DNS Vserver is configured on the NetScaler
Client machines use LB vserver IP for DNS.
After adding a new entry on the backend DNS server, Clients still receive a null response (No such Name)

DIsable DNS caching on DNS Vserver by configuring a new DNS profile or modifiying the Global Default-dns-profile

Example:
add dns profile no-cache -cacheRecords DISABLED -cacheNegativeResponses DISABLED
set lb vserver DNS-Vserver -dnsProfileName no-cache
where "DNS-Vserver" is the name of the DNS LB Vserver.

This will disable the default DNS caching on this Vserver. "DNS-Vserver" for both positive and Negative responses. After this, the DNS Vserver should forward all requests to the backend servers for DNS resolution.

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Problem Cause

By default, a DNS Vserver will cache both positive and Negative DNS responses received from the backend server.
As a result, if a Negative response is cached, the NetScaler will continue to respond with the Negative response until the DNS cache is updated/flushed.