draft-ietf-doh-dns-over-https-12.txt   draft-ietf-doh-dns-over-https-13.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: December 29, 2018 Mozilla Expires: February 9, 2019 Mozilla
June 27, 2018 August 08, 2018
DNS Queries over HTTPS (DoH) DNS Queries over HTTPS (DoH)
draft-ietf-doh-dns-over-https-12 draft-ietf-doh-dns-over-https-13
Abstract Abstract
This document describes how to make DNS queries over HTTPS. This document defines a protocol for sending DNS queries and getting
DNS responses over HTTPS. Each DNS query-response pair is mapped
into an HTTP exchange.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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 https://datatracker.ietf.org/drafts/current/. Drafts is at https://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 December 29, 2018. This Internet-Draft will expire on February 9, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 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
(https://trustee.ietf.org/license-info) in effect on the date of (https://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
skipping to change at page 2, line 36 skipping to change at page 2, line 36
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 13 9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 13
9.1. On The Wire . . . . . . . . . . . . . . . . . . . . . . . 13 9.1. On The Wire . . . . . . . . . . . . . . . . . . . . . . . 13
9.2. In The Server . . . . . . . . . . . . . . . . . . . . . . 13 9.2. In The Server . . . . . . . . . . . . . . . . . . . . . . 13
10. Security Considerations . . . . . . . . . . . . . . . . . . . 15 10. Security Considerations . . . . . . . . . . . . . . . . . . . 15
11. Operational Considerations . . . . . . . . . . . . . . . . . 15 11. Operational Considerations . . . . . . . . . . . . . . . . . 15
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
12.1. Normative References . . . . . . . . . . . . . . . . . . 17 12.1. Normative References . . . . . . . . . . . . . . . . . . 17
12.2. Informative References . . . . . . . . . . . . . . . . . 18 12.2. Informative References . . . . . . . . . . . . . . . . . 18
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 20 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 20
Previous Work on DNS over HTTP or in Other Formats . . . . . . . 20 Previous Work on DNS over HTTP or in Other Formats . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction 1. Introduction
This document defines a specific protocol for sending DNS [RFC1035] This document defines a specific protocol for sending DNS [RFC1035]
queries and getting DNS responses over HTTP [RFC7540] using https queries and getting DNS responses over HTTP [RFC7540] using https
URIs (and therefore TLS [RFC5246] security for integrity and URIs (and therefore TLS [RFC5246] security for integrity and
confidentiality). Each DNS query-response pair is mapped into a HTTP confidentiality). Each DNS query-response pair is mapped into an
exchange. HTTP exchange.
The described approach is more than a tunnel over HTTP. It The described approach is more than a tunnel over HTTP. It
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, redirection, proxying, itself with HTTP features such as caching, redirection, proxying,
authentication, and compression. authentication, and compression.
The integration with HTTP provides a transport suitable for both The integration with HTTP provides a transport suitable for both
skipping to change at page 4, line 28 skipping to change at page 4, line 28
o Supporting insecure HTTP o Supporting insecure HTTP
4. Selection of DoH Server 4. Selection of DoH Server
Configuration, discovery, and updating of the URI Template [RFC6570] Configuration, discovery, and updating of the URI Template [RFC6570]
(see Section 5.1) is done out of band from this protocol. Note that (see Section 5.1) is done out of band from this protocol. Note that
configuration might be manual (such as a user typing URI Templates in configuration might be manual (such as a user typing URI Templates in
a user interface for "options") or automatic (such as URI Templates a user interface for "options") or automatic (such as URI Templates
being supplied in responses from DHCP or similar protocols). DoH being supplied in responses from DHCP or similar protocols). DoH
Servers MAY support more than one URI. This allows the different Servers MAY support more than one URI Template. This allows the
endpoints to have different properties such as different different endpoints to have different properties such as different
authentication requirements or service level guarantees. authentication requirements or service level guarantees.
A DoH client uses configuration to select the URI, and thus the DoH A DoH client uses configuration to select the URI, and thus the DoH
server, that is to be used for resolution. [RFC2818] defines how server, that is to be used for resolution. [RFC2818] defines how
HTTPS verifies the DoH server's identity. HTTPS verifies the DoH server's identity.
A DoH client MUST NOT use a different URI simply because it was A DoH client MUST NOT use a different URI simply because it was
discovered outside of the client's configuration, or because a server discovered outside of the client's configuration, or because a server
offers an unsolicited response that appears to be a valid answer to a offers an unsolicited response that appears to be a valid answer to a
DNS query. This specification does not extend DNS resolution DNS query. This specification does not extend DNS resolution
skipping to change at page 5, line 17 skipping to change at page 5, line 17
the single variable "dns" is defined as the content of the DNS the single variable "dns" is defined as the content of the DNS
request (as described in Section 7), encoded with base64url request (as described in Section 7), encoded with base64url
[RFC4648]. [RFC4648].
