DNS Propagation and Caching

Any time you move a domain to a different IP address, you will run into at least some DNS propagation. While it cannot be avoided entirely, its effects can be minimized.

DNS propagation is simply the amount of time it takes for updates to a DNS record to propagate, or spread, throughout the world’s entire DNS infrastructure. If there was no DNS caching involved, there would be no such thing as DNS propagation; the entire internet would query your nameserver for every DNS lookup, and would get the changed information instantly. That would come at the cost of greatly increasing the traffic to your nameservers. To keep the strain on nameservers down, the designers of DNS implemented DNS caching.

In DNS caching, the servers that run DNS queries for internet users (known as ‘resolvers’) cache a copy of the domain’s zonefile the first time the user asks for that domain’s information. For a set period of time after that, that server never asks the nameserver for the domain’s zonefile. Instead, it answers all DNS questions using the zonefile that it has cached. This dramatically cuts down on the load and bandwidth of DNS nameservers the world over.

It is this caching that causes propagation. For example, if a resolver caches a zonefile for four hours, and an IP address is changed on the nameserver one hour into that period, the resolver will continue to hand out wrong DNS information for the next three hours, until the cache period is up. Once all the resolvers around the globe have started caching the latest zonefile, then propagation has ended.

A smart feature of this system is that you, the domain owner, can control that set period of time right in the zonefile. The TTL (Time To Live) value is the number of seconds that a DNS resolver is allowed to cache a zonefile before asking for a new copy of that zonefile. Normally, this value is set at 14400 seconds, or 4 hours. If you are going to make a DNS change, Liquid Web recommends that the TTL be set at 300 seconds, or 5 minutes, a full 24 hours before the DNS change is actually made. Full instructions on how to lower TTLs can be found here: How To: Lowering Your DNS TTLs.

One final point: some ISPs (Internet Service Providers) do not honor TTLs, and instead refresh their resolvers’ DNS cache once per day. Unfortunately, lowering TTLs does not lower the propagation time for these resolvers.

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Liquid Web’s Heroic Support is always available to assist customers with this or any other issue. If you need our assistance please contact us:
Toll Free 1.800.580.4985
International 517.322.0434
support@liquidweb.com
https://manage.liquidweb.com/

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