DNS

A and AAAA: the IPv4 and IPv6 addresses of a host. Without one of these, nothing resolves.

CNAME: an alias pointing one name at another. It cannot coexist with other records on the same name, which is why an apex domain usually cannot be a CNAME.

MX: where mail for the domain should be delivered. Absent MX records mean mail bounces, whatever the website does.

TXT: free-form text, used in practice for SPF, DKIM and DMARC, and for domain-ownership verification.

NS: which nameservers are authoritative. A mismatch between these and what the registrar delegates is a classic source of intermittent failure.

CAA: which certificate authorities may issue for the domain.

Each record carries a TTL — how long a resolver may cache it. Change a record and resolvers keep serving the old value until their copy expires.

"Propagation" is therefore not a spreading wave: it is caches expiring at different times. There is nothing to accelerate and no central state.

The practical consequence: lower the TTL before a planned change, not after. Lowering it afterwards has no effect on caches already holding the old value.

`dig example.com A +short`, `dig example.com MX +short`, `dig example.com TXT +short` return the current values. `nslookup` does the same on systems without dig.

`dig example.com NS` shows the authoritative nameservers; compare with what the registrar lists.

Query more than one resolver — `dig @1.1.1.1` and `dig @8.8.8.8` — to see whether an inconsistency is yours or a cache's.

Nameserver mismatch: the registrar delegates to one provider while the records live at another. The site works or fails depending on which resolver answers.

An A record pointing at a decommissioned server, still cached by a long TTL.

Multiple SPF records on one domain, which is invalid and causes evaluation to fail — the single most common email misconfiguration.

No IPv6, which is not a defect but is increasingly visible on mobile networks.

Three parties are usually involved, and confusing them is why DNS changes get made in the wrong control panel.

The registrar holds the domain registration and the delegation: which nameservers are authoritative. Changing nameservers happens here and nowhere else.

The DNS host operates those nameservers and holds the records themselves. It is often the registrar, often the CDN, and sometimes a third party — and the records only take effect if the registrar delegates to it.

The web host runs the server the A record points at. It frequently offers a DNS panel too, which may be editing records nobody is querying.

The diagnostic that settles it: `dig example.com NS` returns the nameservers actually answering for the domain. Records edited anywhere other than those nameservers have no effect at all, however correct they look.