< draft-hoffman-dns-over-https-00.txt   draft-hoffman-dns-over-https-01.txt >
Network Working Group P. Hoffman Network Working Group P. Hoffman
Internet-Draft ICANN Internet-Draft ICANN
Intended status: Standards Track P. McManus Intended status: Standards Track P. McManus
Expires: November 4, 2017 Mozilla Expires: December 8, 2017 Mozilla
May 03, 2017 June 06, 2017
DNS Queries over HTTPS DNS Queries over HTTPS
draft-hoffman-dns-over-https-00 draft-hoffman-dns-over-https-01
Abstract Abstract
DNS queries sometimes experience problems with end to end DNS queries sometimes experience problems with end to end
connectivity at times and places where HTTPS flows freely. connectivity at times and places where HTTPS flows freely.
HTTPS provides the most practical mechanism for reliable end to end HTTPS provides the most practical mechanism for reliable end to end
communication. Its use of TLS provides integrity and confidentiality communication. Its use of TLS provides integrity and confidentiality
guarantees and its use of HTTP allows it to interoperate with guarantees and its use of HTTP allows it to interoperate with
proxies, firewalls, and authentication systems where required for proxies, firewalls, and authentication systems where required for
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 4, 2017. This Internet-Draft will expire on December 8, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Protocol Requirements . . . . . . . . . . . . . . . . . . . . 4 4. Protocol Requirements . . . . . . . . . . . . . . . . . . . . 4
4.1. Non-requirements . . . . . . . . . . . . . . . . . . . . 4 4.1. Non-requirements . . . . . . . . . . . . . . . . . . . . 4
5. The HTTP Request . . . . . . . . . . . . . . . . . . . . . . 4 5. The HTTP Request . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 5 5.1. DNS Wire Format . . . . . . . . . . . . . . . . . . . . . 5
5.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 6
6. The HTTP Response . . . . . . . . . . . . . . . . . . . . . . 6 6. The HTTP Response . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 7 6.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. HTTP Integration . . . . . . . . . . . . . . . . . . . . . . 7 7. HTTP Integration . . . . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8 8.1. Registration of Well-Known URI . . . . . . . . . . . . . 8
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 8.2. Registration of application/dns-udpwireformat Media Type 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 8 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . 9 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Previous Work on DNS over HTTP or in Other Formats . 10 11.1. Normative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 11.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Previous Work on DNS over HTTP or in Other Formats . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
The Internet does not always provide end to end reachability for The Internet does not always provide end to end reachability for
native DNS. On-path network devices may spoof DNS responses, block native DNS. On-path network devices may spoof DNS responses, block
DNS requests, or just redirect DNS queries to different DNS servers DNS requests, or just redirect DNS queries to different DNS servers
that give less-than-honest answers. that give less-than-honest answers.
Over time, there have been many proposals for using HTTP and HTTPS as Over time, there have been many proposals for using HTTP and HTTPS as
a substrate for DNS queries and responses. To date, none of those a substrate for DNS queries and responses. To date, none of those
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establishes default media formatting types for requests and responses establishes default media formatting types for requests and responses
but uses normal HTTP content negotiation mechanisms for selecting but uses normal HTTP content negotiation mechanisms for selecting
alternatives that endpoints may prefer in anticipation of serving new alternatives that endpoints may prefer in anticipation of serving new
use cases. In addition to this media type negotiation, it aligns use cases. In addition to this media type negotiation, it aligns
itself with HTTP features such as caching, proxying, and compression. itself with HTTP features such as caching, proxying, and compression.
The integration with HTTP provides a transport suitable for both The integration with HTTP provides a transport suitable for both
traditional DNS clients and native web applications seeking access to traditional DNS clients and native web applications seeking access to
the DNS. the DNS.
2. Terminology
A server that supports this protocol is called a "DNS API server" to A server that supports this protocol is called a "DNS API server" to
differentiate it from a "DNS server" (one that uses the regular DNS differentiate it from a "DNS server" (one that uses the regular DNS
protocol). Similarly, a client that supports this protocol is called protocol). Similarly, a client that supports this protocol is called
a "DNS API client". a "DNS API client".
