Traditionally, in the Authoritative DNS tree, the data for a zone was stored in a Master file on the primary DNS server. However, the fact that many IP numbers are getting dynamically assigned through DHCP has raised the need to be able to remotely edit zones, which is why DNS Update was invented.

When using DNS update, changes for zones that use DNS Update are no longer done in the master file, but through DNS messages, and though the the primary DNS server still is the only server that physically stores the contents of the zone, this may no longer happen in a master file.

The way it works is that any client can send a DNS Update query to the primary DNS server1). Then, the primary, through some kind of access control system, determines whether the client is allowed to do the update specified, and if so, it commits the changes, sends out notifications to the slaves, and serves the new zone contents in zone transfers.

For example, DHCP servers such as ISCs DHCP can send DNS Update messages whenever they assign an IP number to a client, thus automatically pointing the domain name for the client to the right IP number. Posadis’ Zoneedit tool can be used to remotely administer zones through DNS Update. The syntax of DNS Update messages is described in the DNS Messages article.