Future specifications for new media types MUST define the variables Future specifications for new media types MUST define the variables
used for URI Template processing with this protocol. used for URI Template processing with this protocol.
DoH servers MUST implement both the POST and GET methods. DoH servers MUST implement both the POST and GET methods.
When using the POST method the DNS query is included as the message When using the POST method the DNS query is included as the message
body of the HTTP request and the Content-Type request header body of the HTTP request and the Content-Type request header field
indicates the media type of the message. POST-ed requests are indicates the media type of the message. POST-ed requests are
smaller than their GET equivalents. smaller than their GET equivalents.
Using the GET method is friendlier to many HTTP cache Using the GET method is friendlier to many HTTP cache
implementations. implementations.
The DoH client SHOULD include an HTTP "Accept" request header to The DoH client SHOULD include an HTTP "Accept" request header field
indicate what type of content can be understood in response. to indicate what type of content can be understood in response.
Irrespective of the value of the Accept request header, the client Irrespective of the value of the Accept request header field, the
MUST be prepared to process "application/dns-message" (as described client MUST be prepared to process "application/dns-message" (as
in Section 7) responses but MAY also process any other type it described in Section 7) responses but MAY also process any other type
receives. it receives.
In order to maximize cache friendliness, DoH clients using media In order to maximize cache friendliness, DoH clients using media
formats that include DNS ID, such as application/dns-message, SHOULD formats that include DNS ID, such as application/dns-message, SHOULD
use a DNS ID of 0 in every DNS request. HTTP correlates the request use a DNS ID of 0 in every DNS request. HTTP correlates the request
and response, thus eliminating the need for the ID in a media type and response, thus eliminating the need for the ID in a media type
such as application/dns-message. The use of a varying DNS ID can such as application/dns-message. The use of a varying DNS ID can
cause semantically equivalent DNS queries to be cached separately. cause semantically equivalent DNS queries to be cached separately.
DoH clients can use HTTP/2 padding and compression in the same way DoH clients can use HTTP/2 padding and compression in the same way
that other HTTP/2 clients use (or don't use) them. that other HTTP/2 clients use (or don't use) them.
skipping to change at page 6, line 26 skipping to change at page 6, line 26
:path = /dns-query :path = /dns-query
accept = application/dns-message accept = application/dns-message
content-type = application/dns-message content-type = application/dns-message
content-length = 33 content-length = 33
<33 bytes represented by the following hex encoding> <33 bytes represented by the following hex encoding>
00 00 01 00 00 01 00 00 00 00 00 00 03 77 77 77 00 00 01 00 00 01 00 00 00 00 00 00 03 77 77 77
07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00
01 01
In this example, the 33 bytes are the DNS message in DNS wire format.
Finally, a GET based query for a.62characterlabel-makes-base64url- Finally, a GET based query for a.62characterlabel-makes-base64url-
distinct-from-standard-base64.example.com is shown as an example to distinct-from-standard-base64.example.com is shown as an example to
emphasize that the encoding alphabet of base64url is different than emphasize that the encoding alphabet of base64url is different than
regular base64 and that padding is omitted. regular base64 and that padding is omitted.
The DNS query is 94 bytes represented by the following hex encoding The DNS query, expressed in DNS wire format, is 94 bytes represented
by the following:
00 00 01 00 00 01 00 00 00 00 00 00 01 61 3e 36 00 00 01 00 00 01 00 00 00 00 00 00 01 61 3e 36
32 63 68 61 72 61 63 74 65 72 6c 61 62 65 6c 2d 32 63 68 61 72 61 63 74 65 72 6c 61 62 65 6c 2d
6d 61 6b 65 73 2d 62 61 73 65 36 34 75 72 6c 2d 6d 61 6b 65 73 2d 62 61 73 65 36 34 75 72 6c 2d
64 69 73 74 69 6e 63 74 2d 66 72 6f 6d 2d 73 74 64 69 73 74 69 6e 63 74 2d 66 72 6f 6d 2d 73 74
61 6e 64 61 72 64 2d 62 61 73 65 36 34 07 65 78 61 6e 64 61 72 64 2d 62 61 73 65 36 34 07 65 78
61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 01 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 01
:method = GET :method = GET
:scheme = https :scheme = https
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DoH client retries the query with the same DoH server, such as DoH client retries the query with the same DoH server, such as
authorization failures (HTTP status code 401 [RFC7235] Section 3.1). authorization failures (HTTP status code 401 [RFC7235] Section 3.1).
It could also mean that the DoH client retries with a different DoH It could also mean that the DoH client retries with a different DoH
server, such as for unsupported media types (HTTP status code 415, server, such as for unsupported media types (HTTP status code 415,
[RFC7231] Section 6.5.13), or where the server cannot generate a [RFC7231] Section 6.5.13), or where the server cannot generate a
representation suitable for the client (HTTP status code 406, representation suitable for the client (HTTP status code 406,
[RFC7231] Section 6.5.6), and so on. [RFC7231] Section 6.5.6), and so on.
