DNS zone transfer, also sometimes known by its (most common) opcode mnemonic AXFR, is a type of DNS transaction. It is one of the many mechanisms available for administrators to employ for replicating the databases containing the DNS data across a set of DNS servers. Zone transfer comes in two flavors, full (opcode AXFR) and incremental (IXFR).

Nearly universal at one time, it is now becoming less popular in favor of the use of other database replication mechanisms that modern DNS server packages provide. Zone transfer operates on top of the Transmission Control Protocol (TCP), and takes the form of a client-server transaction. The parties involved in a zone transfer are a client (the "slave" requesting the data from a portion of the database to be transferred to it) and a server (the "master" supplying those data from its database).

Some sources refer to the slave as a "secondary" server and the master as a "primary" server. The portion of the database that is replicated is a "zone". Zone transfer comprises a preamble followed by the actual data transfer.

The preamble comprises a lookup of the SOA (Start of Authority) 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 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 numbers differ, 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.

Similar to 0 things you like: