| Network Working Group | P. Hoffman |
| Internet-Draft | ICANN |
| Intended status: Standards Track | P. McManus |
| Expires: October 11, 2018 | Mozilla |
| April 09, 2018 |
DNS Queries over HTTPS
draft-ietf-doh-dns-over-https-06
This document describes how to run DNS service over HTTP (DOH) using https:// URIs.
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/.
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This Internet-Draft will expire on October 11, 2018.
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This document defines a specific protocol for sending DNS [RFC1035] queries and getting DNS responses over HTTP [RFC7540] using https:// (and therefore TLS [RFC5246] security for integrity and confidentiality). Each DNS query-response pair is mapped into a HTTP exchange.
The described approach is more than a tunnel over HTTP. It establishes default media formatting types for requests and responses but uses normal HTTP content negotiation mechanisms for selecting alternatives that endpoints may prefer in anticipation of serving new use cases. In addition to this media type negotiation, it aligns itself with HTTP features such as caching, redirection, proxying, authentication, and compression.
The integration with HTTP provides a transport suitable for both traditional DNS clients and native web applications seeking access to the DNS.
Two primary uses cases were considered during this protocol’s development. They included preventing on-path devices from interfering with DNS operations and allowing web applications to access DNS information via existing browser APIs in a safe way consistent with Cross Origin Resource Sharing (CORS) [CORS]. There are certainly other uses for this work.
A server that supports this protocol on one or more URIs is called a “DNS API server” to differentiate it from a “DNS server” (one that uses the regular DNS protocol). Similarly, a client that supports this protocol is called a “DNS API client”.
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14, RFC8174 [RFC8174] when, and only when, they appear in all capitals, as shown here.
The protocol described here bases its design on the following protocol requirements:
A DNS API client encodes a single DNS query into an HTTP request using either the HTTP GET or POST method and the other requirements of this section. The DNS API server defines the URI used by the request through the use of a URI Template [RFC6570]. Configuration and discovery of the URI Template is done out of band from this protocol.
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 MUST define the variables used for URI Template processing with this protocol.
DNS API servers MUST implement both the POST and GET methods.
When using the POST method the DNS query is included as the message body of the HTTP request and the Content-Type request header indicates the media type of the message. POST-ed requests are smaller than their GET equivalents.
Using the GET method is friendlier to many HTTP cache implementations.
The DNS API client SHOULD include an HTTP “Accept” request header to indicate what type of content can be understood in response. Irrespective of the value of the Accept request header, the client MUST be prepared to process “message/dns” (as described in Section 6) responses but MAY also process any other type it receives.
In order to maximize cache friendliness, DNS API clients using media formats that include DNS ID, such as message/dns, 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 message/dns. The use of a varying DNS ID can cause semantically equivalent DNS queries to be cached separately.
DNS API clients can use HTTP/2 padding and compression in the same way that other HTTP/2 clients use (or don’t use) them.
These examples use HTTP/2 style formatting from [RFC7540].
These examples use a DNS API service with a URI Template of “https://dnsserver.example.net/dns-query{?dns}” to resolve IN A records.
The requests are represented as message/dns typed bodies.
The first example request uses GET to request www.example.com
:method = GET :scheme = https :authority = dnsserver.example.net :path = /dns-query?dns=AAABAAABAAAAAAAAA3d3dwdleGFtcGxlA2NvbQAAAQAB accept = message/dns
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 = message/dns content-type = message/dns 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 = message/dns
An HTTP response with a 2xx status code ([RFC7231] Section 6.3) indicates a valid DNS response to the query made in the HTTP request. A valid DNS response includes both success and failure responses. For example, a DNS failure response such as SERVFAIL or NXDOMAIN will be the message in a successful 2xx HTTP response even though there was a failure at the DNS layer. Responses with non-successful HTTP status codes do not contain DNS answers to the question in the corresponding request. Some of these non-successful HTTP responses (e.g., redirects or authentication failures) could allow clients to make new requests to satisfy the original question.
Different response media types will provide more or less information from a DNS response. For example, one response type might include the information from the DNS header bytes while another might omit it. The amount and type of information that a media type gives is solely up to the format, and not defined in this protocol.
At the time this is published, the response types are works in progress. The only response type defined in this document is “message/dns”, but it is possible that other response formats will be defined in the future.
The DNS response for “message/dns” in Section 6 MAY have one or more EDNS options, depending on the extension definition of the extensions given in the DNS request.
Each DNS request-response pair is matched to one HTTP exchange. The responses may be processed and transported in any order using HTTP’s multi-streaming functionality ([RFC7540] Section 5).
