Fixed portion of EDNS(0) OPT pseudo-RR

Data type representing extension fields of a version 0 EDNS message. When a single EDNS(0) OPT pseudo-RR is present in the additional section of a DNS message, it is processed as an EDNS(0) extension header. The OPT pseudo-RR@ is then elided from the additional section of the decoded message.

The EDNS OPT pseudo-RR augments the message error status with an 8-bit field that together with the 4-bit RCODE from the unextended DNS header forms the full 12-bit extended RCODE. In order to avoid potential misinterpretation of the response RCODE, when the OPT record is decoded, the upper eight bits of the error status are combined with the RCODE of the basic message header to form a single 12-bit result. The decoded EDNS pseudo-header, omits the extended RCODE bits, they are instead found in the upper eight bits of the message RCODE.

Likewise, when decoding EDNS messages the extension flags are folded into the upper 16-bits of an extended 32-bit flags field in the message header. Consequently, the EDNS extension header record needs no extension RCODE or flags fields.

The reverse process occurs when encoding messages. The low four bits of the message header RCODE are encoded into the basic DNS header, while the upper eight bits are encoded as part of the EDNS OPT pseudo-RR. Similarly, the high 16 bits of the flags are also encoded in the OPT pseudo-RR. Encoding of messages with an RCODE larger than 15 or any extension flags set fails unless EDNS is enabled.

When encoding messages for transmission, the EDNS extension header is used to generate the additional OPT record. Do not add explicit OPT records to the additional section, instead configure EDNS via the message ednsHeader field.

The fixed part of an OPT pseudo-RR is structured as follows (RFC891 6.1.2):

+------------+--------------+------------------------------+
| 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 extended RCODE and flags, which OPT stores in the RR Time to Live (TTL) field, are structured as follows (RFC6891 6.1.3):

+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|          EXTENDED-RCODE       |             VERSION           |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| DO|                             Z                             |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

The default EDNS pseudo-header for queries. In accordance with the recommendation in RFC9715, Appedix A the UDP buffer size defaults to 1400 bytes, this should result in replies that fit into both the IPv4 and IPv6 MTU in typical Internet-connected networks.

A small minority of IPv6 networks are rumoured to have smaller MTUs of around 1280 bytes, and the corresponding DNS UDP size might then be 1232 bytes.

Since this library is a stub resolver, it is expected that the configured iterative resolvers are "near" enough to not require pessimistic UDP size limits. With a loopback conenction to a local resolver it may even make sense to set the UDP size limit at the 16KB maximum.

There is no single best value for the buffer size, too large risks fragmentation issues, while too small risks TCP fallback which is more costly and may fail.

defaultEDNS = EDNS
    { ednsVersion = 0      -- The default EDNS version is 0
    , ednsUdpSize = 1400   -- RFC9715 recommended value
    , ednsOptions = []     -- No EDNS options by default
    }

Maximum UDP size that can be advertised. If the ednsUdpSize of EDNS is larger, then this value is sent instead. This value is likely to work only for local nameservers on the loopback network. Servers generally enforce a smaller limit.

>>> maxUdpSize
16384

Minimum UDP size to advertise. If ednsUdpSize of EDNS is smaller, then this value is sent instead.

>>> minUdpSize
512