Internet Engineering Task Force (IETF) P. Hoffman

Internet Engineering Task Force (IETF) P. Hoffman
Request for Comments: 8484 ICANN
Category: Standards Track P. McManus
ISSN: 2070-1721 Mozilla
October 2018

DNS Queries over HTTPS (DoH)

Abstract

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

This is an Internet Standards Track document.

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). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8484.

Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Selection of DoH Server . . . . . . . . . . . . . . . . . . . 4
4. The HTTP Exchange . . . . . . . . . . . . . . . . . . . . . . 4
4.1. The HTTP Request . . . . . . . . . . . . . . . . . . . . 4
4.1.1. HTTP Request Examples . . . . . . . . . . . . . . . . 5
4.2. The HTTP Response . . . . . . . . . . . . . . . . . . . . 7
4.2.1. Handling DNS and HTTP Errors . . . . . . . . . . . . 7
4.2.2. HTTP Response Example . . . . . . . . . . . . . . . . 8
5. HTTP Integration . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Cache Interaction . . . . . . . . . . . . . . . . . . . . 8
5.2. HTTP/2 . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.3. Server Push . . . . . . . . . . . . . . . . . . . . . . . 10
5.4. Content Negotiation . . . . . . . . . . . . . . . . . . . 10
6. Definition of the "application/dns-message" Media Type . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7.1. Registration of the "application/dns-message" Media Type 11
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 12
8.1. On the Wire . . . . . . . . . . . . . . . . . . . . . . . 12
8.2. In the Server . . . . . . . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 14
10. Operational Considerations . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Protocol Development . . . . . . . . . . . . . . . . 20
Appendix B. Previous Work on DNS over HTTP or in Other Formats . 20
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21

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1. Introduction

This document defines a specific protocol, DNS over HTTPS (DoH), for
sending DNS [RFC1035] queries and getting DNS responses over HTTP
[RFC7540] using https [RFC2818] URIs (and therefore TLS [RFC8446]
security for integrity and confidentiality). Each DNS query-response
pair is mapped into an HTTP exchange.

The described approach is more than a tunnel over HTTP. It
establishes default media formatting types for requests and responses
but uses normal HTTP content negotiation mechanisms for selecting
alternatives that endpoints may prefer in anticipation of serving new
use cases. In addition to this media type negotiation, it aligns
itself with HTTP features such as caching, redirection, proxying,
authentication, and compression.

The integration with HTTP provides a transport suitable for both
existing DNS clients and native web applications seeking access to
the DNS.

Two primary use cases were considered during this protocol's
development. These use cases are preventing on-path devices from
interfering with DNS operations, and also allowing web applications
to access DNS information via existing browser APIs in a safe way
consistent with Cross Origin Resource Sharing (CORS) [FETCH]. No
special effort has been taken to enable or prevent application to
other use cases. This document focuses on communication between DNS
clients (such as operating system stub resolvers) and recursive
resolvers.

2. Terminology

A server that supports this protocol is called a "DoH server" to
differentiate it from a "DNS server" (one that only provides DNS
service over one or more of the other transport protocols
standardized for DNS). Similarly, a client that supports this
protocol is called a "DoH client".

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

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3. Selection of DoH Server

The DoH client is configured with a URI Template [RFC6570], which
describes how to construct the URL to use for resolution.
Configuration, discovery, and updating of the URI Template is done
out of band from this protocol. Note that configuration might be
manual (such as a user typing URI Templates in a user interface for
"options") or automatic (such as URI Templates being supplied in
responses from DHCP or similar protocols). DoH servers MAY support
more than one URI Template. This allows the different endpoints to
have different properties, such as different authentication
requirements or service-level guarantees.

A DoH client uses configuration to select the URI, and thus the DoH
server, that is to be used for resolution. [RFC2818] defines how
HTTPS verifies the DoH server's identity.

A DoH client MUST NOT use a different URI simply because it was
discovered outside of the client's configuration (such as through
HTTP/2 server push) or because a server offers an unsolicited
response that appears to be a valid answer to a DNS query. This
specification does not extend DNS resolution privileges to URIs that
are not recognized by the DoH client as configured URIs. Such
scenarios may create additional operational, tracking, and security
hazards that require limitations for safe usage. A future
specification may support this use case.

