Internet Engineering Task Force (IETF) P. Faltstrom
Request for Comments: 7553 Netnod
Category: Informational O. Kolkman
ISSN: 2070-1721 ISOC
The Uniform Resource Identifier (URI) DNS Resource Record
This document describes the already registered DNS resource record
(RR) type, called the Uniform Resource Identifier (URI) RR, that is
used for publishing mappings from hostnames to URIs.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Applicability Statement . . . . . . . . . . . . . . . . . . . 3
3. DNS Considerations . . . . . . . . . . . . . . . . . . . . . 4
4. The Format of the URI RR . . . . . . . . . . . . . . . . . . 4
4.1. Owner Name, Class, and Type . . . . . . . . . . . . . . . 4
4.2. Priority . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Weight . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.4. Target . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.5. URI RDATA Wire Format . . . . . . . . . . . . . . . . . . 6
5. Usages . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Example: FTP Server in the example.com Domain . . . . . . 6
5.2. Relation to S-NAPTR . . . . . . . . . . . . . . . . . . . 7
5.3. Relation to U-NAPTR . . . . . . . . . . . . . . . . . . . 7
5.4. Relation to SRV . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6.1. Registration of the URI Resource Record Type . . . . . . 7
6.2. Registration of Services . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. The Original RRTYPE Allocation Request . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
This document explains the use of the Domain Name System (DNS) for
the storage of URIs [RFC3986] and how to resolve hostnames to such
URIs that can be used by various applications using the URI resource
record type. For resolution, the application needs to know both the
hostname and the protocol that the URI is to be used for. The
protocol is registered by IANA.
Historically, uses of the DNS to map a domain name to a URL have
relied on the Naming Authority Pointer (NAPTR) RRTYPEs [RFC2915] and
then on the Dynamic Delegation Discovery System (DDDS) [RFC3401]
application framework with the DNS as a database as specified in RFC
3404 [RFC3404]. This has a number of implications such as the fact
the RRSet returned will contain all URIs "connected" with the owner,
and not only the ones related to a specific service.
The URI resource record specified in this document enables the
querying party to do the equivalent of selecting which of the NAPTR
records one is interested in and have only those returned. This is
possible because data in the service field of the NAPTR record is
included in the owner part of the URI resource record type. It is
also the case that as the URI resource record type includes the
target URI directly as part of the RDATA, it is very easy to extract
the correct target URI, instead of applying rewrite rules as in
Querying for URI resource records is not replacing querying for NAPTR
resource records (or use of S-NAPTR [RFC3958]). Instead, the URI
resource record type provides a complementary mechanism to be used
when one already knows what service field is interesting. With it,
one can directly query for the specific subset of the otherwise
possibly large RRSet returned when querying for NAPTR resource
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 BCP 14, RFC 2119
2. Applicability Statement
In general, it is expected that URI records will be used by clients
for applications where the relevant protocol to be used is known,
but, for example, an extra abstraction is needed in order to separate
a domain name from a point of service (as addressed by the URI). One
example of such a situation is when an organization has many domain
names but only one official web page.
Applications need to know the specific service to prepend the
hostname with. Using repetitive queries for URI records is not a
replacement for querying for NAPTR records according to the NAPTR
(DDDS) or S-NAPTR algorithms. NAPTR records serve the purpose of
discovering the various services or the URIs (for looking up access
points for a given service). These are two very different kinds of
3. DNS Considerations
Using prefix labels, such as underscored service tags, for a specific
owner name may cause a counter-intuitive effect when the owner name
is a wildcard name. For example, _s2._s1.*.example.net is not a
wildcard name and cannot be used to return a synthesized answer for a
query name of _s2._s1.a.example.net. See Section 4.5 of RFC 4592
[RFC4592] for more details. Besides, underscored service tags used
for the URI RR (based on the "Service Name and Transport Protocol
Port Number Registry") may have slightly different semantics than
service tags used for underscored prefix labels that are used in
combination with other (yet unspecified) RR types. This may cause
subtle management problems when delegation structure that has
developed within the context of URI RRs is also to be used for other
RR types. Because the service labels might be overloaded,
applications should carefully check that the application-level
protocol is indeed the protocol they expect.
