Obsolete RR types

Shared wire-format representation for the RFC 1035 RR types whose RDATA is a single domain name: NS (section 3.3.11), CNAME (section 3.3.1), PTR (section 3.3.12), and the obsolete mailbox-pointer types MB, MD, MF, MG, MR (RFC 1035 sections 3.3.3-3.3.8). The type parameter n (one of N_ns, N_cname, N_ptr, N_mb, N_md, N_mf, N_mg, N_mr) determines the RR type. Each has its own type synonym (T_ns, T_cname, ...) and matching pattern synonym (T_NS, T_CNAME, ...).

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/                 DOMAINNAME                    /
/                                               /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

Although all these RR types share a common underlying representation, the constructors are not shared and the types are not mutually coercible — this is deliberate, to catch RR-type confusion at compile time.

The target domain is subject to wire-form name compression on encode (RFC 3597 section 4) and canonicalises to lower case (RFC 4034 section 6.2). The Eq and Ord instances compare in canonical wire form (via equalWireHost / compareWireHost), so Ord is canonical. Presentation preserves the original case.

X_domain specialised to MD records.

X_domain specialised to MF records.

X_domain specialised to MB records.

X_domain specialised to MG records.

X_domain specialised to MR records.

The WKS resource record (RFC 1035 section 3.4.2), mapping an IPv4 address and a WksProto protocol number to the set of TCP/UDP port numbers (16-bit) on which the named host accepts connections. Ports are encoded on the wire as a packed bitmap whose length implies the maximum port carried.

 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 |                    ADDRESS                    |
 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 |       PROTOCOL        |                       |
 +--+--+--+--+--+--+--+--+                       |
 |                                               |
 /                   <BIT MAP>                   /
 /                                               /
 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

The Ord instance compares by address, then protocol, then the port set in descending order — matching the byte-wise comparison of the wire-form port bitmap, so it agrees with the canonical RR-content ordering of RFC 4034 section 6.2.

IP protocol number used in the T_wks header byte. Bidirectional patterns TCP (6) and UDP (17) cover the two protocols WKS was ever realistically used for; any other protocol number presents as its decimal value.

The MINFO resource record (RFC 1035 section 3.3.7) — mailing-list request and owner addresses: two Domain fields, the request mailbox and the owner (bounce) mailbox.

 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 /                    RMAILBX                    /
 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 /                    EMAILBX                    /
 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

Both fields are subject to wire-form name compression on encode (RFC 3597 section 4) and canonicalise to lower case (RFC 4034 section 6.2). The Eq and Ord instances compare both domain fields in canonical wire form (via equalWireHost / compareWireHost), so Ord is canonical.

The X25 resource record (RFC 1183 section 3.1) — an X.25 PSDN address held as a single DNS character-string. RFC 1183 calls for at least four ASCII digits, but the constructor does not enforce that — callers may store any byte sequence that fits in a character-string (up to 255 bytes).

The Ord instance compares the payload as a DNS character-string (length-prefixed lexicographic), so it agrees with the canonical wire-form ordering of RFC 4034 section 6.2.

The ISDN resource record (RFC 1183 section 3.2) — the ISDN number of the owner host, with an optional Direct-Dial-In subaddress. Both fields are DNS character-strings; only the address is mandatory.

The Ord instance compares both fields as DNS character-strings. With the trailing DDI absent, the wire form is shorter than any encoding with a present DDI, so Ord agrees with the canonical wire-form ordering of RFC 4034 section 6.2.

The RT resource record (RFC 1183 section 3.3) — Route-Through: a 16-bit preference and a Domain naming an intermediate host that will route packets to the owner. Used in the X.25 and ISDN era for hosts without their own wide-area addresses.

The route-through domain is not subject to wire-form name compression on encode but compression is tolerated on decode (RFC 3597 section 4). It canonicalises to lower case (RFC 4034 section 6.2). The Eq and Ord instances compare the domain in canonical wire form (via equalWireHost / compareWireHost), so Ord is canonical.

