Once you have installed libpam-duo or login-duo, you will need to configure
the /etc/security/pam_duo.conf or /etc/security/login_duo.conf files
respectively with the Duo Security keys.

To test the login_duo configuration, run as a regular user:

    $ login_duo echo YOU ROCK

If this is successful, make sure the pam_duo configuration setting
are valid, then you can enable the PAM module by updating the pam
auth stack, and selecting it as the root user:

    # pam-auth-update

To not skip this PAM module when using SSH, either enable
"ChallengeResponseAuthentication=yes" in /etc/ssh/sshd_config or see
"man duo", "man pam_duo", and "man login_duo" for more details.