One document matched: draft-josefsson-dns-url-08.txt
Differences from draft-josefsson-dns-url-07.txt
Network Working Group S. Josefsson
Internet-Draft April 2003
Expires: September 30, 2003
Domain Name System Uniform Resource Identifiers
draft-josefsson-dns-url-08
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
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This Internet-Draft will expire on September 30, 2003.
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 . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
Normative References . . . . . . . . . . . . . . . . . . . . . 7
Informative References . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 8
A. Revision Changes . . . . . . . . . . . . . . . . . . . . . . . 8
A.1 Changes since -06 . . . . . . . . . . . . . . . . . . . . . . 8
A.2 Changes since -07 . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 10
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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 and
certificate revocation lists in the DNS [9].
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 [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, 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, similar to what is done for FTP.
The core part of this document is the URI Registration Template
according to [11].
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].
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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 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
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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 are to be considered an
internationalized domain name (IDN) unaware domain name slot, in the
terminology of [13]. (The reason for this is that making these
fields be IDN aware by, e.g., specifying that they are UTF-8 [6]
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 [13] 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
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
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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].
Security considerations: See below.
Contact: simon@josefsson.org
Author/Change Controller: simon@josefsson.org
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
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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 [12] 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 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 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 [11].
Acknowledgments
Thanks to Stuart Cheshire, Donald Eastlake, Pasi Eronen, Ted Hardie,
Larry Masinter, Michael Mealling, and Steve Mattson for comments and
suggestions. The author acknowledges the RSA Laboratories for
supporting the work that lead 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
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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] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.
[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] Petke, R. and I. King, "Registration Procedures for URL Scheme
Names", BCP 35, RFC 2717, November 1999.
[12] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369,
August 2002.
[13] 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
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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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Intellectual Property Statement
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Full Copyright Statement
Copyright (C) Simon Josefsson (2003). All Rights Reserved.
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