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.
auth required pam_google_authenticator.so nullok
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 “/home/nishv/.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, a new token is generated every 30 seconds by the mobile app. 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. This allows for a time skew of up to 30 seconds between authentication server and client. If you experience problems with poor time synchronization, you can increase the window from its default size of 3 permitted codes (one previous code, the current code, the next code) to 17 permitted codes (the 8 previous codes, the current code, and the 8 next codes). This will permit for a time skew of up to 4 minutes between client and server. Do you want to do so? (y/n) y
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.