5.2.2. HTTP Response Example 5.2.2. HTTP Response Example
This is an example response for a query for the IN A records for This is an example response for a query for the IN AAAA records for
"www.example.com" with recursion turned on. The response bears one "www.example.com" with recursion turned on. The response bears one
record with an address of 192.0.2.1 and a TTL of 128 seconds. answer record with an address of 2001:db8:abcd:12:1:2:3:4 and a TTL
of 3709 seconds.
:status = 200 :status = 200
content-type = application/dns-message content-type = application/dns-message
content-length = 64 content-length = 61
cache-control = max-age=128 cache-control = max-age=3709
<64 bytes represented by the following hex encoding> <61 bytes represented by the following hex encoding>
00 00 81 80 00 01 00 01 00 00 00 00 03 77 77 77 00 00 81 80 00 01 00 01 00 00 00 00 03 77 77 77
07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 00 1c 00
01 03 77 77 77 07 65 78 61 6d 70 6c 65 03 63 6f 01 c0 0c 00 1c 00 01 00 00 0e 7d 00 10 20 01 0d
6d 00 00 01 00 01 00 00 00 80 00 04 C0 00 02 01 b8 ab cd 00 12 00 01 00 02 00 03 00 04
6. HTTP Integration 6. HTTP Integration
This protocol MUST be used with the https scheme URI [RFC7230]. This protocol MUST be used with the https scheme URI [RFC7230].
Section 9 and Section 10 discuss additional considerations for the Section 9 and Section 10 discuss additional considerations for the
integration with HTTP. integration with HTTP.
6.1. Cache Interaction 6.1. Cache Interaction
A DoH exchange can pass through a hierarchy of caches that include A DoH exchange can pass through a hierarchy of caches that include
both HTTP and DNS specific caches. These caches may exist beteen the both HTTP- and DNS-specific caches. These caches may exist between
DoH server and client, or on the DoH client itself. HTTP caches are the DoH server and client, or on the DoH client itself. HTTP caches
by design generic; that is, they do not understand this protocol. are by design generic; that is, they do not understand this protocol.
Even if a DoH client has modified its cache implementation to be Even if a DoH client has modified its cache implementation to be
aware of DoH semantics, it does not follow that all upstream caches aware of DoH semantics, it does not follow that all upstream caches
(for example, inline proxies, server-side gateways and Content (for example, inline proxies, server-side gateways and Content
Delivery Networks) will be. Delivery Networks) will be.
As a result, DoH servers need to carefully consider the HTTP caching As a result, DoH servers need to carefully consider the HTTP caching
metadata they send in response to GET requests (POST requests are not metadata they send in response to GET requests (POST requests are not
cacheable unless specific response headers are sent; this is not cacheable unless specific response header fields are sent; this is
widely implemented, and not advised for DoH). not widely implemented, and not advised for DoH).
In particular, DoH servers SHOULD assign an explicit freshness In particular, DoH servers SHOULD assign an explicit freshness
lifetime ([RFC7234] Section 4.2) so that the DoH client is more lifetime ([RFC7234] Section 4.2) so that the DoH client is more
likely to use fresh DNS data. This requirement is due to HTTP caches likely to use fresh DNS data. This requirement is due to HTTP caches
being able to assign their own heuristic freshness (such as that being able to assign their own heuristic freshness (such as that
described in [RFC7234] Section 4.2.2), which would take control of described in [RFC7234] Section 4.2.2), which would take control of
the cache contents out of the hands of the DoH server. the cache contents out of the hands of the DoH server.
The assigned freshness lifetime of a DoH HTTP response SHOULD be the The assigned freshness lifetime of a DoH HTTP response MUST be less
smallest TTL in the Answer section of the DNS response. For example, than or equal to the smallest TTL in the Answer section of the DNS
if a HTTP response carries three RRsets with TTLs of 30, 600, and response. A freshness lifetime equal to the smallest TTL in the
300, the HTTP freshness lifetime should be 30 seconds (which could be Answer section is RECOMMENDED. For example, if a HTTP response
specified as "Cache-Control: max-age=30"). The assigned freshness carries three RRsets with TTLs of 30, 600, and 300, the HTTP
lifetime MUST NOT be greater than the smallest TTL in the Answer freshness lifetime should be 30 seconds (which could be specified as
section of the DNS response. This requirement helps assure that none "Cache-Control: max-age=30"). This requirement helps assure that
of the RRsets contained in a DNS response are served stale from an none of the RRsets contained in a DNS response are served stale from
HTTP cache. an HTTP cache.