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED", In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
[RFC2119]. [RFC2119].
3. Use Cases 3. Use Cases
There are two primary use cases for this protocol. There are two primary use cases for this protocol.
The primary one is to prevent on-path network devices from The primary one is to prevent on-path network devices from
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instead of over DNS itself. The use cases in this document all instead of over DNS itself. The use cases in this document all
involve query origination instead of proxying. involve query origination instead of proxying.
4. Protocol Requirements 4. Protocol Requirements
The protocol described here bases its design on the following The protocol described here bases its design on the following
protocol requirements: protocol requirements:
o The protocol must use normal HTTP semantics. o The protocol must use normal HTTP semantics.
o The query format must be able to be flexible enough to express o The queries and responses must be able to be flexible enough to
every normal DNS query. express every normal DNS query.
o The protocol must allow implementations to use HTTP's content o The protocol must allow implementations to use HTTP's content
negotiation mechanism. negotiation mechanism.
o The protocol must ensure interoperable media formats through a o The protocol must ensure interoperable media formats through a
mandatory to implement format wherein a query must be able to mandatory to implement format wherein a query must be able to
contain one or more EDNS extensions, including those not yet contain one or more EDNS extensions, including those not yet
defined. defined.
o The protocol must use a secure transport that meets the o The protocol must use a secure transport that meets the
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4.1. Non-requirements 4.1. Non-requirements
o Supporting network-specific DNS64 [RFC6147] o Supporting network-specific DNS64 [RFC6147]
o Supporting other network-specific inferences from plaintext DNS o Supporting other network-specific inferences from plaintext DNS
queries queries
o Supporting insecure HTTP o Supporting insecure HTTP
o Supporitng legacy HTTP versions o Supporting legacy HTTP versions
5. The HTTP Request 5. The HTTP Request
The URI scheme MUST be https. The URI scheme MUST be https.
The path SHOULD be "/.well-known/dns-query" but a different path can The path SHOULD be "/.well-known/dns-query" but a different path can
be used if the DNS API Client has prior knowledge about a DNS API be used if the DNS API Client has prior knowledge about a DNS API
service on a different path at the origin being used. (See Section 8 service on a different path at the origin being used. (See Section 8
for the registration of this in the well-known URI registry.) Using for the registration of this in the well-known URI registry.) Using
the well-known path allows automated discovery of a DNS API Service, the well-known path allows automated discovery of a DNS API Service,
and also helps contextualize DNS Query requests pushed over an active and also helps contextualize DNS Query requests pushed over an active
HTTP/2 connection. HTTP/2 connection.
Some forms of the request may also include a HTTP query as defined by A DNS API Client encodes the DNS query into the HTTP request using
[RFC3986]. either the HTTP GET or POST methods.
A DNS API Client encodes the DNS query into the HTTP request in one
of two different methods:
GET: Using the on-the-wire representation of a DNS message defined
in [RFC1035] as the query string portion of the URI, encoded as
necessary with [RFC3986], using the GET HTTP method. This is the
preferred approach because it is friendlier to HTTP caches.
POST: Including the DNS query as the message body of the HTTP When using the POST method, the DNS query is included as the message
request, with the request using the POST method. The body MUST body of the HTTP request and the Content-Type request header
use a media type selected by the DNS API Client, and that media indicates the media type of the message. POST-ed requests are
type MUST be indicated by the request's Content-Type header. smaller than their GET equivalents.
The DNS request MAY have one or more EDNS(0) extensions [RFC6891]. When using the GET method, the URI path MUST contain a query
parameter of the form content-type=TTT and another of the form
body=BBBB, where "TTT" is the media type of the format used for the
body parameter, and "BBB" is the content of the body encoded with
base64url [RFC4648]. Using the GET method is friendlier to many HTTP
cache implementations.
