wdiff draft-ietf-doh-dns-over-https-07.txt draft-ietf-doh-dns-over-https-08.txt

Network Working Group P. Hoffman
Internet-Draft ICANN
Intended status: Standards Track P. McManus
Expires: October 13, November 17, 2018 Mozilla
April 11,
May 16, 2018

DNS Queries over HTTPS
draft-ietf-doh-dns-over-https-07 (DOH)
draft-ietf-doh-dns-over-https-08

Abstract

This document describes how to run make DNS service queries over HTTP (DOH) using
https:// URIs. HTTPS.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on October 13, November 17, 2018.

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
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described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Requirements . . . . . . . . . . . . . . . . . . . . 3
3.1. Non-requirements . . . . . . . . . . . . . . . . . . . . 4
4. Selection of DNS API Server . . . . . . . . . . . . . . . . . 4
5. The HTTP Exchange . . . . . . . . . . . . . . . . . . . . . . 4
4.1.
5.1. The HTTP Request . . . . . . . . . . . . . . . . . . . . 4
4.1.1.
5.1.1. HTTP Request Examples . . . . . . . . . . . . . . . . 5
4.2.
5.2. The HTTP Response . . . . . . . . . . . . . . . . . . . . 6
4.2.1.
5.2.1. HTTP Response Example . . . . . . . . . . . . . . . . 7
5.
6. HTTP Integration . . . . . . . . . . . . . . . . . . . . . . 7
5.1. 8
6.1. Cache Interaction . . . . . . . . . . . . . . . . . . . . 7
5.2. 8
6.2. HTTP/2 . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.3. 10
6.3. Server Push . . . . . . . . . . . . . . . . . . . . . . . 9
5.4. 10
6.4. Content Negotiation . . . . . . . . . . . . . . . . . . . 9
6. 10
7. DNS Wire Format . . . . . . . . . . . . . . . . . . . . . . . 9
7. 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7.1. 11
8.1. Registration of application/dns-message Media Type . . . 10
8. 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. 13
10. Operational Considerations . . . . . . . . . . . . . . . . . 13
10. Acknowledgments 14
11. References . . . . . . . . . . . . . . . . . . . . . . . 14
11. . . 15
11.1. Normative References . . . . . . . . . . . . . . . . . . 15
11.2. Informative References . . . . . . . 14
11.1. Normative References . . . . . . . . . . 16
Acknowledgments . . . . . . . . 14
11.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. 17
Previous Work on DNS over HTTP or in Other Formats . 16 . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 18

1. Introduction

This document defines a specific protocol for sending DNS [RFC1035]
queries and getting DNS responses over HTTP [RFC7540] using https:// https
URIs (and therefore TLS [RFC5246] security for integrity and
confidentiality). Each DNS query-response pair is mapped into a 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
traditional
existing DNS clients and native web applications seeking access to
the DNS.

Two primary uses cases were considered during this protocol's
development. They included preventing on-path devices from
interfering with DNS operations and allowing web applications to
access DNS information via existing browser APIs in a safe way
consistent with Cross Origin Resource Sharing (CORS) [CORS]. There
are certainly No
special effort has been taken to enable or prevent application to
other uses for this work. 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 on one or more URIs is called a "DNS API server" to
differentiate it from a "DNS server" (one that
uses the regular only provides DNS protocol).
service over one or more of the other transport protocols
standardized for DNS). Similarly, a client that supports this
protocol is called a "DNS API 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, RFC8174
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Protocol Requirements

[[ RFC Editor: Please remove this entire section before publication.
]]

The protocol described here bases 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.

3.1. Non-requirements

o Supporting network-specific DNS64 [RFC6147]

o Supporting other network-specific inferences from plaintext DNS
queries

o Supporting insecure HTTP

4. Selection of DNS API Server

Before using a DNS API server for DNS resolution, the client MUST
establish that the HTTP request URI is a trusted service for the DOH
query, in other words, a DNS API client MUST only use a DNS API
server that is configured as trustworthy.

A client MUST NOT use a DNS API server simply because it was
discovered, or because the client was told to use the DNS API server
by an untrusted party.

This specification does not extend DNS resolution privileges to URIs
that are not recognized by the DNS API client as trusted DNS API
servers. As such, use of untrusted servers is out of scope of this
document.

5. The HTTP Exchange

4.1.

5.1. The HTTP Request

A DNS API 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 DNS API server defines the URI used by the
request through the use of a URI Template [RFC6570].

Configuration and discovery of the URI Template is done out of band
from this protocol. DNS API Servers MAY support more than one URI.
This allows the different endpoints to have different properties such
as different authentication requirements or service level guarantees.

