The Expiration Interval (EXPIRE) is a field found in the Start of Authority (SOA) record of a DNS (Domain Name System) zone. It specifies the maximum amount of time (in seconds) that a secondary DNS server should wait before considering its copy of the zone data as expired or stale if it cannot successfully refresh the zone from the primary DNS server. In essence, it acts as a safety mechanism to prevent outdated data from being served by secondary servers in case they lose contact with the primary server.
Here’s what the EXPIRE field in the SOA record signifies:
- Zone Refresh: Secondary DNS servers periodically contact the primary DNS server to refresh their copy of the zone data. This ensures that the secondary servers have up-to-date information. The EXPIRE value comes into play if a secondary server cannot refresh the zone within this interval.
- Safety Net: If a secondary server fails to refresh the zone from the primary server within the EXPIRE interval, it assumes that its copy of the zone data may be outdated. In this case, it will continue to respond to DNS queries using its current zone data for a period of time defined by the Maximum TTL (MAXTTL) value in the SOA record.
- Preventing Stale Data: The EXPIRE value helps prevent the dissemination of stale or outdated DNS data in cases where communication between primary and secondary servers is disrupted or delayed. It ensures that even if a secondary server cannot refresh the data, it will eventually stop serving it.
In summary, the EXPIRE field in the SOA record provides a mechanism for secondary DNS servers to detect when they should consider their copy of the zone data as potentially outdated. This helps maintain the integrity and currency of DNS information while providing a safety net in cases of network issues or server failures.