

Network Working Group                                       S. Josefsson
Internet-Draft                                          October 26, 2003
Expires: April 25, 2004


            Domain Name System Uniform Resource Identifiers
                       draft-josefsson-dns-url-09

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

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














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Table of Contents

   1.  Introduction and Background  . . . . . . . . . . . . . . . . .  3
   2.  DNS URI Registration . . . . . . . . . . . . . . . . . . . . .  4
   3.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
       Normative References . . . . . . . . . . . . . . . . . . . . .  9
       Informative References . . . . . . . . . . . . . . . . . . . .  9
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 10
   A.  Revision Changes . . . . . . . . . . . . . . . . . . . . . . . 10
   A.1 Changes since -06  . . . . . . . . . . . . . . . . . . . . . . 10
   A.2 Changes since -07  . . . . . . . . . . . . . . . . . . . . . . 10
   A.3 Changes since -08  . . . . . . . . . . . . . . . . . . . . . . 10
       Intellectual Property and Copyright Statements . . . . . . . . 12




































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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.
   Recent work has added support for storing certificates and
   certificate revocation lists in the DNS [10].

   The primary motivation behind defining a Uniform Resource Identifier
   (URI) for DNS resources, instead of using another non-URI syntax that
   embed the domain, type value and class value, is that applications
   that stores or retrieve certificates today uses URIs for this
   purpose.  Thus, defining a URI scheme for DNS resources allows these
   existing protocols to be used with certificates in the DNS without
   having to add DNS specific modifications to said protocols.  In order
   to not introduce interoperability or security considerations,
   protocols that uses these URIs naturally must have been written to
   allow for future, as of writing yet undefined, URIs to be used.

   A few examples of protocols that may utilize DNS URIs:

   o  The OpenPGP Message Format [8], where an end-user may indicate the
      location of a copy of any updates to her key, using the "preferred
      key server" field.

   o  The X.509 Online Certificate Status Protocol [11], where the OCSP
      responder can indicate where a CRL is found, using the
      id-pkix-ocsp-crl extension.

   The DNS URI scheme defined here can, of course, be used to reference
   any DNS data, and is not limited to only certificates.  The purpose
   of this specification is to define a generic DNS URI, not a specific
   DNS solution for certificates stored in the DNS.  Browsers may
   implement support for DNS URIs by forming DNS queries and render DNS
   responses using HTML [14], similar to what is done for the FTP [5].

   The core part of this document is the URI Registration Template
   according to [13].

   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 in RFC 2119 [6].










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2. DNS URI Registration

   URL scheme name: "dns".

   URL scheme syntax: A DNS URI designates a DNS resource record set
   that can be referenced by domain name, type, class and optionally the
   authority.  The DNS URI follows the generic syntax from RFC 2396 [4],
   and is described using ABNF [3].  Strings are not case sensitive and
   free insertion of linear-white-space is not permitted.

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

   dnsauthority    = hostport
                                ; See RFC 2396 for "hostport" definition.

   dnsname         = *pchar
                                ; See RFC 2396 for "pchar" definition.
                                ; NB! Can be empty.

   dnsquery        = dnsqueryelement [";" dnsquery]
                                ; First matching element MUST be used.
                                ; E.g., dns:host.example.org?TYPE=A;TYPE=TXT
                                ; means type A.

   dnsqueryelement = ( "CLASS=" dnsclassval ) / ( "TYPE=" dnstypeval ) /
                     ( 1*alphanum "=" 1*alphanum )

   dnsclassval     = 1*digit / "IN" / "CH" / ...
                                ; Any IANA registered DNS class expressed as
                                ; mnemonic or as decimal integer.

   dnstypeval      = 1*digit / "A" / "NS" / "MD" / ...
                                ; Any IANA registered DNS type expressed as
                                ; mnemonic or as decimal integer.

   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].

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

   To resolve a DNS URI using the DNS protocol [2] a query is formed by
   using the dnsname, dnsclassval and dnstypeval from the URI string (or
   the previously mentioned default values if some value missing from



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   the string).  If 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.  DNS
   URIs does not require the use of the DNS protocol, although it is
   expected to be the typical usage. This 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 is able to correctly
   parse the answer.  A typical example of a client that would not need
   to check dnsclassval and dnstypeval would be a proxy that just treat
   the answer as opaque data.

   Character encoding considerations: The characters are encoded as per
   the "URI Generic Syntax" RFC [4].  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 reason for this is that making these fields
   be IDN aware by, e.g., specifying that they are UTF-8 [7] strings,
   would require further encoding mechanisms to be able to express all
   valid DNS domain names. This is because the DNS allows all octet
   sequences to be used as domain labels, so UTF-8 strings do not cover
   all possibilities.  Instead of defining further encoding mechanisms,
   we point applications with internationalization needs at the ASCII
   encoding described in [16] which should be satisfactory.) The
   considerations for "hostport" are discussed in [4]

   To encode a "." that is part of a DNS label the "escaped" encoding
   MUST be used, and a label delimiter MUST be encoded as ".".  That is,
   the only way to encode a label delimiter is ".", and the only way to
   encode a "." as part of label is "%2e".  This approach was chosen to
   minimize the modifications users will have to do when manually
   translating a domain name string into the URI form.

   This URI specification allows all possible domain names to be encoded
   (of course following the encoding rules of [4]), however certain
   applications may restrict the set of valid characters and 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



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   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.  It may be of interest to auxilliary DNS related software
   too.

   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 type or class mnemonic but 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 could use integers for all mnemonics not defined in
   [2].

   Interaction with Binary Labels [12], or other extended label types,
   has not been analyzed. However, they appear to be infrequently used
   in practice.

   Security considerations: See below.

   Contact: simon@josefsson.org

   Author/Change Controller: simon@josefsson.org






















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3. Examples

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

   dns:[//authority/]domain[?type=TYPE;class=CLASS]

   The following illustrate a URI for a resource with the 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:

   dns:www.example.org

   The following illustrate 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 illustrate 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 (i.e., when DNS is used, the query is sent to that server):

   dns://192.168.1.1/ftp.example.org?type=A

   The following illustrate a strange, albeit valid, DNS resource.  Note
   the encoding of "." and 0x00, and the use of a named dnsauthority:

   dns://internal-dns.example.org/*.%3f%20%00%2e%25+?type=TXT


















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4. 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
   environment.

   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), 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 "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 accepts unknown
   dnsqueryelement values.

   This draft does not modify the security considerations related to the
   DNS or URIs in general.

5. IANA Considerations

   The IANA is asked to register the DNS URI scheme, using the template
   in section 2, in accordance with RFC 2717 [13].



















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Acknowledgments

   Thanks to Stuart Cheshire, Donald Eastlake, Pasi Eronen, Ted Hardie,
   Peter Koch, Andrew Main, Larry Masinter, Michael Mealling, Steve
   Mattson, and Paul Vixie for comments and suggestions.  The author
   acknowledges the RSA Laboratories for supporting the work that led to
   this document.

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]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [4]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
        Identifiers (URI): Generic Syntax", RFC 2396, August 1998.

Informative References

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

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

   [7]   Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
         2279, January 1998.

   [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.

   [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.



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   [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.


Author's Address

   Simon Josefsson

   EMail: simon@josefsson.org

Appendix A. Revision Changes

   Note to RFC editor: This appendix is to be removed on 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.

   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



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   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.













































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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgment

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











































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