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WoJ
Asked: 2011-11-05 13:01:43 +0800 CST

ssh with passphrase, fallback to Google Authenticator

  • 772

With the introduction of Google Authenticator and the ability to use it with ssh I was wondering if someone has gone through a sshd_config setup which would

  • first expect a key
  • if this fails, fall back to an authentication with Google Authenticator

The idea being to usually connect seamlessly with a key and, usually in less friendly environments, connect with a two factor mechanism.

ssh google-authenticator

1 Answers

  1. Best Answer

    Yes, I have a setup where I can ssh to my server using public key authentication, with a fallback to two-factor authentication with Google Authenticator + password when my private key is not available. These are the steps you can use to set it up.

    Installing Google Authenticator

    My server is running Ubuntu Bionic Beaver (18.04.1). You can install Google Authenticator using apt:

    $ sudo apt install libpam-google-authenticator

    Configuring sshd

    Open /etc/pam.d/sshd and add the following line at the top:

    auth optional pam_google_authenticator.so

    Open /etc/ssh/sshd_config and change one line. The existing line is

    ChallengeResponseAuthentication no

    and you should change it to

    ChallengeResponseAuthentication yes

    Configuring Google Authenticator for Your Account

    The next step is to turn on Google Authenticator for your account. You do this by simply running:

    $ google-authenticator

    Make sure you run this as the user who will be making ssh connections, not root. Make a note of your new secret key and your emergency scratch codes. The wizard will ask you several questions to configure the security settings for your account.

    Configuring Your Mobile App

    I use the Google Authenticator app for iPhone. This app has a [+] button that allows me to add a new Time Based Token using the secret key I obtained from the google-authenticator command on my server. It was trivial to set up. I can't help you with apps on any other platform, but I imagine the process is equally simple.

    Pulling the Trigger

    The last thing you need to do is restart sshd.

    $ sudo /etc/init.d/ssh restart

    At this point, when you try to connect to the server when your private key is available, authentication just works. When your private key is not available, you will get a prompt for a verification code, then your account password.

    Bingo, two-factor authentication.

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