Reverse-DNS name

A name a producer mints for itself, with nobody to ask, by reversing a domain it controls. mgba.io becomes io.mgba, and every name under io.mgba belongs to whoever holds the domain. The specifications use this wherever a value has to be globally unique and the spec cannot enumerate the possibilities: extension keys, catalog resolver names, the identity of a producing app, and the open vocabularies a producer can add to, such as a part’s kind or role.

A special vendor label x is used for names that must be referred to for which there is no DNS associated.

This page is normative. When a specification says a field holds a reverse-DNS name, every rule here applies to that field.

Grammar

A reverse-DNS name is two or more labels joined by ., respecting the following rules:

• Lowercase ASCII only. A name carrying an uppercase letter is malformed, and consumers do not case-fold to rescue it.
• The first label is the reversed top-level domain, so it is letters only, and two or more of them. The single-letter label x is the one exception, reserved below.
• Every other label holds letters, digits and -, starts and ends with a letter or a digit, and runs at most 63 characters.
• - is the only punctuation allowed inside a label. _ is not, so that reversing any name yields something that is still a syntactically valid domain.
• The whole name runs at most 255 characters.
• A domain with non-ASCII labels is written in its punycode form, so münchen.de reverses to de.xn--mnchen-3ya.

Nothing checks that the domain exists, resolves, or still belongs to the producer that minted the name. The convention buys uniqueness, not authentication.

The following regular expression can validate a name:

(x|[a-z]{2,63})(\.[a-z0-9]([a-z0-9-]{0,61}[a-z0-9])?)+

Comparison and the tree

Two names are equal when their bytes are equal.

A name is a path rather than an opaque string. io.mgba.rtc sits under io.mgba, which sits under io. Whoever holds a name holds every name below it and needs no further registration to start using them, which is the whole point of minting names this way.

Prefix matching happens on label boundaries. io.mgba matches io.mgba and io.mgba.rtc, and does not match io.mgbahax. A consumer asking “is this name under X” MUST compare label by label; a plain string prefix test is wrong and will match names belonging to somebody else.

A name can be in use and be a parent of other names at once. x.1sav.rtc is an extension key carrying a clock reading, and x.1sav.rtc.s3511a is a different key under it, so matching the parent as a prefix collects both. Sitting under a name says who holds it and nothing about what it means. A consumer that groups by prefix MUST NOT treat what it collected as one kind of thing.

The x tree

Plenty of producers don’t own a domain. An emulator core is often a repository and nothing more, and asking it to buy a domain to name itself is absurd.

The top-level label x is reserved by these specifications for those producers. A single-letter top-level domain cannot exist: ICANN requires an applied-for ASCII string to run three characters or more, since the two-character space is reserved for country codes. So a name in this tree cannot collide with a reversed real one, and no future round of top-level domains can make it start colliding.

A name in the x tree is exactly two labels, x.<vendor>, and the <vendor> label is assigned in the Vendor Names registry. Below its own name a vendor is on its own: whoever holds x.fceumm holds x.fceumm.anything, with nothing further to register.

A producer that controls a domain SHOULD reverse that domain instead of asking for an x name. The tree is there for the ones with nothing to reverse.

x.example is reserved for documentation and is never assigned to anyone.

Examples

Name	Held by	Used as
io.mgba	mGBA, from mgba.io	producer identity
com.1retro.forge	1retro, from 1retro.com	extension key
org.hasheous	Hasheous, from hasheous.org	catalog resolver name
x.example	nobody, reserved for docs	producer identity