DNS propagation - how long it takes and how to check it

DNS propagation is the time between changing a record in DNS (for example moving a site to a new server) and the moment the whole internet starts seeing the new value. If you ask what DNS propagation is, picture a rumor in a small town: you already know something has changed, but the neighbors only find out once someone tells them. On the internet those "neighbors" are thousands of DNS servers around the world that keep answers in a cache.

The key to everything is TTL (Time To Live) - the number of seconds DNS servers are allowed to remember the old answer before asking again. If a record has a TTL of 3600 (one hour), a server that fetched it a minute before your change will keep serving the old value for another 59 minutes. That is why DNS propagation takes anywhere from a few minutes to a few hours, and in extreme cases up to 24-48 hours - for example when changing the NS servers of a whole domain, or when the TTL was set to a full day.

What does the time depend on? Mainly three things: the TTL value of the record being changed, the type of change (a plain A record propagates quickly, an NS delegation change is the slowest) and how aggressively operators cache (some internet providers ignore low TTLs). Your own computer has a cache too - which is why sometimes "it works for everyone except you".

How do you speed up propagation? The only trick that works must happen BEFORE the change: 24-48 hours in advance, lower the TTL of the record to, say, 300 seconds (5 minutes). The old, long TTL will expire in time, and the actual change will spread within minutes. Afterwards you can raise the TTL back up. How do you check DNS propagation? Use the free DNS Lookup tool at /dns - you will see the current record values along with their TTLs and instantly know whether the world already sees the new configuration or still the old one.