What is the SOA record?

There are numerous types of DNS records, and for importance to understand DNS, we have to know how they work and their purpose. The SOA record is one of them, and it is one of the most common records. Let’s explain what it is and why it is essential to have it.

What is the SOA record?

The SOA record is a fundamental DNS record. It indicates the start of authority. It will point to the nameserver that will contain the original zone file. This server will have all the important information about the zone, and it will be the authoritative DNS server. The SOA record is the first that a zone file contains and establishes the general properties of the zone.

Typically, DNS servers operate together in a cluster. All of them are required to synchronize their zone file. To achieve that purpose, they need to perform a zone transfer. The SOA record is like a control record. It has a serial number and shows which is the newest update. Secondary servers, also called slave servers, see that the serial number changes. Then, they update and get the latest data from the authoritative server. 

Zone transfer

The Domain name typically has more than one DNS server. One is the primary DNS server, and the others are secondary DNS servers.

The primary has the original zone file, and you can make all the changes you want inside it. The information from it is going to be propagated to the rest. This is possible through a zone transfer. The zone transfer simply is a process of updating the zone file in the secondary DNS servers. It could be through IXFR zone transfer (partial transfer of changes only) or AXFR zone transfer (complete transfer of all DNS records).

Why do you need an SOA record?

The SOA record is required when you want to indicate the authoritative name server and to achieve a successful zone transfer. It is important for every zone to have an SOA record. Also, you must know that each zone should contain only one SOA record. In case that the zone does not have an SOA record or if you insert there are more than one, your zone will not work. So, be careful and don’t make such a mistake. 

Structure

The SOA record contains inside the following elements:

  • Name – The name of the zone that the DNS admin has put.
  • Type – The type of the DNS record, which in this case will be SOA.
  • Primary name server – The hostname of the authoritative DNS server for that zone.
  • Admin’s email – It shows the email of the DNS administrator for that zone.
  • Serial number – The serial number of the zone that was mentioned before. The secondary DNS servers check this number and determine whether to update their DNS records or not.
  • Refresh rate – This number will show in seconds how frequently the secondary DNS server needs to re-visit the authoritative DNS server and check for changes.
  • Retry rate – If the zone transfer between the secondary DNS servers and the primary fails, this is the time the secondary servers will remain trying to update. If the time expires, the secondary servers’ data will be outdated, and they will stop answering queries. 
  • Default TTL – The number is a time period showing how long the DNS records are valid. After the time runs out, the secondary DNS servers must discard them and perform a new zone transfer again.