Duo provides simple two-factor authentication as a service via:

    1.  Phone callback
    2.  SMS-delivered one-time passcodes
    3.  Duo mobile app to generate one-time passcodes
    4.  Duo mobile app for smartphone push authentication
    5.  Duo hardware token to generate one-time passcodes

Duo two-factor authentication for OpenVPN

WWW: https://github.com/duosecurity/duo_openvpn