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One-Time Passwords For Two-Factor Authentication With SSH for Improved Security

No matter what kind of data you’re hosting, securing access to your Linode is a critical step in preventing your information from falling into the wrong hands. By default, you will need a password to log in, and you may also configure a key pair for even greater security. However, another option exists to complement these methods: time-based one-time passwords (TOTPs).

TOTPs allow you to enable two-factor authentication for SSH with single-use passwords that change every 30 seconds. By combining this method with a regular password or publickey (or both), you can add an extra layer of security, further ensuring your server is sufficiently protected.

This guide will explain how to install the necessary software, configure your system to use two-factor authentication (2FA), and use your new time-based one-time password (TOTP) in combination with existing security features.

Before you begin, make sure you have completed our Securing Your Server guide to create a standard user account, and remove unnecessary network services. This guide will explain a different way to harden SSH access, but you may use a keypair in addition for even greater protection.

You will need a smartphone or another client device with an authentication application such as Google Authenticator or Authy. Many other options exist, and this guide should be compatible with nearly all of them.

Install Google Authenticator

In this section, we’ll install the Google Authenticator package, which is included in the default repositories of both Ubuntu 16.04 and Debian 8. This will also work with Ubuntu 18.04 LTS.  This software will generate keys on your Linux server, which will then be paired with an app on a client device (often a smartphone) to generate single-use passwords that expire after a set period of time.

To install Google Authenticator:

sudo apt-get install libpam-google-authenticator

Although we are using the Google Authenticator package, the keys it generates are compatible with other authentication apps.

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Generating the keys

Now that you’ve installed the package, you’ll use it to generate keys. These keys are then used by software on client devices to generate time-based one-time passwords (TOTPs). To understand the difference between these passwords and the ones you already use, let’s break down the concept of a TOTP:

  • Time-based – The generated password will change every 30-60 seconds. This means that if an attacker tries to use brute force, they’ll almost certainly run out of time before new credentials are needed to gain access.
  • One-time – The password will be valid for a single authentication only, thus minimizing the risk of a replay attack. Even if your TOTP is intercepted upon sending it to the server, it will no longer be valid after you’ve logged in.

The following instructions will generate a password for the user running the commands. If you are configuring two-factor authentication for multiple users, perform these steps for each user.

Step 1. Run the google-authenticator program

google-authenticator

A prompt will appear asking you to specify whether you’d like to use a time-based authentication (as opposed to counter-based). Choose “yes” by entering y at the prompt.

You should see a QR code in your terminal, scan the QR code with the google-authenticator app on your mobile device. A new entry should be added to your authenticator app in the format username@systemname.

You’ll also see a “secret key” below the QR code. You may enter this into the app manually, instead of scanning the QR code, to add your account.

Step 2. Record your emergency scratch codes in a secure location.

These codes can be used for authentication if you lose your device, but be aware that each code is only valid once.

You’ll be prompted to answer the following questions:

Do you want me to update your "/home/exampleuser/.google_authenticator" file (y/n)

This specifies whether the authentication settings will be set for this user. Answer y to create the file that stores these settings.

Do you want to disallow multiple uses of the same authentication
token? This restricts you to one login about every 30s, but it increases your chances to notice or even prevent man-in-the-middle attacks (y/n)

This makes your token a true one-time password, preventing the same password from being used twice. For example, if you set this to “no,” and your password was intercepted while you logged in, someone may be able to gain entry to your server by entering it before the time expires. We strongly recommend answering y.

By default, tokens are good for 30 seconds and in order to compensate for possible time-skew between the client and the server, we allow an extra token before and after the current time. If you experience problems with poor time synchronization, you can increase the window from its default size of 1:30min to about 4min. Do you want to do so (y/n)

This setting accounts for time syncing issues across devices. Unless you have reason to believe that your phone or device may not sync properly, answer n.

If the computer that you are logging into isn't hardened against brute-force login attempts, you can enable rate-limiting for the authentication module. By default, this limits attackers to no more than 3 login attempts every 30s. Do you want to enable rate-limiting (y/n)

This setting prevents attackers from using brute force to guess your token. Although the time limit should be enough to prevent most attacks, this will ensure that an attacker only has three chances per 30 seconds to guess your password. We recommend answering y.

Congratulations! You have finished generating your key and adding it to your client, but some additional configuration is needed before these settings will go into effect. Carefully read the following section in this guide for instructions on how to require two-factor authentication for all SSH login attempts.

Configure Authentication Settings

The TOTP authentication methods in this guide use PAM, or Pluggable Authentication Modules. PAM integrates low-level authentication mechanisms into modules that can be configured for different applications and services. Because you’re using additional software (i.e., programs that aren’t built into the Linux distro), you’ll need to configure PAM to properly authenticate users.

Open /etc/pam.d/sshd with sudo privileges, and add the following lines to the end of the file:

auth required pam_unix.so no_warn try_first_pass
auth required pam_google_authenticator.so

The first line tells PAM to authenticate with a normal Unix user password before other methods. The second line specifies an additional method of authentication, which in this case, is the TOTP software we installed earlier.

Edit /etc/ssh/sshd_config to include the following lines, replacing example-user with any system user for which you’d like to enable two-factor authentication. Comments (preceded by #) are included here, but should not be added to your actual configuration file:

# This line already exists in the file, and should be changed from 'no' to 'yes'
ChallengeResponseAuthentication yes

...

# These lines should be added to the end of the file
Match User example-user
AuthenticationMethods keyboard-interactive

If you created TOTPs for multiple users, and you’d like to have them all use two-factor authentication, create additional Match User blocks for each user, duplicating the command format shown above.

Restart the SSH daemon to apply these changes:

sudo systemctl restart ssh

Combine Two-Factor and Public Key Authentication

This section is optional. If you’d like to use public key authentication instead of a password with TOTP, perform these steps:

Set PasswordAuthentication to no and modify the AuthenticationMethods line in /etc/ssh/sshd_config:

PasswordAuthentication no
...
Match User example-user
AuthenticationMethods publickey,keyboard-interactive

Configure this setting in the AuthenticationMethods directive for each user as appropriate. When any of these users log in, they will need to provide their SSH key and they will be authenticated via TOTP, as well. Be sure to restart your SSH daemon to apply these changes.

Next, you’ll need to make changes to your PAM configuration. Comment out or omit the following lines in your /etc/pam.d/sshd file:

# @include common-auth
...
# auth required pam_unix.so no_warn try_first_pass

That’s it! You should now be able to log in using your SSH key as the first method of authentication and your verification code as the second. To test your configuration, log out and try to log in again via SSH. You should be asked for your 6-digit verification code only, since the key authentication will not produce a prompt.

 

Next Steps

First, be sure you have followed our guide to Securing Your Server. Although there is no single, foolproof method to protecting your data, firewalls and services like Fail2Ban are a great way to minimize risk.

When you use two-factor authentication with TOTPs, an important point to consider is the physical security of the device on which you’ve configured your authenticator app. Be sure your phone or device is secured with a passphrase, so that even if it falls into the wrong hands, it can’t easily be used to compromise your server.

While two-factor authentication may be a valuable security feature, total security is an ongoing process, not an end goal that can be achieved by adding extra layers of authentication. To provide the best protection for your data, take care to follow security best practices at all times.