The Google Authenticator project provides a two-step authentication procedure using one-time passcodes (OTP). The OTP generator application is available for iOS, Android and Blackberry. Similar to S/KEY_Authentication the authentication mechanism integrates into the Linux PAM system. This guide shows the installation and configuration of this mechanism.
The required software is available in the YaourtAUR package in the AUR. It is most conveniently installed using
$ yaourt -S google-authenticator-libpam-git
Setting up the PAM
Usually one demands two-pass authentication only for remote login. The corresponding PAM configuration file is
/etc/pam.d/sshd. In case you want to use Google Authenticator globally you would need to change
/etc/pam.d/system-auth, however, in this case proceed with extreme caution to not lock yourself out.
In this guide we proceed with editing
/etc/pam.d/sshd which is most safely (but not necessarly) be done in a local session.
To enter both, your unix password and your OTP add
pam_google_authenticator.so the following way.
auth required pam_google_authenticator.so auth required pam_unix.so auth required pam_env.so
This will ask for the OTP before prompting for your unix password. Changing the order of the two modules will reverse this order.
To allow login with either the OTP or your unix password use
auth sufficient pam_google_authenticator.so auth sufficient pam_unix.so auth required pam_env.so
Finally enable challenge-response authentication in
# systemctl reload sshd
Generating a secret key file
Every user who wants to use two-pass authentication needs to generate a secret key file in his home folder.
This can very easily be done using
google-authenticator as included in AUR.
$ google-authenticator Do you want authentication tokens to be time-based (y/n) y https://www.google.com/chart?chs=200x200&chld=M%7C0&cht=qr&chl=otpauth://totp/username@hostname%3Fsecret%3DZVZG5UZU4D7MY4DH Your new secret key is: ZVZG5UZU4D7MY4DH Your verification code is 269371 Your emergency scratch codes are: 70058954 97277505 99684896 56514332 82717798 Do you want me to update your "/home/username/.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) y By default, tokens are good for 30 seconds and 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 1:30min to about 4min. 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) y
It's recommended to store the emergency scratch codes savely (print them out and keep them in a save location) as they are your only way to log in (via SSH) when you lost your mobile phone (i.e. your OTP-generator). They are also stored in
~/.google_authenticator, so you can look them up any time as long as you are logged in.
Setting up your OTP-generator
Install the corresponding generator application on your mobile phone from the corresponding store by browsing to http://m.google.com/authenticator from your mobile device. In the applications menu click the corresponding button to create a new account and either scan the QR code from the URL you were told when generating the secret key file, or enter the secret key (in the example above 'ZVZG5UZU4D7MY4DH') manually.
Now you should see a new passcode token beeing generated every 30 seconds on your phone.
SSH to your host from another machine or from another terminal window and check if it works.
$ssh username@hostname login as: username Verification code: Password: $