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RFC 7828 / SIGNAL DNS IDLE TIME

Offer persistence only on an eligible response

02

A DNS client sends a query; the server may reply with its current idle timeout. OPT support in that query gives the server room to signal the option.

DIAGRAMRead RFC section 3.2.1 ↗
RFC 7828 · § 3.2.1EXAMPLE
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Offer persistence only on an eligible responseClient and server use TCP connection C1. First the client sends OPT with an empty code11 option; the server replies on C1 with its current example TIMEOUT600. Separate query cases show that OPT without code11 still permits a server offer, while no request OPT stops response OPT insertion at the eligibility gate. The blocked path concerns inserting an option, not delivery of an ordinary DNS answer.Server signal · existing TCP connection C1Separate TCP query: NO OPTEarlier persistence does not override thisResponse OPT eligibility gateSTOP: no response OPT insertionThe DNS question may still be answeredPeriodic query OPT permits later timeout signals
04
A query without OPT closes the option-insertion gate.

Separate query: no OPT is present. The server can answer the DNS question, but cannot add an OPT record or this option to that response. Earlier keepalive use does not override this rule.

Source for this drawing: RFC 7828 · Read RFC section 3.2.1 ↗

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KEY POINT

The server selects the timeout and needs request OPT.

  • A client MAY include the empty option in the first or later TCP queries. A server receiving it over TCP MAY adjust local idle time if resources permit.
  • A TCP query containing OPT, with or without the keepalive option, permits the server to include keepalive in its response. This is a MAY, not a requirement to accept a client’s preference.
  • Without OPT in the query, the server MUST NOT put OPT in the response, even on a previously negotiated persistent connection. A client that initially asks for keepalive SHOULD periodically include OPT in subsequent queries so updates can be signalled; no interval algorithm is specified.
DETAILS

OPT is an EDNS pseudo-record that extends one DNS message; it is not an ordinary cached answer record. The response and timeout concern the same TCP connection. Ordinary DNS answers can still be returned when this option is absent or ineligible. Periodic OPT inclusion means using later DNS queries, not manufacturing heartbeat probes. A successful signal is permission for possible persistence, not a connection-lifetime guarantee.

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