The DNS API Client SHOULD include an HTTP "Accept:" request header to The DNS API Client SHOULD include an HTTP "Accept:" request header to
say what type of content can be understood in response. The client say what type of content can be understood in response. The client
MUST be prepared to process "application/dns-udpwireformat" responses MUST be prepared to process "application/dns-udpwireformat"
but MAY process any other type it receives. {{dnswire} responses but MAY process any other type it receives.
In order to maximize cache friendliness, DNS API Clients SHOULD use In order to maximize cache friendliness, DNS API clients using media
the same ID (the first two bytes of the header) for every DNS formats that include DNS ID, such as application/dns-udpwireformat,
request. The exact mechanism for doing so is dependent on the media should use a DNS ID of 0 in every DNS request. HTTP semantics
type in use. HTTP semantics correlate the request and response which correlate the request and response, thus eliminating the need for the
eliminates the need for the ID in a media type such as application/ ID in a media type such as application/dns-udpwireformat.
dns-udpwireformat. Using a constant value greatly increases the
opportunity for successful caching.
DNS API clients can use HTTP/2 padding and compression in the same DNS API clients can use HTTP/2 padding and compression in the same
way that other HTTP/2 clients use (or don't use) them. way that other HTTP/2 clients use (or don't use) them.
5.1. Example 5.1. DNS Wire Format
The media type is "application/dns-udpwireformat". The body is the
DNS on-the-wire format is defined in [RFC1035]. The body MUST be
encoded with base64url [RFC4648]. Padding characters for base64url
MUST NOT be included.
DNS API clients using the DNS wire format MAY have one or more
EDNS(0) extensions [RFC6891] in the request.
5.2. Examples
For example, assume a DNS API server is following this specification For example, assume a DNS API server is following this specification
on origin https://dnsserver.example.net/ and the well-known path. on origin https://dnsserver.example.net/ and the well-known path.
The example uses HTTP/2 formatting from [RFC7540]. The DNS API client chooses to send its requests in appliation/dns-
udpwirefomat but indicates it can parse replies in that format or as
a JSON-based content type.
The examples uses HTTP/2 formatting from [RFC7540].
A query for the IN A records for "www.example.com" with recursion A query for the IN A records for "www.example.com" with recursion
turned on using the GET approach would be: turned on using the GET method and a wireformat request would be:
:method = GET :method = GET
:scheme = https :scheme = https
:authority = dnsserver.example.net :authority = dnsserver.example.net
:path = /.well-known/dns-query?%ab%cd%01%00%00%01%00%00%00%00 (no CR) :path = /.well-known/dns-query? (no CR)
%00%00%03www%07example%03com%00%00%01%00%01 content-type=application/dns-udpwireformat& (no CR)
accept = application/dns-udpwireformat, application/dns-futureJsonDns body=q80BAAABAAAAAAAAA3d3dwdleGFtcGxlA2NvbQAAAQAB
accept = application/dns-udpwireformat, application/simpledns+json
The same DNS query, using the second method of HTTP encoding would The same DNS query, using the POST method would be:
be:
:method = POST :method = POST
:scheme = https :scheme = https
:authority = dnsserver.example.net :authority = dnsserver.example.net
:path = /.well-known/dns-query :path = /.well-known/dns-query
accept = application/dns-udpwireformat, application/dns-futureJsonDns accept = application/dns-udpwireformat, application/simpledns+json
content-type = application/dns-udpwireformat content-type = application/dns-udpwireformat
content-length = 33 content-length = 33
<33 bytes represented by the following hex encoding> <33 bytes represented by the following hex encoding>
abcd 0100 0001 0000 0000 0000 0377 7777 abcd 0100 0001 0000 0000 0000 0377 7777
0765 7861 6d70 6c65 0363 6f6d 0000 0100 0765 7861 6d70 6c65 0363 6f6d 0000 0100
01 01
6. The HTTP Response 6. The HTTP Response
Different response media types will provide more or less information Different response media types will provide more or less information
from a DNS response. For example, one response type might include from a DNS response. For example, one response type might include
the information from the DNS header bytes while another might omit the information from the DNS header bytes while another might omit
it. The amount and type of information that a media type gives is it. The amount and type of information that a media type gives is
solely up to the format, and not defined in this protocol. solely up to the format, and not defined in this protocol.
