6. The OPT Pseudo-RR

6. The OPT Pseudo-RR

The OPT pseudo-RR (sometimes called a meta-RR) MAY be added to the additional data section of a request.

The OPT RR has RR type 41.

If an OPT record is present in a received request, compliant responders MUST include an OPT record in their respective responses.

An OPT record does not carry any DNS data. It is used only to contain control information pertaining to the question-and-answer sequence of a specific transaction. OPT RRs MUST NOT be cached, forwarded, or stored in or loaded from zone files.

The OPT RR MAY be placed anywhere within the additional data section. When an OPT RR is included within any DNS message, it MUST be the only OPT RR in that message. If a query message with more than one OPT RR is received, a FORMERR (RCODE=1) MUST be returned. The placement flexibility for the OPT RR does not override the need for the TSIG or SIG(0) RRs to be the last in the additional section whenever they are present.