What “DNS propagation” really is
A DNS change is not distributed. It applies on your name servers immediately. What takes time is forgetting: every resolver has cached the old answer for the length of its TTL and only asks again afterwards.
- The TTL sets the waiting time. If the old record had a TTL of 86400 (one day), resolvers may serve the old value for up to a day. With 300 seconds it is five minutes at most.
- Tip before a move: lower the TTL to 300 a day before, then switch, and raise it again afterwards. The switch then completes in minutes.
- New records are usually visible immediately. Exception: if the name was queried before it existed, resolvers remember the “does not exist” for the negative TTL from the SOA record.
- Name server changes at the registrar take longer. The delegation in the parent zone (e.g. .com) often has a TTL of a day or more.
How to read the result
At the top you see how many resolvers already return the value your authoritative name servers are serving right now. The tool queries them directly, without a cache. If a resolver still answers with the old value, the remaining TTL next to it shows how long it will keep it at most.
If even the authoritative name servers disagree, the problem is on your side: usually a secondary name server is out of sync. Then compare the SOA serial of all name servers (type “SOA”).
Local cache
If all resolvers here show the new value but your computer still shows the old one, the local cache is to blame. Flush it with ipconfig /flushdns (Windows), sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder (macOS) or resolvectl flush-caches (Linux with systemd-resolved). Browsers sometimes have their own cache; in Chrome at chrome://net-internals/#dns.
The DNS lookup shows all records of a name at once.
FAQ
How long does DNS propagation take?
As long as the TTL of the old record, usually between five minutes and a day. The often quoted “up to 48 hours” only applies to name server changes or very high TTL values.
Why does a resolver still show the old value after the TTL has expired?
Some resolvers consist of many servers with separate caches, and your query may land on a different one depending on location. Some providers also keep records a little longer than specified.
Can I speed up propagation?
Only beforehand, by lowering the TTL in time. Some providers also let you flush the cache for a name, e.g. Google (developers.google.com/speed/public-dns/cache) and Cloudflare (1.1.1.1/purge-cache).
Why do the name servers say “unreachable over TCP”?
The tool queries the name servers over TCP because UDP is blocked on the web server. Name servers must support TCP according to RFC 7766, but not all do. Then the majority of resolvers is used for comparison.
Are my queries stored?
No. The names are only passed to the resolvers for the query and are not logged anywhere.