ZT (Zone Transfer)

DNS zone transfer, also sometimes known by the inducing DNS query type AXFR, is a type of DNS transaction. It is one of the many mechanisms available for administrators to replicate DNS databases across a set of DNS servers.

A zone transfer uses the Transmission Control Protocol (TCP) for transport, and takes the form of a client–server transaction. The client requesting a zone transfer may be a slave server or secondary server, requesting data from a master server, sometimes called a primary server. The portion of the database that is replicated is a zone.

Zone transfer consists of a preamble, followed by the actual data transfer. The preamble comprises a lookup of the Start of Authority (SOA) resource record for the “zone apex”, the node of the DNS namespace that is at the top of the “zone”. The fields of this SOA resource record, in particular the “serial number”, determine whether the actual data transfer need to occur at all. The client compares the serial number of the SOA resource record with the serial number in the last copy of that resource record that it has. If the serial number of the record being transferred is greater, the data in the zone are deemed to have “changed” (in some fashion) and the slave proceeds to request the actual zone data transfer. If the serial numbers are identical, the data in the zone are deemed not to have “changed”, and the client may continue to use the copy of the database that it already has, if it has one.

Related Articles