Every computer and network device participating in the Internet communicates with each other using an IP address (Internet Protocol). To make it user-friendly and easier to remember, we use domain names to identify those devices. The Domain Name System (DNS) is utilized to map domain names to IP addresses. Therefore, when wanting to connect to machines, users can simply use a memorable string of characters (domain name) such as: www.microsoft.com, www.ibm.com, instead of using a lengthy numeric IP address that is hard to remember.
A Domain Name Server (DNS Server) is a server that is configured to provide DNS name resolution services. DNS servers are categorized into two types as follows:
Primary DNS Server (PDS)
The Primary DNS Server (PDS) is the official source of authentication for the domain names it is authorized to manage. Information about a domain managed by the PDS is stored here and can later be transferred to Secondary DNS Servers (SDS).
Domains managed by the PDS are created and modified at the PDS before being updated to the SDS.
Secondary DNS Server (SDS)
It is recommended to use at least two DNS servers to store addresses for each zone. The PDS manages the zones, and the SDS is used to provide backup storage for the zone, as well as for the PDS. While the SDS is not mandatory, it is advisable to use one. The SDS is allowed to manage domain names, but the data about the domains is not created from the SDS; it is obtained from the PDS.
The SDS can perform operations in a load-free manner on the network. When the volume of zone queries increases significantly, the PDS can offload some requests to the SDS (this process is also known as load balancing), or if the PDS encounters an issue, the SDS will take over until the PDS is operational again.
SDS is typically used in locations close to client machines to facilitate easy query servicing. However, it is not advisable to install the SDS on the same subnet or the same connection as the PDS. This would be a good solution for backing up the PDS, as when the connection to the PDS fails, it does not affect the SDS.
Additionally, the PDS always maintains a large amount of data that frequently changes or adds new addresses to zones. Therefore, the DNS server employs a mechanism to transfer information from the PDS to the SDS and store it on disk. When it is necessary to restore data about the zones, we can utilize a full retrieval approach or only retrieve the incremental changes.