Slave/Secondary DNS Server (Read Only DNS Server) How AXFR & IXFR Works in DNS for Zone Transfer ?



Slave/Secondary  DNS Server (Read Only DNS Server)

How AXFR & IXFR Works in DNS for Zone Transfer  ?

The term slave simply indicates that a name server will obtain the zone records using zone transfer operations but will answer authoritatively for the zone for as long as it has valid zone data (defined by the expiry field of the zone’s SOA RR). The term slave in no sense implies priority of access. As previously described, slave servers will be accessed, in general, just as frequently as any master name server. Slave servers may also transfer their zone files to other slave servers.

 


The Zone Transfer between Master Server and Secondary Server
What is AXFR ?

AXFR Transfer: The DNS polling process is accomplished by the slave name server sending a query to the zone master requesting the SOA RR. If the SOA RR’s serial number is greater than the current one maintained by the slave name server, a full zone transfer (AXFR) is requested by the slave DNS. This is the reason it is vital to be disciplined about updating the SOA serial number every time anything changes in any of the zone records


The Serial number format may be used as follows ;

YYYYMMDDSS:
For e.g 2014010800 -- > Serial number of a  zone training.com
           
If there is any change of Record for e.g Change of A Record or update of Record of the zone training.com
The new Serial number will be -à  2014011000
Next Serial number will be -à 2014011001(the Last 2 Digit will increase by 1 number if there are multiple change of record for the same zone file in one day)


What is  IXFR Transfer(Incremental Zone Transfer):

IXFR Transfer(Incremental Zone Transfer): Transferring very large zone files can take a long time and waste bandwidth and other resources. It is especially wasteful if only a single record has been changed! RFC 1995 introduced the incremental zonetransfer (IXFR), which (as the name suggests) allows the slave name server and master name server to transfer only those records that have changed.The process works as for AXFR. The slave name server sends a query for the domain’s SOA RR to the zone master every refresh interval. If the serial number of the SOA RR is greater than the one currently stored by the slave, the name server requests a zone transfer and indicates whether or not it is capable of accepting an IXFR. If both master and slave name servers support the feature, an IXFR takes place; otherwise, an AXFR takes place. IXFRs use TCP on port 53


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