Section 5.1 discusses the relationship between DNS and HTTP response caching.
A DNS API server MUST be able to process message/dns request messages.
A DNS API server SHOULD respond with HTTP status code 415 (Unsupported Media Type) upon receiving a media type it is unable to process.
This is an example response for a query for the IN A records for “www.example.com” with recursion turned on. The response bears one record with an address of 192.0.2.1 and a TTL of 128 seconds.
:status = 200 content-type = message/dns 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
This protocol MUST be used with the https scheme URI [RFC7230].
A DNS API client may utilize a hierarchy of caches that include both HTTP and DNS specific caches. HTTP cache entries may be bypassed with HTTP mechanisms such as the “Cache-Control no-cache” directive; however DNS caches do not have a similar mechanism.
The Answer section of a DNS response can contain zero or more RRsets. (RRsets are defined in [RFC7719].) According to [RFC2181], each resource record in an RRset has Time To Live (TTL) freshness information. Different RRsets in the Answer section can have different TTLs, although it is only possible for the HTTP response to have a single freshness lifetime. The HTTP response freshness lifetime ([RFC7234] Section 4.2) should be coordinated with the RRset with the smallest TTL in the Answer section of the response. Specifically, the HTTP freshness lifetime SHOULD be set to expire at the same time any of the DNS resource records in the Answer section reach a 0 TTL. The response freshness lifetime MUST NOT be greater than that indicated by the DNS resoruce record with the smallest TTL in the response.
If the DNS response has no records in the Answer section, and the DNS response has an SOA record in the Authority section, the response freshness lifetime MUST NOT be greater than the MINIMUM field from that SOA record. (See [RFC2308].) Otherwise, the HTTP response MUST set a freshness lifetime ([RFC7234] Section 4.2) of 0 by using a mechanism such as “Cache-Control: no-cache” ([RFC7234] Section 5.2.1.4).
A DNS API client that receives a response without an explicit freshness lifetime MUST NOT assign that response a heuristic freshness ([RFC7234] Section 4.2.2.) greater than that indicated by the DNS Record with the smallest TTL in the response.
A DOH response that was previously stored in an HTTP cache will contain the [RFC7234] Age response header indicating the elapsed time between when the entry was placed in the HTTP cache and the current DOH response. DNS API clients should subtract this time from the DNS TTL if they are re-sharing the information in a non HTTP context (e.g., their own DNS cache) to determine the remaining time to live of the DNS record.
HTTP revalidation (e.g., via If-None-Match request headers) of cached DNS information 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. DNS API servers may wish to consider whether providing these validation enabling response headers is worthwhile.
The stale-while-revalidate and stale-if-error cache control directives may be well suited to a DOH implementation when allowed by server policy. Those mechanisms allow a client, at the server’s discretion, to reuse a cache entry that is no longer fresh under some extenuating circumstances defined in [RFC5861].
All HTTP servers, including DNS API servers, need to consider cache interaction when they generate responses that are not globally valid. For instance, if a DNS API server customized a response based on the client’s identity then it would not want to globally allow reuse of that response. This could be accomplished through a variety of HTTP techniques such as a Cache-Control max-age of 0, or perhaps by the Vary response header.
The minimum version of HTTP used by DOH SHOULD be HTTP/2 [RFC7540].
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 is a trusted service for the DOH query. For HTTP requests initiated by the DNS API client this trust is implicit in the selection of URI. For HTTP server push ([RFC7540] Section 8.2) extra care must be taken to ensure that the pushed URI is one that the client would have directed the same query to if the client had initiated the request. This specification does not extend DNS resolution privileges to URIs that are not recognized by the client as trusted DNS API servers.
In order to maximize interoperability, DNS API clients and DNS API servers MUST support the “message/dns” media type. Other media types MAY be used as defined by HTTP Content Negotiation ([RFC7231] Section 3.4).
The data payload is the DNS on-the-wire format defined in [RFC1035]. The format is for DNS over UDP. Note that this is different than the wire format used in [RFC7858]. Also note that while [RFC1035] says “Messages carried by UDP are restricted to 512 bytes”, that was later updated by [RFC6891], and this protocol allows DNS on-the-wire format payloads of any size.
When using the GET method, the data payload MUST be encoded with base64url [RFC4648] and then provided as a variable named “dns” to the URI Template expansion. Padding characters for base64url MUST NOT be included.
When using the POST method, the data payload MUST NOT be encoded and is used directly as the HTTP message body.
DNS API clients using the DNS wire format MAY have one or more EDNS options [RFC6891] in the request.
The media type is “message/dns”.