4. The HTTP Exchange

4.1. The HTTP Request

A DoH client encodes a single DNS query into an HTTP request using
either the HTTP GET or POST method and the other requirements of this
section. The DoH server defines the URI used by the request through
the use of a URI Template.

The URI Template defined in this document is processed without any
variables when the HTTP method is POST. When the HTTP method is GET,
the single variable "dns" is defined as the content of the DNS
request (as described in Section 6), encoded with base64url
[RFC4648].

Future specifications for new media types for DoH MUST define the
variables used for URI Template processing with this protocol.

DoH servers MUST implement both the POST and GET methods.

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When using the POST method, the DNS query is included as the message
body of the HTTP request, and the Content-Type request header field
indicates the media type of the message. POSTed requests are
generally smaller than their GET equivalents.

Using the GET method is friendlier to many HTTP cache
implementations.

The DoH client SHOULD include an HTTP Accept request header field to
indicate what type of content can be understood in response.
Irrespective of the value of the Accept request header field, the
client MUST be prepared to process "application/dns-message" (as
described in Section 6) responses but MAY also process other DNS-
related media types it receives.

In order to maximize HTTP cache friendliness, DoH clients using media
formats that include the ID field from the DNS message header, such
as "application/dns-message", SHOULD 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 such as "application/dns-
message". The use of a varying DNS ID can cause semantically
equivalent DNS queries to be cached separately.

DoH clients can use HTTP/2 padding and compression [RFC7540] in the
same way that other HTTP/2 clients use (or don't use) them.

4.1.1. HTTP Request Examples

These examples use HTTP/2-style formatting from [RFC7540].

These examples use a DoH service with a URI Template of
"https://dnsserver.example.net/dns-query{?dns}" to resolve IN A
records.

The requests are represented as bodies with media type "application/
dns-message".

The first example request uses GET to request "www.example.com".

:method = GET
:scheme = https
:authority = dnsserver.example.net
:path = /dns-query?dns=AAABAAABAAAAAAAAA3d3dwdleGFtcGxlA2NvbQAAAQAB
accept = application/dns-message

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The same DNS query for "www.example.com", using the POST method would
be:

:method = POST
:scheme = https
:authority = dnsserver.example.net
:path = /dns-query
accept = application/dns-message
content-type = application/dns-message
content-length = 33

<33 bytes represented by the following hex encoding>
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
01

In this example, the 33 bytes are the DNS message in DNS wire format
[RFC1035], starting with the DNS header.

Finally, a GET-based query for "a.62characterlabel-makes-base64url-
distinct-from-standard-base64.example.com" is shown as an example to
emphasize that the encoding alphabet of base64url is different than
regular base64 and that padding is omitted.

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
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
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 6d 70 6c 65 03 63 6f 6d 00 00 01 00 01

:method = GET
:scheme = https
:authority = dnsserver.example.net
:path = /dns-query? (no space or Carriage Return (CR))
dns=AAABAAABAAAAAAAAAWE-NjJjaGFyYWN0ZXJsYWJl (no space or CR)
bC1tYWtlcy1iYXNlNjR1cmwtZGlzdGluY3QtZnJvbS1z (no space or CR)
dGFuZGFyZC1iYXNlNjQHZXhhbXBsZQNjb20AAAEAAQ
accept = application/dns-message

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4.2. The HTTP Response

The only response type defined in this document is "application/dns-
message", but it is possible that other response formats will be
defined in the future. A DoH server MUST be able to process
"application/dns-message" request messages.

Different response media types will provide more or less information
from a DNS response. For example, one response type might include
information from the DNS header bytes while another might omit it.
The amount and type of information that a media type gives are solely
up to the format, which is not defined in this protocol.

Each DNS request-response pair is mapped to one HTTP exchange. The
responses may be processed and transported in any order using HTTP's
multi-streaming functionality (see Section 5 of [RFC7540]).

