Issue description #
There may be cases in which DNS resolution works without issues but the Citrix ADC appliance (Netscaler) shows the Name Server objects in down state. The reason for this is
Root cause #
From the NetScaler trace files there could be a latency of more than 6 seconds for the monitor probe packets sent out by NetScaler to DNS servers which are ICMP packets. There is a timeout of 2 seconds on the monitor probe packets sent out of NetScaler. Since the monitor probe response does not reach NetScaler within 2 secs, NetScaler is marking the DNS servers as DOWN.
To resolve this issue, create a DNS service on NetScaler with an increased response timeout custom monitor (in this example 10 seconds) and bind it to the DNS service.