DNS propagation

“DNS propagation” is an informal phrase for the period after DNS data changes when different people may still receive different answers. DNS does not push a new record to every resolver, the service that looks up answers, around the world. Instead, the nameservers responsible for publishing the DNS data provide the updated records. Temporary stores called caches may continue to reuse earlier answers until those answers expire.

Why can an old answer remain visible?

When a resolver looks up a record, it can cache the answer for the record's TTL, or time to live. If a website owner changes an address record, a resolver that cached the old address may keep using it until the cached copy expires. Another resolver that has no old copy can retrieve the updated address right away. This is why two people may temporarily see different results after the same DNS change.

For example, a shop changes its A record at 10:00. A customer's home resolver cached the previous address at 09:55 with a one-hour TTL, while a phone on another network has no cached answer. The phone may get the new address first; the home connection can keep using its cached answer until that cache entry expires. The exact time varies with what each resolver cached and when.

What should you check after changing DNS?

First confirm that the record was changed in the set of DNS records published by the domain's current nameservers. Then compare the authoritative answer with answers returned by the resolver used on the affected network. A change entered at an old provider may not matter if the domain's nameserver delegation points somewhere else.

If the record is correct at the authoritative source but a particular resolver still returns the old answer, its cache may not have expired yet. A resolver may also have saved an earlier answer saying that a name was missing. This is called a negative cache entry. This is why the phrase “wait for propagation” is not a reliable diagnosis by itself: the underlying cause may be a cached answer, an edit to the wrong zone, or another configuration issue.

Related terms

• DNS cache — stores answers that may outlive a recent change
• TTL — limits how long an ordinary DNS answer can be reused
• DNS delegation — determines which authoritative nameservers are asked