6.2 Rendering Replies

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The outcome-to-response-code table and the rules for CNAME, RA, AA and AD are PSPU §6.8. This article is how resolvd builds each reply.

6.2.1 The header and question #

  • The reply copies the query's ID, opcode, RD and CD bits and its question section.
  • QR and RA are set. AA, AD and TC are clear, except that TC is set on a truncated UDP reply (below).
  • No bit from an upstream server's reply reaches the stub reply.

6.2.2 By outcome #

6.2.2.1 found replies #

  • The response code is NOERROR, and the answer section holds the engine's records in the order the engine has them (§4.8).
  • When the answer was found at a candidate other than the question's name — compared case-insensitively — the section begins with a CNAME from the question's name, in the query's case, to the candidate. Its TTL is the least TTL among the records when any are present.
  • A found with no records at an expanded name is NOERROR with only that leading CNAME. Its TTL is the negative cache lifetime: the lesser of the enclosing upstream SOA record's TTL and minimum field, capped at 300 seconds and at the TTL of any CNAME path the reply followed (§4.5). A cache hit uses the remaining lifetime, rounded down to whole seconds. With no enclosing SOA, the TTL is zero; no lifetime is invented. The authority section remains empty. An unexpanded empty found has no CNAME.

6.2.2.2 notfound replies #

NXDOMAIN, with empty answer and authority sections. No SOA is supplied, so a client has nothing from which to cache the absence.

6.2.2.3 unavailable replies #

SERVFAIL, with empty sections.

6.2.3 EDNS in stub replies #

When the query carried an OPT record, the reply carries one advertising a payload size of 1 232 bytes, with DO clear and no options. A query without OPT gets a reply without one.

6.2.4 Size and truncation #

  • Over UDP the reply's size limit is the payload size the query's OPT advertised, or 512 bytes when that is smaller or there is no OPT.
  • A reply over the limit is replaced by a truncated one, which keeps the same header with TC set, and the question.
  • The truncated reply keeps the OPT record when the reply has one.
  • The truncated reply carries no records: nothing is partially included.
  • A reply that cannot be encoded at all is not sent.
  • Over TCP the whole reply is sent, whatever its size up to the 65 535 bytes a DNS message can hold.
  • A TCP reply is written with its two-byte length, without blocking. Unfinished writes are queued with a fixed one-second deadline and a cap of 256 queued TCP replies (§2.5); a failed or expired write is not logged.

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