Subtle management issues may also arise when the delegations from
service to sub-service labels involve several parties and different
4. The Format of the URI RR
This is the presentation format of the URI RR:
Owner name TTL Class URI Priority Weight Target
The URI RR does not cause any kind of Additional Section processing.
4.1. Owner Name, Class, and Type
The URI owner name is subject to special conventions.
Just like the SRV RR [RFC2782], the URI RR has service information
encoded in its owner name. In order to encode the service for a
specific owner name, one uses service parameters. Valid service
parameters are those registered by IANA in the "Service Name and
Transport Protocol Port Number Registry" [RFC6335] or as "Enumservice
Registrations [RFC6117]. The Enumservice Registration parameters are
reversed (i.e., subtype(s) before type), prepended with an underscore
(_), and prepended to the owner name in separate labels. The
underscore is prepended to the service parameters to avoid collisions
with DNS labels that occur in nature, and the order is reversed to
make it possible to do delegations, if needed, to different zones
(and therefore providers of DNS).
For example, suppose we are looking for the URI for a service with
ENUM Service Parameter "A:B:C" for host example.com. Then we would
query for (QNAME,QTYPE)=("_C._B._A.example.com","URI").
As another example, suppose we are looking for the URI for a service
with Service Name "A" and Transport Protocol "B" for host
example.com. Then we would query for
The type number for the URI record is 256.
The URI resource record is class independent.
The URI RR has no special Time-to-Live (TTL) requirements.
This field holds the priority of the target URI in this RR. Its
range is 0-65535. A client MUST attempt to contact the URI with the
lowest-numbered priority it can reach; URIs with the same priority
SHOULD be selected according to probabilities defined by the weight
This field holds the server selection mechanism. The weight field
specifies a relative weight for entries with the same priority.
Larger weights SHOULD be given a proportionately higher probability
of being selected. The range of this number is 0-65535.
This field holds the URI of the target, enclosed in double-quote
characters ('"'), where the URI is as specified in RFC 3986
[RFC3986]. Resolution of the URI is according to the definitions for
the Scheme of the URI.
Since the URI will not be encoded as a <character-string> (see
Section 3.3 of RFC 1035 [RFC1035]), there is no 255-character size
The Target MUST NOT be an empty URI ("").
4.5. URI RDATA Wire Format
The RDATA for a URI RR consists of a 2-octet Priority field, a
2-octet Weight field, and a variable-length Target field.
Priority and Weight are unsigned integers in network byte order.
The remaining data in the RDATA contains the Target field. The
Target field contains the URI as a sequence of octets (without the
enclosing double-quote characters used in the presentation format).
The length of the Target field MUST be greater than zero.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Priority | Weight |
/ Target /
5.1. Example: FTP Server in the example.com Domain
An organization has the domain names example.com and example.net, and
their FTP archive is at ftp://ftp1.example.com/public. Given the
service name "ftp" and transport protocol "tcp" (from the IANA
"Service Name and Transport Protocol Port Number Registry"), the
following URI resource records could be made available in the
respective zones (example.com and example.net):
_ftp._tcp IN URI 10 1 "ftp://ftp1.example.com/public"
_ftp._tcp IN URI 10 1 "ftp://ftp1.example.com/public"
5.2. Relation to S-NAPTR
The URI resource record type is not a replacement for the S-NAPTR.
It is instead an extension and the second step of the S-NAPTR
resolution can resolve a URI resource record instead of using SRV
records and yet another algorithm for how to use SRV records for the
;; order pref flags
IN NAPTR 100 10 "D" "EM:ProtA" ( ; service
"" ; regexp
_http._tcp.example.com. ) ; replacement
_http._tcp IN URI 10 1 "http://www.example.com/path"
5.3. Relation to U-NAPTR
The URI resource record type, together with S-NAPTR, can be viewed as
a replacement for U-NAPTR [RFC4848]. The URI resource record type is
only interesting when one know a base domain name, a protocol, and a
service so that one can compose the record to look up. NAPTR records
of any kind are used to look up what services exist for a certain
domain, which is one step before the URI resource record is used.
