Two Factors Authentication With OpenSSH
Configure openSSH to use PAM
OpenSSH allows us to configure PAM to handle authentication duties, working nicely with thepackage.
First, configure the SSH daemon to use PAM authentication:
AuthenticationMethods publickey,keyboard-interactive ChallengeResponseAuthentication yes PermitRootLogin yes UsePAM yes
Then, configure pam by adding the following lines, enabling google-authenticator as the package handling authentication:
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 user root:
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 notice or even 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 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 +-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
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.
Download Google Authenticator app from your App Store. Startup Google Authenticator app and enter manually your <secret> key.
You should see the last lines saying:
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 successfull and now the verifier is asking for the verification code generated from the Google Authenticator app.