If the DNS response has no records in the Answer section, and the DNS If the DNS response has no records in the Answer section, and the DNS
response has an SOA record in the Authority section, the response response has an SOA record in the Authority section, the response
freshness lifetime MUST NOT be greater than the MINIMUM field from freshness lifetime MUST NOT be greater than the MINIMUM field from
that SOA record (see [RFC2308]). that SOA record (see [RFC2308]).
The stale-while-revalidate and stale-if-error Cache-Control The stale-while-revalidate and stale-if-error Cache-Control
directives ([RFC5861]) could be well suited to a DoH implementation directives ([RFC5861]) could be well-suited to a DoH implementation
when allowed by server policy. Those mechanisms allow a client, at when allowed by server policy. Those mechanisms allow a client, at
the server's discretion, to reuse a cache entry that is no longer the server's discretion, to reuse a cache entry that is no longer
fresh. In such a case, the client reuses all of a cached entry, or fresh. In such a case, the client reuses all of a cached entry, or
none of it. none of it.
DoH servers also need to consider caching when generating responses DoH servers also need to consider caching when generating responses
that are not globally valid. For instance, if a DoH server that are not globally valid. For instance, if a DoH server
customizes a response based on the client's identity, it would not customizes a response based on the client's identity, it would not
want to allow global reuse of that response. This could be want to allow global reuse of that response. This could be
accomplished through a variety of HTTP techniques such as a Cache- accomplished through a variety of HTTP techniques such as a Cache-
Control max-age of 0, or by using the Vary response header ([RFC7231] Control max-age of 0, or by using the Vary response header field
Section 7.1.4) to establish a secondary cache key ([RFC7234] ([RFC7231] Section 7.1.4) to establish a secondary cache key
Section 4.1). ([RFC7234] Section 4.1).
DoH clients MUST account for the Age response header's value DoH clients MUST account for the Age response header field's value
([RFC7234]) when calculating the DNS TTL of a response. For example, ([RFC7234]) when calculating the DNS TTL of a response. For example,
if a RRset is received with a DNS TTL of 600, but the Age header if an RRset is received with a DNS TTL of 600, but the Age header
indicates that the response has been cached for 250 seconds, the field indicates that the response has been cached for 250 seconds,
remaining lifetime of the RRset is 350 seconds. the remaining lifetime of the RRset is 350 seconds. This requirement
applies to both DoH client HTTP caches and DoH client DNS caches.
DoH clients can request an uncached copy of a response by using the DoH clients can request an uncached copy of a response by using the
"no-cache" request cache control directive ([RFC7234], "no-cache" request cache control directive ([RFC7234],
Section 5.2.1.4) and similar controls. Note that some caches might Section 5.2.1.4) and similar controls. Note that some caches might
not honor these directives, either due to configuration or not honor these directives, either due to configuration or
interaction with traditional DNS caches that do not have such a interaction with traditional DNS caches that do not have such a
mechanism. mechanism.
HTTP conditional requests ([RFC7232]) may be of limited value to DoH, HTTP conditional requests ([RFC7232]) may be of limited value to DoH,
as revalidation provides only a bandwidth benefit and DNS as revalidation provides only a bandwidth benefit and DNS
transactions are normally latency bound. Furthermore, the HTTP transactions are normally latency bound. Furthermore, the HTTP
response headers that enable revalidation (such as "Last-Modified" response header fields that enable revalidation (such as "Last-
and "Etag") are often fairly large when compared to the overall DNS Modified" and "Etag") are often fairly large when compared to the
response size, and have a variable nature that creates constant overall DNS response size, and have a variable nature that creates
pressure on the HTTP/2 compression dictionary [RFC7541]. Other types constant pressure on the HTTP/2 compression dictionary [RFC7541].
of DNS data, such as zone transfers, may be larger and benefit more Other types of DNS data, such as zone transfers, may be larger and
from revalidation. benefit more from revalidation.
6.2. HTTP/2 6.2. HTTP/2
HTTP/2 [RFC7540] is the minimum RECOMMENDED version of HTTP for use HTTP/2 [RFC7540] is the minimum RECOMMENDED version of HTTP for use
with DoH. with DoH.
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, parallelism, priority, and header compression to support reordering, parallelism, priority, and header compression
to achieve similar performance. Those features were introduced to to achieve similar performance. Those features were introduced to
HTTP in HTTP/2 [RFC7540]. Earlier versions of HTTP are capable of HTTP in HTTP/2 [RFC7540]. Earlier versions of HTTP are capable of
conveying the semantic requirements of DoH but may result in very conveying the semantic requirements of DoH but may result in very
poor performance. poor performance.
6.3. Server Push 6.3. Server Push
Before using DoH response data for DNS resolution, the client MUST Before using DoH response data for DNS resolution, the client MUST
establish that the HTTP request URI may be used for the DoH query. establish that the HTTP request URI can be used for the DoH query.
