It is always best to have Two-factor authentication (2FA) to any method of access control. The following post will guide you to enable 2FA on Debian Linux environment.
It is assumed that we will be using Password Authentication in conjunction with 2FA.
Install Google Authenticator
apt-get install libpam-google-authenticator
Edit /etc/pam.d/sshd and add the following.
# Google Authenticator auth required pam_google_authenticator.so
Edit the file /etc/ssh/sshd_config and make sure you have the following enabled.
UsePAM yes ChallengeResponseAuthentication yes
Run Google Authenticator from the account.
Add the account to your Google Authenticator app and save the emergency codes.
Do you want me to update your “/root/.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) y
By default, tokens are good for 30 seconds. In order 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 +-1min (window size of 3) to about +-4min (window size of
17 acceptable tokens).
Do you want to do so? (y/n) 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) y
You should now be able to login by using Two-factor authentication.
If you are using SecureCRT, make sure to use Keyboard Interactive authentication.