Use Google Authenticator For Two-Factor SSH Authentication in Linux
From scottlinux.com
For a very secure and convenient way to add two-factor ssh auth to your Linux server, you can use Google Authenticator. I’ll show you how to set this up!
1. In Ubuntu 13.04:
$ sudo apt-get install libpam-google-authenticator
2. Then, from an open bash shell with your user account on that server and run the following:
$ google-authenticator
Scan the bar code with your Android phone or iPhone to add the key. (Or alternatively, open the URL provided on your phone to add the key.)
Note: save the emergency scratch codes in case you need to access the server without your phone!