Knowledgebase: DNS
Where are DNS records cached, and how can this be changed?
Posted by Darren Radsmith on 16 July 2010 01:22 PM

There are multiple locations where DNS records can be cached.

The first, and probably the most important is the resolving name server. A resolving name server is typically provided by an end client's ISP. This is the server that will recieve the request from the client's computer, ask DNS Made Easy for the answer, store the answer, and then return this to the client's computer. The length of time that this record is held in cache (in memory) on the resolving name server is set by the TTL that you define in DNS Made Easy. A resolving name server or "resolver" will abide by this TTL and expire the cached lookup once the TTL expires. It is not possible to clear the cache of a client's resolving name server unless they operate their own internal resolving name server.

The TTL is the amount of time that your record will cache in remote systems / resolving name servers. So the longer the TTL the less frequent remote systems will lookup your DNS record and the less query traffic you receive. The shorter the configured TTL, the faster DNS changes propagate in servers that have cache data, however the higher the volume of query traffic you receive.

DNS Made Easy suggests all records that are using Failover or on Dynamic IPs have TTL's set anywhere from 180 to 600 (3 to 10 minutes cache). Records that are static and don't change often should have TTL's set between 1800 (being on the low end) to 86400 seconds (30 minutes to 1 day cache). Any added NS records or glue records for vanity DNS configuration should have high TTL's set. You can always lower the TTL before you make a record change and then raise it back up again.

Another location that DNS records can be cached is in the client's application or operating system. Many operating systems have independant internal cache so a request to the resolving name server for every single lookup is not required. In Windows based operating systems for example, this internal cache is stored in the windows registry. The Microsoft knowledgebase article KB 318803 includes details about internal cache for some Windows applications / operating systems.

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