Two Factor Authentication With OpenSSH

Configure OpenSSH to use PAM

OpenSSH allows us to configure PAM to handle authentication duties, working nicely with the google-authenticator package.

# apk add google-authenticator openssh-server-pam

First, configure the SSH daemon to use PAM authentication:

# cat /etc/ssh/sshd_config

AuthenticationMethods publickey,keyboard-interactive
ChallengeResponseAuthentication yes
PermitRootLogin yes
UsePAM yes

Note: This configuration does NOT allow password authentication globally

Then, configure PAM by adding the following lines, enabling google-authenticator as the package handling authentication:

For Alpine 3.14 or newer:

# cat /etc/pam.d/sshd.pam #create the file if needed

For Alpine 3.13 or older:

# cat /etc/pam.d/sshd #create the file if needed

account		include				base-account

auth		required			pam_env.so
auth		required			pam_nologin.so	successok
auth		include				google-authenticator

Time-based One Time Password authentication (TOTP RFC 6238)

As the root user:

# google-authenticator

Note: Please take note of <secret>

Do you want authentication tokens to be time-based (y/n) y
https://www.google.com/<pruned>
Your new secret key is: <secret>
Your verification code is <pruned>
Your emergency scratch codes are:
  <pruned>
  <pruned>
  <pruned>
  <pruned>
  <pruned>

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 detect or prevent man-in-the-middle attacks (y/n) n

By default, tokens are good for 30 seconds. In order to compensate for
possible time-skew between the client and 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 ±1min (window size of 3) to about ±4min (window size of
17 acceptable tokens).
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) n

Tip: You might want to answer questions 2, 3 and 4 differently based on your paranoia level and firewall settings :)

Re-run google-authenticator for each user that needs to login via SSH. Don't forget to include .google_authenticator files in your LBU if you're running from RAM.

Authentication token

Download the Google Authenticator app from the App Store. Startup Google Authenticator and manually enter your <secret> key.

Login

$ ssh -v root@yourbox

The last lines should say:

Authenticated with partial success.
debug1: Authentications that can continue: keyboard-interactive
debug1: Next authentication method: keyboard-interactive
Verification code:

Authenticated with partial success means that pubkey authentication was successful and now the verifier is asking for the verification code generated from the Google Authenticator app.