Editing Time to live (section)

==DNS records==
[[File:TTL of a DNS answer resolving google.com, seen in Ubuntu Linux Wireshark.png|thumb|TTL of a DNS answer resolving google.com, seen in [[Wireshark]]]]
TTLs also occur in the [[Domain Name System]] (DNS), where they are set by an [[Domain Name System#Authoritative name server|authoritative name server]] for a particular resource record. When a caching (recursive) [[name server|nameserver]] queries the authoritative nameserver for a resource record, it will cache that record for the time (in seconds) specified by the TTL. If a [[stub resolver]] queries the caching nameserver for the same record before the TTL has expired, the caching server will simply reply with the already cached resource record rather than retrieve it from the authoritative nameserver again. TTL for NXDOMAIN (non-existent domain) responses is set from the minimum of the MINIMUM field of the [[List of DNS record types#SOA|SOA]] record and the TTL of the SOA itself, and indicates how long a resolver may cache the negative answer.{{ref RFC|2308}}{{Technical statement|date=November 2014}}

Shorter TTLs can cause heavier loads on an authoritative name server, but can be useful when changing the address of critical services like [[web servers]] or [[MX record]]s, and therefore are often lowered by the DNS administrator prior to a service being moved, in order to reduce possible disruptions.

The units used are seconds. An older common TTL value for DNS was 86400 seconds, which is 24 hours. A TTL value of 86400 would mean that, if a DNS record was changed on the authoritative nameserver, DNS servers around the world could still be showing the old value from their cache for up to 24 hours after the last update by client.

Newer DNS methods that are part of a [[IT disaster recovery|disaster recovery]] (DR) system may have some records deliberately set extremely low on TTL. For example, a 300-second TTL would help key records expire in 5 minutes to help ensure these records are flushed quickly worldwide. This gives administrators the ability to edit and update records in a timely manner. TTL values are "per record" and setting this value on specific records is sometimes honored automatically by all standard DNS systems worldwide. However, a problem persists in that some caching DNS nameservers set their own TTLs regardless of the authoritative records, thus it cannot be guaranteed that all downstream DNS servers have the new records after the TTL has expired.