Multi-factor Authentication with Google Authenticator on Ubuntu Servers

Introduction

This tutorial describes how to set up multi-factor authentication with Google Authenticator on your Ubuntu server when using SSH keys.

I like using SSH keys instead of passwords but wanted an extra level of security. So implementing a second factor to confirm the user's identity was the logical choice. We will be using Google Authenticator in this tutorial to generate one-time passwords that need to be provided during the login process.

Requirements

The following equipment is needed:

• Ubuntu 16.04 LTS or newer
• a mobile phone / tablet with the Google Authenticator app installed

1. Android: https://play.google.com/store/apps/details?id=com.google.android.apps.authenticator2&hl=en&gl=US
2. iOS: https://apps.apple.com/us/app/google-authenticator/id388497605

There are no other requirements. Basically any server or VPS from netcup can be used. I have successfully implemented this and am currently using it on a VPS 1000 G9 and an older VPS 1000 G8. The G9 is running Ubuntu 20.04 LTS.

I am assuming that you already have a working SSH server setup and are just looking to add an extra level of security. Take a look at the other tutorials as well, using SSH keys instead of passwords is also recommended.

Please also take a snapshot before attempting this tutorial in order to be able to rollback any changes made! If something goes wrong, you could loose access to your server.

Step 1 - Installing packages

Log in and install the required packages. We will be using the Google Authenticator PAM module. (More info can be found on the project's github: https://github.com/google/google-authenticator-libpam)

Install it via the following command:

sudo apt install libpam-google-authenticator

Step 2 - Configuration

Step 2.1 - SSH

In order to use the newly installed module we will have to edit our sshd file. I use nano as my editor.

sudo nano /etc/pam.d/sshd

Add one of the following lines to the file:

auth required pam_google_authenticator.so

To allow login for users who have not yet created a secret key, use

 auth required pam_google_authenticator.so nullok

instead of the line above. I always configure secrets for my users right away, so I prefer option 1.

Close and save.

In order to apply the changes, restart the sshd daemon by running:

sudo systemctl restart sshd.service

Step 2.2 - SSH daemon

We need to make some changes to /etc/ssh/sshd_config by running:

sudo nano /etc/ssh/sshd_config

Look for the line "ChallengeResponseAuthentication". You want to change it from "no" to "yes".

In the end it should look like this:

## Change to yes to enable challenge-response passwords (beware issues with
## some PAM modules and threads)
ChallengeResponseAuthentication yes

Close and save.

Step 2.3 - Setting up Google Authenticator

Run the following command with each user you want to use mfa with.

google-authenticator

You will be asked several questions. I choose to answer them like this, but feel free to make changes depending on your needs:

• Make tokens time-base: yes
• Update the .google_authenticator file: yes
• disallow multiple uses: yes
• increase the original generation time limit: no
• enable rate-limiting: yes

You should now see a QR code, a secret key, a verification key, and several emergency codes. Write the emergency scratch codes down and store them in a safe place. You will need them if you loose access to your device or the app.

Step 2.4 - Using the Google Authenticator app

Install the Google Authenticator app from your App Store of choice. In order to generate tokens for your server, you can either scan the QR code or enter the secret key. On most of my machines the QR code tends to get distorted due to the size of my shell, so I usually go with the manual entry method. If you choose manual mode, you will be asked for a name. Choose something that you easily recognize. If you have several servers, as I do, you want to have names that easily correlate to the servers, otherwise you might end up entering the wrong tokens. Then enter the secret key generated in Step 2.3. Now you should see the generated code being displayed in the Authenticator app. The little circle on the right indicates the time remaining before a new token is generated.

Step 2.5

We are all done. Reboot your server and try to log in. After entering your password, you should now be asked for a token.

Conclusion

Every time you try to log in you will now be asked for a second factor. Keep your phone / tablet with you, otherwise you won't be able to log in.

License

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