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IETF RFC 4501
Domain Name System Uniform Resource Identifiers
Last modified on Wednesday, May 3rd, 2006
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Network Working Group S. Josefsson
Request for Comments: 4501 SJD
Category: Standards Track May 2006
Domain Name System Uniform Resource Identifiers
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2006).
Abstract
This document defines Uniform Resource Identifiers for Domain Name
System resources.
Table of Contents
1. Introduction and Background . . . . . . . . . . . . . . . . . 2
2. Usage Model . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. DNS URI Registration . . . . . . . . . . . . . . . . . . . . . 3
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Copying Conditions . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Josefsson Standards Track PAGE 1 
RFC 4501 DNS URI May 2006
1. Introduction and Background
The Domain Name System (DNS) [1] [2] is a widely deployed system
used, among other things, to translate host names into IP addresses.
Several protocols use Uniform Resource Identifiers (URIs) to refer to
data. By defining a URI scheme for DNS data, the gap between these
two worlds is 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
rendering DNS responses using HTML [12], which is similar to what is
commonly done for FTP [6] resources. Applications that are
Multipurpose Internet Mail Extensions (MIME) [7] aware may tag DNS
data retrieved using this scheme with the text/dns or application/dns
types as specified in [15].
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].
2. Usage Model
Refer to section 1 of [5] for an in-depth discussion of URI
classifications. In particular, the reader is assumed to be familiar
with the distinction between "name" and "locator". This section
describes how the DNS URI scheme is intended to be used and outlines
future work that may be required to use URIs with the DNS for some
applications.
The URI scheme described in this document focuses 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 intended so that the URI
may be used even in situations where the DNS protocol is not used
directly. Two examples for this are 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 does not accommodate all
protocol parameters within the DNS protocol. It is expected that,
for certain applications, a more detailed URI syntax that maps more
closely to the DNS protocol may be required. However, such a URI
definition is not included in this document. This document specifies
a URI that is primarily intended to name DNS resources, but it can
also be used to locate said resources for simple, yet common,
applications.
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RFC 4501 DNS URI May 2006
3. DNS URI Registration
This section contains the registration template for the DNS URI
scheme in accordance with [11].
URL scheme name: "dns".
URL scheme syntax: A DNS URI designates 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 dnsnames.
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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RFC 4501 DNS URI May 2006
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 occurrence 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 it 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 a degenerative form of a relative name, which refers 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 indicates 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 illustrates 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: Characters are encoded as per RFC
3986 [5]. The DNS protocol does 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 RFC 3940 [14].
The considerations for "host" and "port" are discussed in [5].
Josefsson Standards Track PAGE 4
RFC 4501 DNS URI May 2006
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 [2]. For
example, a DNS label of "exa.mple" can be escaped as "exa\.mple" or
"exa\046mple". However, the URI specification disallows the "\"
character from occurring 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 interchangeable.
This URI specification allows all possible domain names to be
encoded, provided the encoding rules are observed per [5]). However,
certain applications may restrict the set of valid characters. Care
should be taken so that invalid characters in these contexts do not
cause harm. In particular, host names in the DNS have certain
restrictions. It is up to these applications to limit this subset;
this URI scheme places no restrictions.
Intended usage: Whenever it is useful for DNS resources to be
referenced by protocol-independent identifiers. Often, this occurs
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 that use this scheme include
Security-related software, DNS administration tools, and 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.
Interoperability problems may occur if one entity understands a new
DNS class/type mnemonic that another entity does not. This is an
interoperability problem for DNS software in general, although it is
not a major practical problem for current DNS deployments, 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 [10] or other extended label types has
not been analyzed. However, binary labels appear to be infrequently
used in practice.
Josefsson Standards Track PAGE 5
RFC 4501 DNS URI May 2006
Contact: simon@josefsson.org
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 illustrates 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 illustrates 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 illustrates 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 illustrates various escaping techniques. The owner
name would be "world wide web.example\.domain.org", where "\."
denotes the character "." as part of a label, and "." denotes the
label separator:
dns:world%20wide%20web.example%5c.domain.org?TYPE=TXT
The following illustrates a strange but valid DNS resource:
dns://fw.example.org/*.%20%00.example?type=TXT
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RFC 4501 DNS URI May 2006
5. Acknowledgements
Thanks to Stuart Cheshire, Donald Eastlake, Pasi Eronen, Bill Fenner,
Ted Hardie, Russ Housley, Peter Koch, Andrew Main, Larry Masinter,
Michael Mealling, Steve Mattson, Marcos Sanz, Jason Sloderbeck, Paul
Vixie, Sam Weiler, and Bert Wijnen for comments and suggestions. The
author acknowledges 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 referenced should be
handled using the same considerations that apply to DNS data in the
"internal" environment.
If information referenced by DNS URIs are used to make security
decisions (such data includes, but is not limited to, certificates
stored in the DNS [9]), implementations may need to employ security
techniques such as Secure DNS [16], CMS [13], 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 in a URI (e.g.,
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 accept unknown
dnsqueryelement values.
Slight variations, such as the 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 has registered the DNS URI scheme, using the template in
section 3, in accordance with RFC 2717 [11].
Josefsson Standards Track PAGE 7
RFC 4501 DNS URI May 2006
8. Copying Conditions
Copyright (c) 2000, 2001, 2002, 2003, 2004, 2005, 2006 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 4234, October 2005.
[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 3rd, D. and O. Gudmundsson, "Storing Certificates in
the Domain Name System (DNS)", RFC 2538, March 1999.
Josefsson Standards Track PAGE 8
RFC 4501 DNS URI May 2006
[10] Crawford, M., "Binary Labels in the Domain Name System", RFC
2673, August 1999.
[11] Petke, R. and I. King, "Registration Procedures for URL Scheme
Names", BCP 35, RFC 2717, November 1999.
[12] Connolly, D. and L. Masinter, "The 'text/html' Media Type", RFC
2854, June 2000.
[13] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852,
July 2004.
[14] Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing
Domain Names in Applications (IDNA)", RFC 3490, March 2003.
[15] Josefsson, S., "Domain Name System Media Types", RFC 4027, April
2005.
[16] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033, March
2005.
Author's Address
Simon Josefsson
SJD
EMail: simon@josefsson.org
Josefsson Standards Track PAGE 9
RFC 4501 DNS URI May 2006
Full Copyright Statement
Copyright © The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
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Acknowledgement
Funding for the RFC Editor function is provided by the IETF
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Josefsson Standards Track PAGE 10
Domain Name System Uniform Resource Identifiers
RFC TOTAL SIZE: 20990 bytes
PUBLICATION DATE: Wednesday, May 3rd, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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