I missed the cut-off date, but meanwhile it is available from: http://josefsson.org/draft-josefsson-mime-dns.txt The forms where part of earlier versions of draft-josefsson-dns-url, and there has been two versions of draft-josefsson-mime-dns published at IETF before, so the types may have been discussed here before; I don't recall. Still, this version has been improved in many ways, so I'd appreciate more review. Thanks, Simon 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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 25, 2004. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document define Uniform Resource Identifiers for Domain Name System resources. Josefsson Expires April 25, 2004 [Page 1] Internet-Draft DNS URI October 2003 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 Josefsson Expires April 25, 2004 [Page 2] Internet-Draft DNS URI October 2003 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]. Josefsson Expires April 25, 2004 [Page 3] Internet-Draft DNS URI October 2003 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 Josefsson Expires April 25, 2004 [Page 4] Internet-Draft DNS URI October 2003 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 Josefsson Expires April 25, 2004 [Page 5] Internet-Draft DNS URI October 2003 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 Josefsson Expires April 25, 2004 [Page 6] Internet-Draft DNS URI October 2003 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 Josefsson Expires April 25, 2004 [Page 7] Internet-Draft DNS URI October 2003 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]. Josefsson Expires April 25, 2004 [Page 8] Internet-Draft DNS URI October 2003 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. Josefsson Expires April 25, 2004 [Page 9] Internet-Draft DNS URI October 2003 [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 Josefsson Expires April 25, 2004 [Page 10] Internet-Draft DNS URI October 2003 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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