What is DNS round robin?

DNS round robin is a traffic distribution method that assigns multiple IP addresses to a single domain name and rotates those addresses for DNS requests. It’s designed to spread traffic across different servers, which can help reduce reliance on a single server.

How does DNS round robin work?

A network administrator creates multiple DNS records for the same domain, each pointing to a different server IP address. When a DNS server responds to a request for that domain, it returns all the addresses but shifts their order with each response.

For example, a domain with three servers might return its addresses in this order:

First lookup: 192.0.2.1, 192.0.2.2, 192.0.2.3
Second lookup: 192.0.2.2, 192.0.2.3, 192.0.2.1
Third lookup: 192.0.2.3, 192.0.2.1, 192.0.2.2

Devices may connect to whichever IP address appears first, so rotating the order is intended to balance requests across all available servers. How DNS round robin distributes traffic across three servers by rotating IP address order.

Why is DNS round robin important?

DNS round robin provides a simple form of load balancing, directing traffic to multiple servers instead of overwhelming one. Because it’s set up using standard DNS configuration, it doesn't require additional hardware or specialized software.

It also adds basic redundancy. If one server becomes unavailable, other servers in the rotation can still receive requests. This doesn't guarantee that traffic will automatically avoid the failed server, but it can reduce reliance on a single system.

Downsides of DNS round robin

Not true load balancing: DNS round robin doesn't consider server health, response time, or current load. Systems that need traffic directed based on real-time conditions require a dedicated load balancer.
Caching can skew distribution: Recursive resolvers cache DNS records, meaning they’ll serve the same IP address order until the DNS record’s time to live (TTL) expires. This can funnel a disproportionate share of traffic to one server.
Limited session persistence: Consecutive lookups for the same domain can return different address orders. Applications that depend on a user returning to the same server, such as those storing login sessions locally, may not work reliably with DNS round robin.
Client-side behavior varies: Not all devices connect to the first address in the list. Some operating systems reorder addresses based on how closely a server's IP matches the device's own network range. This can override the intended rotation and concentrate traffic on specific servers.
Fast flux abuse: Fast flux is a method used to prevent malicious domains from being identified by their IP addresses. Although DNS round robin is commonly used for legitimate traffic distribution, attackers may abuse these techniques to achieve fast flux.

FAQ

Is DNS round robin the same as load balancing?

DNS round robin is a basic form of load balancing. It rotates IP addresses in DNS responses, but it doesn’t consider server health, capacity, or location when distributing traffic.

Can DNS round robin detect server failures?

No. DNS round robin doesn’t check whether a server is online before returning its IP address. If a server fails or is taken down, its address may still be returned until the DNS records are updated.

Does DNS caching affect DNS round robin?

Yes. DNS responses are often cached for a period of time. While cached, multiple users may receive the same address order, which can send more traffic to one server.

Is DNS round robin secure enough for critical services?

DNS round robin doesn’t monitor servers or adjust routing automatically. Critical services usually require systems that can detect failures and direct traffic accordingly.

What is the difference between DNS round robin and GeoDNS?

DNS round robin rotates IP addresses in order. GeoDNS returns different IP addresses based on the user’s location. This allows GeoDNS to direct users to nearby servers, while DNS round robin spreads traffic across all servers.