Table of Contents
1. Why Do I Need MFA for SSH?
If you are a sysadmin of a server anywhere (that includes at home!), you may want an added layer of protection against intruders. This is not a replacement for other security measures, such as:
- Disable root SSH
- Disable SSH password authentication
- Allow only certain users to login via SSH
- Allow SSH only from certain IPs
However, MFA can be added as an additional security measure to ensure that your server is protected. This is especially important if you need to allow password authentication for SSH.
For more guidance on server security measures, see my other post: Hardening a Public-Facing Home Server.
2. Install MFA PAM Module
PAM, which stands for Pluggable Authentication Module, is an
authentication infrastructure used on Linux systems to authenticate a
user. In order to use this technology, let's install the
libpam-google-authenticator
package:
sudo apt-get update
sudo apt-get install libpam-google-authenticator
3. Initialize the PAM Module
3.1. Interactive Method
Once the package is installed, initialize it and following the interactive prompts to generate your OTP or TOTP:
google-authenticator
If you are not sure how to answer, read the prompts carefully and think about having to how each situation would affect your normal login attempts. If you are still not sure, use my default responses below.
OUTPUT Do you want authentication tokens to be time-based (y/n) y
At this point, use an authenticator app somewhere one of your devices to scan the QR code. Any future login attempts after our upcoming configuration changes will require that TOTP.
OUTPUT Do you want me to update your "/home/user/.google_authenticator" file? (y/n) y
OUTPUT 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
OUTPUT 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
OUTPUT 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
3.2. Non-Interactive Method
If you need to do this quickly, know your responses to the prompts, or are setting this up for numerous users, the non-interactive method can be much faster:
google-authenticator -t -d -f -r 3 -R 30 -w 3
The options referenced above are as follows:
google-authenticator [<options>] -h, --help Print this message -c, --counter-based Set up counter-based (HOTP) verification -t, --time-based Set up time-based (TOTP) verification -d, --disallow-reuse Disallow reuse of previously used TOTP tokens -D, --allow-reuse Allow reuse of previously used TOTP tokens -f, --force Write file without first confirming with user -l, --label=<label> Override the default label in "otpauth://" URL -i, --issuer=<issuer> Override the default issuer in "otpauth://" URL -q, --quiet Quiet mode -Q, --qr-mode={NONE,ANSI,UTF8} QRCode output mode -r, --rate-limit=N Limit logins to N per every M seconds -R, --rate-time=M Limit logins to N per every M seconds -u, --no-rate-limit Disable rate-limiting -s, --secret=<file> Specify a non-standard file location -S, --step-size=S Set interval between token refreshes -w, --window-size=W Set window of concurrently valid codes -W, --minimal-window Disable window of concurrently valid codes -e, --emergency-codes=N Number of emergency codes to generate
This fully configures the authenticator, saves it to a file, and then
outputs the secret key, QR code, and recovery codes. (If you add the
flag -q
, then there won't be any output). If you use this command in
an automated fashion, make sure your script captures the secret key
and/or recovery codes and makes them available to the user.
4. PAM Configuration Settings
Once you've enabled the T/OTP and have it saved to an MFA app on your
phone or other device, open the PAM sshd
file:
sudo nano /etc/pam.d/sshd
You need to do two things in this file. First, add the following lines to the bottom of the file:
auth required pam_google_authenticator.so nullok auth required pam_permit.so
Second, comment-out the following line near the top of the file.
If you leave this line uncommented, every SSH login attempt will ask for the following three authentication factors:
- Publickey
- Password
- T/OTP code
#@include common-auth
5. SSH Configuration Settings
Finally, edit the sshd_config
file again:
sudo nano /etc/ssh/sshd_config
You'll need to change ChallengeResponseAuthentication
to yes and add
the AuthenticationMethods
line to the bottom of the file.
ChallengeResponseAuthentication yes AuthenticationMethods publickey,password publickey,keyboard-interactive
Finally, restart the ssh
service:
sudo systemctl restart sshd.service
The next time you log in, you should be greeted with a verification code request!