Domain Name Servers (DNS) are the Internet's equivalent of a phone book. They maintain a directory of domain names and translate them to Internet Protocol (IP) addresses.

When you request DNS resolution of a domain name, the NetScaler appliance uses the configured Load Balancing method to select a DNS service. The DNS server to which the service is bound then resolves the domain name and returns the IP address as the response. The appliance can also cache DNS responses and use the cached information to respond to future requests for resolution of the same domain name. Load Balancing DNS servers improves DNS response times.

The NetScaler also allows caching of negative responses for a domain. A negative response indicates that information about a requested domain does not exist, or that the server cannot provide an answer for the query. The storage of this information is called negative caching. Negative caching helps speed up responses to queries on a domain. 

The advantage of DNS cache is retrieving an IP address from the local cache is almost instantaneous. This eliminates many delays and speeds up response time for the end user.

Instructions

To configure a basic DNS Load Balancing setup:

1. Go to System, Traffic Management, Load Balancing and Services.
   

2. Add a new DNS service by selecting the protocol as DNS under Basic Settings.
   

3. Go to System, Traffic Management, Load Balancing and add a new Virtual Servers of type DNS by selecting the protocol as DNS under Basic Settings.
   

4. Click “Load Balancing virtual Server Service Binding” under Services and Services Groups.
   

5. Click on “Click to select” and select the DNS service from the list.
   
   

6. Click Bind to bind the DNS service to a DNS Virtual Server.
   

Once the NetScaler is configured as a DNS proxy, it returns the DNS records in the order in which it receives the records from the server. 

Note: The above steps indicates how we can create a UDP based DNS Load Balancing virtual server.
In some cases,  you may also consider to create a TCP based DNS Load Balancing virtual server at the same time in case that your DNS requests transparently switch to TCP (typically for large DNS response).
The steps to create a TCP based DNS Load Balancing virtual server is same as above, except that you need to choose DNS_TCP under Protocol in step 2 and step 3. The IP and port needs to be same with the UDP based DNS load balancing virtual server.

For more information about Load Balancing feature in NetScaler, please refer to the following link:
Set up basic load balancing