Network Working Group S. Josefsson
Internet-Draft September 2, 2004
Expires: March 3, 2005
Domain Name System Uniform Resource Identifiers
draft-josefsson-dns-url-10
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Copyright Notice
Copyright (C) The Internet Society (2004).
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
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1 Normative References . . . . . . . . . . . . . . . . . . . . 9
6.2 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 . . . . . . . . . . . . . . . . . . . . 11
A.4 Changes since -09 . . . . . . . . . . . . . . . . . . . . 11
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 (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.
A few examples of protocols that may utilize 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 X.509 Online Certificate Status Protocol [10], 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 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 [5] resources.
The core part of this document is the URI Registration Template in
accordance with [12].
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 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 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.
; 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 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.
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
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
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"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
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 [15]. The considerations for "hostport" are discussed
in [4]
Because "." is used as the DNS label separator, an escaping mechanism
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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 [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
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
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[?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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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 [8], or even CMS [14] 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.
5. IANA Considerations
The IANA is asked to register the DNS URI scheme, using the template
in section 2, in accordance with RFC 2717 [12].
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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.
6. References
6.1 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] 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.
6.2 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] 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,
"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.
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[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., "Cryptographic Message Syntax (CMS)", RFC 3369,
August 2002.
[15] Faltstrom, P., Hoffman, P. and A. Costello, "Internationalizing
Domain Names in Applications (IDNA)", RFC 3490, March 2003.
[16] 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.
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.
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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.
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