Section 5.1 discusses the relationship between DNS and HTTP response
caching.

4.2.1. Handling DNS and HTTP Errors

DNS response codes indicate either success or failure for the DNS
query. A successful HTTP response with a 2xx status code (see
Section 6.3 of [RFC7231]) is used for any valid DNS response,
regardless of the DNS response code. For example, a successful 2xx
HTTP status code is used even with a DNS message whose DNS response
code indicates failure, such as SERVFAIL or NXDOMAIN.

HTTP responses with non-successful HTTP status codes do not contain
replies to the original DNS question in the HTTP request. DoH
clients need to use the same semantic processing of non-successful
HTTP status codes as other HTTP clients. This might mean that the
DoH client retries the query with the same DoH server, such as if
there are authorization failures (HTTP status code 401; see
Section 3.1 of [RFC7235]). It could also mean that the DoH client
retries with a different DoH server, such as for unsupported media
types (HTTP status code 415; see Section 6.5.13 of [RFC7231]), or
where the server cannot generate a representation suitable for the
client (HTTP status code 406; see Section 6.5.6 of [RFC7231]), and so
on.

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4.2.2. HTTP Response Example

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
answer record with an address of 2001:db8:abcd:12:1:2:3:4 and a TTL
of 3709 seconds.

:status = 200
content-type = application/dns-message
content-length = 61
cache-control = max-age=3709

<61 bytes represented by the following hex encoding>
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 1c 00
01 c0 0c 00 1c 00 01 00 00 0e 7d 00 10 20 01 0d
b8 ab cd 00 12 00 01 00 02 00 03 00 04

5. HTTP Integration

This protocol MUST be used with the https URI scheme [RFC7230].

Sections 8 and 9 discuss additional considerations for the
integration with HTTP.

5.1. Cache Interaction

A DoH exchange can pass through a hierarchy of caches that include
both HTTP- and DNS-specific caches. These caches may exist between
the DoH server and client, or they may exist on the DoH client
itself. HTTP caches are generic by design; that is, they do not
understand this protocol. Even if a DoH client has modified its
cache implementation to be aware of DoH semantics, it does not follow
that all upstream caches (for example, inline proxies, server-side
gateways, and content delivery networks) will be.

As a result, DoH servers need to carefully consider the HTTP caching
metadata they send in response to GET requests (responses to POST
requests are not cacheable unless specific response header fields are
sent; this is not widely implemented and is not advised for DoH).

In particular, DoH servers SHOULD assign an explicit HTTP freshness
lifetime (see Section 4.2 of [RFC7234]) so that the DoH client is
more likely to use fresh DNS data. This requirement is due to HTTP
caches being able to assign their own heuristic freshness (such as
that described in Section 4.2.2 of [RFC7234]), which would take
control of the cache contents out of the hands of the DoH server.

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The assigned freshness lifetime of a DoH HTTP response MUST be less
than or equal to the smallest TTL in the Answer section of the DNS
response. A freshness lifetime equal to the smallest TTL in the
Answer section is RECOMMENDED. For example, if a HTTP response
carries three RRsets with TTLs of 30, 600, and 300, the HTTP
freshness lifetime should be 30 seconds (which could be specified as
"Cache-Control: max-age=30"). This requirement helps prevent expired
RRsets in messages in an HTTP cache from unintentionally being
served.

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
freshness lifetime MUST NOT be greater than the MINIMUM field from
that SOA record (see [RFC2308]).

The stale-while-revalidate and stale-if-error Cache-Control
directives [RFC5861] could be well suited to a DoH implementation
when allowed by server policy. Those mechanisms allow a client, at
the server's discretion, to reuse an HTTP cache entry that is no
longer fresh. In such a case, the client reuses either all of a
cached entry or none of it.

DoH servers also need to consider HTTP caching when generating
responses that are not globally valid. For instance, if a DoH server
customizes a response based on the client's identity, it would not
want to allow global reuse of that response. This could be
accomplished through a variety of HTTP techniques, such as a Cache-
Control max-age of 0, or by using the Vary response header field (see
Section 7.1.4 of [RFC7231]) to establish a secondary cache key (see
Section 4.1 of [RFC7234]).

