There are three communication ports on the customer network that can be used. These ports need to be open outbound. Outbound firewall rules define the traffic that is allowed to leave the server, on which ports, and to which destinations.
Port 443 - Port 443 is the standard TCP port used for websites that use SSL. Websites that use https at the beginning of their URL (or web address) are connecting to port 443.
UDP 123 - UDP port 123 is the port for SNTP (Simple Network Time Protocol) that is used to synchronize computer clocks on a network. It provides time synch between computers and network systems and assists in database management, auth schemes, and audit/logging accuracy.
UDP 53 - Protocol / Name: domain. Port Description: Domain Name Server (DNS). DNS servers offer different services on TCP and UDP. TCP is used for "zone transfers" of full name record databases, while UDP is used for individual lookups. The DNS uses TCP Port 53 for maintaining coherence between the DNS database and the server. UDP protocol is used when a client sends a query to the DNS server. For example, port 53 converts pdk.io to its IP address. The VPN needs to connect to a cloud link to know what the IP address is.