DNS Load Balancing

How DNS load balancing works
When the request comes to the DNS server to resolve the domain name, it gives out one of the several canonical names in a rotated order. This redirects the request to one of the several servers in a server group. Once the BIND feature of DNS resolves the domain to one of the servers, subsequent requests from the same client are sent to the same server.

DNS load balancing implementation (Multiple CNAMES)
This approach works for BIND 4 name servers, where multiple CNAMES are not considered as a configuration error. Assuming there are 4 web servers in the cluster configured with IP addresses 123.45.67.[1-4], add all of them to the DNS with Address records (A Names) as below. The srv[1-4] can be set to any name you want, such as foo[1-4], but should match the next step.

The DNS server will resolve the www.domain.com to one of the listed servers in a rotated manner. That will spread the requests over the group of servers.

Note: The requests sent to http://domain.com (without www) should be forwarded to http://www.domain.com in this case to work. For BIND 8 name servers, the above approach will throw an error for multiple CNAMES. This can be avoided by an explicit multiple CNAME configuration option as shown below.

DNS load balancing implementation (Multiple A Records)

This above approach with multiple CNAMES for one domain name is not a valid DNS server configuration for BIND 9 and above. In this case, multiple A records are used.