DNS Propagation Checker

"DNS propagation" is a slightly misleading name for what is actually happening. Nothing spreads outward from your nameserver. When you change a record, your authoritative nameserver is updated instantly — but every resolver that already asked for the old value is holding it in cache until the TTL you set on that record expires. What feels like a change rippling across the internet is really thousands of independent caches quietly timing out at different moments.

That mechanic explains the symptom that drives people to a tool like this: you moved a site to a new host, it loads perfectly for you, and a colleague two streets away still sees the old server. Neither of you is wrong. Your resolver had a stale entry that has since expired; theirs has not. There is no way to force someone else's resolver to forget early, which is why the only real fix is planning — lowering a record's TTL to a few minutes a day or two *before* a migration, so the cached copies everyone is holding are short-lived by the time you cut over.

This checker queries A, AAAA, MX, TXT, and NS records from three independent public resolvers at the same instant: Cloudflare, Google, and Quad9. Because they operate separate cache infrastructure, disagreement between them is direct evidence that a change is still working through. Agreement across all three is a reasonable signal that most of the internet has caught up, though it is a sample, not a guarantee.

It queries each resolver over DNS-over-HTTPS rather than traditional port 53, because the serverless platform this runs on blocks outbound UDP. The records returned are the same; only the transport differs. One consequence worth knowing: DoH endpoints answer from the resolver's own cache, which is exactly what you want here — you are trying to observe cache state, not bypass it.