Caching and time to live

Because of the huge volume of requests generated by a system like DNS, the designers provided a mechanism to reduce the load on individual DNS servers. When a DNS resolver (that is, a client) receives a DNS response, it caches that response for a specific time, called the time to live, or TTL. The administrator of the DNS server that is handing out the response sets the value of TTL. After a response goes into the cache, the resolver consults its cached (stored) answer. Only when the TTL expires (or when an administrator manually flushes the response from the resolver's memory) does the resolver contact the DNS server for the same information again.

Start of Authority (SOA) record parameters

Generally, the time to live is specified in the Start of Authority (SOA) record. SOA parameters are:

• Serial: The revision number of this zone file. Increment this number each time the zone file is changed so that the changes are distributed to any secondary DNS servers.
• Refresh: The amount of time in seconds that a secondary name server needs to wait to check for a new copy of a DNS zone from the domain's primary name server. If a zone file changed, then the secondary DNS server updates its copy of the zone to match the primary DNS server's zone.
• Retry: The amount of time in seconds that a domain's primary name server (or servers) needs to wait if an attempt to refresh by a secondary name server failed before you attempt to refresh a domain's zone with that secondary name server again.
• Expire: The amount of time in seconds that a secondary name server (or servers) holds a zone before it is no longer considered authoritative.
• Minimum: The amount of time in seconds that a domain's resource records are valid. The minimum is also known as a minimum TTL, and can be overridden by an individual resource record's TTL.
• TTL (time to live): The number of seconds a domain name is cached locally before expiration and return to authoritative name servers for updated information.