Internet Engineering Task Force (IETF) P. Hoffman Request for Comments: 8484 ICANN Category: Standards Track P. McManus ISSN: 2070-1721 Mozilla October 2018
DNS Queries over HTTPS (DoH)
Abstract
This document defines a protocol for sending DNS queries and getting DNS responses over HTTPS. Each DNS query-response pair is mapped into an HTTP exchange.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8484.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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RFC 8484 DNS Queries over HTTPS (DoH) October 2018
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.
A server that supports this protocol is called a "DoH server" to differentiate it from a "DNS server" (one that only provides DNS service over one or more of the other transport protocols standardized for DNS). Similarly, a client that supports this protocol is called a "DoH client".
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
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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.
A DoH client encodes a single DNS query into an HTTP request using either the HTTP GET or POST method and the other requirements of this section. The DoH server defines the URI used by the request through the use of a URI Template.
The URI Template defined in this document is processed without any variables when the HTTP method is POST. When the HTTP method is GET, the single variable "dns" is defined as the content of the DNS request (as described in Section 6), encoded with base64url [RFC4648].
Future specifications for new media types for DoH MUST define the variables used for URI Template processing with this protocol.
DoH servers MUST implement both the POST and GET methods.
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When using the POST method, the DNS query is included as the message body of the HTTP request, and the Content-Type request header field indicates the media type of the message. POSTed requests are generally smaller than their GET equivalents.
Using the GET method is friendlier to many HTTP cache implementations.
The DoH client SHOULD include an HTTP Accept request header field to indicate what type of content can be understood in response. Irrespective of the value of the Accept request header field, the client MUST be prepared to process "application/dns-message" (as described in Section 6) responses but MAY also process other DNS- related media types it receives.
In order to maximize HTTP cache friendliness, DoH clients using media formats that include the ID field from the DNS message header, such as "application/dns-message", SHOULD use a DNS ID of 0 in every DNS request. HTTP correlates the request and response, thus eliminating the need for the ID in a media type such as "application/dns- message". The use of a varying DNS ID can cause semantically equivalent DNS queries to be cached separately.
DoH clients can use HTTP/2 padding and compression [RFC7540] in the same way that other HTTP/2 clients use (or don't use) them.
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:
:method = GET :scheme = https :authority = dnsserver.example.net :path = /dns-query? (no space or Carriage Return (CR)) dns=AAABAAABAAAAAAAAAWE-NjJjaGFyYWN0ZXJsYWJl (no space or CR) bC1tYWtlcy1iYXNlNjR1cmwtZGlzdGluY3QtZnJvbS1z (no space or CR) dGFuZGFyZC1iYXNlNjQHZXhhbXBsZQNjb20AAAEAAQ accept = application/dns-message
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The only response type defined in this document is "application/dns- message", but it is possible that other response formats will be defined in the future. A DoH server MUST be able to process "application/dns-message" request messages.
Different response media types will provide more or less information from a DNS response. For example, one response type might include information from the DNS header bytes while another might omit it. The amount and type of information that a media type gives are solely up to the format, which is not defined in this protocol.
Each DNS request-response pair is mapped to one HTTP exchange. The responses may be processed and transported in any order using HTTP's multi-streaming functionality (see Section 5 of [RFC7540]).
Section 5.1 discusses the relationship between DNS and HTTP response caching.
DNS response codes indicate either success or failure for the DNS query. A successful HTTP response with a 2xx status code (see Section 6.3 of [RFC7231]) is used for any valid DNS response, regardless of the DNS response code. For example, a successful 2xx HTTP status code is used even with a DNS message whose DNS response code indicates failure, such as SERVFAIL or NXDOMAIN.
HTTP responses with non-successful HTTP status codes do not contain replies to the original DNS question in the HTTP request. DoH clients need to use the same semantic processing of non-successful HTTP status codes as other HTTP clients. This might mean that the DoH client retries the query with the same DoH server, such as if there are authorization failures (HTTP status code 401; see Section 3.1 of [RFC7235]). It could also mean that the DoH client retries with a different DoH server, such as for unsupported media types (HTTP status code 415; see Section 6.5.13 of [RFC7231]), or where the server cannot generate a representation suitable for the client (HTTP status code 406; see Section 6.5.6 of [RFC7231]), and so on.
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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.
A DoH exchange can pass through a hierarchy of caches that include both HTTP- and DNS-specific caches. These caches may exist between the DoH server and client, or they may exist on the DoH client itself. HTTP caches are generic by design; that is, they do not understand this protocol. Even if a DoH client has modified its cache implementation to be aware of DoH semantics, it does not follow that all upstream caches (for example, inline proxies, server-side gateways, and content delivery networks) will be.