DoH clients MUST account for the Age response header field's value
[RFC7234] when calculating the DNS TTL of a response. For example,
if an RRset is received with a DNS TTL of 600, but the Age header
field indicates that the response has been cached for 250 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 HTTP response by using
the "no-cache" request Cache-Control directive (see Section 5.2.1.4
of [RFC7234]) and similar controls. Note that some caches might not
honor these directives, either due to configuration or interaction
with traditional DNS caches that do not have such a mechanism.

HTTP conditional requests [RFC7232] may be of limited value to DoH,
as revalidation provides only a bandwidth benefit and DNS
transactions are normally latency bound. Furthermore, the HTTP
response header fields that enable revalidation (such as "Last-

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Modified" and "Etag") are often fairly large when compared to the
overall DNS response size and have a variable nature that creates
constant pressure on the HTTP/2 compression dictionary [RFC7541].
Other types of DNS data, such as zone transfers, may be larger and
benefit more from revalidation.

5.2. HTTP/2

HTTP/2 [RFC7540] is the minimum RECOMMENDED version of HTTP for use
with DoH.

The messages in classic UDP-based DNS [RFC1035] are inherently
unordered and have low overhead. A competitive HTTP transport needs
to support reordering, parallelism, priority, and header compression
to achieve similar performance. Those features were introduced to
HTTP in HTTP/2 [RFC7540]. Earlier versions of HTTP are capable of
conveying the semantic requirements of DoH but may result in very
poor performance.

5.3. Server Push

Before using DoH response data for DNS resolution, the client MUST
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 selection of URI. For HTTP server push (see Section 8.2 of
[RFC7540]), extra care must be taken to ensure that the pushed URI is
one that the client would have directed the same query to if the
client had initiated the request (in addition to the other security
checks normally needed for server push).

5.4. Content Negotiation

In order to maximize interoperability, DoH clients and DoH servers
MUST support the "application/dns-message" media type. Other media
types MAY be used as defined by HTTP Content Negotiation (see
Section 3.4 of [RFC7231]). Those media types MUST be flexible enough
to express every DNS query that would normally be sent in DNS over
UDP (including queries and responses that use DNS extensions, but not
those that require multiple responses).

6. Definition of the "application/dns-message" Media Type

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
of [RFC1035], which in turn refers to the full wire format defined in
Section 4.1 of that RFC.

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Although [RFC1035] says "Messages carried by UDP are restricted to
512 bytes", that was later updated by [RFC6891]. This media type
restricts the maximum size of the DNS message to 65535 bytes.

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 Extension
Mechanisms for DNS (EDNS) options [RFC6891] in the request. DoH
servers using this media type MUST 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
be encoded with base64url [RFC4648] and then provided as a variable
named "dns" to the URI Template expansion. Padding characters for
base64url MUST NOT be included.

When using the POST method, the data payload for this media type MUST
NOT be encoded and is used directly as the HTTP message body.

7. IANA Considerations

7.1. Registration of the "application/dns-message" Media Type

Type name: application

Subtype name: dns-message

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: See RFC 8484. The content is a DNS message
and thus not executable code.

Interoperability considerations: None.

Published specification: RFC 8484.

Applications that use this media type:
Systems that want to exchange full DNS messages.

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Additional information:

Deprecated alias names for this type: N/A
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 <[email protected]>

Intended usage: COMMON

Restrictions on usage: N/A

Author: Paul Hoffman <[email protected]>

Change controller: IESG

8. Privacy Considerations

[RFC7626] discusses DNS privacy considerations in both "on the wire"
(Section 2.4 of [RFC7626]) and "in the server" (Section 2.5 of
[RFC7626]) contexts. This is also a useful framing for DoH's privacy
considerations.

