Struct trust_dns_proto::rr::rdata::opt::OPT

source · [−]

pub struct OPT { /* private fields */ }

Expand description

The OPT record type is used for ExtendedDNS records.

These allow for additional information to be associated with the DNS request that otherwise would require changes to the DNS protocol.

RFC 6891, EDNS(0) Extensions, April 2013

6.1.  OPT Record Definition

6.1.1.  Basic Elements

   An 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.

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      |
       +------------+--------------+------------------------------+

                               OPT RR Format

   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 behaviour 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.

6.1.3.  OPT Record TTL Field Use

   The extended RCODE and flags, which OPT stores in the RR Time to Live
   (TTL) field, are structured as follows:

                  +0 (MSB)                            +1 (LSB)
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    0: |         EXTENDED-RCODE        |            VERSION            |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    2: | DO|                           Z                               |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   EXTENDED-RCODE
      Forms the upper 8 bits of extended 12-bit RCODE (together with the
      4 bits defined in [RFC1035].  Note that EXTENDED-RCODE value 0
      indicates that an unextended RCODE is in use (values 0 through
      15).

   VERSION
      Indicates the implementation level of the setter.  Full
      conformance with this specification is indicated by version '0'.
      Requestors are encouraged to set this to the lowest implemented
      level capable of expressing a transaction, to minimise the
      responder and network load of discovering the greatest common
      implementation level between requestor and responder.  A
      requestor's version numbering strategy MAY ideally be a run-time
      configuration option.
      If a responder does not implement the VERSION level of the
      request, then it MUST respond with RCODE=BADVERS.  All responses
      MUST be limited in format to the VERSION level of the request, but
      the VERSION of each response SHOULD be the highest implementation
      level of the responder.  In this way, a requestor will learn the
      implementation level of a responder as a side effect of every
      response, including error responses and including RCODE=BADVERS.

6.1.4.  Flags

   DO
      DNSSEC OK bit as defined by [RFC3225].

   Z
      Set to zero by senders and ignored by receivers, unless modified
      in a subsequent specification.