If your Horizon implementation uses a Unified Access Gateway appliance instead of a security server, users use client drive redirection with the PCoIP display protocol, and the Horizon Client and Horizon Agent machines are on different networks, the UDP Tunnel Server must be enabled for the Unified Access Gateway appliance.
To enable the UDP Tunnel Server, in the Unified Access Gateway admin UI, set the UDP Tunnel Server Enabled setting to Yes.
If you do not enable the UDP Tunnel Server, users cannot use the client drive redirection feature with the PCoIP display protocol. Client drive redirection works with the VMware Blast display protocol, regardless of whether the UDP Tunnel Server is enabled.
For more information, see the Unified Access Gateway documentation.