Tasks of the DNS server...

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Check our cache for answer
All nameservers do this

If answer is in cache
All nameservers remember answers previously obtained

Retention period determined by TTL

TTL is set by administrator of remote zone
Return answer to client
Answers returned from cache are considered non-authoritative

If answer is not in cache
Find authoritative nameserver by traversing the namespace to learn the answer

Cache all the answers along the way
Return the answer to the client
Answers from an authoritative nameserver (primary or secondary) are considered to be authoritative