Network Working Group S. Josefsson
Internet-Draft August 5, 2005
Expires: February 6, 2006
Domain Name System Uniform Resource Identifiers
draft-josefsson-dns-url-13
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document define Uniform Resource Identifiers for Domain Name
System resources.
See <http://josefsson.org/dns-url/> for more information.
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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
A.7 Changes since -12 . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . 14
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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.
Several protocols are using Uniform Resource Identifier (URI) to
refer to data. By defining a URI scheme for DNS data, the gap
between these two worlds are 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 render
DNS responses using HTML [14], similar to what is commonly done for
FTP [6] resources. Applications that are Multipurpose Internet Mail
Extension (MIME) [7] aware may tag DNS data retrieve using this
scheme with the text/dns or application/dns types as specified in
[18].
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].
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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.
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3. DNS URI Registration
The section contain the registration template for the DNS URI scheme
in accordance with [13].
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" /
<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 Address
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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]
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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 [12], or other extended label types,
has not been analyzed. However, they appear to be infrequently used
in practice.
Contact: simon@josefsson.org
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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
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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 [10]), 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.
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 [13].
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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] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures",
BCP 13, RFC 2048, November 1996.
[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.
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[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.
[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.
[17] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
STD 63, RFC 3629, November 2003.
[18] Josefsson, S., "Domain Name System Media Types", RFC 4027,
April 2005.
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.
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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
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 "...".
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A.7 Changes since -12
Reference MIME and RFC 4027. IESG feedback: Do not mention OpenPGP/
X.509 as illustrative examples in the introduction section.
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