Soa updating zone files
If the master has a more recent copy of the zone than the slave then the slave requests a transfer.
For this scheme to work the serial number of each zone must increase monotonically.
Additional information is available in RFC 1034, Domain Names—Concepts and Facilities section 3.6.1, and RFC 1035, Domain Names—Implementation and Specification section 5.
You’ll start by dissecting a domain name, and then you’ll learn about the mechanics of DNS resolution, including name servers, zone files, and individual DNS records.
Note A zone file uses a standard format known as BIND to represent resource record sets in a text format.