If the application that is published from a XenApp server can be launched successfully however VDA from a XenDesktop server fails to launch then ensure that DNS port is not blocked between NetScaler Gateway and internal DNS server. Run the following command to disable the DNS resolution in the XenDesktop server. Set-BrokerSite –DnsResolutionEnabled 0
XenDesktop server returns the VDA messages to STA server and STA server forwards the VDA messages to the NetScaler Gateway when the client launches the VDA.
Run the command Get-BrokerSite in the XenDesktop server PowerShell.
The DnsResolutionEnabled setting changes depending on whether ICA files are returned by a broker service to a user device that contains the IP address or the DNS name of the VDA.