GlossaryDNS
DNS propagation
Definition
“DNS propagation” is the everyday name for the delay between a DNS change and the moment everybody sees it. The word is misleading: nothing is being pushed anywhere. Each resolver simply keeps its old answer until its TTL expires, then asks for a fresh one.
Why the distinction changes what you do
If something really were being pushed from server to server, all you could do is wait without knowing for how long. Because it is caches expiring one after another, the wait has an upper bound and you can calculate it: it cannot exceed the TTL that was in force before the change. Nothing about the rest is random.
The second consequence is that contradictory tests are normal. A colleague already seeing the new site while you still see the old one is just two resolvers whose caches expired at different moments. So the useful question is not whether it has propagated, but what the domain's own name server answers — that one is always current.
What actually happens, minute by minute
An address change on a record whose TTL was 3,600 seconds.
t+0 min The new address is published on the domain's name server.
t+0 min A resolver with nothing cached reads it: it already sees the new site.
t+12 min A resolver that asked at t-48 min still serves the old one.
t+60 min The last cache filled just before the change expires.
t+60 min No resolver can serve the old answer any more.There is no wave rolling forward: there are caches expiring, each on its own schedule, all of them before the original TTL runs out. That is why a change can be instantly visible in one place and invisible in another with nothing at all going wrong.
The usual mistake
Making the change again because it “does not look like it went through”, then making it again after that. Each attempt resets the clock for resolvers that have just read the value, and you end up publishing a wrong address in a hurry.
How to avoid it: Change it once, then verify at the source by querying a name server declared for the domain. If it answers with the right value, the work is done: all that is left is the wait that no amount of clicking shortens.
Where the sources disagree: The delays hosts quote — “up to 24 hours”, sometimes 48 or 72 — are commercial margins, not standardized values. The only delay you can calculate is the TTL that was in force before the change.
Source: Caching and cache expiry are described in RFC 1034 and RFC 1035. The word “propagation” appears in neither: it is industry shorthand, not a term from the standard.
Related terms
Where to go next
Knowing the words is half the job
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