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Network Working Group                                       S. JosefssonRequest for Comments: 4501                                           SJDCategory: Standards Track                                       May 2006Domain Name System Uniform Resource IdentifiersStatus 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).Abstract   This document defines Uniform Resource Identifiers for Domain Name   System resources.Table of Contents1.  Introduction and Background  . . . . . . . . . . . . . . . . .22.  Usage Model  . . . . . . . . . . . . . . . . . . . . . . . . .23.  DNS URI Registration . . . . . . . . . . . . . . . . . . . . .34.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .65.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .76.  Security Considerations  . . . . . . . . . . . . . . . . . . .77.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .78.  Copying Conditions . . . . . . . . . . . . . . . . . . . . . .89.  References . . . . . . . . . . . . . . . . . . . . . . . . . .89.1.  Normative References . . . . . . . . . . . . . . . . . .89.2.  Informative References . . . . . . . . . . . . . . . . .8Josefsson                   Standards Track                     [Page 1]

RFC 4501                        DNS URI                         May 20061.  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",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 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   applications.   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,   applications.Josefsson                   Standards Track                     [Page 2]

RFC 4501                        DNS URI                         May 20063.  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 fromRFC 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 ]                                ; SeeRFC 3986 for the                                ; definition of "host" and "port".   dnsname         = *pchar                                ; SeeRFC 3986 for the                                ; definition of "pchar".                                ; The "dnsname" field may be a                                ; "relative" or "absolute" name,                                ; as perRFC 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 AddressJosefsson                   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 perRFC3986 [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 ofRFC 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 insection 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   [2].   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   Contact: simon@josefsson.org   Author/Change Controller: simon@josefsson.org4.  Examples   A DNS URI is of the following general form.  This is intended to   illustrate, not define, the scheme:   dns:[//authority/]domain[?CLASS=class;TYPE=type]   The following illustrates a URI for a resource with the absolute name   "www.example.org.", the Internet (IN) class, and the Address (A)   type:   dns:www.example.org.?clAsS=IN;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:   dns:www.example.org   The following illustrates a URI for a resource with the name   "simon.example.org" for the CERT type in the Internet (IN) class:   dns:simon.example.org?type=CERT   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 192.168.1.1 instead of the default   authority:   dns://192.168.1.1/ftp.example.org?type=A   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:   dns:world%20wide%20web.example%5c.domain.org?TYPE=TXT   The following illustrates a strange but valid DNS resource:   dns://fw.example.org/*.%20%00.example?type=TXTJosefsson                   Standards Track                     [Page 6]

RFC 4501                        DNS URI                         May 20065.  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   information.7.  IANA Considerations   The IANA has registered the DNS URI scheme, using the template insection 3, in accordance withRFC 2717 [11].Josefsson                   Standards Track                     [Page 7]

RFC 4501                        DNS URI                         May 20068.  Copying Conditions   Copyright (c) 2000, 2001, 2002, 2003, 2004, 2005, 2006 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.  References9.1.  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,RFC959, October 1985.   [7]  Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet        Mail Extensions (MIME) Part Four: Registration Procedures",BCP13,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",RFC2673, 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",RFC2854, 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        2005.   [16] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,        "DNS Security Introduction and Requirements",RFC 4033, March        2005.Author's Address   Simon Josefsson   SJD   EMail: simon@josefsson.orgJosefsson                   Standards Track                     [Page 9]

RFC 4501                        DNS URI                         May 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Josefsson                   Standards Track                    [Page 10]