For HTTP requests initiated by the DoH client this is implicit in the For HTTP requests initiated by the DoH client, this is implicit in
selection of URI. For HTTP server push ([RFC7540] Section 8.2) extra the selection of URI. For HTTP server push ([RFC7540] Section 8.2)
care must be taken to ensure that the pushed URI is one that the extra care must be taken to ensure that the pushed URI is one that
client would have directed the same query to if the client had the client would have directed the same query to if the client had
initiated the request. initiated the request.
6.4. Content Negotiation 6.4. Content Negotiation
In order to maximize interoperability, DoH clients and DoH servers In order to maximize interoperability, DoH clients and DoH servers
MUST support the "application/dns-message" media type. Other media MUST support the "application/dns-message" media type. Other media
types MAY be used as defined by HTTP Content Negotiation ([RFC7231] types MAY be used as defined by HTTP Content Negotiation ([RFC7231]
Section 3.4). Those media types MUST be flexible enough to express Section 3.4). Those media types MUST be flexible enough to express
every DNS query that would normally be sent in DNS over UDP every DNS query that would normally be sent in DNS over UDP
(including queries and responses that use DNS extensions, but not (including queries and responses that use DNS extensions, but not
those that require multiple responses). those that require multiple responses).
7. Definition of the application/dns-message media type 7. Definition of the application/dns-message media type
The data payload for the application/dns-message media type is a The data payload for the application/dns-message media type is a
single message of the DNS on-the-wire format defined in Section 4.2.1 single message of the DNS on-the-wire format defined in Section 4.2.1
of [RFC1035]. The format was originally for DNS over UDP. Although of [RFC1035], which in turn refers to the full wire format defined in
[RFC1035] says "Messages carried by UDP are restricted to 512 bytes", Section 4.1 of that RFC.
that was later updated by [RFC6891]. This media type restricts the
maximum size of the DNS message to 65535 bytes. Note that the wire Although [RFC1035] says "Messages carried by UDP are restricted to
format used in this media type is different than the wire format used 512 bytes", that was later updated by [RFC6891]. This media type
in [RFC7858] (which uses the format defined in Section 4.2.2 of restricts the maximum size of the DNS message to 65535 bytes.
[RFC1035]).
Note that the wire format used in this media type is different than
the wire format used in [RFC7858] (which uses the format defined in
Section 4.2.2 of [RFC1035] that includes two length bytes).
DoH clients using this media type MAY have one or more EDNS options DoH clients using this media type MAY have one or more EDNS options
[RFC6891] in the request. DoH servers using this media type MUST [RFC6891] in the request. DoH servers using this media type MUST
ignore the value given for the EDNS UDP payload size in DNS requests. ignore the value given for the EDNS UDP payload size in DNS requests.
When using the GET method, the data payload for this media type MUST When using the GET method, the data payload for this media type MUST
be encoded with base64url [RFC4648] and then provided as a variable be encoded with base64url [RFC4648] and then provided as a variable
named "dns" to the URI Template expansion. Padding characters for named "dns" to the URI Template expansion. Padding characters for
base64url MUST NOT be included. base64url MUST NOT be included.
skipping to change at page 13, line 7 skipping to change at page 13, line 7
Intended usage: COMMON Intended usage: COMMON
Restrictions on usage: n/a Restrictions on usage: n/a
Author: Paul Hoffman, paul.hoffman@icann.org Author: Paul Hoffman, paul.hoffman@icann.org
Change controller: IESG Change controller: IESG
9. Privacy Considerations 9. Privacy Considerations
[RFC7626] discusses DNS Privacy Considerations in both "On the wire" [RFC7626] discusses DNS privacy considerations in both "On the wire"
(Section 2.4), and "In the server" (Section 2.5) contexts. This is (Section 2.4), and "In the server" (Section 2.5) contexts. This is
also a useful framing for DoH's privacy considerations. also a useful framing for DoH's privacy considerations.
9.1. On The Wire 9.1. On The Wire
DoH encrypts DNS traffic and requires authentication of the server. DoH encrypts DNS traffic and requires authentication of the server.
This mitigates both passive surveillance [RFC7258] and active attacks This mitigates both passive surveillance [RFC7258] and active attacks
that attempt to divert DNS traffic to rogue servers ([RFC7626] that attempt to divert DNS traffic to rogue servers ([RFC7626]
Section 2.5.1). DNS over TLS [RFC7858] provides similar protections, Section 2.5.1). DNS over TLS [RFC7858] provides similar protections,
while direct UDP and TCP based transports are vulnerable to this while direct UDP and TCP based transports are vulnerable to this
class of attack. class of attack.
Additionally, the use of the HTTPS default port 443 and the ability Additionally, the use of the HTTPS default port 443 and the ability
to mix DoH traffic with other HTTPS traffic on the same connection to mix DoH traffic with other HTTPS traffic on the same connection
can deter unprivileged on-path devices from interfering with DNS can deter unprivileged on-path devices from interfering with DNS
operations and make DNS traffic analysis more difficult. operations and make DNS traffic analysis more difficult.
