DNS propagation check
Query the same record against six DNS resolvers in parallel. Disagreement usually means recent changes haven't propagated yet.
About propagation
When you change a DNS record, the new value doesn't reach every resolver simultaneously. Each resolver has its own cache, and the cache lifetime is controlled by the TTL on your old record. If your previous TTL was 86400 (24 hours), some resolvers will keep serving the old value for up to a full day after you update.
This tool queries six major public resolvers in parallel and shows what each returns. If they all agree, your change has fully propagated. If they disagree, you can see which ones are still caching the old value.
Persistent disagreement (more than a few hours after a change) usually points to one of: a higher-than-expected TTL on the previous record, split-horizon DNS serving different answers to different networks, or DNSSEC validation failures at one of the resolvers.