Check the following:
1. Status of DNS Name Server under Traffic Management > DNS > Name Server.
2. If the DNS Name Server is Down check using nstcpdump or nstrace if the ADC is sending any probe to DNS server or not.
3. If it is not sending delete the DNS name Servers and create LB services for all the name Servers on protocol DNS and port 53.
4. If the DNS service is down and monito error is : “No MIP/SNIP configured to send probe”, create a net-profile for a particular SNIP i.e. reachable and bind the same to DNS service.
5. Once the DNS service is UP create a DNS LB Vsevrer (preferrably non-addressable as it won’t need any new IP-Address) and bind the DNS service to it.
6. Bind the DNS LB Vserver as DNS name server.
Check the status of the DNS based service and is should be UP.
If it is still Down, please contact Citrix HelpDesk.