// Tutorial //

How To Protect SSH With Two-Factor Authentication

Published on April 29, 2013
Default avatar
By Tim Kornhuber
Developer and author at DigitalOcean.
How To Protect SSH With Two-Factor Authentication

Status: Deprecated

This article is deprecated and no longer maintained.

Reason

This article duplicates more recent, accurate content.

See Instead

This article may still be useful as a reference, but may not work or follow best practices.

Please see How To Set Up Multi-Factor Authentication for SSH on Ubuntu 16.04 instead.

Introduction

To protect your SSH server with an two-factor authentication, you can use the Google Authenticator PAM module.

Every time you connect you have to enter the code from your smartphone.

Attention: If you activate the google-authenticator for a normal user but not for root you can’t login with the root user directly anymore. You will need to login as the new user first, then switch to the super user with the su command to get root.

Before you do anything on your VPS, install the Google Authenticator application, it is available for Android, iOS and BlackBerry. Install the App using the market or use your mobile browser to go to m.google.com/authenticator. After this connect to your VPS and switch to the root user.

Step One - Install Dependencies

sudo apt-get install libpam-google-authenticator

libqrencode3 will be installed automatically and will allow you to use the camera of your phone to scan the qr-code directly from the console.

Step Two - Edit the Configuration Files

To use the module you have to edit two configuration files.

nano /etc/pam.d/sshd

Add the following line on top of the file:

auth required pam_google_authenticator.so

One more file to edit:

nano /etc/ssh/sshd_config

Find and change the following line:

ChallengeResponseAuthentication yes

Step Three - Activate the Two-Factor Authentication For a User

You can activate the google-authenticator for the root user or any other user. Switch to the user who should use the two-factor authentication and type in:

google-authenticator

You will be prompted to answer a few questions; answer the first two questions with yes (y):

Do you want authentication tokens to be time-based (y/n) y
Do you want me to update your "/home/USERNAME/.google_authenticator" file (y/n) y

You can answer the next questions according to your needs.

You can use the Google Authenticator app to scan the qr-code, or add the account using the secret key and the verification code. Do not forget to print out the emergency scratch codes and store them in a safe place!

Now switch back to root and restart the SSH server. If you added the two-factor authentication for the root user you can skip the next step.

su root 

Finally restart the SSH server.

/etc/init.d/ssh restart

That’s it! You should now have a SSH server with an two-factor authentication!

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It should be noted that when doing something like this, you should always keep the terminal/SSH client in which you originally executed these commands open, until you successfully authenticate (on a NEW terminal) with the new method. That way you’re still “in” if the new setup fails.

I had the issue with the access denied messages. I found I had to run

```instead of the **sudo /etc/init.d/ssh restart** command.


Also, if 
```auth required pam_google_authenticator.so
```is at the top of **/etc/pam.d/sshd** then you type in the Google Authenticator code BEFORE the password.  If
```auth required pam_google_authenticator.so
```is at the bottom of **/etc/pam.d/sshd** then you type in the Google Authenticator code AFTER the password.  I think I prefer the latter!

Having followed the above instructions I could not open new ssh sessions on my ubuntu 14.04 server; the normal password request was shown but no chance to give a token, and the login failed with the correct address.

However, as per http://ubuntuhandbook.org/index.php/2013/08/install-setup-google-authenticator-in-ubuntu-13-04-12-04/ I moved the

auth required pam_google_authenticator.so

line to the end of /etc/pam.d/sshd and it works.

Well this does… nothing. I can just login like before…

This comment has been deleted

Hi.

I am wondering if anybody can help me out.

  1. I’d like to generate the QR code on a web page, instead of through CLI (you’d have to consider the width/height of your SSH windows beforehand), how can I do this?

  2. when I did a two-factor authentication with GMail, Microsoft or LogMeIn, the code generated would show the company name above and my email address below the six digit code…how can I do this?

Sorry, I know this is not a support forum, but I’ve been wrecking my brain about these two obstacles.

Any pointer is greatly appreciated!

Ok, it works but with some notes.

  1. As mentioned in comments, you should add the line ‘auth required pam_google_authenticator.so’ to the END of /etc/pam.d/sshd.
  2. If you use SFTP, for example in Filezilla you have to change the Auth method to ‘Interactive’.

If you want to use Google Auth + SSH Keys, see this link: https://wiki.archlinux.org/index.php/SSH_keys#Two-factor_authentication_and_public_keys

Short version: Add ‘AuthenticationMethods publickey,keyboard-interactive:pam’ right after ‘ChallengeResponseAuthentication yes’ in /etc/ssh/sshd_config

I’m now unable to sign in to users wich does’t have the google authenticator enabled.

Any solutions?

Hi,

In case of emergency, how to disable the Two factor Authentication?