Time to Live for Queries

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Time to Live for Queries

A name server caches all the information it receives when it resolves queries outside its own zone files. These cached responses can then be used to answer queries for the same information in the future. These cached entries don’t stay on the DNS Server forever, however. There is a time to live (TTL) for the responses to make sure the DNS Server doesn’t keep information for so long that it becomes out-of-date. The time to live for the cache can be set on each DNS Server.

There are two competing factors to consider when setting the time to live. One is the accuracy of the cached information. If the TTL is short, then the likelihood of having old information goes down considerably. If the TTL is long, then the cached responses could become outdated, meaning the DNS Server could give false answers to queries. The accuracy of the responses is also dependent on how stable your environment is. If names change often, then a short TTL is necessary. It is important, however, to consider the load on the server and the network. If the TTL is large, the server can answer more queries from cache and doesn’t use local resources or network bandwidth to send out additional queries.

If a query is answered with an entry from cache, the TTL of the entry is also passed with the response. This way other DNS Servers that receive the response know how long the entry is valid. Other DNS Servers honor the TTL from the responding server; they don’t set it again based on their own TTL. Thus entries truly expire rather than live in perpetuity as they move from server to server with an updated TTL. Resolvers (clients) also have a cache, and honor the TTL received from a name server that answered a query from the name server’s cache.

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