Tags

Here is a non-exhaustive list of known tags:

High word	Low word	Name in English	Validity	Description	Value structure
ffff-ffff-ffff-ffff	0000-0000-0000-0000	placeholder	anywhere	used for examples and testing	unspecified
edcc-1a0a-8817-f0f6	4711-bbca-83f3-6f5d	self-proposed name	anywhere	name given by a zone/service to itself	localised text
	0d0b-c1d2-fe54-36e8	edge pointer	metadata service	endorses another zone and gives it a name	TODO
	0b40-40b1-1f4a-ded4	authoritative source	metadata service	other tags in this record indicate where to find a complete copy of the zone	empty
	ba58-942b-c175-b313	maintainer info	metadata service	name/contact details for zone owner	localised text
	9310-2a53-98f4-a870	cache duration	metadata service	maximum number of seconds for which the zone may be cached	u16
	e8ed-44a7-a476-0a51	surrogate hostname	anywhere	DNS-style hostname to use in protocols that require one	string
849f-4d8f-a556-6e1e	df4e-0109-3265-d87b	IPv4	anywhere	IPv4 address	[4]u8
	527f-0be4-8e93-a502	IPv6		IPv6 address	[16]u8
	77e3-4e50-6dab-3c1e	TCP		TCP port number	u16
	b231-d968-259f-5edf	UDP		UDP port number	u16
	7bd4-b12b-7ba9-f2fc	QUIC		QUIC port number	u16
	0987-da54-8679-e3fe	TLS		TLS certificate hash(?)	TODO
	02bc-6f4b-ac77-1630	IMAP		TODO	TODO
	6971-6c69-d21d-64fb	SMTP		TODO	TODO
	9082-761f-c79d-433f	POP		TODO	TODO
	7dcc-0481-3e5d-891c	DNS		TODO	TODO
	fccd-66df-c927-9ac4	HTTP		HTTP base path	string starting with `/`
	12b7-ba1f-4cba-a18a	Gemini		Gemini base path
	59e9-725e-59de-27d6	Gopher		Gopher base path

pawsd-fennel has a more easily machine-readable version of the first two columns.

Within values, there are a few common constructs:

localised text: language code length (u8), ISO-639 language code ([_]u8), raw UTF-8 string (no length prefix or terminator)
string: raw UTF-8 string (no length prefix or terminator)

Note that strings don't need any indication of their length, because this is already given by the structure of the record itself, which includes a “value length” field.