4.2 Synthetic Names

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PSPU §6.7 lists the names a resolver answers before any network. resolvd checks them in a fixed order, on the name as asked and before any search expansion, and the first match answers the task.

4.2.1 The order #

1. localhost, or any name under it. 127.0.0.1 for A, ::1 for AAAA, both for ANY. Source synthetic.
2. A static name. An exact, case-insensitive match against the Hosts\ values (§2.3). The name's addresses of the asked family — IPv4 for A, IPv6 for AAAA, both for ANY. Source hosts.
3. The machine's hostname. An exact, case-insensitive match against the hostname (§3.3), answered with the machine's own addresses (below). Source synthetic.
4. .local. The name local or any name under it is notfound, whatever the type asked. Source synthetic. No query is sent.
5. A reverse-mapping name, for PTR or ANY only. See below.

A name that matches none of these goes on to search expansion and the network.

4.2.1.1 Static names beat the hostname and .local #

Because the order is fixed, a static name beats the hostname and .local: a Hosts\ value named printer.local is answered with its addresses, and one named after the machine overrides the machine's own addresses.

4.2.1.2 localhost beats a static name #

localhost is checked first, so a Hosts\ value named localhost, or a name under it such as printer.localhost, never answers a forward question: the synthetic loopback answer is given instead. Its addresses still answer reverse questions through the static-name row below, except loopback addresses, which the loopback row takes first.

4.2.1.3 Record types #

For the three address-bearing kinds, the answer is found whatever type was asked; a type other than A, AAAA or ANY, or a family the name has no address in, gives found with no records. localhost asked for MX is an empty found, not a notfound, and is not forwarded.

4.2.1.4 The machine's own addresses #

• They are every address of every scope at addressed or better, whether or not the scope has servers, in snapshot order, link-local addresses included and loopback addresses excluded.
• Repeated addresses are removed only when they are adjacent in that list, so an address on two interfaces that are not consecutive in the snapshot appears twice.
• When there are none, the hostname is answered with 127.0.0.1 and ::1.

4.2.2 Reverse names #

4.2.2.1 Reverse-mapping names #

A reverse-mapping name is one under in-addr.arpa with exactly four labels before the suffix, or one under ip6.arpa with exactly 32. The suffix is matched case-insensitively.

• Each in-addr.arpa label parses as a decimal number from 0 to 255. Leading zeros and a leading + are accepted, so 1.0.0.0127.in-addr.arpa and +1.0.0.127.in-addr.arpa both name 127.0.0.1.
• Each ip6.arpa label is a single hexadecimal digit, in either case.

A name of either shape whose labels do not parse is not a reverse-mapping name.

4.2.2.2 Synthetic reverse answers #

Asked for PTR or ANY, a reverse-mapping name is answered synthetically when its address is:

The rows are tried in that order.

4.2.2.3 An address shared by static names #

When several static names list the same address, resolvd returns one PTR: the name whose ASCII-lowercased presentation form sorts first in byte order. For example, alpha.home wins over beta.home. The answer is independent of registry insertion order, hash-table order, reloads and restarts while the set of matching names stays the same. It changes when that set changes and another name sorts first. The selected name retains its configured spelling in the reply. This rule applies to IPv4 and IPv6, for both PTR and ANY questions.

4.2.2.4 Other reverse questions #

A reverse name asked for any other type, and a reverse name whose address matches none of the rows, goes to the network.

4.2.3 Properties of every synthetic answer #

• Every synthetic record has TTL 0.
• Synthetic answers are never cached, and the no_cache flag has no effect on them.
• Each one adds one to the synthetic counter (§4.10).

4.2.4 Expanded candidates are not checked #

Only the name as asked is checked. An expanded candidate is never compared with the synthetic names, so with a static name printer.lan and the search domain lan, a question for printer is expanded and sent to the network as printer.lan.

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