Preguntas Frecuentes - FAQ

Two-Factor Authentication (2FA) adds a second layer of protection to SSH: even if your password or key were compromised, an attacker still couldn't log in without a time-based code from your phone. This guide uses Google Authenticator PAM module, which works with any TOTP app (Google Authenticator, Authy, 1Password, etc.).

Prerequisites

• Ubuntu 22.04/24.04 or Debian 11/12 VPS
• Root or sudo access
• An authenticator app on your phone

Step 1 — Install the PAM Module

sudo apt update
sudo apt install libpam-google-authenticator -y

Step 2 — Generate a Token for Your User

google-authenticator

Answer the setup prompts:

• Generate time-based tokens: Yes
• Update the configuration file: Yes
• Disallow multiple uses of the same token: Yes
• Increase brute-force protection window: Yes
• Enable rate limiting: Yes

Scan the displayed QR code with your authenticator app, and save the printed emergency recovery codes somewhere safe.

Step 3 — Configure PAM

sudo nano /etc/pam.d/sshd

Add this line at the end:

auth required pam_google_authenticator.so

Step 4 — Configure SSH

sudo nano /etc/ssh/sshd_config

Make sure these are set:

KbdInteractiveAuthentication yes
UsePAM yes

If you use SSH key + 2FA together (recommended), also add:

AuthenticationMethods publickey,keyboard-interactive

Step 5 — Restart SSH

sudo systemctl restart ssh

Step 6 — Test in a New Session

Without closing your current session, open a new terminal and connect. You should be prompted for your SSH key (if configured) and then a verification code.

Common Errors

Locked out after enabling 2FA — use your VPS provider's console to log in and remove the pam_google_authenticator.so line from /etc/pam.d/sshd.

Codes rejected even though correct — your server's clock may be out of sync. Check with timedatectl and enable NTP.

Best Practices

• Always keep the emergency recovery codes in a secure, offline location
• Combine 2FA with SSH keys (AuthenticationMethods publickey,keyboard-interactive) rather than passwords + 2FA
• Test 2FA on a new session before closing your existing one

FAQ

Does this work with any authenticator app?
Yes, any TOTP-compatible app works: Google Authenticator, Microsoft Authenticator, Authy, 1Password, etc.

What if I lose my phone?
Use one of the emergency recovery codes generated during setup, or your provider's console access.

Continue Reading

• SSH Hardening: Change the Port, Disable Root Login & Use SSH Keys
• How to Install and Configure Fail2Ban
• VPS Security Checklist for Beginners

Browse more articles in Server Security & Hardening.