As a result, DoH servers need to carefully consider the HTTP caching metadata they send in response to GET requests (responses to POST requests are not cacheable unless specific response header fields are sent; this is not widely implemented and is not advised for DoH).
In particular, DoH servers SHOULD assign an explicit HTTP freshness lifetime (see Section 4.2 of [RFC7234]) so that the DoH client is more likely to use fresh DNS data. This requirement is due to HTTP caches being able to assign their own heuristic freshness (such as that described in Section 4.2.2 of [RFC7234]), which would take control of the cache contents out of the hands of the DoH server.
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The assigned freshness lifetime of a DoH HTTP response MUST be less than or equal to the smallest TTL in the Answer section of the DNS response. A freshness lifetime equal to the smallest TTL in the Answer section is RECOMMENDED. For example, if a HTTP response carries three RRsets with TTLs of 30, 600, and 300, the HTTP freshness lifetime should be 30 seconds (which could be specified as "Cache-Control: max-age=30"). This requirement helps prevent expired RRsets in messages in an HTTP cache from unintentionally being served.
If the DNS response has no records in the Answer section, and the DNS response has an SOA record in the Authority section, the response freshness lifetime MUST NOT be greater than the MINIMUM field from that SOA record (see [RFC2308]).
The stale-while-revalidate and stale-if-error Cache-Control directives [RFC5861] could be well suited to a DoH implementation when allowed by server policy. Those mechanisms allow a client, at the server's discretion, to reuse an HTTP cache entry that is no longer fresh. In such a case, the client reuses either all of a cached entry or none of it.
DoH servers also need to consider HTTP caching when generating responses that are not globally valid. For instance, if a DoH server customizes a response based on the client's identity, it would not want to allow global reuse of that response. This could be accomplished through a variety of HTTP techniques, such as a Cache- Control max-age of 0, or by using the Vary response header field (see Section 7.1.4 of [RFC7231]) to establish a secondary cache key (see Section 4.1 of [RFC7234]).
DoH clients MUST account for the Age response header field's value [RFC7234] when calculating the DNS TTL of a response. For example, if an RRset is received with a DNS TTL of 600, but the Age header field indicates that the response has been cached for 250 seconds, the remaining lifetime of the RRset is 350 seconds. This requirement applies to both DoH client HTTP caches and DoH client DNS caches.
DoH clients can request an uncached copy of a HTTP response by using the "no-cache" request Cache-Control directive (see Section 5.2.1.4 of [RFC7234]) and similar controls. Note that some caches might not honor these directives, either due to configuration or interaction with traditional DNS caches that do not have such a mechanism.
HTTP conditional requests [RFC7232] may be of limited value to DoH, as revalidation provides only a bandwidth benefit and DNS transactions are normally latency bound. Furthermore, the HTTP response header fields that enable revalidation (such as "Last-
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Modified" and "Etag") are often fairly large when compared to the overall DNS response size and have a variable nature that creates constant pressure on the HTTP/2 compression dictionary [RFC7541]. Other types of DNS data, such as zone transfers, may be larger and benefit more from revalidation.
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.
Before using DoH response data for DNS resolution, the client MUST establish that the HTTP request URI can be used for the DoH query. For HTTP requests initiated by the DoH client, this is implicit in the selection of URI. For HTTP server push (see Section 8.2 of [RFC7540]), extra care must be taken to ensure that the pushed URI is one that the client would have directed the same query to if the client had initiated the request (in addition to the other security checks normally needed for server push).
In order to maximize interoperability, DoH clients and DoH servers MUST support the "application/dns-message" media type. Other media types MAY be used as defined by HTTP Content Negotiation (see Section 3.4 of [RFC7231]). Those media types MUST be flexible enough to express every DNS query that would normally be sent in DNS over UDP (including queries and responses that use DNS extensions, but not those that require multiple responses).
6. Definition of the "application/dns-message" Media Type
The data payload for the "application/dns-message" media type is a single message of the DNS on-the-wire format defined in Section 4.2.1 of [RFC1035], which in turn refers to the full wire format defined in Section 4.1 of that RFC.
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Although [RFC1035] says "Messages carried by UDP are restricted to 512 bytes", that was later updated by [RFC6891]. This media type restricts the maximum size of the DNS message to 65535 bytes.
