Most of the servers I manage used to be locked down using certificates so it's really hard to break in; this had the advantage of being more secure than a password but, as time went on, the big disadvantage of not being able to log in from PCs which lacked the certs.
In time I've went back on my decision to only allow certificate based logins. The downside to this change is that I felt the the server was still exposed despite using a strong password.
Fortunately Google offers a nice Authenticator application for smartphones as well as offering a pluggable authentication module (PAM) which ties in with the Authenticator app for two-factor authentication on login. This means that besides knowing the username and password, a user would need to have a time-based one-time password generated by the app in order to log in to the server.
Installing google-authenticator PAM
I'm running Debian so this guide applies to Debian and Debian-based distributions.
First of all we're going to need to install the build-essential package which will allow us to actually install the software.
sudo apt-get install build-essential
After we've got them installed we're going to install some dependencies needed for the PAM module to work (libpam-dev) as well as to retrieve (git) and easily configure the software (libqrencode-dev).
sudo apt-get install git libpam-dev libqrencode-dev libpam0g-dev
Now let's go ahead and download the code:
git clone https://code.google.com/p/google-authenticator/
This grabs the files from Google's server and creates a local copy. After that's done navigate to the google-authenticator/libpam directory and run
sudo make install
Now that we've got everything installed it's time to actually set up the application. This is extremely easy, just run
google-authenticator which will generate a QR code (thanks to the libqrencode-dev package); scan this code in the Google Authenticator mobile app (or enter the information by hand) and head back to your PC to finish setting up the software by answering the questions it asks.
Setting up the OS to use Challenge Response Authentication
Once the previous step is done we need to let the OS know that is has to use the newly installed PAM module. To do this we need to edit the
/etc/pam.d/common-auth (by running
sudo vim /etc/pam.d/common-auth for example) and adding the following line:
auth required pam_google_authenticator.so
Next up is the ssh_config file in order to allow SSH to send challenge requests:
sudo vim /etc/ssh/sshd_config
Find the line containing
ChallengeResponseAuthentication and set it to yes (also uncomment it if it's commented out) so that it looks like:
Now all there's left to do is restart the SSH server and to test the change:
sudo /etc/init.d/ssh restart
The next time you'll log in you should be greeted with a message asking for a verification code after entering the username and password.
One advantage to this is the fact that even if you log in using a certificate (which bypasses the two-factor authentication) you are still asked for a verification code when trying to elevate to root via sudo for example.