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#+date: <2022-03-26>
#+title: Enable TOTP MFA for SSH
#+description:
#+slug: ssh-mfa
* 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/][Hardening a Public-Facing
Home Server]].
* 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:
#+begin_src sh
sudo apt-get update
#+end_src
#+begin_src sh
sudo apt-get install libpam-google-authenticator
#+end_src
* Initialize the PAM Module
** Interactive Method
Once the package is installed, initialize it and following the
interactive prompts to generate your OTP or TOTP:
#+begin_src sh
google-authenticator
#+end_src
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.
#+begin_src txt
OUTPUT
Do you want authentication tokens to be time-based (y/n) y
#+end_src
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.
#+begin_src txt
OUTPUT
Do you want me to update your "/home/user/.google_authenticator" file? (y/n) y
#+end_src
#+begin_src txt
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
#+end_src
#+begin_src txt
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
#+end_src
#+begin_src txt
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
#+end_src
** 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:
#+begin_src sh
google-authenticator -t -d -f -r 3 -R 30 -w 3
#+end_src
The options referenced above are as follows:
#+begin_src txt
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
#+end_src
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.
* 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:
#+begin_src sh
sudo nano /etc/pam.d/sshd
#+end_src
You need to do two things in this file. First, add the following lines
to the bottom of the file:
#+begin_src config
auth required pam_google_authenticator.so nullok
auth required pam_permit.so
#+end_src
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:
1. Publickey
2. Password
3. T/OTP code
#+begin_src config
#@include common-auth
#+end_src
* SSH Configuration Settings
Finally, edit the =sshd_config= file again:
#+begin_src sh
sudo nano /etc/ssh/sshd_config
#+end_src
You'll need to change =ChallengeResponseAuthentication= to yes and add
the =AuthenticationMethods= line to the bottom of the file.
#+begin_src config
ChallengeResponseAuthentication yes
AuthenticationMethods publickey,password publickey,keyboard-interactive
#+end_src
Finally, restart the =ssh= service:
#+begin_src sh
sudo systemctl restart sshd.service
#+end_src
The next time you log in, you should be greeted with a verification code
request!
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