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Internet Engineering Task Force (IETF)                        P. HoffmanRequest for Comments: 8484                                         ICANNCategory: Standards Track                                     P. McManusISSN: 2070-1721                                                  Mozilla                                                            October 2018DNS 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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://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 toBCP 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.Hoffman & McManus            Standards Track                    [Page 1]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .32.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .33.  Selection of DoH Server . . . . . . . . . . . . . . . . . . .44.  The HTTP Exchange . . . . . . . . . . . . . . . . . . . . . .44.1.  The HTTP Request  . . . . . . . . . . . . . . . . . . . .44.1.1.  HTTP Request Examples . . . . . . . . . . . . . . . .54.2.  The HTTP Response . . . . . . . . . . . . . . . . . . . .74.2.1.  Handling DNS and HTTP Errors  . . . . . . . . . . . .74.2.2.  HTTP Response Example . . . . . . . . . . . . . . . .85.  HTTP Integration  . . . . . . . . . . . . . . . . . . . . . .85.1.  Cache Interaction . . . . . . . . . . . . . . . . . . . .85.2.  HTTP/2  . . . . . . . . . . . . . . . . . . . . . . . . .105.3.  Server Push . . . . . . . . . . . . . . . . . . . . . . .105.4.  Content Negotiation . . . . . . . . . . . . . . . . . . .106.  Definition of the "application/dns-message" Media Type  . . .107.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .11     7.1.  Registration of the "application/dns-message" Media Type   118.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .128.1.  On the Wire . . . . . . . . . . . . . . . . . . . . . . .128.2.  In the Server . . . . . . . . . . . . . . . . . . . . . .129.  Security Considerations . . . . . . . . . . . . . . . . . . .1410. Operational Considerations  . . . . . . . . . . . . . . . . .1511. References  . . . . . . . . . . . . . . . . . . . . . . . . .1611.1.  Normative References . . . . . . . . . . . . . . . . . .1611.2.  Informative References . . . . . . . . . . . . . . . . .18Appendix A.  Protocol Development . . . . . . . . . . . . . . . .20Appendix B.  Previous Work on DNS over HTTP or in Other Formats .  20   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .21   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .21Hoffman & McManus            Standards Track                    [Page 2]

RFC 8484              DNS Queries over HTTPS (DoH)          October 20181.  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 inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.Hoffman & McManus            Standards Track                    [Page 3]

RFC 8484              DNS Queries over HTTPS (DoH)          October 20183.  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 Exchange4.1.  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 inSection 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.Hoffman & McManus            Standards Track                    [Page 4]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018   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 inSection 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-messageHoffman & McManus            Standards Track                    [Page 5]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018   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-messageHoffman & McManus            Standards Track                    [Page 6]

RFC 8484              DNS Queries over HTTPS (DoH)          October 20184.2.  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 (seeSection 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 (seeSection 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; seeSection 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; seeSection 6.5.13 of [RFC7231]), or   where the server cannot generate a representation suitable for the   client (HTTP status code 406; seeSection 6.5.6 of [RFC7231]), and so   on.Hoffman & McManus            Standards Track                    [Page 7]

RFC 8484              DNS Queries over HTTPS (DoH)          October 20184.2.2.  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 00 045.  HTTP Integration   This protocol MUST be used with the https URI scheme [RFC7230].   Sections8 and9 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 (seeSection 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 inSection 4.2.2 of [RFC7234]), which would take   control of the cache contents out of the hands of the DoH server.Hoffman & McManus            Standards Track                    [Page 8]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018   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 (seeSection 7.1.4 of [RFC7231]) to establish a secondary cache key (seeSection 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 (seeSection 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-Hoffman & McManus            Standards Track                    [Page 9]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018   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 (seeSection 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 (seeSection 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 inSection 4.2.1   of [RFC1035], which in turn refers to the full wire format defined inSection 4.1 of that RFC.Hoffman & McManus            Standards Track                   [Page 10]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018   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 inSection 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 Considerations7.1.  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 inRFC 1035.  The format used here is for      DNS over UDP, which is the format defined in the diagrams inRFC 1035.   Security considerations: SeeRFC 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.Hoffman & McManus            Standards Track                   [Page 11]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018   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 <paul.hoffman@icann.org>   Intended usage: COMMON   Restrictions on usage: N/A   Author: Paul Hoffman <paul.hoffman@icann.org>   Change controller: IESG8.  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 (seeSection 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.Hoffman & McManus            Standards Track                   [Page 12]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018   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 asSection 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.Hoffman & McManus            Standards Track                   [Page 13]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018   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 inSection 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 (seeSection 10.6 of [RFC7540]) and padding (seeSection 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 inSection 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.Hoffman & McManus            Standards Track                   [Page 14]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018Section 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 (seeSection 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 usingSection 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.Hoffman & McManus            Standards Track                   [Page 15]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018   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.  References11.1.  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>.Hoffman & McManus            Standards Track                   [Page 16]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018   [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>.Hoffman & McManus            Standards Track                   [Page 17]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018   [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 inRFC2119 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>.Hoffman & McManus            Standards Track                   [Page 18]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018   [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>.Hoffman & McManus            Standards Track                   [Page 19]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018Appendix 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 HTTPAppendix 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>Hoffman & McManus            Standards Track                   [Page 20]

RFC 8484              DNS Queries over HTTPS (DoH)          October 2018   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: paul.hoffman@icann.org   Patrick McManus   Mozilla   Email: mcmanus@ducksong.comHoffman & McManus            Standards Track                   [Page 21]