Network Working Group P. Hoffman Internet-Draft ICANN Intended status: Standards Track P. McManus Expires: December 29, 2018 Mozilla June 27, 2018 DNS Queries over HTTPS (DoH) draft-ietf-doh-dns-over-https-12 Abstract This document describes how to make DNS queries over 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- Drafts is at https://datatracker.ietf.org/drafts/current/. 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 December 29, 2018. 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. Hoffman & McManus Expires December 29, 2018 [Page 1] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Protocol Requirements . . . . . . . . . . . . . . . . . . . . 3 3.1. Non-requirements . . . . . . . . . . . . . . . . . . . . 4 4. Selection of DoH Server . . . . . . . . . . . . . . . . . . . 4 5. The HTTP Exchange . . . . . . . . . . . . . . . . . . . . . . 4 5.1. The HTTP Request . . . . . . . . . . . . . . . . . . . . 4 5.1.1. HTTP Request Examples . . . . . . . . . . . . . . . . 5 5.2. The HTTP Response . . . . . . . . . . . . . . . . . . . . 7 5.2.1. Handling DNS and HTTP Errors . . . . . . . . . . . . 7 5.2.2. HTTP Response Example . . . . . . . . . . . . . . . . 7 6. HTTP Integration . . . . . . . . . . . . . . . . . . . . . . 8 6.1. Cache Interaction . . . . . . . . . . . . . . . . . . . . 8 6.2. HTTP/2 . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.3. Server Push . . . . . . . . . . . . . . . . . . . . . . . 10 6.4. Content Negotiation . . . . . . . . . . . . . . . . . . . 10 7. Definition of the application/dns-message media type . . . . 10 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 8.1. Registration of application/dns-message Media Type . . . 11 9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 13 9.1. On The Wire . . . . . . . . . . . . . . . . . . . . . . . 13 9.2. In The Server . . . . . . . . . . . . . . . . . . . . . . 13 10. Security Considerations . . . . . . . . . . . . . . . . . . . 15 11. Operational Considerations . . . . . . . . . . . . . . . . . 15 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 12.1. Normative References . . . . . . . . . . . . . . . . . . 17 12.2. Informative References . . . . . . . . . . . . . . . . . 18 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 20 Previous Work on DNS over HTTP or in Other Formats . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction This document defines a specific protocol for sending DNS [RFC1035] queries and getting DNS responses over HTTP [RFC7540] using 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. Hoffman & McManus Expires December 29, 2018 [Page 2] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 The integration with HTTP provides a transport suitable for both 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]. 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. 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. Hoffman & McManus Expires December 29, 2018 [Page 3] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 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 DoH Server Configuration, discovery, and updating of the URI Template [RFC6570] (see Section 5.1) is done out of band from this protocol. Note that configuration might be manual (such as a user typing URI Templates in 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. 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, 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. 5. The HTTP Exchange 5.1. 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. Hoffman & McManus Expires December 29, 2018 [Page 4] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 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 7), encoded with base64url [RFC4648]. Future specifications for new media types MUST define the variables used for URI Template processing with this protocol. DoH 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 DoH 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 7) responses but MAY also process any other type it receives. In order to maximize cache friendliness, DoH 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. DoH clients can use HTTP/2 padding and compression in the same way that other HTTP/2 clients use (or don't use) them. 5.1.1. 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 application/dns-message typed bodies. The first example request uses GET to request www.example.com Hoffman & McManus Expires December 29, 2018 [Page 5] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 :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 Hoffman & McManus Expires December 29, 2018 [Page 6] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 5.2. 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 is solely up to the format, and not defined in this protocol. 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 6.1 discusses the relationship between DNS and HTTP response caching. 5.2.1. 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 ([RFC7231] Section 6.3) can be 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 authorization failures (HTTP status code 401 [RFC7235] Section 3.1). It could also mean that the DoH client retries with a different DoH server, such as for unsupported media types (HTTP status code 415, [RFC7231] Section 6.5.13), or where the server cannot generate a representation suitable for the client (HTTP status code 406, [RFC7231] Section 6.5.6), and so on. 5.2.2. 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. Hoffman & McManus Expires December 29, 2018 [Page 7] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 :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]. Section 9 and Section 10 discuss additional considerations for the integration with HTTP. 6.1. Cache Interaction A DoH exchange can pass through a hierarchy of caches that include both HTTP and DNS specific caches. These caches may exist beteen the DoH server and client, or on the DoH client itself. HTTP caches are by design generic; 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 (POST requests are not cacheable unless specific response headers are sent; this is not widely implemented, and not advised for DoH). In particular, DoH servers SHOULD assign an explicit freshness lifetime ([RFC7234] Section 4.2) so that the DoH client is more likely to use fresh DNS data. This requirement is due to HTTP caches being able to assign their own heuristic freshness (such as that described in [RFC7234] Section 4.2.2), which would take control of the cache contents out of the hands of the DoH server. The assigned freshness lifetime of a DoH HTTP response SHOULD be the smallest TTL in the Answer section of the DNS response. 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"). The assigned freshness lifetime MUST NOT be greater than the smallest TTL in the Answer section of the DNS response. This requirement helps assure that none Hoffman & McManus Expires December 29, 2018 [Page 8] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 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]). 