A Secondary Zone is a type of DNS (Domain Name System) zone that contains a copy of the DNS records from a Primary Zone. Secondary Zones are used to provide redundancy, load distribution, and fault tolerance for DNS resolution. They help distribute the load of DNS queries and ensure that if the primary DNS server hosting the Primary Zone becomes unavailable, other DNS servers can still respond to queries for that domain’s records.

Here’s how Secondary Zones work:

  1. Zone Copy: The DNS records from a Primary Zone are periodically copied to the Secondary Zone. The Secondary Zone’s DNS server requests and receives updates from the Primary Zone’s DNS server.
  2. Read-Only Copies: Secondary Zones are read-only copies of the Primary Zone. They do not allow modifications to the DNS records. Any updates or changes to the DNS records must be made on the Primary Zone.
  3. Redundancy and Load Distribution: By having multiple DNS servers hosting Secondary Zones, the DNS queries for a domain can be distributed among different servers. This improves the load distribution and reduces the chances of a single point of failure.
  4. Fault Tolerance: If the Primary Zone’s DNS server becomes unavailable due to maintenance, network issues, or other reasons, the Secondary Zones can continue to respond to DNS queries, ensuring continued availability of the domain’s DNS records.
  5. Synchronization: Secondary Zones are synchronized periodically with the Primary Zone to ensure that the DNS records in the Secondary Zone are up to date. This synchronization process is known as a zone transfer.
  6. Name Server Configuration: A Secondary Zone is typically configured with a list of authoritative name servers (usually the Primary DNS server and additional secondary DNS servers) that are allowed to transfer the zone’s records. These name servers are responsible for obtaining updates from the Primary Zone.

Secondary Zones are commonly used to improve the reliability and availability of DNS services for domains with high traffic or critical importance. They are especially valuable for organizations that want to ensure uninterrupted DNS resolution even in the event of primary server failures.

In summary, a Secondary Zone is a read-only copy of a Primary Zone’s DNS records that provides redundancy, load distribution, and fault tolerance for DNS resolution.