My VBox guest is Ubuntu, and the network mode is NAT. I cannot use Bridge Network mode. I tried to connect to my VPN from the guest, but failed.
I can connect to the VPN on another VBox guest, which network is bridged. So I think the NAT mode is causing the problem. I think it is because I did not configure the
port-forwarding rules correctly. But I have no idea which port the PPTP program uses.