Incremental zone transfer

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The incremental zone transfer (from the English Incremental Zone Transfer , abbreviated to IXFR ) is a procedure that is used in the domain name system of the Internet to update zone files . It regulates the communication between a master name server, which has the current data, and the slave name servers, whose data is to be updated.

aims

The aim of the incremental zone transfer is to minimize the amount of data to be transferred. In contrast to the complete Asynchronous Xfer Full Range (AXFR) zone transfer , only the changed records are copied from the master to the slaves . The savings can be significant , especially with the Notify process . In a zone with 10,000 resource records , which are changed frequently and have to be distributed by a master to four slaves, the effort to transfer the entire zone with each change with AXFR would be disproportionately high.

In order to ensure that there are no inconsistencies in the database between the master and slaves, complex bookkeeping of all changes is required on the master. Information about new or deleted RRs is first collected and saved in a file. There are different procedures for doing this, depending on the type of DNS server. The free DNS server BIND, for example, saves all (dynamic) changes in so-called journal files.

restrictions

The incremental zone transfer is practically only useful with the notify method. When using BIND name servers, it should be noted that only dynamic updates are included. In the case of manual changes to a zone file, a BIND server must be reset, which entails a complete zone transfer.

more details

The IXFR is initiated by the slave with a UDP message (DNS query type 251). The master then sends a list of all changes. A list element contains the SOA RR and the changed RR (possibly also several RRs). In the case of a deleted RR, the SOA RR is used before the deletion and in the case of an added RR the SOA is used after the addition. The slave processes the received list and thus synchronizes itself with the master.

A list element with at least two RRs is transmitted for each change. If there are a lot of changes, an incremental zone transfer can be more extensive than a complete one. In such cases, a master may respond to an IXFR request from a slave with an AXFR. In other cases, changes cancel. It can happen, for example, that a new name is added and removed again a little later. The master can then omit these irrelevant intermediate steps when transmitting the list.

Web links

  • RFC 1995 - Incremental Zone Transfer in DNS