What are primary versus secondary DNS servers: What’s the difference?

A zone file is a text file that contains all the DNS records for a particular domain (or “zone”). Zone data includes DNS records like IP addresses, mail server exchange records and name server records. Essentially, it contains the instructions that direct the internet to your website, email servers, subdomains and other resources.

Within a DNS infrastructure, DNS records are the coin of the realm. They are central to everything that occurs, especially the lookup process administrators used to handle DNS queries—with entered hostnames and corresponding IP addresses.

Keeping up with all the related domains and subdomains involved requires effort. So, enterprises employ two types of management devices to maintain DNS service.

Primary DNS servers maintain the ultimate source of truth for a domain’s DNS records. These authoritative DNS servers hold the master copy of the zone file.

Should a domain administrator need to modify the zone file, those alterations are made directly to the primary zone of the authoritative name server. Access controls are implemented on the primary server to ensure that only authorized personnel can enact changes to zone files.

However, secondary DNS servers primarily serve in a backup capacity—springing into action in case the primary server suddenly goes offline. While it’s technically true that a primary DNS server can be operated without a secondary DNS server, such practice is highly discouraged.

If a failover occurs, secondary zone servers can step in and handle DNS query traffic without having to sacrifice any significant amount of uptime. Without a secondary DNS server, that’s a single point of failure just waiting to happen.

Beyond that important function, secondary DNS servers also assist in the twin causes of load balancing and redundancy. Load balancing helps redirect query traffic as needed to create a type of resource-usage equilibrium. Meanwhile, redundancy ensures that a reliable version of the truth continues to exist, regardless of what’s going on with a particular server. 

For the system administrator, setting up primary and secondary DNS servers can be a challenging affair. Fortunately, many useful tutorials can guide personnel as they implement DNS servers, configure servers for operation and manage servers for optimal results.

It should be noted that one key user of DNS servers is applications. When apps require specific resources, those apps operate as if they were systems sequencing DNS queries. This process consists of taking advantage of the primary server while keeping the secondary server ready for emergency duty should the primary server experience failover.

There are several related technologies associated with the use of primary DNS and secondary DNS.

Active Directory (AD) is Microsoft’s directory service for Windows-domain networks, which the company created to address the need for centralized management of Windows user accounts, computer resources and customized security policies.

AD works in close conjunction with DNS, especially in name resolution. Client devices are configured with both a primary DNS server and a secondary DNS server for backup continuity. These servers are housed in DNS zones that are within Active Directory. Because this zone data has been loaded to Active Directory, it is accessible to anyone with domain control capabilities.

The Dynamic Host Configuration Protocol (DHCP) governs how devices operating on a computer network interact and automatically receive primary and secondary DNS server addresses (along with IP addresses and necessary DNS settings).

DHCP and DNS cooperate in various ways, starting from the moment a device (for example, a phone, laptop or router) connects to a network. The device interacts with a DHCP server, issuing a DHCP request. In response, the DHCP server counters by assigning an IP address along with other key data, such as the IP addresses of primary and secondary DNS servers.

Later steps see the resolution of domain names by the primary DNS, which attaches them to a matching IP address. If the primary DNS server becomes unavailable for service, then the device is going to seek out the secondary DNS server.

Domain name system security extensions (DNSSEC) are a security enhancement that allows DNS providers to ramp up authentication efforts for DNS data. The main technique for it is the enforced use of digital signatures, helping block attacks like DNS cache poisoning and DNS spoofing.

DNSSEC requires that the primary authoritative DNS server uses digital signatures to “sign” and authorize the DNS zone. Further, DNSSEC demands that the primary authoritative server and secondary authoritative server each maintain the signed DNS records. These records can then be sent to recursive servers and outward for further distribution and data validation.

Both sets of servers—primary and secondary—operate as the authoritative sources for that signed DNS zone. They provide signed resource records to recursive resolvers, which then authenticate and validate the data contained in those records.

An Internet Protocol (IPv4) address is a distinct numeric sequence, constituted as four sets of numbers separated by periods. The IPv4 address is formulated in a strictly numeric fashion because this way it lets devices communicate over a network, including the internet.

IPv4 addresses contain 32 bits, which means this format offers approximately 4.3 billion unique addresses possible (based on that many distinct mathematical permutations). At one point, it was thought that it would provide enough unique addresses, but that time came and went and the need for more IP addresses continued to grow.

Even though IPv4 addresses are most certainly still in use, the IPv4 format had to be expanded to offer an even greater number of possible IP addresses. Enter IPv6, the updated format that’s four times as large, with IPv6 addresses weighing in at 128 bits. The step up in scale means that billions of trillions of IPv6 addresses are now possible.

The open source operating systems that comprise Linux function differently regarding their relationship to primary and secondary DNS servers. With Linux, there is no strict adherence to the primary/secondary DNS server model.

Instead, a list of server IP addresses is provided. The client then selects a DNS server based on either the order of server listings or some round-robin arrangement that supports a more equalized distribution of search queries.

The exact role that a Linux machine plays can vary and depends largely on the exact intended context. For example, a home Linux PC serves as a DNS client—it’s configured for use with a particular ISP name server or public DNS.

Meanwhile, if a Linux server is hosting a website, it’s then considered a DNS server and can be treated as the primary server for that domain. (If there’s more than one Linux server being used, extra servers can be set up to operate in a secondary, backup role, for increased redundancy.)