6.1.2. Wire Format

An OPT RR has a fixed part and a variable set of options expressed as {attribute, value} pairs. The fixed part holds some DNS metadata, and also a small collection of basic extension elements that we expect to be so popular that it would be a waste of wire space to encode them as {attribute, value} pairs.

The fixed part of an OPT RR is structured as follows:

    +------------+--------------+------------------------------+
    | Field Name | Field Type   | Description                  |
    +------------+--------------+------------------------------+
    | NAME       | domain name  | MUST be 0 (root domain)      |
    | TYPE       | u_int16_t    | OPT (41)                     |
    | CLASS      | u_int16_t    | requestor's UDP payload size |
    | TTL        | u_int32_t    | extended RCODE and flags     |
    | RDLEN      | u_int16_t    | length of all RDATA          |
    | RDATA      | octet stream | {attribute,value} pairs      |
    +------------+--------------+------------------------------+

The variable part of an OPT RR may contain zero or more options in the RDATA. Each option MUST be treated as a bit field. Each option is encoded as:

                +0 (MSB)                            +1 (LSB)
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  0: |                          OPTION-CODE                          |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  2: |                         OPTION-LENGTH                         |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  4: |                                                               |
     /                          OPTION-DATA                          /
     /                                                               /
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

OPTION-CODE : Assigned by the Expert Review process as defined by the DNSEXT working group and the IESG.

OPTION-LENGTH : Size (in octets) of OPTION-DATA.

OPTION-DATA : Varies per OPTION-CODE. MUST be treated as a bit field.

The order of appearance of option tuples is not defined. If one option modifies the behavior of another or multiple options are related to one another in some way, they have the same effect regardless of ordering in the RDATA wire encoding.

Any OPTION-CODE values not understood by a responder or requestor MUST be ignored. Specifications of such options might wish to include some kind of signaled acknowledgement. For example, an option specification might say that if a responder sees and supports option XYZ, it MUST include option XYZ in its response.

6.1.2. Wire Format​

6.1.2. Wire Format