The NSAP resource record (RFC 1706 section 5; deprecated by RFC 9121) — mapped a domain name to an OSI Network Service Access Point address. An opaque byte string carrying the NSAP value verbatim; presented in zone-file syntax as a 0x-prefixed hex literal.

Derived Ord compares the raw bytes, which matches the canonical wire-form ordering of RFC 4034 section 6.2.

The NSAPPTR resource record (RFC 1348; obsoleted by PTR in RFC 1706 section 6, deprecated by RFC 9121) — the OSI counterpart of PTR, mapping an NSAP-derived owner name to a domain name.

The target domain is not subject to wire-form name compression (RFC 3597 section 4) and is not in the RFC 4034 section 6.2 list of types that lower-case their RDATA names. Derived Eq and Ord therefore compare the wire bytes verbatim (case-sensitively).

The PX resource record (RFC 2163 section 4; deprecated by RFC 9121) — points at X.400/RFC 822 mapping information: a 16-bit preference and two Domain fields naming the RFC 822 (SMTP) and X.400 sides of the mapping.

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                  PREFERENCE                   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/                    MAP822                     /
/                                               /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/                    MAPX400                    /
/                                               /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

Neither domain field is subject to wire-form name compression on encode but compression is tolerated on decode (RFC 3597 section 4). Both canonicalise to lower case (RFC 4034 section 6.2). The Eq and Ord instances compare both domains in canonical wire form (via equalWireHost / compareWireHost), so Ord is canonical.

The GPOS resource record (RFC 1712 section 3) — an early geographical-location record: longitude, latitude, and altitude as three DNS character-strings holding decimal floating-point text. Superseded by LOC-style records and not used in modern zone data.

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/                 LONGITUDE                  /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/                  LATITUDE                  /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/                  ALTITUDE                  /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

The Ord instance compares the three fields as DNS character-strings, agreeing with the canonical wire-form ordering of RFC 4034 section 6.2.

The KX resource record (RFC 2230 section 3.1) — names a Key Exchange host for the owner: a 16-bit preference and a Domain naming the exchanger. Defined for the DNSSEC precursor work; never widely deployed.

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                  PREFERENCE                   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/                   EXCHANGER                   /
/                                               /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

The exchanger field is not subject to wire-form name compression (RFC 3597 section 4) and canonicalises to lower case (RFC 4034 section 6.2). The Eq and Ord instances compare the exchanger in canonical wire form (via equalWireHost / compareWireHost), so Ord is canonical.

The A6 resource record (RFC 2874 section 3.1; obsoleted by RFC 6563) — an experimental chained IPv6 addressing scheme. Each record carries a prefix length, an address suffix containing the low-order bits, and a Domain naming where to look up the remaining prefix bits. Resolution walked the chain to assemble the full address. The deployment experience reported in RFC 6563 led to A6 being abandoned in favour of plain T_aaaa records.

+-----------+------------------+-------------------+
|Prefix len.|  Address suffix  |    Prefix name    |
| (1 octet) |  (0..16 octets)  |  (0..255 octets)  |
+-----------+------------------+-------------------+

The wire encoding rules are:

• The prefix length is an unsigned octet between 0 and 128.
• The address-suffix field carries exactly enough octets to hold 128 - prefix-length bits, with up to seven leading pad bits set to zero so the field is an integral number of bytes.
• The prefix-name field is a wire-form Domain. It is absent when the prefix length is zero (the suffix already holds the whole address); the suffix field is absent when the prefix length is 128 (the whole address comes from the chain).
• The prefix-name is not subject to wire-form name compression (RFC 3597 section 4) and canonicalises to lower case (RFC 4034 section 6.2).

The Eq and Ord instances compare the prefix-name field in canonical wire form (via toHost), so Ord is canonical.