1. sudo apt-get install pptpd

  2. sudo nano /etc/pptpd.conf

  3. Add server IP and client IP at the end of the file. You can add like below: 
    localip 192.168.2.100  
remoteip 192.168.2.101-200

    localip is the ip address of the server in the network and remoteip is the range you specify for vpn clients to join your network.

  4. sudo nano /etc/ppp/pptpd-options

  5. Uncomment the ms-dns and add google like below or OpenDNS 
    ms-dns 8.8.8.8  
ms-dns 8.8.4.4
  6. sudo nano /etc/ppp/chap-secrets 
    # client        server  secret                  IP addresses  
username * myPassword *
  7. sudo /etc/init.d/pptpd restart

If you are setting your iphone to use this server set the “Encryption Level” to “Auto”.

If your modem has DMZ setting you don’t have to forward any port but in case you can forward 1723/TCP and 47/UDP to the server.