8.1. On the Wire

DoH encrypts DNS traffic and requires authentication of the server.
This mitigates both passive surveillance [RFC7258] and active attacks
that attempt to divert DNS traffic to rogue servers (see
Section 2.5.1 of [RFC7626]). DNS over TLS [RFC7858] provides similar
protections, while direct UDP- and TCP-based transports are
vulnerable to this class of attack. An experimental effort to offer
guidance on choosing the padding length can be found in [RFC8467].

Additionally, the use of the HTTPS default port 443 and the ability
to mix DoH traffic with other HTTPS traffic on the same connection
can deter unprivileged on-path devices from interfering with DNS
operations and make DNS traffic analysis more difficult.

8.2. In the Server

The DNS wire format [RFC1035] contains no client identifiers;
however, various transports of DNS queries and responses do provide
data that can be used to correlate requests. HTTPS presents new
considerations for correlation, such as explicit HTTP cookies and
implicit fingerprinting of the unique set and ordering of HTTP
request header fields.

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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
queries to the same identity. DNS transports will generally carry
the same privacy properties of the layers used to implement them.
For example, the properties of IP, TCP, and TLS apply to
implementations of DNS over TLS.

The privacy considerations of using the HTTPS layer in DoH are
incremental to those of DNS over TLS. DoH is not known to introduce
new concerns beyond those associated with HTTPS.

At the IP level, the client address provides obvious correlation
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
DNS server that can correlate real-time addressing information with
other personal identifiers, such as when a DNS server and DHCP server
are operated by the same entity.

DNS implementations that use one TCP connection for multiple DNS
requests directly group those requests. Long-lived connections have
better performance behaviors than short-lived connections; however,
they group more requests, which can expose more information to
correlation and consolidation. TCP-based solutions may also seek
performance through the use of TCP Fast Open [RFC7413]. The cookies
used in TCP Fast Open allow servers to correlate TCP sessions.

TLS-based implementations often achieve better handshake performance
through the use of some form of session resumption mechanism, such as
Section 2.2 of [RFC8446]. Session resumption creates trivial
mechanisms for a server to correlate TLS connections together.

HTTP's feature set can also be used for identification and tracking
in a number of different ways. For example, Authentication request
header fields explicitly identify profiles in use, and HTTP cookies
are designed as an explicit state-tracking mechanism between the
client and serving site and often are used as an authentication
mechanism.

Additionally, the User-Agent and Accept-Language request header
fields often convey specific information about the client version or
locale. This facilitates content negotiation and operational work-
arounds for implementation bugs. Request header fields that control
caching can expose state information about a subset of the client's
history. Mixing DoH requests with other HTTP requests on the same
connection also provides an opportunity for richer data correlation.

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The DoH protocol design allows applications to fully leverage the
HTTP ecosystem, including features that are not enumerated here.
Utilizing the full set of HTTP features enables DoH to be more than
an HTTP tunnel, but it is at the cost of opening up implementations
to the full set of privacy considerations of HTTP.

Implementations of DoH clients and servers need to consider the
benefit and privacy impact of these features, and their deployment
context, when deciding whether or not to enable them.
Implementations are advised to expose the minimal set of data needed
to achieve the desired feature set.

Determining whether or not a DoH implementation requires HTTP cookie
[RFC6265] support is particularly important because HTTP cookies are
the primary state tracking mechanism in HTTP. HTTP cookies SHOULD
NOT be accepted by DOH clients unless they are explicitly required by
a use case.

9. Security Considerations

Running DNS over HTTPS relies on the security of the underlying HTTP
transport. This mitigates classic amplification attacks for UDP-
based DNS. Implementations utilizing HTTP/2 benefit from the TLS
profile defined in Section 9.2 of [RFC7540].

Session-level encryption has well-known weaknesses with respect to
traffic analysis, which might be particularly acute when dealing with
DNS queries. HTTP/2 provides further advice about the use of
compression (see Section 10.6 of [RFC7540]) and padding (see
Section 10.7 of [RFC7540]). DoH servers can also add DNS padding
[RFC7830] if the DoH client requests it in the DNS query. An
experimental effort to offer guidance on choosing the padding length
can be found in [RFC8467].

