pub struct OPT { /* fields omitted */ }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 master 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.
Creates a new OPT record data.
options - A map of the codes and record types
The newly created OPT data
The entire map of options
Get a single option based on the code
Insert a new option, the key is derived from the EdnsOption
Returns the "default value" for a type. Read more
Formats the value using the given formatter. Read more
This method tests for self and other values to be equal, and is used by ==. Read more
This method tests for !=.
Performs copy-assignment from source. Read more