABNF not to use "...".

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A.7  Changes since -12

   Reference MIME and RFC 4027.  IESG feedback: Do not mention OpenPGP/
   X.509 as illustrative examples in the introduction section.

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Intellectual Property Statement

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   Intellectual Property Rights or other rights that might be claimed to
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   this document or the extent to which any license under such rights
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Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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