The HTTPS connection provides transport security for the interaction
between the DoH server and client, but it does not provide the
response integrity of DNS data provided by DNSSEC. DNSSEC and DoH
are independent and fully compatible protocols, each solving
different 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
perform full DNSSEC validation of answers or to trust the DoH server
to do DNSSEC validation and inspect the AD (Authentic Data) bit in
the returned message to determine whether an answer was authentic or
not. As noted in Section 4.2, different response media types will
provide more or less information from a DNS response, so this choice
may be affected by the response media type.

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Section 5.1 describes the interaction of this protocol with HTTP
caching. An adversary that can control the cache used by the client
can affect that client's view of the DNS. This is no different than
the security implications of HTTP caching for other protocols that
use HTTP.

In the absence of DNSSEC information, a DoH server can give a client
invalid data in response to a DNS query. Section 3 disallows the use
of DoH DNS responses that do not originate from configured servers.
This prohibition does not guarantee protection against invalid data,
but it does reduce the risk.

10. Operational Considerations

Local policy considerations and similar factors mean different DNS
servers may provide different results to the same query, for
instance, in split DNS configurations [RFC6950]. It logically
follows that the 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 queries. For example, in the case of DNS64 [RFC6147],
the choice could affect whether IPv6/IPv4 translation will work at
all.

The HTTPS channel used by this specification establishes secure two-
party communication between the DoH client and the DoH server.
Filtering or inspection systems that rely on unsecured transport of
DNS will not function in a DNS over HTTPS environment due to the
confidentiality and integrity protection provided by TLS.

Some HTTPS client implementations perform real time third-party
checks of the revocation status of the certificates being used by
TLS. If this check is done as part of the DoH server connection
procedure and the check itself requires DNS resolution to connect to
the third party, a deadlock can occur. The use of Online Certificate
Status Protocol (OCSP) [RFC6960] servers or Authority Information
Access (AIA) for Certificate Revocation List (CRL) fetching (see
Section 4.2.2.1 of [RFC5280]) are examples of how this deadlock can
happen. To mitigate the possibility of deadlock, the authentication
given DoH servers SHOULD NOT rely on DNS-based references to external
resources in the TLS handshake. For OCSP, the server can bundle the
certificate status as part of the handshake using a mechanism
appropriate to the version of TLS, such as using Section 4.4.2.1 of
[RFC8446] for TLS version 1.3. AIA deadlocks can be avoided by
providing intermediate certificates that might otherwise be obtained
through additional requests. Note that these deadlocks also need to
be considered for servers that a DoH server might redirect to.

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A DoH client may face a similar bootstrapping problem when the HTTP
request needs to resolve the hostname portion of the DNS URI. Just
as the address of a traditional DNS nameserver cannot be originally
determined from that same server, a DoH client cannot use its DoH
server to initially resolve the server's host name into an address.
Alternative strategies a client might employ include 1) making the
initial resolution part of the configuration, 2) IP-based URIs and
corresponding IP-based certificates for HTTPS, or 3) resolving the
DNS API server's hostname via traditional DNS or another DoH server
while still authenticating the resulting connection via HTTPS.

HTTP [RFC7230] is a stateless application-level protocol, and
therefore DoH implementations do not provide stateful ordering
guarantees between different requests. DoH cannot be used as a
transport for other protocols that require strict ordering.

A DoH server is allowed to answer queries with any valid DNS
response. For example, a valid DNS response might have the TC
(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
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
example, a DoH server could use an HTTP error instead of a non-error
response that has the TC bit set.

Many extensions to DNS, using [RFC6891], have been defined over the
years. Extensions that are specific to the choice of transport, such
as [RFC7828], are not applicable to DoH.

11. References

11.1. Normative References

[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://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,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
<https://www.rfc-editor.org/info/rfc2308>.

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[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.

[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.

[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/info/rfc6570>.

[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.

[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.

[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
DOI 10.17487/RFC7232, June 2014,
<https://www.rfc-editor.org/info/rfc7232>.

[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.

[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235,
DOI 10.17487/RFC7235, June 2014,
<https://www.rfc-editor.org/info/rfc7235>.

[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.

[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<https://www.rfc-editor.org/info/rfc7541>.

