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A DNS record’s TTL (Time To Live) sets the time the record may be cached. During this time, clients don’t ask the authoritative server for information. Frequent values vary from 1 hour to 1 day. Very small TTL values (below 300s) may be ignored by resolvers.
It is important to note, that TTL values are stored per-record, not per-zone (domain).
A DNS entry will be resolved for the client by a DNS resolver. A DNS update can have the following effects:
- If the resolver was not asked the record before the update, it performs a recursion and returns the new value.
- If the resolver was asked the record before the update and the record did not exist at that time, the negative cache TTL is in effect. (This is the last parameter of the SOA record.) The client will be returned a servfail.
- If the resolver was asked the record before the update and the record did exist at that time, the record’s TTL value is in effect. The client will be returned to the old setting.