Note that the wire format used in this media type is different than the wire format used in [RFC7858] (which uses the format defined in Section 4.2.2 of [RFC1035] that includes two length bytes).
DoH clients using this media type MAY have one or more Extension Mechanisms for DNS (EDNS) options [RFC6891] in the request. DoH servers using this media type MUST ignore the value given for the EDNS UDP payload size in DNS requests.
When using the GET method, the data payload for this media type MUST be encoded with base64url [RFC4648] and then provided as a variable named "dns" to the URI Template expansion. Padding characters for base64url MUST NOT be included.
When using the POST method, the data payload for this media type MUST NOT be encoded and is used directly as the HTTP message body.
7.1. Registration of the "application/dns-message" Media Type
Type name: application
Subtype name: dns-message
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: This is a binary format. The contents are a DNS message as defined in RFC 1035. The format used here is for DNS over UDP, which is the format defined in the diagrams in RFC 1035.
Security considerations: See RFC 8484. The content is a DNS message and thus not executable code.
[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.
DoH encrypts DNS traffic and requires authentication of the server. This mitigates both passive surveillance [RFC7258] and active attacks that attempt to divert DNS traffic to rogue servers (see Section 2.5.1 of [RFC7626]). DNS over TLS [RFC7858] provides similar protections, while direct UDP- and TCP-based transports are vulnerable to this class of attack. An experimental effort to offer guidance on choosing the padding length can be found in [RFC8467].
Additionally, the use of the HTTPS default port 443 and the ability to mix DoH traffic with other HTTPS traffic on the same connection can deter unprivileged on-path devices from interfering with DNS operations and make DNS traffic analysis more difficult.
The DNS wire format [RFC1035] contains no client identifiers; however, various transports of DNS queries and responses do provide data that can be used to correlate requests. HTTPS presents new considerations for correlation, such as explicit HTTP cookies and implicit fingerprinting of the unique set and ordering of HTTP request header fields.
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A DoH implementation is built on IP, TCP, TLS, and HTTP. Each layer contains one or more common features that can be used to correlate queries to the same identity. DNS transports will generally carry the same privacy properties of the layers used to implement them. For example, the properties of IP, TCP, and TLS apply to implementations of DNS over TLS.
The privacy considerations of using the HTTPS layer in DoH are incremental to those of DNS over TLS. DoH is not known to introduce new concerns beyond those associated with HTTPS.
At the IP level, the client address provides obvious correlation information. This can be mitigated by use of a NAT, proxy, VPN, or simple address rotation over time. It may be aggravated by use of a DNS server that can correlate real-time addressing information with other personal identifiers, such as when a DNS server and DHCP server are operated by the same entity.
DNS implementations that use one TCP connection for multiple DNS requests directly group those requests. Long-lived connections have better performance behaviors than short-lived connections; however, they group more requests, which can expose more information to correlation and consolidation. TCP-based solutions may also seek performance through the use of TCP Fast Open [RFC7413]. The cookies used in TCP Fast Open allow servers to correlate TCP sessions.
TLS-based implementations often achieve better handshake performance through the use of some form of session resumption mechanism, such as Section 2.2 of [RFC8446]. Session resumption creates trivial mechanisms for a server to correlate TLS connections together.
HTTP's feature set can also be used for identification and tracking in a number of different ways. For example, Authentication request header fields explicitly identify profiles in use, and HTTP cookies are designed as an explicit state-tracking mechanism between the client and serving site and often are used as an authentication mechanism.
Additionally, the User-Agent and Accept-Language request header fields often convey specific information about the client version or locale. This facilitates content negotiation and operational work- arounds for implementation bugs. Request header fields that control caching can expose state information about a subset of the client's history. Mixing DoH requests with other HTTP requests on the same connection also provides an opportunity for richer data correlation.
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The DoH protocol design allows applications to fully leverage the HTTP ecosystem, including features that are not enumerated here. Utilizing the full set of HTTP features enables DoH to be more than an HTTP tunnel, but it is at the cost of opening up implementations to the full set of privacy considerations of HTTP.
Implementations of DoH clients and servers need to consider the benefit and privacy impact of these features, and their deployment context, when deciding whether or not to enable them. Implementations are advised to expose the minimal set of data needed to achieve the desired feature set.
Determining whether or not a DoH implementation requires HTTP cookie [RFC6265] support is particularly important because HTTP cookies are the primary state tracking mechanism in HTTP. HTTP cookies SHOULD NOT be accepted by DOH clients unless they are explicitly required by a use case.
