MX, SRV, and SOA Records — Byte-Exact RDATA

Beyond A/AAAA/CNAME, three RR types matter the most in real ops: MX for mail routing, SRV for service discovery, and SOA for zone administration. Their RDATA formats are precise — let's encode them down to the byte.

MX (Mail Exchange) — RFC 1035 §3.3.9

+--+--+
|  PREFERENCE  |
+--+--+
|     EXCHANGE  (encoded name)
+--/--+

• PREFERENCE (16-bit, big-endian): smaller = higher priority. Senders try low pref first.
• EXCHANGE: the hostname of the mail server (e.g. mail.example.com).

Example: 10 mail.example.com →

00 0a 04 6d 61 69 6c 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00

SRV (Service Locator) — RFC 2782

+--+--+--+--+--+--+
| PRIORITY | WEIGHT | PORT |
+--+--+--+--+--+--+
|        TARGET (encoded name)
+--/--+

• PRIORITY (16-bit): clients try lower priority first.
• WEIGHT (16-bit): tie-breaker within the same priority — relative random selection.
• PORT (16-bit): TCP/UDP port number.
• TARGET: hostname providing the service.

Used for: SIP, XMPP, Kerberos, LDAP. Lookup form: _service._proto.name, e.g. _sip._tcp.example.com.

SOA (Start of Authority) — RFC 1035 §3.3.13

+--+--+--+--+--+--+--+--+--+--+--+
|  MNAME (encoded name)                |
+--+--+--+--+--+--+--+--+--+--+--+
|  RNAME (encoded name)                |
+--+--+--+--+--+--+--+--+--+--+--+
|       SERIAL (32)                    |
+--+--+--+--+--+--+--+--+--+--+--+
|       REFRESH (32)                   |
+--+--+--+--+--+--+--+--+--+--+--+
|       RETRY (32)                     |
+--+--+--+--+--+--+--+--+--+--+--+
|       EXPIRE (32)                    |
+--+--+--+--+--+--+--+--+--+--+--+
|       MINIMUM (32, negative TTL)     |
+--+--+--+--+--+--+--+--+--+--+--+

• MNAME: primary name server.
• RNAME: admin email — @ becomes a dot in the wire (e.g. [email protected] → admin.example.com).
• SERIAL: zone version. Secondaries reload when serial increases (typically YYYYMMDDnn).
• REFRESH: how often secondaries poll (seconds).
• RETRY: if refresh fails, retry interval.
• EXPIRE: secondaries discard the zone after this without contact.
• MINIMUM: per RFC 2308, this is the negative-cache TTL (NOT the default record TTL — that was the original meaning, then redefined).