Which record to look at
Each record type answers one question. If a site does not load, start with A and AAAA. If mail bounces, look at MX and TXT. If a certificate cannot be issued, look at CAA.
- A and AAAA: the IPv4 and IPv6 addresses of the name.
- CNAME: the name is an alias of another name (common for CDNs and SaaS).
- MX: the servers that receive mail for the domain.
- TXT: SPF, DKIM, DMARC and domain verification strings.
- NS: the name servers that hold the zone.
- CAA: which certificate authorities may issue certificates for the domain.
When the answer is not what you expect
A change you just made may not show yet. Resolvers keep the old answer until its TTL runs out, often one hour, sometimes a day. Wait, or lower the TTL before the next change.
No record at all usually means the record was created in the wrong zone, or the domain's NS records point to another provider than the one you edited. An expired domain also stops resolving.
A CNAME cannot share its name with other records, so it never works on the bare domain: most providers refuse it, a few accept it and break mail. Use an ALIAS or a flattened record there, if your provider offers one.
Questions
Which resolver do you use?
Cloudflare (1.1.1.1), then Google (8.8.8.8) if it fails. You see what most visitors see, not what your own network has cached.
How long does a DNS change take?
As long as the TTL of the old record. Most providers use 300 seconds to 1 hour.
Can I be alerted when a record changes?
Yes. A SolidPing DNS check resolves the name every minute and alerts you when it stops resolving or no longer returns the value you expect.