Network Working Group S. Josefsson Internet-Draft April 20, 2003 Expires: October 19, 2003 Domain Name System Uniform Resource Identifiers draft-josefsson-dns-url-07 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 October 19, 2003. Abstract This document define Uniform Resource Identifiers for Domain Name System resources. Josefsson Expires October 19, 2003 [Page 1] Internet-Draft DNS URI April 2003 Table of Contents 1. Introduction and Background . . . . . . . . . . . . . . . . . 3 1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 Changes since -06 . . . . . . . . . . . . . . . . . . . . . . 3 2. DNS URI Registration . . . . . . . . . . . . . . . . . . . . . 4 3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 Normative References . . . . . . . . . . . . . . . . . . . . . 8 Informative References . . . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 8 Intellectual Property and Copyright Statements . . . . . . . . 9 Josefsson Expires October 19, 2003 [Page 2] Internet-Draft DNS URI April 2003 1. Introduction and Background The Domain Name System (DNS) [1][2] is a widely deployed protocol used to, among other things, translate host names into IP addresses. Recent work has added support for storing certificates in DNS [8]. To be able to locate and retrieve certificates via a network, Uniform Resource Identifiers (URIs) are often used. This document define a URI scheme for DNS information. The DNS URI scheme defined here can, of course, be used to reference any DNS data, and is not limited to only certificates. The core part of this document is the URI Registration Template according to [9]. 1.1 Terminology 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 [5]. 1.2 Changes since -06 Note to RFC editor: This section is to be removed on publication. 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 October 19, 2003 [Page 3] Internet-Draft DNS URI April 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 server. 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:" [ "//" hostport "/" ] dnsname ["?" dnsquery] ; See RFC 2396 for "hostport" definition dnsname = *pchar ; See RFC 2396 for "pchar" definition dnsquery = dnsqueryelement [";" dnsquery] dnsqueryelement = ( "CLASS=" dnsclassval ) | ( "TYPE=" dnstypeval ) | ( 1*alphanum "=" 1*alphanum ) dnsclassval = 1*digit / "IN" / "CH" / ... ; Any standard DNS class expressed as ; mnemonic or as decimal integer dnstypeval = 1*digit / "A" / "NS" / "MD" / ... ; Any standard 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 of MUST recognize the mnemonics defined in [2]. Unless specified in the URI, the server ("hostport") is assumed to be locally known, "dnsclassval" to be the Internet class ("IN"), and "dnstypeval" to be the Address (A) type. 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 the string). If server ("hostport") is given in the URI string, this server should receive the DNS query, otherwise the default DNS server should receive it. A client MAY want to check that it understands the dnsclassval and dnstypeval before sending a query, so that it is able to correctly Josefsson Expires October 19, 2003 [Page 4] Internet-Draft DNS URI April 2003 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. 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 DNS 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 DNS have certain restrictions. It is up to these application to limit this subset, this URI scheme places no restrictions. Intended usage: Broad usage. Applications and/or protocols which use this scheme: DNS related software. 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 a 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]. Security considerations: See below. Contact: simon@josefsson.org Author/Change Controller: IESG Josefsson Expires October 19, 2003 [Page 5] Internet-Draft DNS URI April 2003 3. Examples A DNS URI is of the following general form. This is intended to illustrate, not define, the scheme. dns:[//server/]domain[?type=TYPE;class=CLASS] The following illustrate a DNS query for "www.example.org" for the Internet (IN) class and the Address (A) type: dns:www.example.org?class=IN;type=A The following illustrate a DNS query for "simon.example.org" for the CERT type in the Internet (IN) class: dns:simon.example.org?type=CERT The following illustrate a DNS query for "ftp.example.org" from the DNS server "internal-dns.example.org" server, in the Internet (IN) class and the address (A) type: dns://internal-dns.example.org/ftp.example.org?type=A The following illustrate a strange, albeit valid, DNS query. Note the encoding of "." and 0x00: dns://internal-dns.example.org/*.%3f%20%00%2e%25+?type=TXT Josefsson Expires October 19, 2003 [Page 6] Internet-Draft DNS URI April 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 DNS), implementations may need to employ security techniques such as Secure DNS [7], or even CMS [10] or OpenPGP [6], 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 ill-formed DNS URIs and try to attach meaning to them (e.g., accepting the URI "dns:www.example.org?secret=value"), a covert channel used to "leak" information may be enabled. The implications of covert channels should be understood by applications that accepts ill-formed DNS URIs. This draft does not modify the security considerations related to DNS itself. 5. IANA Considerations The IANA is asked to register the DNS URI scheme, using the template in section 2, in accordance with RFC 2717 [9]. Josefsson Expires October 19, 2003 [Page 7] Internet-Draft DNS URI April 2003 Acknowledgements Thanks to Stuart Cheshire, Donald Eastlake, Pasi Eronen, Ted Hardie, Michael Mealling, and Steve Mattson for comments and suggestions. 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] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [6] Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP Message Format", RFC 2440, November 1998. [7] Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999. [8] Eastlake, D. and O. Gudmundsson, "Storing Certificates in the Domain Name System (DNS)", RFC 2538, March 1999. [9] Petke, R. and I. King, "Registration Procedures for URL Scheme Names", BCP 35, RFC 2717, November 1999. [10] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369, August 2002. Author's Address Simon Josefsson EMail: simon@josefsson.org Josefsson Expires October 19, 2003 [Page 8] Internet-Draft DNS URI April 2003 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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