Install a VPN Server (PPTP) on Debian

Step 1:
nano /etc/apt/sources.list

deb http://ftp.us.debian.org/debian stable main contrib non-free
deb http://ftp.debian.org/debian/ jessie-updates main contrib non-free
deb http://security.debian.org/ jessie/updates main contrib non-free
deb http://packages.dotdeb.org jessie all    
deb-src http://packages.dotdeb.org jessie all
apt-get update
apt-get install pptpd


pico -w /etc/pptpd.conf
(or use your favorite text editor, like vim)

Add the local and remote IP pool and the end of file:

localip 172.16.5.133 (server ip address)
remoteip 172.16.5.150-160 (client ip adress)

pico -w /etc/ppp/pptpd-options
add the following at the end of file:

name pptpd
 refuse-pap
 refuse-chap
 refuse-mschap
 require-mschap-v2
 require-mppe-128
 ms-dns 8.8.8.8
 #ms-dns 8.8.4.4
 proxyarp
 nodefaultroute
 lock
 nobsdcomp
 mtu 1490
 mru 1490

 nano /etc/ppp/chap-secrets
# client        server                    secret                  IP addresses

ali                       *                  123Pass                  172.16.5.151


Edit /etc/sysctl.conf and enable ipv4 forwarding by un-commenting the line (removing the # sign) and changing 0 to 1 so it looks like this

net.ipv4.ip_forward=1

Save & exit the editor, then run:

sysctl -p

and you should apply destination nat to 1723 port at outside interface.