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), 7), encoded with base64url
[RFC4648].

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

DNS API servers MUST implement both the POST and GET methods.

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

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

The DNS API client SHOULD include an HTTP "Accept" request header to
indicate what type of content can be understood in response.
Irrespective of the value of the Accept request header, the client
MUST be prepared to process "application/dns-message" (as described
in Section 6) 7) responses but MAY also process any other type it
receives.

In order to maximize cache friendliness, DNS API clients using media
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
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.

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.

4.1.1.

5.1.1. HTTP Request Examples

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

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

The requests are represented as application/dns-message typed bodies.

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
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

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 is 94 bytes represented by the following hex encoding

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 CR)
dns=AAABAAABAAAAAAAAAWE-NjJjaGFyYWN0ZXJsYWJl (no space or CR)
bC1tYWtlcy1iYXNlNjR1cmwtZGlzdGluY3QtZnJvbS1z (no space or CR)
dGFuZGFyZC1iYXNlNjQHZXhhbXBsZQNjb20AAAEAAQ
accept = application/dns-message

4.2.

5.2. The HTTP Response

An HTTP response with a 2xx status code ([RFC7231] Section 6.3)
indicates a valid DNS response to the query made in the HTTP request.
A valid DNS response includes both success and failure responses.
For example, a DNS failure response such as SERVFAIL or NXDOMAIN will
be the message in a successful 2xx HTTP response even though there
was a failure at the DNS layer. Responses with non-successful HTTP
status codes do not contain DNS answers to the question in the
corresponding request. Some of these non-successful HTTP responses
(e.g., redirects or authentication failures) could allow mean that clients
need to make new requests to satisfy the original question.

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

At the time this is published, the response types are works in
progress.

The only response type defined in this document is
"application/dns-message", "application/dns-
message", but it is possible that other response formats will be
defined in the future.

The DNS response for "application/dns-message" in Section 6 7 MAY have
one or more EDNS options, options [RFC6891], depending on the extension
definition of the extensions given in the DNS request.

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

Section 5.1 6.1 discusses the relationship between DNS and HTTP response
caching.

A DNS API server MUST be able to process application/dns-message
request messages.

A DNS API server SHOULD respond with HTTP status code 415
(Unsupported Media Type) upon receiving a media type it is unable to
process.

4.2.1.

5.2.1. HTTP Response Example

This is an example response for a query for the IN A records for
"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.

:status = 200
content-type = application/dns-message
content-length = 64
cache-control = max-age=128

<64 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 01 00
01 03 77 77 77 07 65 78 61 6d 70 6c 65 03 63 6f
6d 00 00 01 00 01 00 00 00 80 00 04 C0 00 02 01

6. HTTP Integration

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

5.1.

6.1. Cache Interaction

A DNS API client may utilize DOH exchange can pass through a hierarchy of caches that include
both HTTP and DNS specific caches. HTTP cache entries These caches may be bypassed
with HTTP mechanisms such as exist beteen the
DNS API server and client, or on the "Cache-Control no-cache" directive;
however DNS API client itself. HTTP
caches are by design generic; that is, they do not have understand this
protocol. Even if a similar mechanism.

The Answer section DNS API 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, DNS response can contain zero or more RRsets.
(RRsets are defined in [RFC7719].) According API servers need to [RFC2181], each
resource record carefully consider the HTTP
caching metadata they send in response to GET requests (POST requests
are not cacheable unless specific response headers are sent; this is
not widely implemented, and not advised for DOH).

In particular, DNS API servers SHOULD assign an RRset has Time To Live (TTL) explicit freshness
information. Different RRsets in
lifetime ([RFC7234] Section 4.2) so that the Answer section can have
different TTLs, although it DNS API client is only possible for the more
likely to use fresh DNS data. This requirement is due to HTTP response caches
being able to
have a single assign their own heuristic freshness lifetime. (such as that
described in [RFC7234] Section 4.2.2), which would take control of
the cache contents out of the hands of the DNS API server.

The HTTP response assigned freshness lifetime ([RFC7234] Section 4.2) should of a DOH HTTP response SHOULD be coordinated with the RRset
with the
smallest TTL in the Answer section of the DNS response.
Specifically, For example,
if a HTTP response carries three RRsets with TTLs of 30, 600, and
300, the HTTP freshness lifetime SHOULD should be set to expire at
the same time any of the DNS resource records in the Answer section
reach a 0 TTL. 30 seconds (which could be
specified as "Cache-Control: max-age=30"). The response assigned freshness
lifetime MUST NOT be greater than that indicated by the DNS resoruce record with the smallest TTL in the Answer
section of the DNS response. This requirement helps assure that none
of the RRsets contained in a DNS response are served stale from an
HTTP cache.

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].) Otherwise, the HTTP response MUST
set record (see [RFC2308]).

The stale-while-revalidate and stale-if-error Cache-Control
directives ([RFC5861]) could be well suited to a freshness lifetime ([RFC7234] Section 4.2) of 0 DOH implementation
when allowed by using server policy. Those mechanisms allow a
mechanism client, at
the server's discretion, to reuse a cache entry that is no longer
fresh. In such as "Cache-Control: no-cache" ([RFC7234]
Section 5.2.1.4).

A DNS API a case, the client that receives reuses all of a response without an explicit
freshness lifetime MUST NOT assign cached entry, or
none of it.

DNS API servers also need to consider caching when generating
responses that response are not globally valid. For instance, if a heuristic
freshness ([RFC7234] Section 4.2.2.) greater than that indicated by
the DNS Record with the smallest TTL in the response.

A DOH API
server customizes a response based on the client's identity, it would
not want to allow global reuse of that was previously stored in an response. This could be
accomplished through a variety of HTTP cache will
contain techniques such as a Cache-
Control max-age of 0, or by using the [RFC7234] Age Vary response header indicating the elapsed time
between when the entry was placed in the HTTP ([RFC7231]
Section 7.1.4) to establish a secondary cache and the current
DOH response. key ([RFC7234]
Section 4.1).

DNS API clients should subtract this time from MUST account for the Age response header's value
([RFC7234]) when calculating the DNS TTL of a response. For example,
if they are re-sharing the information in a non HTTP context
(e.g., their own RRset is received with a DNS cache) to determine TTL of 600, but the Age header
indicates that the response has been cached for 250 seconds, the
remaining time to live lifetime of the RRset is 350 seconds.

DNS record.

HTTP revalidation (e.g., via If-None-Match API clients can request headers) an uncached copy of cached a response by using
the "no-cache" request cache control directive ([RFC7234],
Section 5.2.1.4) and similar controls. Note that some caches might
not honor these directives, either due to configuration or
interaction with traditional DNS information caches that do not have such a
mechanism.

HTTP conditional requests ([RFC7232]) may be of limited value to DOH DOH,
as revalidation provides only a bandwidth benefit and DNS
transactions are normally latency bound. Furthermore, the HTTP
response headers that enable revalidation (such as "Last-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.
DNS API servers may wish to consider whether providing these
validation enabling response headers

6.2. HTTP/2

HTTP/2 [RFC7540] is worthwhile.

The stale-while-revalidate and stale-if-error cache control
directives may be well suited to a DOH implementation when allowed by
server policy. Those mechanisms allow a client, at the server's
discretion, to reuse a cache entry that is no longer fresh under some
extenuating circumstances defined in [RFC5861].

All HTTP servers, including DNS API servers, need to consider cache
interaction when they generate responses that are not globally valid.
For instance, if a DNS API server customized a response based on the
client's identity then it would not want to globally allow reuse of
that response. This could be accomplished through a variety of HTTP
techniques such as a Cache-Control max-age of 0, or perhaps by the
Vary response header.

5.2. HTTP/2

The minimum RECOMMENDED version of HTTP used by DOH SHOULD be HTTP/2 [RFC7540]. 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.

6.3. Server Push

Before using DOH response data for DNS resolution, the client MUST
establish that the HTTP request URI is a trusted service may be used for the DOH query.
For HTTP requests initiated by the DNS API client this trust is implicit in
the selection of URI. For HTTP server push ([RFC7540] Section 8.2)
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. This specification does not extend
DNS resolution privileges to URIs that are not recognized by the
client as trusted DNS API servers.

5.4.

6.4. Content Negotiation

In order to maximize interoperability, DNS API clients and DNS API
servers MUST support the "application/dns-message" media type. Other
media types MAY be used as defined by HTTP Content Negotiation
([RFC7231] Section 3.4).

6. 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).

7. DNS Wire Format

The data payload is the DNS on-the-wire format defined in [RFC1035].
The format is for DNS over UDP. Note that this is different than the
wire format used in [RFC7858]. Also note that while [RFC1035] says
"Messages carried by UDP are restricted to 512 bytes", that was later
updated by [RFC6891], and this [RFC6891]. This protocol allows DNS on-the-wire format
payloads of any size.

When using the GET method, the data payload 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 MUST NOT be encoded and
is used directly as the HTTP message body.

DNS API clients using the DNS wire format MAY have one or more EDNS
options [RFC6891] in the request.

The media type is "application/dns-message".