At the time this is published, the response types are works in At the time this is published, the response types are works in
progress. The only known response type is "application/dns- progress. The only known response type is "application/dns-
udpwireformat", but it is likely that at least one JSON-based udpwireformat", but it is likely that at least one JSON-based
response format might be defined in the future. response format will be defined in the future.
The DNS response MAY have one or more EDNS(0) extensions, depending The DNS response for "application/dns-udpwireformat" in Section 5.1
on the extension definition of the extensions given in the DNS MAY have one or more EDNS(0) extensions, depending on the extension
request. definition of the extensions given in the DNS request.
Native HTTP methods are used to correlate requests and responses. Native HTTP methods are used to correlate requests and responses.
Responses may be returned in a different temporal order than requests Responses may be returned in a different temporal order than requests
were made using the protocols native multistreaming functionality. were made using the protocols native multi-streaming functionality.
In the HTTP responses, the HTTP cache headers SHOULD be set to expire In the HTTP responses, the HTTP cache headers SHOULD be set to expire
at the same time as the shortest DNS TTL in the response. Because at the same time as the shortest DNS TTL in the response. Because
DNS provides only caching but not revalidation semantics, DNS over DNS provides only caching but not revalidation semantics, DNS over
HTTP responses should not carry revalidation response headers (such HTTP responses should not carry revalidation response headers (such
as Last-Modified: or Etag:) or return 304 responses. as Last-Modified: or Etag:) or return 304 responses.
A DNS API Server MUST be able to process application/dns- A DNS API Server MUST be able to process application/dns-
udpwireformat request messages. udpwireformat request messages.
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:status = 200 :status = 200
content-type = application/dns-udpwireformat content-type = application/dns-udpwireformat
content-length = 64 content-length = 64
cache-control = max-age=128 cache-control = max-age=128
<64 bytes represented by the following hex encoding> <64 bytes represented by the following hex encoding>
abcd 8180 0001 0001 0000 0000 0377 7777 abcd 8180 0001 0001 0000 0000 0377 7777
0765 7861 6d70 6c65 0363 6f6d 0000 0100 0765 7861 6d70 6c65 0363 6f6d 0000 0100
0103 7777 7707 6578 616d 706c 6503 636f 0103 7777 7707 6578 616d 706c 6503 636f
6d00 0001 0001 0000 0080 0004 5Db8 d822 6d00 0001 0001 0000 0080 0004 5db8 d822
7. HTTP Integration 7. HTTP Integration
In order to satisfy the security requirements of DNS over HTTPS, this In order to satisfy the security requirements of DNS over HTTPS, this
protocol MUST use HTTP/2 [RFC7540] or its successors. HTTP/2 protocol MUST use HTTP/2 [RFC7540] or its successors. HTTP/2
enforces a modern TLS profile necessary for achieving the security enforces a modern TLS profile necessary for achieving the security
requirements of this protocol. requirements of this protocol.
This protocol MUST be used with https scheme URI [RFC7230]. This protocol MUST be used with https scheme URI [RFC7230].
The messages in classic UDP based DNS [RFC1035] are inherently The messages in classic UDP based DNS [RFC1035] are inherently
unordered and have low overhead. A competitive HTTP transport needs unordered and have low overhead. A competitive HTTP transport needs
to support reordering, priority, parallelism, and header compression. to support reordering, priority, parallelism, and header compression.
For this additional reason, this protocol MUST use HTTP/2 [RFC7540] For this additional reason, this protocol MUST use HTTP/2 [RFC7540]
or its successors. or its successors.
