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A zone transfer is the process of copying a DNS (Domain Name System) database, or "zone file," from one authoritative DNS server to another. The goal is to ensure that a domain's DNS records, which map domain names to IP addresses, are consistent and synchronized across multiple servers.

How zone transfers work: The process involves a primary (or master) DNS server that holds the main, editable copy of a zone file and one or more secondary (or slave) DNS servers that hold read-only copies.

Triggering the transfer: A zone transfer is typically initiated by a secondary server, which regularly checks the primary server for updates. The primary server can also send a notification to secondary servers when a change occurs, prompting them to initiate a transfer.

SOA record check: The secondary server compares the "serial number" in the primary server's Start of Authority (SOA) record to its own. If the primary server's number is higher, the secondary server knows it needs an update.

Data transfer: The secondary server requests the updated zone data, which is sent from the primary server to the secondary server over TCP port 53.