DNS on Windows Server 2003, 3rd Edition

Choosing Between Authoritative Name Servers

Some of the card-carrying Mensa members in our reading audience may be wondering how the name server that receives the recursive query chooses between the name servers authoritative for the zone. For example, we said that there are 13 root name servers on the Internet today. Does the name server simply query the one that appears first in the referral? Does it choose randomly?

The Microsoft DNS Server uses roundtrip time (RTT) to choose between name servers authoritative for the same zone. Roundtrip time is a measurement of how long a remote name server takes to respond to queries. Each time a Microsoft DNS Server sends a query to a remote name server, it starts an internal stopwatch. When it receives a response, it stops the stopwatch and makes a note of how long that remote name server took to respond. When the name server must choose which of a group of authoritative name servers to query, it simply chooses the one with the lowest roundtrip time.

Before a Microsoft DNS Server has queried a name server, it ranks it according to how many octets its IP address has in common with the local host’s. This is designed to favor remote name servers on the same or nearby networks.

On the whole, this simple but elegant algorithm allows Microsoft DNS Servers to “lock on” to the closest name servers quickly and without the overhead of an out-of-band mechanism to measure performance.