| Network Working Group | P. Hoffman |
| Internet-Draft | ICANN |
| Intended status: Standards Track | P. McManus |
| Expires: December 3, 2018 | Mozilla |
| June 01, 2018 |
DNS Queries over HTTPS (DOH)
draft-ietf-doh-dns-over-https-10
This document describes how to make DNS queries over HTTPS.
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 3, 2018.
Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
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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.
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.
A server that supports this protocol is called a “DNS API 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 “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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
[[ RFC Editor: Please remove this entire section before publication. ]]
The protocol described here bases its design on the following protocol requirements:
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). DNS API Servers MAY support more than one URI. This allows the different endpoints to have different properties such as different authentication requirements or service level guarantees.
A DNS API client uses configuration to select the URI, and thus the DNS API server, that is to be used for resolution. [RFC2818] defines how HTTPS verifies the DNS API server’s identity.
A DNS API 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 DNS API 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 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.
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.
DNS API servers MUST implement both the POST and GET methods.
When using the POST method the DNS query is included as the message body of the HTTP request and the Content-Type request header indicates the media type of the message. POST-ed requests are smaller than their GET equivalents.
Using the GET method is friendlier to many HTTP cache implementations.
The DNS API client SHOULD include an HTTP “Accept” request header to indicate what type of content can be understood in response. Irrespective of the value of the Accept request header, the client MUST be prepared to process “application/dns-message” (as described in Section 7) responses but MAY also process any other type it receives.
In order to maximize cache friendliness, DNS API clients using media formats that include DNS ID, such as application/dns-message, SHOULD use a DNS ID of 0 in every DNS request. HTTP correlates the request and response, thus eliminating the need for the ID in a media type such as application/dns-message. The use of a varying DNS ID can cause semantically equivalent DNS queries to be cached separately.
DNS API clients can use HTTP/2 padding and compression in the same way that other HTTP/2 clients use (or don’t use) them.
These examples use HTTP/2 style formatting from [RFC7540].
These examples use a DNS API service with a URI Template of “https://dnsserver.example.net/dns-query{?dns}” to resolve IN A records.
The requests are represented as application/dns-message typed bodies.
The first example request uses GET to request www.example.com
:method = GET :scheme = https :authority = dnsserver.example.net :path = /dns-query?dns=AAABAAABAAAAAAAAA3d3dwdleGFtcGxlA2NvbQAAAQAB accept = application/dns-message
The same DNS query for www.example.com, using the POST method would be:
:method = POST :scheme = https :authority = dnsserver.example.net :path = /dns-query accept = application/dns-message content-type = application/dns-message content-length = 33 <33 bytes represented by the following hex encoding> 00 00 01 00 00 01 00 00 00 00 00 00 03 77 77 77 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 01
Finally, a GET based query for a.62characterlabel-makes-base64url-distinct-from-standard-base64.example.com is shown as an example to emphasize that the encoding alphabet of base64url is different than regular base64 and that padding is omitted.
The DNS query is 94 bytes represented by the following hex encoding
00 00 01 00 00 01 00 00 00 00 00 00 01 61 3e 36
32 63 68 61 72 61 63 74 65 72 6c 61 62 65 6c 2d
6d 61 6b 65 73 2d 62 61 73 65 36 34 75 72 6c 2d
64 69 73 74 69 6e 63 74 2d 66 72 6f 6d 2d 73 74
61 6e 64 61 72 64 2d 62 61 73 65 36 34 07 65 78
61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 01
:method = GET
:scheme = https
:authority = dnsserver.example.net
:path = /dns-query? (no space or CR)
dns=AAABAAABAAAAAAAAAWE-NjJjaGFyYWN0ZXJsYWJl (no space or CR)
bC1tYWtlcy1iYXNlNjR1cmwtZGlzdGluY3QtZnJvbS1z (no space or CR)
dGFuZGFyZC1iYXNlNjQHZXhhbXBsZQNjb20AAAEAAQ
accept = application/dns-message
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 mean that clients need to make new requests to satisfy the original question.
Different response media types will provide more or less information from a DNS response. For example, one response type might include the information from the DNS header bytes while another might omit it. The amount and type of information that a media type gives is solely up to the format, and not defined in this protocol.
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.
The DNS response for “application/dns-message” in Section 7 MAY have one or more EDNS options [RFC6891], depending on the extension definition of the extensions given in the DNS request.
Each DNS request-response pair is matched to one HTTP exchange. The responses may be processed and transported in any order using HTTP’s multi-streaming functionality ([RFC7540] Section 5).
Section 6.1 discusses the relationship between DNS and HTTP response caching.
A DNS API server MUST be able to process application/dns-message request messages.
A DNS API server SHOULD respond with HTTP status code 415 (Unsupported Media Type) upon receiving a media type it is unable to process.
This is an example response for a query for the IN A records for “www.example.com” with recursion turned on. The response bears one record with an address of 192.0.2.1 and a TTL of 128 seconds.
