Resource Records

Answer / Authority / Additional sections all use the same Resource Record (RR) format:

+----------+
| NAME     |   encoded name (often a pointer)
+----------+
| TYPE     |   16-bit
+----------+
| CLASS    |   16-bit (= IN normally)
+----------+
| TTL      |   32-bit seconds
+----------+
| RDLENGTH |   16-bit length of RDATA
+----------+
| RDATA    |   variable; format depends on TYPE
+----------+

RDATA per type:

• A (1): 4 bytes (the IPv4 address)
• AAAA (28): 16 bytes (the IPv6 address)
• NS (2): an encoded domain name
• CNAME (5): an encoded domain name
• MX (15): 16-bit preference + encoded domain name
• TXT (16): one or more length-prefixed text strings
• SOA (6): primary NS + admin email + serial + refresh + retry + expire + minimum TTL

The TTL governs caching: how many seconds the answer is valid before re-querying. Typical: 300s (5 min). Production DNS for a high-traffic site might be 60s or even 30s.

For this lesson: encoding/decoding A and CNAME records.