The Minimum TTL (Time to Live) is a field found in the Start of Authority (SOA) record of a DNS (Domain Name System) zone. It specifies the default time duration in seconds for which resource records (RRs) in that zone should be cached by DNS resolvers and other DNS servers if no specific TTL value is set for those RRs.

Here’s what the Minimum TTL (MINIMUM) field in the SOA record signifies:

  1. Default TTL: When a DNS server or resolver receives a DNS response that includes resource records (such as A records, MX records, CNAME records, etc.) from a particular zone and those records don’t have their own TTL values set, they inherit the TTL specified in the Minimum TTL field of the SOA record. This means the Minimum TTL acts as a default TTL value for records in the zone.
  2. Caching: The TTL values associated with DNS records control how long the records are cached by DNS resolvers and servers. If, for example, a DNS resolver queries a DNS server for an A record and receives a response with a TTL of 3600 seconds (1 hour) for that record, it will cache that record for 1 hour before expiring it. If no specific TTL is set for the record, and the Minimum TTL in the SOA record is 900 seconds (15 minutes), the DNS resolver will use 900 seconds as the default TTL for that record.
  3. Propagation: The Minimum TTL also plays a role in DNS record propagation. When changes are made to DNS records in a zone (e.g., updating an IP address), DNS servers across the internet will cache these changes according to the TTL values specified. The Minimum TTL ensures that even if no specific TTL is set for a record, there is a limit to how long outdated information is cached.

In summary, the Minimum TTL in the SOA record helps control how DNS records are cached and how long cached information remains valid. It’s an important element for DNS management, especially when ensuring timely updates and changes to DNS records across the internet.