4.9 Lookup and Reverse

3 min read

4.9.1 lookup #

A lookup (PSPU §6.5) asks for A, AAAA, or both:

4.9.1.1 Each task is an ordinary question #

Each task is an ordinary question (§4.1): it has its own synthetic check, its own expansion, its own routing per candidate, its own cache lookups and its own attempts. The two tasks of an any lookup run at the same time and can be answered from different sources. The reply is built when the last of them is done.

4.9.1.2 The outcome #

An A answer and an AAAA that is unavailable is found, with IPv4 addresses only.

4.9.1.3 Chasing CNAMEs #

For each found task, resolvd looks for addresses in that task's answer records, starting at the candidate the answer was found at. Names are compared case-insensitively.

1. If the first record at the current name is a CNAME, and no record of the asked type is at the current name, the chase moves to the CNAME's target.
2. This repeats at most 16 times; a loop stops there.
3. The addresses are the A (for the A task) or AAAA (for the AAAA task) records at the name the chase ended at, in answer order, each with its record's TTL.

4.9.1.4 The chase asks nothing further #

Only the records in the task's own answer are used. A chain that leaves them — a CNAME whose target's addresses the server did not include — ends with no addresses, and no further question is asked.

4.9.1.5 The reply fields #

• addresses lists each finished task's addresses in turn, in the order the tasks finished, so whether IPv4 or IPv6 addresses come first depends on which family was answered first.
• canonical and source come from the same selected found task. Prefer a task that produced addresses; when both or neither did, prefer A over AAAA. Completion order does not affect this choice. canonical is the name that task's CNAME chase ended at, after search expansion. Without a found task it is the name asked, as parsed, in presentation form without a trailing dot (the root is .).
• A found with no addresses still reports the expanded or chased name.
• A name that does not parse is answered at once with outcome notfound, canonical ., no addresses and source local; no task is started.
• source is the source of the selected task, and local when no task was found. It describes that task; the other family's records can have a different source.
• validation is unvalidated.
• A lookup adds one to queries however many tasks it makes.

4.9.2 reverse becomes a PTR question #

A reverse turns its address into the reverse-mapping name — the four octets reversed under in-addr.arpa for IPv4, the 32 nibbles reversed, in lower-case hexadecimal, under ip6.arpa for IPv6 — and resolves that name for PTR exactly as a resolve without no_cache would. The reply is an ordinary answer (§5.3).

Edit this page