DNS Multiple QTYPEs

Introduction A commonly requested DNS feature is the ability to receive multiple related resource records (RRs) in a single DNS response. For example, it may be desirable to receive the A, AAAA and HTTPS records for a domain name together, rather than having to issue multiple queries. The DNS wire protocol in theory supported having multiple questions in a single packet, but in practise this does not work. In , is updated to only permit a single question in a QUERY (OpCode=0) request. Sending QTYPE=ANY does not guarantee that all RRsets will be returned. specifies that responders may return a single RRset of their choosing. This document provides a solution for those cases where only the QTYPE varies by specifying a new option for the Extension Mechanisms for DNS (EDNS ) that contains an additional list of QTYPE values that the client wishes to receive in addition to the single QTYPE appearing in the question section. A different EDNS option is used in response packets as protection against DNS middleboxes that echo EDNS options verbatim. The specification described herein is applicable both for queries from a stub resolver to recursive servers, and from recursive resolvers to authoritative servers. It does not apply to Multicast DNS queries , which are already designed to allow requesting multiple records in a single query.

Description

Multiple QTYPE EDNS Options Format The overall format of an EDNS option is shown for reference below, per , followed by the option specific data: OPTION-CODE: MQTYPE-Query (TBD1) in queries and MQTYPE-Response (TBD2) in responses. OPTION-LENGTH: Size (in octets) of OPTION-DATA. OPTION-DATA: Option specific, as below: QT: a (potentially empty) list of 2 byte fields (QTx) in network order (MSB first) each specifying a DNS RRTYPE. The RRTYPEs MUST be for real resource records, and MUST NOT refer to Meta RRTYPEs such as "OPT" and those from the reserved range 128 - 255, e.g. "IXFR", "TSIG", "*", etc.

Server Handling

Request Parsing If MQTYPE-Query is received in any inbound DNS message with an OpCode other than QUERY (0) the server MUST return a FORMERR response. A server that receives an MQTYPE-Response option in any inbound DNS message MUST return a FORMERR response. A server that receives more than one MQTYPE-Query option in a query MUST return a FORMERR response. If MQTYPE-Query is received in a query that contains no primary question (i.e. QDCOUNT=0) the server MUST return a FORMERR response. If any duplicate QTx (or one duplicating the primary QTYPE field) is contained in a query the server MUST return a FORMERR response. If any invalid QTx is received in the query (e.g. one corresponding to a Meta RRTYPE) the server MUST return a FORMERR response.

Response Generation A conforming server that receives an MQTYPE-Query option in a query MUST return an MQTYPE-Response option in its response, even if that response is truncated (TC=1). The server MUST first start constructing a response for the primary (QNAME, QCLASS, QTYPE) tuple specified in the Question section per the existing DNS sections. The RCODE and all other flags (e.g. AA, AD, etc) MUST be determined at this time. If this initial response results in truncation (TC=1) then the additional queries specified in MQTYPE-Query MUST NOT be processed. After the initial response is prepared, the server MUST attempt to combine the responses for individual (QNAME, QCLASS, QTx) combinations into the response for the first query. For each individual combination the server MUST evaluate the resulting RCODE and other flags and check that they all match the values generated from the primary query. If any mismatch is detected the mismatching additional response MUST NOT be included in the final combined response and its QTx value MUST NOT be included in the MQTYPE-Response list. This might happen, for example, if the primary query resulted in a NOERROR response but a QTx query resulted in a SERVFAIL, or if the primary response has AA=0 but a QTx response has AA=1, such as might happen if the NS and DS records were both requested at the parent side of a zone cut. If no mismatches are detected the server MUST attempt to combine the individual RRs into their respective sections. The server MUST detect duplicate RRs and keep only a single copy of each RR in its respective section. Duplicates can occur e.g. in Answer section if a CNAME chain is involved, or e.g. Authority section if multiple QTYPEs don't exist etc. Note that RRs can be legitimately duplicated in different sections, e.g. for (SOA, TYPE12345) combination on apex where TYPE12345 is not present. If message size (or other) limits do not allow all of the data obtained by querying for an additional QTx to be included in the final response then the server MUST NOT include the respective QTx in the MQTYPE-Response list and MAY stop processing further QTx combinations. If all RRs for a single QTx combination fit into the message then the server MUST include the respective QTx in the MQTYPE-Response list to indicate that the given query type was completely processed.

Client Response Processing Recursive resolvers MAY use this method to obtain multiple records from an authoritative server. For the purposes of Section 5.4.1 of any authoritative answers received MUST be ranked the same as the answer for the primary question. If the response to a query containing an MQTYPE-Query option does not contain an MQTYPE-Response option, or if it erroneously contains an MQTYPE-Query option, the client MUST treat the response as if this option is unsupported by the server and SHOULD process the response as if the MQTYPE-Query option had not been used. If the MQTYPE-Response option is present more than once or if a QTx value is duplicated (or duplicates the primary QTYPE field) the client MUST treat the answer as invalid (equivalent to FORMERR) The Question section and the list of types present in the MQTYPE-Response option indicates the list of (QNAME, QCLASS, qtypes) combinations which are completely contained within the received response. The answers to all query combinations share the same RCODE and all other flags. All RRs required by existing DNS specifications are expected to be present in the respective sections of the DNS message, including proofs of nonexistence where required. The client MUST NOT rely on any particular order of RRs in the message sections. Clients MUST take into account that individual RRs might originate from different DNS zones and that proofs of non-existence might have been produced by different signers. Absence of QTx values which were requested by client but are not present in MQTYPE-Response option indicates that:

the server was unwilling to process the request (e.g. because a limit was exceeded), and/or
the individual responses could not be combined into one message because of RCODE or other flag mismatches, and/or
the message size limit would be exceeded

The client SHOULD subsequently initiate standalone queries (e.g. without using the MQTYPE-Query option) for any QTx value which was requested but is missing in the response.