
Zone transfer is the process of replicating a zone database file to multiple DNS name servers. It is achieved by copying the zone database file information from a master server to the secondary server. A master server is the source of the zone information, and can be either a primary or secondary server in that zone.
The zone transfer process is initiated when one of the following occurs:
The two methods for replicating zone information are:
Most DNS implementations use and support AXFR. When the refresh interval elapses on a secondary server, it queries its master server by using an AXFR query. The secondary server detects whether its local copy of a zone is the same as its master server's copy by comparing serial numbers for the zone. If the secondary server detects that its copy of the zone database file is not current, it copies the entire contents of the zone database file from its master server.
IXFR is a recent, RFC-defined DNS implementation that is included in Windows 2000, and can reduce the amount of zone data that is transferred during replication. IXFR also uses serial numbers to determine if changes have been made to a zone database file. If changes have been made, only the resource records that have changed are transferred, rather than the entire zone database file. Changes and additions are kept in the cache until the secondary server has received all of the updated information.
| Note Windows NT 4.0 supports AXFR only, but Windows 2000 supports both AXFR and IXFR. |
RFC 1995.