8. IANA Considerations

7.1.

8.1. Registration of application/dns-message Media Type
To: ietf-types@iana.org
Subject: Registration of MIME media type
application/dns-message

MIME media type name: application

MIME 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: 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

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 [RFC7540] Section 9.2.

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 (Section 10.6 of [RFC7540]) ([RFC7540] Section 10.6) and padding (Section ([RFC7540]
Section 10.7 of
[RFC7540]). ). DNS API Servers can also add DNS padding [RFC7830]
if the DNS API requests it in the DNS query.

The HTTPS connection provides transport security for the interaction
between the DNS API server and client, but does not inherently ensure provide the authenticity
response integrity of DNS data. A DNS API 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 may also to either
perform full DNSSEC validation of answers received from a DNS API server or it may
choose to trust answers from a particular the DNS API server, much as a
DNS client might choose
server to trust answers 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 5.2, different response media
types will provide more or less information from its recursive a DNS
resolver. This capability might response so
this choice may be affected by the response media type.

Section 5.1 6.1 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.

A server that is acting both as a normal web server and

In the absence of DNSSEC information, a DNS API server is in a position to choose which DNS names it forces can give a
client invalid data in response 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. query. A client MUST NOT trust a DNS API server simply because it was discovered, or
because the client was told to trust the
use arbitrary DNS API server by an
untrusted party. servers. Instead, a client MUST only trust use DNS
API server
that is configured servers specified using mechanisms such as trustworthy. explicit
configuration. This does not guarantee protection against invalid
data but reduces the risk.

A client can use DNS over HTTPS as one of multiple mechanisms to
obtain DNS data. If a client of this protocol encounters an HTTP
error after sending a DNS query, and then falls back to a different
DNS retrieval mechanism, doing so can weaken the privacy and
authenticity expected by the user of the client.

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 which 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 DNS API client and the DNS API
server. Filtering or inspection systems that rely on unsecured
transport of DNS will not function in a DNS over HTTPS environment.

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 DNS API server connection
procedure and the check itself requires DNS resolution to connect to
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
examples of how this deadlock can happen. To mitigate the
possibility of deadlock, DNS API 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
[RFC6066] Section 8 for TLS version 1.2. 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 server that a DNS API server might
redirect to.

A DNS API 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 DNS API client cannot
use its DNS API server to initially resolve the server's host name
into an address. Alternative strategies a client might employ
include making the initial resolution part of the configuration, IP
based URIs and corresponding IP based certificates for HTTPS, or
resolving the DNS API server's hostname via traditional DNS or
another DNS API 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.

If a

A DNS API server responds is allowed to a DNS API client answer queries with any valid DNS
response. For example, a valid DNS message
that has response might have the TC
(truncation) bit set in the header, that indicates DNS header to indicate that the DNS API server
was not able to retrieve a full answer for the query and but is providing
the best answer it could get. This
protocol does not require that a A DNS API server that cannot get an
untruncated answer send back such an answer; it can instead send back reply to queries
with an HTTP error to indicate for queries that it cannot give fulfill. In this same
example, a useful answer.

10. Acknowledgments

This work required DNS API server could use an HTTP error instead of a high level 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 cooperation between experts in
different technologies. Thank you Ray Bellis, Stephane Bortzmeyer,
Manu Bretelle, Tony Finch, Daniel Kahn Gilmor, Olafur Guomundsson,
Wes Hardaker, Rory Hewitt, Joe Hildebrand, David Lawrence, Eliot
Lear, John Mattson, Alex Mayrhofer, Mark Nottingham, Jim Reid, Adam
Roach, Ben Schwartz, Davey Song, Daniel Stenberg, Andrew Sullivan,
Martin Thomson, and Sam Weiler. 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>.

[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>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

[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>.

[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>.

[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>.

11.2. Informative References

[CORS] "Cross-Origin Resource Sharing", n.d.,
<https://fetch.spec.whatwg.org/#http-cors-protocol>.

[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.

[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>.

[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.

[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>.

[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>.

[RFC7719] Hoffman,

[RFC7828] Wouters, P., Sullivan, A., Abley, J., Dickinson, S., and K. Fujiwara, "DNS
Terminology", R. Bellis, "The
edns-tcp-keepalive EDNS0 Option", RFC 7719, 7828,
DOI 10.17487/RFC7719, December
2015, <https://www.rfc-editor.org/info/rfc7719>. 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>.

Appendix A.

Acknowledgments

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

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

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/

Authors' Addresses

Paul Hoffman
ICANN

Email: paul.hoffman@icann.org

Patrick McManus
Mozilla

Email: mcmanus@ducksong.com