Running DNS over HTTPS relies on the security of the underlying HTTP transport. This mitigates classic amplification attacks for UDP- based DNS. Implementations utilizing HTTP/2 benefit from the TLS profile defined in Section 9.2 of [RFC7540].
Session-level encryption has well-known weaknesses with respect to traffic analysis, which might be particularly acute when dealing with DNS queries. HTTP/2 provides further advice about the use of compression (see Section 10.6 of [RFC7540]) and padding (see Section 10.7 of [RFC7540]). DoH servers can also add DNS padding [RFC7830] if the DoH client requests it in the DNS query. An experimental effort to offer guidance on choosing the padding length can be found in [RFC8467].
The HTTPS connection provides transport security for the interaction between the DoH server and client, but it does not provide the response integrity of DNS data provided by DNSSEC. DNSSEC and DoH are independent and fully compatible protocols, each solving different problems. The use of one does not diminish the need nor the usefulness of the other. It is the choice of a client to either perform full DNSSEC validation of answers or to trust the DoH server to do DNSSEC validation and inspect the AD (Authentic Data) bit in the returned message to determine whether an answer was authentic or not. As noted in Section 4.2, different response media types will provide more or less information from a DNS response, so this choice may be affected by the response media type.
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Section 5.1 describes the interaction of this protocol with HTTP caching. An adversary that can control the cache used by the client can affect that client's view of the DNS. This is no different than the security implications of HTTP caching for other protocols that use HTTP.
In the absence of DNSSEC information, a DoH server can give a client invalid data in response to a DNS query. Section 3 disallows the use of DoH DNS responses that do not originate from configured servers. This prohibition does not guarantee protection against invalid data, but it does reduce the risk.
Local policy considerations and similar factors mean different DNS servers may provide different results to the same query, for instance, in split DNS configurations [RFC6950]. It logically follows that the server that is queried can influence the end result. Therefore, a client's choice of DNS server may affect the responses it gets to its queries. For example, in the case of DNS64 [RFC6147], the choice could affect whether IPv6/IPv4 translation will work at all.
The HTTPS channel used by this specification establishes secure two- party communication between the DoH client and the DoH server. Filtering or inspection systems that rely on unsecured transport of DNS will not function in a DNS over HTTPS environment due to the confidentiality and integrity protection provided by TLS.
Some HTTPS client implementations perform real time third-party checks of the revocation status of the certificates being used by TLS. If this check is done as part of the DoH server connection procedure and the check itself requires DNS resolution to connect to the third party, a deadlock can occur. The use of Online Certificate Status Protocol (OCSP) [RFC6960] servers or Authority Information Access (AIA) for Certificate Revocation List (CRL) fetching (see Section 4.2.2.1 of [RFC5280]) are examples of how this deadlock can happen. To mitigate the possibility of deadlock, the authentication given DoH servers SHOULD NOT rely on DNS-based references to external resources in the TLS handshake. For OCSP, the server can bundle the certificate status as part of the handshake using a mechanism appropriate to the version of TLS, such as using Section 4.4.2.1 of [RFC8446] for TLS version 1.3. AIA deadlocks can be avoided by providing intermediate certificates that might otherwise be obtained through additional requests. Note that these deadlocks also need to be considered for servers that a DoH server might redirect to.
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A DoH client may face a similar bootstrapping problem when the HTTP request needs to resolve the hostname portion of the DNS URI. Just as the address of a traditional DNS nameserver cannot be originally determined from that same server, a DoH client cannot use its DoH server to initially resolve the server's host name into an address. Alternative strategies a client might employ include 1) making the initial resolution part of the configuration, 2) IP-based URIs and corresponding IP-based certificates for HTTPS, or 3) resolving the DNS API server's hostname via traditional DNS or another DoH server while still authenticating the resulting connection via HTTPS.
HTTP [RFC7230] is a stateless application-level protocol, and therefore DoH implementations do not provide stateful ordering guarantees between different requests. DoH cannot be used as a transport for other protocols that require strict ordering.
A DoH server is allowed to answer queries with any valid DNS response. For example, a valid DNS response might have the TC (truncation) bit set in the DNS header to indicate that the server was not able to retrieve a full answer for the query but is providing the best answer it could get. A DoH server can reply to queries with an HTTP error for queries that it cannot fulfill. In this same example, a DoH server could use an HTTP error instead of a non-error response that has the TC bit set.
Many extensions to DNS, using [RFC6891], have been defined over the years. Extensions that are specific to the choice of transport, such as [RFC7828], are not applicable to DoH.
[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>.
[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>.
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>.
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.
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.