11.4 Load Sharing Between Mirrored Servers

The Microsoft DNS Server has a feature called round robin (named after the equivalent feature in the BIND name server): the server rotates address records for the same domain name between responses. For example, if the domain name foo.bar.baz has three address records for IP addresses 192.1.1.1, 192.1.1.2, and 192.1.1.3, the round-robin feature causes the name server to give them out first in the order:

192.1.1.1 192.1.1.2 192.1.1.3

then in the order:

192.1.1.2 192.1.1.3 192.1.1.1

and then in the order:

192.1.1.3 192.1.1.1 192.1.1.2

before starting over again with the first order and repeating the rotation ad infinitum.

This functionality is enormously useful if you have a number of equivalent network resources, such as mirrored FTP servers, web servers, or terminal servers, and you'd like to spread the load among them. You establish one domain name that refers to the group of resources and configure clients to access that domain name, and the name server inverse-multiplexes the accesses between the IP addresses you list.

It's a good idea to reduce the records' TTLs, too. This ensures that, if the addresses are cached on an intermediate name server that doesn't support round robin, they'll time out of the cache quickly. If the intermediate name server looks up the name again, your authoritative name server can round-robin the addresses again.

Note that this is really load sharing, not load balancing: the name server gives out the addresses in a completely deterministic way, without regard to the actual load or capacity of the servers servicing the requests. In our example, the server at address 192.1.1.3 could be a 486 running Linux and the other two servers could be quad-processor Itanium2s, and the Linux box would still get a third of the load.