Network Working Group S. Josefsson Internet-Draft May 25, 2005 Expires: November 26, 2005 Domain Name System Uniform Resource Identifiers draft-josefsson-dns-url-12 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 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 November 26, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document define Uniform Resource Identifiers for Domain Name System resources. See for more information. Josefsson Expires November 26, 2005 [Page 1] Internet-Draft DNS URI May 2005 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 Intellectual Property and Copyright Statements . . . . . . . . 13 Josefsson Expires November 26, 2005 [Page 2] Internet-Draft DNS URI May 2005 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 (CRLs) in the DNS [9]. Several protocols use Uniform Resource Locators (URLs) to point at certificates and CRLs. By defining a Uniform Resource Identifier (URI) scheme for DNS resources, such protocols can reference certificates and CRLs stored in the DNS. Two examples of data structures that may embed DNS URIs: o The OpenPGP Message Format [7], 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 Internet X.509 Public Key Infrastructure [14] format, where the issuer may use a DNS URI in a CRL Distribution Point certificate extension field. The DNS URI scheme defined here can be used to reference any data stored in the DNS, and is not limited to certificates or CRLs. The purpose of this specification is to define a generic DNS URI, not to specify a solution only for certificates stored in the DNS. Data browsers may support DNS URIs by forming DNS queries and render DNS responses using HTML [13], similar to what is commonly done for FTP [6] resources. 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 [3]. Josefsson Expires November 26, 2005 [Page 3] Internet-Draft DNS URI May 2005 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 applications. 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) applications. Josefsson Expires November 26, 2005 [Page 4] Internet-Draft DNS URI May 2005 3. DNS URI Registration The section contain the registration template for the DNS URI scheme in accordance with [12]. 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" / dnstypeval = 1*digit / "A" / "NS" / "MD" / 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 Expires November 26, 2005 [Page 5] Internet-Draft DNS URI May 2005 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] Josefsson Expires November 26, 2005 [Page 6] Internet-Draft DNS URI May 2005 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 [2]. Interaction with Binary Labels [11], or other extended label types, has not been analyzed. However, they appear to be infrequently used in practice. Contact: simon@josefsson.org Josefsson Expires November 26, 2005 [Page 7] Internet-Draft DNS URI May 2005 Author/Change Controller: simon@josefsson.org 4. 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 illustrate 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 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: dns://192.168.1.1/ftp.example.org?type=A The following illustrate various escaping techniques. The owner name would be "world wide web.example\.domain.org" where "\." denote the character "." as part of a label, and "." denote the label separator: dns:world%20wide%20web.example%5c.domain.example?TYPE=TXT The following illustrate a strange, but valid, DNS resource: dns://fw.example.org/*.%20%00.example?type=TXT Josefsson Expires November 26, 2005 [Page 8] Internet-Draft DNS URI May 2005 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 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 [8], or even CMS [15] or OpenPGP [7], 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. Slight variations, such as 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 is asked to register the DNS URI scheme, using the template in section 3, in accordance with RFC 2717 [12]. Josefsson Expires November 26, 2005 [Page 9] Internet-Draft DNS URI May 2005 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] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, "OpenPGP Message Format", RFC 2440, November 1998. [8] Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999. [9] Eastlake, D. and O. Gudmundsson, "Storing Certificates in the Domain Name System (DNS)", RFC 2538, March 1999. [10] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, Josefsson Expires November 26, 2005 [Page 10] Internet-Draft DNS URI May 2005 "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 2560, June 1999. [11] Crawford, M., "Binary Labels in the Domain Name System", RFC 2673, August 1999. [12] Petke, R. and I. King, "Registration Procedures for URL Scheme Names", BCP 35, RFC 2717, November 1999. [13] Connolly, D. and L. Masinter, "The 'text/html' Media Type", RFC 2854, June 2000. [14] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, April 2002. [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. Author's Address Simon Josefsson Email: simon@josefsson.org 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. Security consideration improvements. Josefsson Expires November 26, 2005 [Page 11] Internet-Draft DNS URI May 2005 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 considerations. 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 "...". Josefsson Expires November 26, 2005 [Page 12] Internet-Draft DNS URI May 2005 Intellectual Property Statement 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 in BCP 78 and BCP 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 at http://www.ietf.org/ipr. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Josefsson Expires November 26, 2005 [Page 13]