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How To Set Up Multi-Factor Authentication for SSH on Ubuntu 20.04

Installing Google’s PAM

{
    sudo apt update
    sudo apt install libpam-google-authenticator -y
}
google-authenticator
Do you want authentication tokens to be time-based (y/n) y
Warning: pasting the following URL into your browser exposes the OTP secret to Google:
  ...

...
...
...

Your new secret key is: ...
Enter code from app (-1 to skip): ...
Code confirmed
Your emergency scratch codes are:

...

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

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) y

By default, a new token is generated every 30 seconds by the mobile app.
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. This allows for a
time skew of up to 30 seconds between authentication server and client. If you
experience problems with poor time synchronization, you can increase the window
from its default size of 3 permitted codes (one previous code, the current
code, the next code) to 17 permitted codes (the 8 previous codes, the current
code, and the 8 next codes). This will permit for a time skew of up to 4 minutes
between client and server.
Do you want to do so? (y/n) 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) y

Configuring OpenSSH to Use MFA/2FA

sudo cp /etc/pam.d/sshd /etc/pam.d/sshd.bak
sudo vim /etc/pam.d/sshd
...

# Standard Un*x password updating.
@include common-password
auth required pam_google_authenticator.so nullok
auth required pam_permit.so
sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak
sudo vim /etc/ssh/sshd_config
...

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

...
sudo systemctl restart sshd.service

Making SSH Aware of MFA

sudo vim /etc/ssh/sshd_config
...

AuthenticationMethods publickey,password publickey,keyboard-interactive
sudo vim /etc/pam.d/sshd
  • Comment out @include common-auth
...

# Standard Un*x authentication.
# @include common-auth

...
sudo systemctl restart sshd.service