% sudo apt-get install libpam-google-authenticator

auth    [success=1 default=ignore]      pam_unix.so nullok_secure
auth required pam_google_authenticator.so

% google-authenticator
Do you want me to update your "/home/USER/.google_authenticator" file (y/n) y
Do you want to disallow multiple uses of the same authentication token? This restricts you to one login about every 30s, but it increases your chances to notice or even prevent man-in-the-middle attacks (y/n)
By default, tokens are good for 30 seconds, and to compensate for possible time-skew between the client and the server, we allow an extra token before and after the current time. If you experience problems with poor time synchronization, you can increase the window from its default size of 1:30min to about 4min. Do you want to do so (y/n)
If the computer that you are logging into isn't hardened against brute-force login attempts, you can enable rate-limiting for the authentication module. By default, this limits attackers to no more than 3 login attempts every 30s. Do you want to enable rate-limiting (y/n)