To: ietf-types@iana.org
Subject: Registration of MIME media type
message/dns
MIME media type name: message
MIME subtype name: dns
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
Running DNS over HTTPS relies on the security of the underlying HTTP transport. This mitigates classic amplification attacks for UDP-based DNS. Implementations utilizing HTTP/2 benefit from the TLS profile defined in [RFC7540] Section 9.2.
Session level encryption has well known weaknesses with respect to traffic analysis which might be particularly acute when dealing with DNS queries. HTTP/2 provides further advice about the use of compression (Section 10.6 of [RFC7540]) and padding (Section 10.7 of [RFC7540]).
The HTTPS connection provides transport security for the interaction between the DNS API server and client, but does not inherently ensure the authenticity of DNS data. A DNS API client may also perform full DNSSEC validation of answers received from a DNS API server or it may choose to trust answers from a particular DNS API server, much as a DNS client might choose to trust answers from its recursive DNS resolver. This capability might be affected by the response media type.
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.
A server that is acting both as a normal web server and a DNS API server is in a position to choose which DNS names it forces a client to resolve (through its web service) and also be the one to answer those queries (through its DNS API service). An untrusted DNS API server can thus easily cause damage by poisoning a client’s cache with names that the DNS API server chooses to poison. A client MUST NOT trust a DNS API server simply because it was discovered, or because the client was told to trust the DNS API server by an untrusted party. Instead, a client MUST only trust DNS API server that is configured as trustworthy.
A client can use DNS over HTTPS as one of multiple mechanisms to obtain DNS data. If a client of this protocol encounters an HTTP error after sending a DNS query, and then falls back to a different DNS retrieval mechanism, doing so can weaken the privacy and authenticity expected by the user of the client.
Local policy considerations and similar factors mean different DNS servers may provide different results to the same query: for instance in split DNS configurations [RFC6950]. It logically follows that the server which is queried can influence the end result. Therefore a client’s choice of DNS server may affect the responses it gets to its queries. For example, in the case of DNS64 [RFC6147], the choice could affect whether IPv6/IPv4 translation will work at all.
The HTTPS channel used by this specification establishes secure two party communication between the DNS API client and the DNS API server. Filtering or inspection systems that rely on unsecured transport of DNS will not function in a DNS over HTTPS environment.
Some HTTPS client implementations perform real time third party checks of the revocation status of the certificates being used by TLS. If this check is done as part of the DNS API server connection procedure and the check itself requires DNS resolution to connect to the third party a deadlock can occur. The use of OCSP [RFC6960] servers or AIA for CRL fetching ([RFC5280] Section 4.2.2.1) are examples of how this deadlock can happen. To mitigate the possibility of deadlock, DNS API servers SHOULD NOT rely on DNS based references to external resources in the TLS handshake. For OCSP the server can bundle the certificate status as part of the handshake using a mechanism appropriate to the version of TLS, such as using [RFC6066] Section 8 for TLS version 1.2. AIA deadlocks can be avoided by providing intermediate certificates that might otherwise be obtained through additional requests.
A DNS API client may face a similar bootstrapping problem when the HTTP request needs to resolve the hostname portion of the DNS URI. Just as the address of a traditional DNS nameserver cannot be originally determined from that same server, a DNS API client cannot use its DNS API server to initially resolve the server’s host name into an address. Alternative strategies a client might employ include making the initial resolution part of the configuration, IP based URIs and corresponding IP based certificates for HTTPS, or resolving the DNS API server’s hostname via traditional DNS or another DNS API server while still authenticating the resulting connection via HTTPS.
HTTP [RFC7230] is a stateless application level protocol and therefore DOH implementations do not provide stateful ordering guarantees between different requests. DOH cannot be used as a transport for other protocols that require strict ordering.
If a DNS API server responds to a DNS API client with a DNS message that has the TC (truncation) bit set in the header, that indicates that the DNS API server was not able to retrieve a full answer for the query and is providing the best answer it could get. This protocol does not require that a DNS API server that cannot get an untruncated answer send back such an answer; it can instead send back an HTTP error to indicate that it cannot give a useful answer.
This work required a high level of cooperation between experts in different technologies. Thank you Ray Bellis, Stephane Bortzmeyer, Manu Bretelle, Tony Finch, Daniel Kahn Gilmor, Olafur Guomundsson, Wes Hardaker, Rory Hewitt, Joe Hildebrand, David Lawrence, Eliot Lear, John Mattson, Alex Mayrhofer, Mark Nottingham, Jim Reid, Adam Roach, Ben Schwartz, Davey Song, Daniel Stenberg, Andrew Sullivan, Martin Thomson, and Sam Weiler.
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.