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DNS Guides for Operators

Use these notes with the live lookup on dnschecker.sd9.in. Each section maps to a tool so you can verify the current public answer after a change.

DNS records explained

DNS is a distributed database. A resolver asks nameservers for a type at a name and caches the answer. Common types:

• A — IPv4 address for a host.
• AAAA — IPv6 address for a host.
• CNAME — alias to another name.
• MX — mail exchanger and preference.
• NS — authoritative nameservers.
• TXT — SPF, DKIM, DMARC, and proofs.
• SOA — zone serial and timers.
• CAA — certificate authority authorization.
• PTR — reverse DNS for an IP.

TTL and propagation

TTL is how long a resolver may reuse an answer. If you lower TTL hours before a cutover, more resolvers will refetch quickly. After you raise TTL, old caches can still serve the previous value until they expire. The propagation checker compares Cloudflare, Google, and NextDNS so you can see whether caches have converged.

Email authentication DNS

Mail routing needs MX records that point at hosts with A or AAAA. Sender policy uses SPF TXT on the envelope domain. DMARC is a TXT record at _dmarc.example.com. Start with p=none, watch reports, then move to quarantine or reject. Check them with the SPF and DMARC tools.

Cutover checklist

1. Lower TTL on records you plan to change.
2. Publish the new value and confirm with an A/AAAA or MX lookup.
3. Compare resolvers on the propagation page.
4. Confirm PTR if the host sends mail.
5. Raise TTL once the cutover is stable.