DNS propagation

What the tool does

NetCheck asks the same question to four widely used public DNS resolvers, Google, Cloudflare, Quad9 and AdGuard, and compares their answers. If they all agree, the new value has been picked up. If they differ, some resolvers are still serving an old answer from their cache.

Why it is called “propagation”

The word is misleading: nothing actually propagates. As soon as you save a change, your name servers give the new answer. What takes time is the expiry of the copies cached by thousands of resolvers, each of which queried your domain at a different moment. Each copy lives at most for the TTL written in the record.

Preparing a change with no waiting

1. The day before, lower the TTL of the record concerned to 300 seconds.
2. Wait at least as long as the old TTL, so every cache has picked up the short value.
3. Make the change: it will be visible everywhere in about five minutes.
4. Check with this tool that all four resolvers agree.
5. Raise the TTL back to a normal value (3600 or more) to ease the load on your name servers.

Reading mixed results

Different values across resolvers are normal during the transition. A few cases worth knowing:

• A resolver shows the old value long after the TTL: some apply a minimum cache time, or keep a stale answer when your servers are hard to reach. Be patient, and check that your name servers answer correctly.
• A resolver says “no records” while the others see the value: this is often a negative cache, created if the name was requested before the record existed. Its duration is set by the zone's SOA record.
• A resolver is unavailable: the request failed from your browser, sometimes because of a blocker or a corporate network. It has nothing to do with your domain.

Everything is up to date here, but not on my machine

Your computer, your browser and your router keep their own caches. On Windows, ipconfig /flushdns clears the system cache; on macOS, sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder. In Chrome, the chrome://net-internals/#dns page clears the browser's cache, and restarting the router clears its own. If your provider uses its own resolver, it may keep the old value a little longer than the public resolvers tested here.

Frequently asked questions

How long does propagation take?

At most the TTL of the old record for an ordinary change, so from a few minutes to a few hours. A change of name servers can take up to 48 hours. Lowering the TTL before the change brings the delay down to a few minutes.

Can propagation be sped up?

Not once the change is made: caches expire at their own pace, and you can only clear your own. The only real speed-up is prepared beforehand, by lowering the TTL.

Why does my site work on my phone but not on my computer?

They use different resolvers, which did not cache the answer at the same moment. Clear the DNS cache of the computer and the browser, or wait for the TTL to run out.

Does the tool check every resolver in the world?

No: four representative public resolvers that serve a large share of internet users. Their agreement is a very good indicator, but an internet provider's resolver can occasionally lag behind.