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 a cache entry that is no longer fresh. In such a case, the client reuses all of a cached entry, or none of it. DoH servers also need to consider 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 ([RFC7231] Section 7.1.4) to establish a secondary cache key ([RFC7234] Section 4.1). DoH clients MUST account for the Age response header's value ([RFC7234]) when calculating the DNS TTL of a response. For example, if a RRset is received with a DNS TTL of 600, but the Age header indicates that the response has been cached for 250 seconds, the remaining lifetime of the RRset is 350 seconds. DoH clients can request an uncached copy of 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 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 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. Hoffman & McManus Expires December 29, 2018 [Page 9] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 6.2. 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. 6.3. Server Push Before using DoH response data for DNS resolution, the client MUST establish that the HTTP request URI may 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 ([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. 6.4. 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 ([RFC7231] Section 3.4). 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. 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]. The format was originally for DNS over UDP. 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]). DoH clients using this media type MAY have one or more 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. Hoffman & McManus Expires December 29, 2018 [Page 10] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 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. 8. IANA Considerations 8.1. Registration of application/dns-message Media Type Hoffman & McManus Expires December 29, 2018 [Page 11] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 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 Hoffman & McManus Expires December 29, 2018 [Page 12] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 9. Privacy Considerations [RFC7626] discusses DNS Privacy Considerations in both "On the wire" (Section 2.4), and "In the server" (Section 2.5) contexts. This is also a useful framing for DoH's privacy considerations. 9.1. 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 ([RFC7626] Section 2.5.1). DNS over TLS [RFC7858] provides similar protections, while direct UDP and TCP based transports are vulnerable to this class of attack. 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. 9.2. 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 headers. 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 DNS over TLS implementations. 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. Hoffman & McManus Expires December 29, 2018 [Page 13] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 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, but group more requests. 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 session tickets [RFC5077]. 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. 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 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. Hoffman & McManus Expires December 29, 2018 [Page 14] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 10. 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 ([RFC7540] Section 10.6) and padding ([RFC7540] Section 10.7 ). DoH Servers can also add DNS padding [RFC7830] if the DoH client requests it in the DNS query. The HTTPS connection provides transport security for the interaction between the DoH server and client, but 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 5.2, 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. Section 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. In the absence of DNSSEC information, a DoH server can give a client invalid data in response to a DNS query. Section 4 disallows the use 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. 11. 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. Hoffman & McManus Expires December 29, 2018 [Page 15] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 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. 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 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, 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 [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 DoH server might redirect to. 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 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 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. Hoffman & McManus Expires December 29, 2018 [Page 16] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 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. 12. References 12.1. Normative References [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, . [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, . [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, . [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, DOI 10.17487/RFC6265, April 2011, . [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., and D. Orchard, "URI Template", RFC 6570, DOI 10.17487/RFC6570, March 2012, . [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, . [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, . Hoffman & McManus Expires December 29, 2018 [Page 17] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 [RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests", RFC 7232, DOI 10.17487/RFC7232, June 2014, . [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, . [RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Authentication", RFC 7235, DOI 10.17487/RFC7235, June 2014, . [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015, . [RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015, . [RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626, DOI 10.17487/RFC7626, August 2015, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 12.2. Informative References [CORS] "Cross-Origin Resource Sharing", n.d., . [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/RFC2818, May 2000, . [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 5077, DOI 10.17487/RFC5077, January 2008, . Hoffman & McManus Expires December 29, 2018 [Page 18] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 [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, . [RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale Content", RFC 5861, DOI 10.17487/RFC5861, May 2010, . [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011, . [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, . [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, April 2013, . [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, . [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, . [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014, . [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, . Hoffman & McManus Expires December 29, 2018 [Page 19] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 [RFC7828] Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The edns-tcp-keepalive EDNS0 Option", RFC 7828, DOI 10.17487/RFC7828, April 2016, . [RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830, DOI 10.17487/RFC7830, May 2016, . [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, . 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 Mattsson, 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 Hoffman & McManus Expires December 29, 2018 [Page 20] Internet-Draft DNS Queries over HTTPS (DoH) June 2018 Patrick McManus Mozilla Email: mcmanus@ducksong.com Hoffman & McManus Expires December 29, 2018 [Page 21]