A DNS AXFR zone transfer is a process in which a DNS server can request a copy of the entire zone file from another DNS server. This allows the receiving server to have an up-to-date copy of the DNS records for a particular domain, which it can use to answer DNS queries from clients.
To understand how a DNS AXFR zone transfer works, it's first important to understand the role of DNS servers. DNS, or Domain Name System, is a distributed database that maps human-readable domain names (like example.com) to IP addresses (like 192.168.1.1). When a client, such as a web browser, wants to access a website, it sends a DNS query to a DNS server to get the IP address for the domain name. The DNS server then looks up the IP address in its database and responds to the client with the correct IP address.
A DNS zone is a collection of DNS records for a particular domain. This includes records for subdomains as well as the "apex" domain (the domain without any subdomains, like example.com). The zone file is a text file that contains the DNS records for a particular zone. It's organized into several sections, including a section for the start of authority (SOA) record, which provides information about the DNS server responsible for the zone, and a section for the resource records, which contain the actual DNS records.
When a DNS AXFR zone transfer occurs, one DNS server (the "secondary" server) requests a copy of the zone file from another DNS server (the "primary" server) that is authoritative for the domain. The primary server then sends the entire zone file to the secondary server. The secondary server can then use this information to update its own copy of the zone file and answer DNS queries for the domain.
One important thing to note is that DNS AXFR zone transfers only work between DNS servers that have been specifically configured to allow them. This is because DNS AXFR zone transfers can be a security risk if an unauthorized server is able to request and receive a copy of the zone file. For this reason, it's important to carefully control which DNS servers are allowed to perform AXFR zone transfers.
ZoneWatcher allows you to track your DNS records via AXFR zone transfers (as well as many other DNS providers) and get alerts when records change!