9.2. In The Server 9.2. In The Server
The DNS wire format [RFC1035] contains no client identifiers, however The DNS wire format [RFC1035] contains no client identifiers;
various transports of DNS queries and responses do provide data that however, various transports of DNS queries and responses do provide
can be used to correlate requests. HTTPS presents new considerations data that can be used to correlate requests. HTTPS presents new
for correlation such as explicit HTTP cookies and implicit considerations for correlation, such as explicit HTTP cookies and
fingerprinting of the unique set and ordering of HTTP request implicit fingerprinting of the unique set and ordering of HTTP
headers. request header fields.
A DoH implementation is built on IP, TCP, TLS, and HTTP. Each layer A DoH implementation is built on IP, TCP, TLS, and HTTP. Each layer
contains one or more common features that can be used to correlate contains one or more common features that can be used to correlate
queries to the same identity. DNS transports will generally carry queries to the same identity. DNS transports will generally carry
the same privacy properties of the layers used to implement them. the same privacy properties of the layers used to implement them.
For example, the properties of IP, TCP, and TLS apply to DNS over TLS For example, the properties of IP, TCP, and TLS apply to DNS over TLS
implementations. implementations.
The privacy considerations of using the HTTPS layer in DoH are The privacy considerations of using the HTTPS layer in DoH are
incremental to those of DNS over TLS. DoH is not known to introduce incremental to those of DNS over TLS. DoH is not known to introduce
new concerns beyond those associated with HTTPS. new concerns beyond those associated with HTTPS.
At the IP level, the client address provides obvious correlation At the IP level, the client address provides obvious correlation
information. This can be mitigated by use of a NAT, proxy, VPN, or information. This can be mitigated by use of a NAT, proxy, VPN, or
simple address rotation over time. It may be aggravated by use of a simple address rotation over time. It may be aggravated by use of a
DNS server that can correlate real-time addressing information with DNS server that can correlate real-time addressing information with
other personal identifiers, such as when a DNS server and DHCP server other personal identifiers, such as when a DNS server and DHCP server
are operated by the same entity. are operated by the same entity.
DNS implementations that use one TCP connection for multiple DNS DNS implementations that use one TCP connection for multiple DNS
requests directly group those requests. Long lived connections have requests directly group those requests. Long-lived connections have
better performance behaviors than short lived connections, but group better performance behaviors than short-lived connections, but group
more requests. TCP-based solutions may also seek performance through more requests. TCP-based solutions may also seek performance through
the use of TCP Fast Open [RFC7413]. The cookies used in TCP Fast the use of TCP Fast Open [RFC7413]. The cookies used in TCP Fast
Open allow servers to correlate TCP sessions. Open allow servers to correlate TCP sessions.
TLS based implementations often achieve better handshake performance TLS-based implementations often achieve better handshake performance
through the use of some form of session resumption mechanism such as through the use of some form of session resumption mechanism such as
session tickets [RFC5077]. Session resumption creates trivial session tickets [RFC5077]. Session resumption creates trivial
mechanisms for a server to correlate TLS connections together. mechanisms for a server to correlate TLS connections together.
HTTP's feature set can also be used for identification and tracking HTTP's feature set can also be used for identification and tracking
in a number of different ways. For example, authentication request in a number of different ways. For example, authentication request
header fields explicitly identify profiles in use, and HTTP Cookies header fields explicitly identify profiles in use, and HTTP Cookies
are designed as an explicit state tracking mechanism between the are designed as an explicit state-tracking mechanism between the
client and serving site and often are used as an authentication client and serving site and often are used as an authentication
mechanism. mechanism.
Additionally, the User-Agent and Accept-Language request header Additionally, the User-Agent and Accept-Language request header
fields often convey specific information about the client version or fields often convey specific information about the client version or
locale. This facilitates content negotiation and operational work- locale. This facilitates content negotiation and operational work-
arounds for implementation bugs. Request header fields that control arounds for implementation bugs. Request header fields that control
caching can expose state information about a subset of the client's caching can expose state information about a subset of the client's
history. Mixing DoH requests with other HTTP requests on the same history. Mixing DoH requests with other HTTP requests on the same
connection also provides an opportunity for richer data correlation. connection also provides an opportunity for richer data correlation.
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NOT be accepted by DOH clients unless they are explicitly required by NOT be accepted by DOH clients unless they are explicitly required by
a use case. a use case.
10. Security Considerations 10. Security Considerations
Running DNS over HTTPS relies on the security of the underlying HTTP Running DNS over HTTPS relies on the security of the underlying HTTP
transport. This mitigates classic amplification attacks for UDP- transport. This mitigates classic amplification attacks for UDP-
based DNS. Implementations utilizing HTTP/2 benefit from the TLS based DNS. Implementations utilizing HTTP/2 benefit from the TLS
profile defined in [RFC7540] Section 9.2. profile defined in [RFC7540] Section 9.2.