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[RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
DOI 10.17487/RFC7626, August 2015,
<https://www.rfc-editor.org/info/rfc7626>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

11.2. Informative References

[FETCH] "Fetch Living Standard", August 2018,
<https://fetch.spec.whatwg.org/>.

[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.

[RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale
Content", RFC 5861, DOI 10.17487/RFC5861, May 2010,
<https://www.rfc-editor.org/info/rfc5861>.

[RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
DOI 10.17487/RFC6147, April 2011,
<https://www.rfc-editor.org/info/rfc6147>.

[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<https://www.rfc-editor.org/info/rfc6891>.

[RFC6950] Peterson, J., Kolkman, O., Tschofenig, H., and B. Aboba,
"Architectural Considerations on Application Features in
the DNS", RFC 6950, DOI 10.17487/RFC6950, October 2013,
<https://www.rfc-editor.org/info/rfc6950>.

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[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/info/rfc6960>.

[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.

[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.

[RFC7828] Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
edns-tcp-keepalive EDNS0 Option", RFC 7828,
DOI 10.17487/RFC7828, April 2016,
<https://www.rfc-editor.org/info/rfc7828>.

[RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
DOI 10.17487/RFC7830, May 2016,
<https://www.rfc-editor.org/info/rfc7830>.

[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.

[RFC8467] Mayrhofer, A., "Padding Policies for Extension Mechanisms
for DNS (EDNS(0))", RFC 8467, DOI 10.17487/RFC8467,
October 2018, <https://www.rfc-editor.org/info/rfc8467>.

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Appendix A. Protocol Development

This appendix describes the requirements used to design DoH. These
requirements are listed here to help readers understand the current
protocol, not to limit how the protocol might be developed in the
future. This appendix is non-normative.

The protocol described in this document based its design on the
following protocol requirements:

o The protocol must use normal HTTP semantics.

o The queries and responses must be able to be flexible enough to
express every DNS query that would normally be sent in DNS over
UDP (including queries and responses that use DNS extensions, but
not those that require multiple responses).

o The protocol must permit the addition of new formats for DNS
queries and responses.

o The protocol must ensure interoperability by specifying a single
format for requests and responses that is mandatory to implement.
That format must be able to support future modifications to the
DNS protocol including the inclusion of one or more EDNS options
(including those not yet defined).

o The protocol must use a secure transport that meets the
requirements for HTTPS.

The following were considered non-requirements:

o Supporting network-specific DNS64 [RFC6147]

o Supporting other network-specific inferences from plaintext DNS
queries

o Supporting insecure HTTP

Appendix B. Previous Work on DNS over HTTP or in Other Formats

The following is an incomplete list of earlier work that related to
DNS over HTTP/1 or representing DNS data in other formats.

The list includes links to the tools.ietf.org site (because these
documents are all expired) and web sites of software.

o <https://tools.ietf.org/html/draft-mohan-dns-query-xml>

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o <https://tools.ietf.org/html/draft-daley-dnsxml>

o <https://tools.ietf.org/html/draft-dulaunoy-dnsop-passive-dns-cof>

o <https://tools.ietf.org/html/draft-bortzmeyer-dns-json>

o <https://www.nlnetlabs.nl/projects/dnssec-trigger/>

Acknowledgments

This work required a high level of cooperation between experts in
different technologies. Thank you Ray Bellis, Stephane Bortzmeyer,
Manu Bretelle, Sara Dickinson, Massimiliano Fantuzzi, Tony Finch,
Daniel Kahn Gilmor, Olafur Gudmundsson, Wes Hardaker, Rory Hewitt,
Joe Hildebrand, David Lawrence, Eliot Lear, John Mattsson, Alex
Mayrhofer, Mark Nottingham, Jim Reid, Adam Roach, Ben Schwartz, Davey
Song, Daniel Stenberg, Andrew Sullivan, Martin Thomson, and Sam
Weiler.

Authors' Addresses

Paul Hoffman
ICANN

Email: [email protected]

Patrick McManus
Mozilla

Email: [email protected]

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