Time Synchronization

TOTP requires that the system time be accurate, so synching the server with a Network Time Protocol (NTP) service is recommended. While not a requirement for TOTP to work, NTP will ensure system time synchronization with an external service. By default, Ubuntu Server 22.04 uses systemd-timesyncd for system time synchronization, but it does not have an NTP server configured by default. To determine if the NTP service is active, use the following command:

$ timedatectl status

I recommend adding NIST's NTP service as the primary NTP server, and Ubuntu's own NTP service as a fallback. To do this, uncomment the #NTP= and #FallBackNTP=ntp.ubuntu.com lines in /etc/systemd/timesyncd.conf, and change NTP= to NTP=time.nist.gov. After these configuration changes have been made, restart the timesyncd service with:

# restart systemd-timesyncd.service

The system should now synchronize its time with the NIST NTP service.

Configuration

The OpenSSH server must be configured to use PAM. In /etc/ssh/sshd_config, look for the configuration item UsePAM yes. On Ubuntu Server, sshd is configured to use PAM by default. Next, switch the KbdInteractiveAuthentication option from No to Yes.

Once these configuration changes have been added to sshd_config, you will need to add the Google Authentication module to PAM.

PAM looks for service-specific configuration files within /etc/pam.d/ by default. OpenSSH server has a file located at /etc/pam.d/sshd. Add this configuration item to the bottom of line:

auth required pam_google_authenticator.so nullok

The temporary nullok option, used for testing, allows users that have not generated a secret key to still authenticate with SSH using only their password. Once keys have been added to all SSH users, the nullok option will be removed to enforce MFA.

Finally, reload the SSH server configuration with:

# systemctl reload sshd

Generating a Key

Next, you will need to generate a key for users. If you are logging in with the user account via SSH, you will see Verification code: after entering the password or authenticating with your SSH key. Just click Enter. Because you added nullok during PAM configuration, you will still be authenticated. Once logged in, run google-authenticator to generate a secret key. When you run the command, you will be prompted to choose whether the token is time based; enter y for yes.

A QR code will be displayed, which you can scan with the Google Authenticator app on your phone. A plaintext key will also be displayed, which should be treated with the security of a password. You should write down and securely store the plaintext key or save it to a password manager. This plaintext key can be used to re-add the TOTP key to Google Authenticator if the existing phone is lost, replaced, or broken. After adding the QR code or key to your Google Authenticator app, you can enter the current code displayed in the app for the next prompt in the shell console to verify it is working (you can use -1 to skip this, but it is not recommended). You will also receive several "scratch codes," which can be used in emergencies if the Google Authenticator device is not available. Next, you'll be given several prompts for security options; choose y for yes for all of them (see Figure 1) The rate limiting options are especially important because these options will prevent an attacker from attempting a brute-force attack on a time-based password.

Figure 1: For each security question prompt, enter y.