Install PPTP VPN Server in VICIBOX

I never thought I need to use PPTP VPN server ever again as more advanced and more secure VPN technologies such as OpenVPN are up in the market and can be used. But recently one of my friends wanted to connect his DINSTAR UC2000-VG GSM Gateway to VICIbox 9 Dialer that is using OpenSUSE Leap 15. Dinstar UC2000-VG GSM Gateway just supports PPTP VPN connection so I needed to run PPTP on OpenSUSE Leap. I had some challenges with the setting of the firewall as I just thought I need to open the PPTPD port but actually, I needed to allow GRE as well. So here is how to set up and configure PPTP VPN in Suse Leap. I hope someone finds it useful.
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http://omid-mohajerani.blogspot.com/202 ... TEWAY.html