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. HTTP/2 provides further advice about the use of DNS queries. HTTP/2 provides further advice about the use of
compression ([RFC7540] Section 10.6) and padding ([RFC7540] compression ([RFC7540] Section 10.6) and padding ([RFC7540]
Section 10.7 ). DoH Servers can also add DNS padding [RFC7830] if Section 10.7 ). DoH Servers can also add DNS padding [RFC7830] if
the DoH client requests it in the DNS query. the DoH client requests it in the DNS query. An experimental effort
to offer guidance on choosing the padding length can be found in
[I-D.ietf-dprive-padding-policy].
The HTTPS connection provides transport security for the interaction The HTTPS connection provides transport security for the interaction
between the DoH server and client, but does not provide the response between the DoH server and client, but does not provide the response
integrity of DNS data provided by DNSSEC. DNSSEC and DoH are integrity of DNS data provided by DNSSEC. DNSSEC and DoH are
independent and fully compatible protocols, each solving different independent and fully compatible protocols, each solving different
problems. The use of one does not diminish the need nor the problems. The use of one does not diminish the need nor the
usefulness of the other. It is the choice of a client to either usefulness of the other. It is the choice of a client to either
perform full DNSSEC validation of answers or to trust the DoH server perform full DNSSEC validation of answers or to trust the DoH server
to do DNSSEC validation and inspect the AD (Authentic Data) bit in to do DNSSEC validation and inspect the AD (Authentic Data) bit in
the returned message to determine whether an answer was authentic or the returned message to determine whether an answer was authentic or
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In the absence of DNSSEC information, a DoH server can give a client In the absence of DNSSEC information, a DoH server can give a client
invalid data in response to a DNS query. Section 4 disallows the use invalid data in response to a DNS query. Section 4 disallows the use
of DoH DNS responses that do not originate from configured servers. of DoH DNS responses that do not originate from configured servers.
This prohibition does not guarantee protection against invalid data, This prohibition does not guarantee protection against invalid data,
but it does reduce the risk. but it does reduce the risk.
11. Operational Considerations 11. Operational Considerations
Local policy considerations and similar factors mean different DNS Local policy considerations and similar factors mean different DNS
servers may provide different results to the same query: for instance servers may provide different results to the same query, for instance
in split DNS configurations [RFC6950]. It logically follows that the in split DNS configurations [RFC6950]. It logically follows that the
server which is queried can influence the end result. Therefore a server that is queried can influence the end result. Therefore a
client's choice of DNS server may affect the responses it gets to its client's choice of DNS server may affect the responses it gets to its
queries. For example, in the case of DNS64 [RFC6147], the choice queries. For example, in the case of DNS64 [RFC6147], the choice
could affect whether IPv6/IPv4 translation will work at all. could affect whether IPv6/IPv4 translation will work at all.
The HTTPS channel used by this specification establishes secure two The HTTPS channel used by this specification establishes secure two-
party communication between the DoH client and the DoH server. party communication between the DoH client and the DoH server.
Filtering or inspection systems that rely on unsecured transport of Filtering or inspection systems that rely on unsecured transport of
DNS will not function in a DNS over HTTPS environment. DNS will not function in a DNS over HTTPS environment.
Some HTTPS client implementations perform real time third party Some HTTPS client implementations perform real time third-party
checks of the revocation status of the certificates being used by checks of the revocation status of the certificates being used by
TLS. If this check is done as part of the DoH server connection TLS. If this check is done as part of the DoH server connection
procedure and the check itself requires DNS resolution to connect to procedure and the check itself requires DNS resolution to connect to
the third party a deadlock can occur. The use of OCSP [RFC6960] the third party a deadlock can occur. The use of OCSP [RFC6960]
servers or AIA for CRL fetching ([RFC5280] Section 4.2.2.1) are servers or AIA for CRL fetching ([RFC5280] Section 4.2.2.1) are
examples of how this deadlock can happen. To mitigate the examples of how this deadlock can happen. To mitigate the
possibility of deadlock, DoH servers SHOULD NOT rely on DNS based possibility of deadlock, DoH servers SHOULD NOT rely on DNS-based
references to external resources in the TLS handshake. For OCSP the references to external resources in the TLS handshake. For OCSP, the
server can bundle the certificate status as part of the handshake server can bundle the certificate status as part of the handshake
using a mechanism appropriate to the version of TLS, such as using using a mechanism appropriate to the version of TLS, such as using
[RFC6066] Section 8 for TLS version 1.2. AIA deadlocks can be [RFC6066] Section 8 for TLS version 1.2. AIA deadlocks can be
avoided by providing intermediate certificates that might otherwise avoided by providing intermediate certificates that might otherwise
be obtained through additional requests. Note that these deadlocks be obtained through additional requests. Note that these deadlocks
also need to be considered for server that a DoH server might also need to be considered for server that a DoH server might
redirect to. redirect to.
