A method of updating zone information in newer implementations of the Domain Name System (DNS).
Incremental zone transfer is a more efficient method of propagating zone updates than the earlier standard DNS method of transferring the entire zone file using the AXFR request.
Incremental zone transfer is defined in Request for Comments (RFC) 1995 and uses the IXFR request to transfer only the minimal information needed to keep the DNS servers within a given zone of authority in synchronization.
In incremental zone transfer, every primary or master DNS server maintains a full copy of the up-to-date zone file plus an additional version history that records any changes to resource records that occurred during recent updates of the zone file. When a secondary DNS server makes an IXFR request to a primary or master DNS server, the master server compares the zone version number of the secondary server’s zone to its own current version number.
The zone version number is the serial number stored in the start of authority (SOA) record of the DNS server. If the master server has a newer version number and incremental zone transfers are supported, the master server sends to the secondary server only those changes to resource records that have occurred in the time interval between the two version numbers.
If the version numbers of the master and secondary servers match, no zone transfer takes place. And if incremental zone transfer is not supported, the normal full zone transfer takes place instead.
Incremental zone transfers are supported as part of the dynamic update features of Microsoft Windows 2000.