Time To Live, or TTL for short, is the sort of expiration date that is put on a DNS record. The TTL serves to tell the recursive server or local resolver how long it should keep said record in its cache. The longer the TTL, the longer the resolver holds that information in its cache. The shorter the TTL, the shorter amount of time the resolver holds that information in its cache.
For example, we’ve got example.com. Example.com has an A-record at the apex of the zone to point us to a server. With a TTL of 3600 seconds, or 1 hour, that means that as a recursive server learns about example.com, it will store that information about the A-record at example.com for one hour. Anyone else who uses that same resolver will get the same answer, and on the authoritative side, there will be no query to the server unless the TTL runs out.
The SOA TTLs
At the top of every DNS zone, in the Start of Authority (SOA), there are five TTL values that serve a higher purpose in the DNS.
SOA TTL – The interval at which the SOA record itself is refreshed.
Refresh TTL – The interval at which secondary servers (secondary DNS) are set to refresh the primary zone file from the primary server.
Retry TTL – The rate at which a secondary server will retry to refresh the primary zone file if the initial refresh failed.
Expiry TTL – If Refresh and Retry fail repeatedly, this is the time period after which the primary should be considered gone and no longer authoritative for the given zone.
NX TTL – In the event that requesting the domain results in a non-existent query (NXDOMAIN), this is the amount of time that is respected by the recursor to return the NXDOMAIN response.