What is zone transfer?
Zone transfer is a process of duplicating the DNS information (DNS records) from the Primary DNS zone to the Secondary DNS zone. That way, you are able to establish multiple copies of your DNS records on several name servers. As a result of completing the transfer, you are going to ensure better availability if one of the name servers goes down. In addition, you are going to guarantee faster DNS resolution in case you own an international website with users coming from all around the globe and multiple points of presence (PoPs).
Full zone transfer (AXFR zone transfer)
The Full zone transfer, also named AXFR zone transfer, is a complete transfer of the entire DNS information (DNS records) from the Primary (Master) DNS server to a Secondary (Slave) DNS server. When you utilize this type of transfer, it is going to guarantee that the complete collection of DNS records is updated and fresh. In addition, they are going to be identical to the DNS records inside the Primary DNS zone.
It is best to utilize the AXFR transfer when you are building one or several new Secondary DNS servers, and you wish to duplicate the DNS records straight away.
Another case when you could use full transfer is when you know that you have one or more Secondary DNS servers that haven’t been updated recently.
Partial zone transfer (IXFR zone transfer)
Partial zone transfer, or IXFR transfer, is the alternative to just sharing the latest modifications from the Primary DNS zone to the Secondary DNS zone. It is not necessary to copy all of the DNS data. The IXFR is beneficial when you have modified only a small number of DNS records. For illustration, you have created 2 new A records, while the rest are the same. The Secondary DNS server is able to get only the latest data. It can be assigned to scan after a certain amount of time if there is a difference in the serial numbers of the SOA record. In case the serial number is lower, it is going to request and obtain the modifications from the Primary DNS server.
As you see, the IXFR transfer requires less data transfer, and in most circumstances, specifically, when you have already arranged the Secondary DNS servers, it is the more functional practice.
How does it work?
Every time you make adjustments and changes to your Primary DNS zone, they have to be duplicated to the Secondary DNS servers. Yet, there are some rules about how and when these changes must be transferred. That happens thanks to the SOA (Start of Authority) record. It carries a refresh rate for the Slave DNS servers. It refers to the amount of time (in seconds) Slave DNS servers are going to wait before requesting the Master DNS server for the SOA record to review if it was updated. On the other hand, the serial number, which is also inside, signifies the Slave DNS server if its version requires to be updated. When the serial number changes in a DNS zone file, this alerts Slave DNS servers that they should update their duplicates of the zone file through a DNS transfer.