The incremental zone transfer (also IXFR called) is a procedure, which is used in the Domain Name System of the Internets for updating zone files. It regulates communication between a master name server, which has the current data, and which Slave Nameservern, whose data are to be updated.

A goal of the incremental zone transfer is it to minimize the quantity of the data which can be transmitted. Contrary to the complete zone transfer AXFR therefore only the changed sentences are copied from the master to the Slaves. Savings can particularly with the Notify procedure (see: Zone transfer) substantially its. One imagines for instance a zone with 10000 resource record, which is frequently changed and by a master at four Slaves be distributed must. With each change with AXFR the complete zone files are overacted!

In order to guarantee that between master and Slaves no inconsistencies of the volume of data develop, it is necessary on the master a record keeping of all changes. Information about new or deleted RRs is first collected and in a file secured. Depending upon DNS Servertyp in addition different procedures exist. BIND for example stores all (dynamic) changes in so-called journal files.

Detail

The IXFR is introduced by the Slave with a UDP Message (DNS Query type 251). The master sends thereupon a list with all changes. An element of a list contains thereby the SOA-RR and the changed RR (possibly also several RRs). With a deleted RR the SOA-RR before the deletion is used and with an added RR of the SOA after adding. The Slave processes the received list and synchronizes in such a way with the master.

Per change an element of a list with at least two RRs will thus transfer. When very many changes a incremental zone transfer can be more extensive than more complete. A master may react in such cases to a IXFR Request of a Slaves with a AXFR. In other cases changes waive themselves. So it can occur about that a new name is added and removed a little later again. The master can omit then these irrelevant intermediate steps during the transmission of the list.

Notes

The incremental zone transfer is meaningful practically only with the Notify procedure. With the employment by binding name servers it is to be noted that only dynamic updates are included. When manual changes zone files must a binding server be put back, which draws a complete zone transfer.

Related links

• RFC 1995 - Inc. rem valley zone transfer into DNS

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