8. IANA Considerations 8. IANA Considerations
8.1. Registration of Well-Known URI
This specification registers a Well-Known URI [RFC5785]: This specification registers a Well-Known URI [RFC5785]:
o URI Suffix: dns-query o URI Suffix: dns-query
o Change Controller: IETF o Change Controller: IETF
o Specification Document(s): [this specification] o Specification Document(s): [this specification]
(Note for the -00 draft: a request for the media type application/ 8.2. Registration of application/dns-udpwireformat Media Type
dns-udpwireformat has already been submitted separately from this To: ietf-types@iana.org
draft because it may be useful for other documents as well. That Subject: Registration of MIME media type
application is pending approval.) pplication/dns-udpwireformat
MIME media type name: application
MIME subtype name: dns-udpwireformat
Required parameters: n/a
Optional parameters: n/a
Encoding considerations: This is a binary format. The contents are a
DNS message as defined in RFC 1035. The format used here is for DNS
over UDP, which is the format defined in the diagrams in RFC 1035.
Security considerations: The security considerations for carrying
this data are the same for carrying DNS without encryption.
Interoperability considerations: None.
Published specification: This document.
Applications that use this media type:
Systems that want to exchange full DNS messages.
Additional information:
Magic number(s): n/a
File extension(s): n/a
Macintosh file type code(s): n/a
Person & email address to contact for further information:
Paul Hoffman, paul.hoffman@icann.org
Intended usage: COMMON
Restrictions on usage: n/a
Author: Paul Hoffman, paul.hoffman@icann.org
Change controller: IESG
9. Security Considerations 9. Security Considerations
Running DNS over https:// relies on the security of the underlying Running DNS over https:// relies on the security of the underlying
HTTP connection. By requiring at least [RFC7540] levels of support HTTP connection. By requiring at least [RFC7540] levels of support
for TLS this protocol expects to use current best practices for for TLS this protocol expects to use current best practices for
secure transport. secure transport.
Session level encryption has well known weaknesses with respect to Session level encryption has well known weaknesses with respect to
traffic analysis which might be particularly acute when dealing with traffic analysis which might be particularly acute when dealing with
DNS queries. Sections 10.6 (Compression) and 10.7 (Padding) of DNS queries. Sections 10.6 (Compression) and 10.7 (Padding) of
[RFC7540] provide some further advice on mitigations within an HTTP/2 [RFC7540] provide some further advice on mitigations within an HTTP/2
context. context.
A server that is acting both as a normal web server and a DNS API
server is in a position to choose which DNS names it forces a client
to resolve (through its web service) and also be the one to answer
those queries (through its DNS API service). An untrusted DNS API
server can thus easily cause damage by poisoning a client's cache
with names that the DNS API server chooses to poison. A client MUST
NOT trust a DNS API server simply because it was discovered, or
because the client was told to trust the DNS API server by an
untrusted party. Instead, a client MUST only trust DNS API server
that is configured as trustworthy.
10. Acknowledgments 10. Acknowledgments
Joe Hildebrand contributed lots of material for a different iteration Joe Hildebrand contributed lots of material for a different iteration
of this document. Helpful early comments were given by Ben Schwartz of this document. Helpful early comments were given by Ben Schwartz
and Mark Nottingham. and Mark Nottingham.
11. References 11. References
11.1. Normative References 11.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>. November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Resource Identifier (URI): Generic Syntax", STD 66, Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
RFC 3986, DOI 10.17487/RFC3986, January 2005, <http://www.rfc-editor.org/info/rfc4648>.
<http://www.rfc-editor.org/info/rfc3986>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, (TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008, DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>. <http://www.rfc-editor.org/info/rfc5246>.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785, Uniform Resource Identifiers (URIs)", RFC 5785,
DOI 10.17487/RFC5785, April 2010, DOI 10.17487/RFC5785, April 2010,
<http://www.rfc-editor.org/info/rfc5785>. <http://www.rfc-editor.org/info/rfc5785>.
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