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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

Josefsson Standards Track [Page 1] RFC 4501 DNS URI May 2006

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,

Josefsson Standards Track [Page 2] RFC 4501 DNS URI May 2006

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

Josefsson Standards Track [Page 3] RFC 4501 DNS URI May 2006

 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].

Josefsson Standards Track [Page 4] RFC 4501 DNS URI May 2006

 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.

Josefsson Standards Track [Page 5] RFC 4501 DNS URI May 2006

 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 illustrates a URI for a resource with the absolute name
 "", 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
 "" for the CERT type in the Internet (IN) class:
 The following illustrates 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 illustrates various escaping techniques.  The owner
 name would be "world wide web.example\", where "\."
 denotes the character "." as part of a label, and "." denotes the
 label separator:
 The following illustrates a strange but valid DNS resource:

Josefsson Standards Track [Page 6] RFC 4501 DNS URI May 2006

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 "") 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].

Josefsson Standards Track [Page 7] RFC 4501 DNS URI May 2006

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.

Josefsson Standards Track [Page 8] RFC 4501 DNS URI May 2006

 [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

Josefsson Standards Track [Page 9] RFC 4501 DNS URI May 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 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.
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Josefsson Standards Track [Page 10]

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