DNS is the internet's address book, but it is also a distributed cache with a trust model. Understanding both explains most "it works, then it does not" incidents.
Records you will meet
| Record | Purpose |
|---|---|
| A | Name to IPv4 |
| AAAA | Name to IPv6 |
| CNAME | Alias to another name |
| MX | Mail exchanger |
| TXT | Policy and verification text |
| NS | Authoritative nameservers |
| SOA | Zone authority and timing |
Resolution and caching
A resolver walks the hierarchy:
- Root servers refer the resolver to the TLD.
- The TLD refers it to the authoritative nameserver.
- The authoritative server returns the record.
- The resolver caches it for the TTL.
Caching is why a DNS change can take time to propagate. It is also why an old answer can send users to the wrong place after a migration.
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