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Multiple Name Servers: As described in the previous tutorial on DNS, two to seven name servers can be registered against a domain name. These are DNS servers that contain information on where to find services (such
as www or email) for a particular domain name. There are two main reasons for providing more than one DNS server for each domain. The first is to provide protection against a DNS server going down- if a DNS server failed for any reason (eg. power outage,
communication failure etc.) the domains using this DNS server would be unobtainable since the IP address of these domains could not be retrieved. Instead, having two or more (which should be geographically and topologically seperate) means that if a request
to one of them fails, the name server can be requested again. The name servers will be served in rotation and so the next request will be a different DNS server and the chances of two DNS servers going down independently to each other are very slim. Lets
walk through what happens when one DNS server fails for a request to www.wight-hat.com. The name servers registered against the domain name wight-hat.com are "ns1.wight-hat.com 87.106.82.7" and "ns2.wight-hat.com 64.15.153.83".
1) The ISP requests the name server for wight-hat.com and receives the first name server (it can receive either as they are served in rotation for each request). The name server information is "ns1.wight-hat.com 87.106.82.7".
2) A request is sent to 87.106.82.7 for the location of wight-hat.com but, this DNS server is not online and the request fails.
3) A second request is made for the name server for wight-hat.com and receives the second name server (it can receive either as they are served in rotation for each request). The name server information is "ns2.wight-hat.com 64.15.153.83".
4) The ISP request is sent to 64.15.153.83 which is a different DNS server and receives the required information.
Round Robin configuration: The second reason for multiple name servers is closely related to the first. Knowing that the name servers are delivered in rotation, if the DNS servers point to different IP addresses then each request could be delivered
to different servers in rotation. Suppose a very high traffic website was starting to show a speed lag because the server that the site was on could not handle the amount of requests/ traffic/ processing. An additional server could be employed so that it
would share the load. However, how do you share the load when you have one domain name that everyone is accessing the website through? You configure one DNS server to point to the first web server and the seconds DNS server to point to the second web server.
In this way, each request will receive alternating DNS servers and these will give alternating IP addresses for the website servers thus sharing the load 50/50. With up to seven name server entires it is simple to configure up to seven web servers to share
the load of a very high traffic website. This is known as load balancing.
We discuss a slight variation on this principle next in the tutorial to provide redundancy as well as load balancing using a similar technique.
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