DNS-over-HTTPS (DoH)

Google Public DNS provides two distinct DoH APIs at these endpoints:

  • https://dns.google/dns-query – RFC 8484 (GET and POST)
  • https://dns.google/resolve? – JSON API (GET)

The Secure Transports Overview page has curl command line examples for using both APIs as well as details of TLS and other features common to both DNS over TLS (DoT) and DoH.

Google Public DNS does not support insecure http: URLs for API calls.

HTTP methods

GET
Using the GET method can reduce latency, as it is cached more effectively. RFC 8484 GET requests must have a ?dns= query parameter with a Base64Url encoded DNS message. The GET method is the only method supported for the JSON API.
POST
The POST method is only supported for the RFC 8484 API and uses a binary DNS message with Content-Type application/dns-message in the request body and in the DoH HTTP response.
HEAD
HEAD is not currently supported, and returns a 400 Bad Request error.

Other methods return 501 Not Implemented errors.

HTTP status codes

Google Public DNS DoH returns the following HTTP status codes:

Success

200 OK
HTTP parsing and communication with DNS resolver was successful, and the response body content is a DNS response in either binary or JSON encoding, depending on the query endpoint, Accept header and GET parameters.

Redirections

301 Moved Permanently
Clients should retry at the URL provided in the Location: header. If the original query was a POST request, clients should only retry with GET if the new URL specifies a dns GET parameter argument; otherwise clients should retry with POST.

Other codes such as 302 Found, 307 Temporary Redirect or 308 Permanent Redirect may be used in the future, and DoH clients should handle all four codes.

Responses with the permanent 301 and 308 codes should be cached indefinitely, and if practical, users may be prompted to update their original configuration using the new URL.

POST requests that get 307 or 308 responses should always be retried with POST.

Errors

Error responses will have an explanation of the HTTP status in the body, using either HTML or plain text.

400 Bad Request
Problems parsing the GET parameters, or an invalid DNS request message. For bad GET parameters, the HTTP body should explain the error. Most invalid DNS messages get a 200 OK with a FORMERR; the HTTP error is returned for garbled messages with no Question section, a QR flag indicating a reply, or other nonsensical flag combinations with binary DNS parse errors.
413 Payload Too Large
An RFC 8484 POST request body exceeded the 512 byte maximum message size.
414 URI Too Long
The GET query header was too large or the dns parameter had a Base64Url encoded DNS message exceeding the 512 byte maximum message size.
415 Unsupported Media Type
The POST body did not have an application/dns-message Content-Type header.
429 Too Many Requests
The client has sent too many requests in a given amount of time. Clients should stop sending requests until the time specified in the Retry-After header (a relative time in seconds).
500 Internal Server Error
Google Public DNS internal DoH errors.
501 Not Implemented
Only GET and POST methods are implemented, other methods get this error.
502 Bad Gateway
The DoH service could not contact Google Public DNS resolvers.

In the case of a 502 response, although retrying on an alternate Google Public DNS address might help, a more effective fallback response would be to try another DoH service, or to switch to traditional UDP or TCP DNS at 8.8.8.8.

Benefits of DoH

Using HTTPS, not just TLS encryption, has some practical benefits:

  • Widely available and well-supported HTTPS APIs simplify implementation for both Google Public DNS itself and potential clients.
  • An HTTPS service provides web apps with access to all DNS record types, avoiding the limitations of existing browser and OS DNS APIs, which generally support only host-to-address lookups.
  • Clients that implement QUIC UDP-based HTTPS support can avoid problems like head-of-line blocking that can occur when using TCP transport.

Privacy Best Practices for DoH

Developers of DoH applications should consider the privacy best practices outlined in RFC 8484 and expanded below:

  • Limit use of HTTP Headers

    HTTP headers reveal information about the client's DoH implementation and can be used to deanonymize clients. Headers like Cookie, User-Agent, and Accept-Language are the worst offenders, but even the set of headers sent can be revealing. To minimize this risk, send only the HTTP headers required for DoH: Host, Content-Type (for POST), and if necessary, Accept. User-Agent should be included in any development or testing versions.

  • Use EDNS padding options

    Follow the guidance in RFC 8467 for use of EDNS padding options to pad DoH queries to a few common lengths to protect against traffic analysis. Use of HTTP/2 padding is also possible but unlike EDNS padding, will not elicit padding on responses from DoH servers.

  • Use RFC 8484 POST only for privacy sensitive applications or browser modes

    Using POST for DoH queries reduces the cacheability of responses and can increase DNS latency, so it is not generally recommended. However, reducing caching is probably desirable for privacy sensitive applications, and might protect against timing attacks from web apps trying to determine what domains the user has visited lately.

Issues

To report a bug or request a new feature, please open an issue for DoH.