Introduction to DNS
Introduction to DNS |
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Full zone transfers can consume a great deal of network bandwidth, especially for complex DNS configurations. To solve this problem, RFC 1995 specifies an additional standard, incremental zone transfer. With incremental zone transfer, only the modified part of the zone must be transferred.
Incremental zone transfer works much the same as full zone transfer. The secondary server for the zone still uses the SOA resource record to determine when to poll the master server for the zone, when to retry, and so on. However, if it needs to perform a zone transfer, it sends an incremental zone transfer (IXFR) query instead of an AXFR query, requesting that the master server for the zone perform an incremental zone transfer.
The master server for the zone, meanwhile, maintains a recent version history of the zone, which observes any record changes that occurred in the most recent version updates of the zone. Then, if the master server for the zone has a newer version of the zone, it can forward only those record changes that have occurred between the two different versions of the zone (the current versions on the master and secondary servers) to the secondary server for the zone. The master server sends the oldest updates first and the newest updates last.
When the secondary server receives an incremental zone transfer, it creates a new version of the zone and begins replacing its resource records with the updated resource records, starting with the oldest updates and ending with the newest updates. When all of the updates have been made, the secondary server replaces its old version of the zone with the new version of the zone.
The master server for the zone is not required to perform an incremental transfer. It can perform a full zone transfer if it does not support incremental zone transfer, if it does not have all the necessary data for performing an incremental zone transfer, or if an incremental zone transfer takes more bandwidth than a full zone transfer.
Even though incremental zone transfer saves network bandwidth, it uses space on the server to record the version history. To conserve space, servers can purge the version history.