A DoH client may face a similar bootstrapping problem when the HTTP A DoH client may face a similar bootstrapping problem when the HTTP
request needs to resolve the hostname portion of the DNS URI. Just request needs to resolve the hostname portion of the DNS URI. Just
as the address of a traditional DNS nameserver cannot be originally as the address of a traditional DNS nameserver cannot be originally
determined from that same server, a DoH client cannot use its DoH determined from that same server, a DoH client cannot use its DoH
server to initially resolve the server's host name into an address. server to initially resolve the server's host name into an address.
Alternative strategies a client might employ include making the Alternative strategies a client might employ include making the
initial resolution part of the configuration, IP based URIs and initial resolution part of the configuration, IP-based URIs and
corresponding IP based certificates for HTTPS, or resolving the DNS corresponding IP-based certificates for HTTPS, or resolving the DNS
API server's hostname via traditional DNS or another DoH server while API server's hostname via traditional DNS or another DoH server while
still authenticating the resulting connection via HTTPS. still authenticating the resulting connection via HTTPS.
HTTP [RFC7230] is a stateless application level protocol and HTTP [RFC7230] is a stateless application-level protocol and
therefore DoH implementations do not provide stateful ordering therefore DoH implementations do not provide stateful ordering
guarantees between different requests. DoH cannot be used as a guarantees between different requests. DoH cannot be used as a
transport for other protocols that require strict ordering. transport for other protocols that require strict ordering.
A DoH server is allowed to answer queries with any valid DNS A DoH server is allowed to answer queries with any valid DNS
response. For example, a valid DNS response might have the TC response. For example, a valid DNS response might have the TC
(truncation) bit set in the DNS header to indicate that the server (truncation) bit set in the DNS header to indicate that the server
was not able to retrieve a full answer for the query but is providing was not able to retrieve a full answer for the query but is providing
the best answer it could get. A DoH server can reply to queries with the best answer it could get. A DoH server can reply to queries with
an HTTP error for queries that it cannot fulfill. In this same an HTTP error for queries that it cannot fulfill. In this same
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
12.2. Informative References 12.2. Informative References
[CORS] "Cross-Origin Resource Sharing", n.d., [CORS] "Cross-Origin Resource Sharing", n.d.,
<https://fetch.spec.whatwg.org/#http-cors-protocol>. <https://fetch.spec.whatwg.org/#http-cors-protocol>.
[I-D.ietf-dprive-padding-policy]
Mayrhofer, A., "Padding Policy for EDNS(0)", draft-ietf-
dprive-padding-policy-06 (work in progress), July 2018.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000, DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>. <https://www.rfc-editor.org/info/rfc2818>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without "Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077, Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>. January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
skipping to change at page 20, line 23 skipping to change at page 20, line 31
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>. 2016, <https://www.rfc-editor.org/info/rfc7858>.
Acknowledgments Acknowledgments
This work required a high level of cooperation between experts in This work required a high level of cooperation between experts in
different technologies. Thank you Ray Bellis, Stephane Bortzmeyer, different technologies. Thank you Ray Bellis, Stephane Bortzmeyer,
Manu Bretelle, Sara Dickinson, Tony Finch, Daniel Kahn Gilmor, Olafur Manu Bretelle, Sara Dickinson, Massimiliano Fantuzzi, Tony Finch,
Guomundsson, Wes Hardaker, Rory Hewitt, Joe Hildebrand, David Daniel Kahn Gilmor, Olafur Gudmundsson, Wes Hardaker, Rory Hewitt,
Lawrence, Eliot Lear, John Mattsson, Alex Mayrhofer, Mark Nottingham, Joe Hildebrand, David Lawrence, Eliot Lear, John Mattsson, Alex
Jim Reid, Adam Roach, Ben Schwartz, Davey Song, Daniel Stenberg, Mayrhofer, Mark Nottingham, Jim Reid, Adam Roach, Ben Schwartz, Davey
Andrew Sullivan, Martin Thomson, and Sam Weiler. Song, Daniel Stenberg, Andrew Sullivan, Martin Thomson, and Sam
Weiler.
Previous Work on DNS over HTTP or in Other Formats Previous Work on DNS over HTTP or in Other Formats
The following is an incomplete list of earlier work that related to The following is an incomplete list of earlier work that related to
DNS over HTTP/1 or representing DNS data in other formats. DNS over HTTP/1 or representing DNS data in other formats.
The list includes links to the tools.ietf.org site (because these The list includes links to the tools.ietf.org site (because these
documents are all expired) and web sites of software. documents are all expired) and web sites of software.
o https://tools.ietf.org/html/draft-mohan-dns-query-xml o https://tools.ietf.org/html/draft-mohan-dns-query-xml
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