Use Linux PPTP client to connect Windows VPN server

Example: Server is foo.bar.com, administrator's password is 1234

First, add CHAP info to /etc/ppp/chap-secrets
printf "administrator vpn 1234 *" >> /etc/ppp/chap-secrets
Connect method 1
pppd pty "pptp foo.bar.com --nolaunchpppd" remotename vpn name administrator

Connect method 2, create /etc/ppp/peers/vpn as follow

pty "pptp 192.168.1.231 --nolaunchpppd"
name administrator

ref. PPTP Client | Poptop - The PPTP Server for Linux