:status = 200 content-type = application/dns-message content-length = 64 cache-control = max-age=128 <64 bytes represented by the following hex encoding> 00 00 81 80 00 01 00 01 00 00 00 00 03 77 77 77 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 01 03 77 77 77 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 01 00 00 00 80 00 04 C0 00 02 01
This protocol MUST be used with the https scheme URI [RFC7230].
A DOH exchange can pass through a hierarchy of caches that include both HTTP and DNS specific caches. These caches may exist beteen the DNS API server and client, or on the DNS API client itself. HTTP caches are by design generic; that is, they do not understand this protocol. Even if a DNS API client has modified its cache implementation to be aware of DOH semantics, it does not follow that all upstream caches (for example, inline proxies, server-side gateways and Content Delivery Networks) will be.
As a result, DNS API 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, DNS API servers SHOULD assign an explicit freshness lifetime ([RFC7234] Section 4.2) so that the DNS API 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 DNS API 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 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.
DNS API servers also need to consider caching when generating responses that are not globally valid. For instance, if a DNS API 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).
DNS API 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.
DNS API 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.
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 may be used for the DOH query. For HTTP requests initiated by the DNS API 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.
In order to maximize interoperability, DNS API clients and DNS API servers MUST support the “application/dns-message” media type. Other media types MAY be used as defined by HTTP Content Negotiation ([RFC7231] Section 3.4). 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).
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]. This protocol allows DNS on-the-wire format payloads of any size.
When using the GET method, the data payload MUST be encoded with base64url [RFC4648] and then provided as a variable named “dns” to the URI Template expansion. Padding characters for base64url MUST NOT be included.
When using the POST method, the data payload MUST NOT be encoded and is used directly as the HTTP message body.
DNS API clients using the DNS wire format MAY have one or more EDNS options [RFC6891] in the request.
The media type is “application/dns-message”.
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
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 ). DNS API Servers can also add DNS padding [RFC7830] if the DNS API requests it in the DNS query.
The HTTPS connection provides transport security for the interaction between the DNS API server and client, but does not 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 DNS API 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 DNS API 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.
Local policy considerations and similar factors mean different DNS servers may provide different results to the same query: for instance in split DNS configurations [RFC6950]. It logically follows that the server which is queried can influence the end result. Therefore a client’s choice of DNS server may affect the responses it gets to its queries. For example, in the case of DNS64 [RFC6147], the choice could affect whether IPv6/IPv4 translation will work at all.
The HTTPS channel used by this specification establishes secure two party communication between the DNS API client and the DNS API server. Filtering or inspection systems that rely on unsecured transport of DNS will not function in a DNS over HTTPS environment.
Some HTTPS client implementations perform real time third party checks of the revocation status of the certificates being used by TLS. If this check is done as part of the DNS API server connection procedure and the check itself requires DNS resolution to connect to the third party a deadlock can occur. The use of OCSP [RFC6960] servers or AIA for CRL fetching ([RFC5280] Section 4.2.2.1) are examples of how this deadlock can happen. To mitigate the possibility of deadlock, DNS API servers SHOULD NOT rely on DNS based references to external resources in the TLS handshake. For OCSP the server can bundle the certificate status as part of the handshake using a mechanism appropriate to the version of TLS, such as using [RFC6066] Section 8 for TLS version 1.2. AIA deadlocks can be avoided by providing intermediate certificates that might otherwise be obtained through additional requests. Note that these deadlocks also need to be considered for server that a DNS API server might redirect to.
A DNS API client may face a similar bootstrapping problem when the HTTP request needs to resolve the hostname portion of the DNS URI. Just as the address of a traditional DNS nameserver cannot be originally determined from that same server, a DNS API client cannot use its DNS API server to initially resolve the server’s host name into an address. Alternative strategies a client might employ include making the initial resolution part of the configuration, IP based URIs and corresponding IP based certificates for HTTPS, or resolving the DNS API server’s hostname via traditional DNS or another DNS API server while still authenticating the resulting connection via HTTPS.
HTTP [RFC7230] is a stateless application level protocol and therefore DOH implementations do not provide stateful ordering guarantees between different requests. DOH cannot be used as a transport for other protocols that require strict ordering.
A DNS API 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 DNS API server can reply to queries with an HTTP error for queries that it cannot fulfill. In this same example, a DNS API 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.
This work required a high level of cooperation between experts in different technologies. Thank you Ray Bellis, Stephane Bortzmeyer, Manu Bretelle, Sara Dickinson, Tony Finch, Daniel Kahn Gilmor, Olafur Guomundsson, Wes Hardaker, Rory Hewitt, Joe Hildebrand, David Lawrence, Eliot Lear, John Mattson, Alex Mayrhofer, Mark Nottingham, Jim Reid, Adam Roach, Ben Schwartz, Davey Song, Daniel Stenberg, Andrew Sullivan, Martin Thomson, and Sam Weiler.
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.