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6. Definition of the "application/dns-message" Media Type

  1. 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.

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.

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