Don't have IP protocol 47 (GRE)
Port 42 for name resolution

Don't have IP protocol 47 (GRE)

A PPTP connection will fail if the firewall/router blocks GRE, which is IP protocol 47, and you may receive error 721. If your router does not mention GRE by name or by IP protocol number, look for switches like PPTP pass-through or PPTP Forwarding or even VPN pass-through mode. In some routers, you may have an option to open other ports.

Port 42 for name resolution

To use VPN, you may need to open port 42 for name resolution.