The checks
- At least two nameservers — so the domain survives a server outage.
- Authoritative answers — each server must answer with the authority flag. A server that does not is a "lame delegation" and causes random lookup failures.
- Matching SOA serials — the zone version number. Different serials on one provider mean some servers did not receive your latest change. (Domains served by two different DNS providers normally have different serials.)
- Network diversity — nameservers on different networks protect against a single network failure.
When to use it
- After moving DNS to a new provider or registrar.
- When a website or email works for some visitors but not others.
- Before and after changing nameservers at your registrar.
Frequently asked questions
What is lame delegation?
A nameserver is listed for your domain but does not answer authoritatively for it, usually after a DNS migration that was not completed.
How many nameservers should a domain have?
At least two; most providers give you two to four.
Why do my SOA serials differ?
On a single provider it means servers are out of sync. If you use two providers (for example Route 53 and NS1), different serials are expected.