Enabling Google Authenticator for SSH Logon on Linux Server

This guide explains how to set up Google Authenticator for Two-Factor Authentication (2FA) on a Linux server for SSH logon.

Prerequisites

1. A Linux server (Ubuntu/Debian/CentOS, etc.)
2. SSH access to the server
3. A smartphone with the Google Authenticator app installed

Steps

1. Install Google Authenticator PAM Module

For Ubuntu/Debian-based systems:

sudo apt update
sudo apt install libpam-google-authenticator

For CentOS/RHEL-based systems:

sudo yum install epel-release
sudo yum install google-authenticator

2. Configure the PAM (Pluggable Authentication Modules)

Edit the PAM configuration file for SSH:

sudo vim /etc/pam.d/sshd

Add the following line to the file:

auth required pam_google_authenticator.so

3. Configure SSHD to Allow PAM Authentication

Edit the SSH configuration file:

sudo vim /etc/ssh/sshd_config

Make sure the following settings are configured:

• ChallengeResponseAuthentication should be set to yes:

  ChallengeResponseAuthentication yes

• UsePAM should be set to yes:

  UsePAM yes

4. Restart the SSH Service

After modifying the SSH and PAM configuration files, restart the SSH service for the changes to take effect:

sudo systemctl restart sshd

5. Set Up Google Authenticator for Your User

Run the following command as the user you want to set up 2FA for:

google-authenticator

This will trigger a series of prompts:

• Do you want authentication tokens to be time-based? (Yes)
• Do you want to update your "~/.google_authenticator" file? (Yes)
• Do you want me to disallow multiple uses of the same token? (Yes)
• Do you want to increase the verification time window? (Yes)

It will then generate a QR code and show secret keys. Scan the QR code with the Google Authenticator app on your smartphone.

6. Testing the Setup

To test, open a new SSH session and log in. You will be prompted for:

1. Your regular password.
2. A verification code from your Google Authenticator app.