How does DNS caching work?

A DNS cache keeps recent answers so every request does not need a complete lookup through the naming hierarchy. Caches may exist in an application, the operating system and the recursive resolver. Removing an answer from one of them does not immediately replace copies elsewhere.

What determines the lifetime

A DNS record carries a TTL: the number of seconds for which a received answer may be reused from a cache. A resolver normally counts down the remaining lifetime while it stores the record. If a domain owner changes an address, an existing cached answer can remain valid until its old TTL expires. Lowering the TTL at the same time as the address change cannot shorten the lifetime of copies fetched earlier.

Negative answers may also be cached. A resolver might have received NXDOMAIN for a name that had not yet been created. After the record is added, some users can still see that earlier negative answer for a while. RFC 2308 defines the relevant negative caching behaviour.

Locate the stale answer

Query the domain’s authoritative server, a chosen public recursive resolver and your own device in that order. If the authoritative answer is still old, correct the zone or its delegation; clearing a local cache will not help. If the authoritative answer is new but the local one is old, examine the remaining TTL and retry after it expires. The DNS configuration tool can inspect domain records.

Ask for the precise type you need. A cached A record cannot explain a wrong MX answer, while an AAAA record may direct IPv6 traffic to a different server from IPv4. Check the full hostname, including any subdomain. A browser may use its own DoH resolver, so its result can differ from a system utility.

When to clear a local cache

Clearing a local cache can help after a correction when your device alone keeps using the older result. The exact command depends on the operating system and active resolver; identify that component first. The action cannot flush a provider’s cache, a public resolver or another person’s computer.

Do not confuse a DNS cache with a browser page cache. One holds naming information; the other holds site content. Even a correct IP address does not prove that HTTP, HTTPS or a mail service works. Identify the layer giving the unexpected answer before changing settings.

A concrete example

A site moves from 192.0.2.10 to 192.0.2.20. Its authoritative server gives the new address, while a home router still returns the old cached value. A direct authoritative query shows the new record; a query through the home network shows the earlier one and its remaining TTL. After expiry, the answers should agree unless another configuration problem remains.

These addresses are reserved for documentation, not a real website. In practice, record the change time, old and new values, TTL and answers from named servers. That evidence is more useful than waiting for an unspecified moment when DNS has supposedly “fully propagated”.