5.4. Relation to SRV
The URI resource record type can be viewed as a replacement for the
SRV record. This is because it, like the SRV record, can only be
looked up if one knows the base domain, the protocol, and the
service. It has a similar functionality and uses the same registry
for service names, but instead of returning a hostname and port
number, the URI record returns a full URI. As such, it can be viewed
as a more powerful resource record than SRV.
6. IANA Considerations
6.1. Registration of the URI Resource Record Type
After an expert review in February 2011 (see Appendix A), IANA
allocated RRTYPE 256 for the URI resource record type in the registry
named "Resource Record (RR) TYPEs" (as defined in [BCP42], which was
RFC 6195 at the time but has since been replaced by RFC 6895) located
IANA has updated the reference for this registration to refer to this
6.2. Registration of Services
No new registry is needed for the registration of services as the
Service Name, Transport Protocol Port Numbers, Enumservices and the
DNS SRV Service Type registries are also used for the URI resource
7. Security Considerations
Using the URI resource record together with security mechanisms that
rely on verification of authentication of hostnames, like TLS, makes
it important to choose the correct domain name when doing the
comparison and ensure that the change in the hostname to be used is
secured by DNSSEC so that it can be trusted in a similar way as a
redirect in HTTP using TLS.
If, for example, the URI resource record is not signed with the help
of DNSSEC and then validated successfully, trusting the non-signed
URI will effectively lead to a downgrade attack.
The basic mechanism for successful use of URI works as follows:
1. Announce that example.com is hosted at example.org (with some
URL) in DNS.
2. Secure the URI resource record with DNSSEC. This is best done
by doing validation in the application doing the lookup, but it
could also be done in the local recursive resolver or in the
trusted recursive resolver also doing validation. All are
according to the local trust policy.
3. Verify the TLS (for example) certificate for the connection to
example.org matches, i.e., use the hostname in the URI and not
the hostname used originally when looking up the URI resource
4. If needed, do application-layer authentication, etc., over the
then encrypted connection.
It is also possible that the URI in the resource record type has
errors in it. Applications using the URI resource record type for
resolution should behave similarly as if the user typed (or copied
and pasted) the URI. At least it must be clear to the user that the
error is not due to any error from his side.
One SHOULD NOT include userinfo (see "User Information",
Section 3.2.1 of RFC 3986 [RFC3986]) in a URI that is used in a URI
resource record as DNS data must be viewed as publicly available
8.1. Normative References
[BCP42] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, April 2013,
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
[RFC6117] Hoeneisen, B., Mayrhofer, A., and J. Livingood, "IANA
Registration of Enumservices: Guide, Template, and IANA
Considerations", RFC 6117, DOI 10.17487/RFC6117, March
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
8.2. Informative References
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
[RFC2915] Mealling, M. and R. Daniel, "The Naming Authority Pointer
(NAPTR) DNS Resource Record", RFC 2915,
DOI 10.17487/RFC2915, September 2000,
[RFC3401] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part One: The Comprehensive DDDS", RFC 3401,
DOI 10.17487/RFC3401, October 2002,
[RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database",
RFC 3403, DOI 10.17487/RFC3403, October 2002,
[RFC3404] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Four: The Uniform Resource Identifiers (URI)",
RFC 3404, DOI 10.17487/RFC3404, October 2002,
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
[RFC3958] Daigle, L. and A. Newton, "Domain-Based Application
Service Location Using SRV RRs and the Dynamic Delegation
Discovery Service (DDDS)", RFC 3958, DOI 10.17487/RFC3958,
January 2005, <http://www.rfc-editor.org/info/rfc3958>.
[RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
[RFC4848] Daigle, L., "Domain-Based Application Service Location
Using URIs and the Dynamic Delegation Discovery Service
(DDDS)", RFC 4848, DOI 10.17487/RFC4848, April 2007,
[RFC5507] IAB, Faltstrom, P., Ed., Austein, R., Ed., and P. Koch,
Ed., "Design Choices When Expanding the DNS", RFC 5507,
DOI 10.17487/RFC5507, April 2009,
Appendix A. The Original RRTYPE Allocation Request
On February 22, 2011 IANA assigned RRTYPE 256 for the URI resource
record based on a request that followed the procedure documented in
[BCP42] (which was RFC 6195 at the time but has since been replaced
by RFC 6895). The DNS RRTYPE PARAMETER ALLOCATION form as submitted
to IANA at that time is replicated below for reference.
Note: Although "ownername" should be "owner name", "ownername" has
been preserved below because it was part of the original request form
submitted to IANA.
A. Submission Date:
May 23, 2009
B. Submission Type:
[X] New RRTYPE
[ ] Modification to existing RRTYPE
C. Contact Information for submitter:
Name: Patrik Faltstrom
Email Address: firstname.lastname@example.org
International telephone number: +46-8-6859131
Other contact handles:
(Note: This information will be publicly posted.)
D. Motivation for the new RRTYPE application?
There is no easy way to get from a domain name to a URI. Some
mechanisms exists via use of the NAPTR [RFC3403] resource
record. That implies quite complicated rules that are
simplified via the S-NAPTR [RFC3958] specification. But, the
ability to directly look up a URI still exists. This
specification uses a prefix based naming mechanism originated in
the definition of the SRV [RFC2782] resource record, and the
RDATA is a URI, encoded as one text field.
See also above (Section 1).
E. Description of the proposed RR type.
The format of the URI resource record is as follows:
Ownername TTL Class URI Priority Weight Target
The URI RR has service information encoded in its ownername. In
order to encode the service for a specific ownername one uses
service parameters. Valid service parameters used are either
Enumservice Registrations registered by IANA, or prefixes used
for the SRV resource record.
The wire format of the RDATA is as follows:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Priority | Weight |
/ Target /
F. What existing RRTYPE or RRTYPEs come closest to filling that
need and why are they unsatisfactory?
The RRTYPE that come closest is the NAPTR resource record. It
is for example used in the DDDS and S-NAPTR algorithms. The
main problem with the NAPTR is that selection of what record (or
records) one is interested in is based on data stored in the
RDATA portion of the NAPTR resource record. This, as explained
in RFC 5507 [RFC5507], is not optimal for DNS lookups. Further,
most applications using NAPTR resource records uses regular
expression based rewrite rules for creation of the URI, and that
has shown be complicated to implement.
The second closest RRTYPE is the SRV record that given a
prefixed based naming just like is suggested for the URI
resource record, one get back a port number and domain name.
This can also be used for creation of a URI, but, only URIs
without path components.
G. What mnemonic is requested for the new RRTYPE (optional)?
H. Does the requested RRTYPE make use of any existing IANA Registry
or require the creation of a new IANA sub-registry in DNS
One of the mechanisms to select a service is to use the
Enumservice Registry managed by IANA. Another is to use
services and protocols used for SRV records.
I. Does the proposal require/expect any changes in DNS servers/
resolvers that prevent the new type from being processed as an
unknown RRTYPE (see [RFC3597])?
Ideas on how to split the two different kinds of queries, "What
services exists for this domain name" and "What is the URI for this
service", came from Scott Bradner and Lawrence Conroy. Other people
that have contributed to this document include Richard Barnes, Leslie
Daigle, Victor Dukhovni, Olafur Gudmundsson, Philip Hallam-Baker, Ted
Hardie, Sam Hartman, Evan Hunt, John Klensin, Peter Koch, Eliot Lear,
Andy Newton, Mark Nottingham, Penn Pfautz, Jinmei Tatuya, Willem
Toorop, and Nico Williams.
Cisco is acknowledged as Mr. Faltstrom's employer at the time this
document was developed.
The NLnet Labs is acknowledged as Mr. Kolkman's employer at the time
this document was developed.