This is a guide on how to setup the use of Google Authenticator along with public-key authentication.

First, we clone the Google Authenticator PAM module from Github:

$ git clone https://github.com/google/google-authenticator-libpam.git

To build it, we need a few packages which are not included by default in macOS.

$ brew install autoconf automake libtool

To be able to get QR codes without revealing secrets to Google, you can install
libqrencode:

$ brew install libqrencode

Now we are ready to build Google Authenticator.

$ ./bootstrap
$ ./configure
$ make
$ sudo make install

To install the PAM module, invoke

$ sudo cp /usr/local/lib/security/pam_google_authenticator.so /usr/lib/pam/

Then, we add the line

auth required pam_google_authenticator.so

to the configuration file /etc/pam.d/sshd. It will look something like this:

# sshd: auth account password session

# Not used by me!
# auth       optional       pam_krb5.so use_kcminit
# auth       optional       pam_ntlm.so try_first_pass
# auth       optional       pam_mount.so try_first_pass
# auth       required       pam_opendirectory.so try_first_pass

# Relevant methods of authentication:
auth       required       pam_google_authenticator.so
account    required       pam_nologin.so
account    required       pam_sacl.so sacl_service=ssh
account    required       pam_opendirectory.so
password   required       pam_opendirectory.so
session    required       pam_launchd.so
session    optional       pam_mount.so

I have removed the alternative methods of authentication. In /etc/ssh/sshd_config, we set

PubkeyAuthentication yes
ChallengeResponseAuthentication yes
UsePAM yes
AuthenticationMethods publickey,keyboard-interactive:pam

This will force the user to prove ownership of a valid SSH-key and along with a verification code from Google Authenticator.

Now we are ready to setup the two-factor authentication. Running

$ /usr/local/bin/google-authenticator

generate a shared secret and provide you with a QR code to be used with your phone. You can use TOTP or OTP verification codes. I suggest going with TOTP and small time windows.

Adding some notifications

Another good thing is to add some notifications to your ~/.ssh/rc file (also — if you are in the US, there are a lot of services available for sending SMS, which can be handy, but remember not to send any sensitive data through third parties). For instance:

# Get and sanitize sender address, probably not needed...
ip=`echo $SSH_CONNECTION | cut -d " " -f 1`
ip=${ip//[^a-zA-Z0-9:.]/}

# Sanitize user, probably not needed...
user=`whoami`
user=${user//[^a-zA-Z0-9_]/}

# Show notification
osascript -e 'display notification "SSH connection as '$user' from '$ip'" with title "SSH"'

# Send email
echo "SSH connection as '$user' from '$ip'" | sendmail me@mydomain.com

This will give a notification to you directly if you are at the computer being connected to. Additionally, it will send an email to you just in case you are AFK. The regexp are there to mitigate possible command injection, though I doubt it is possible unless there is severe bug in SSHD and the script above is running as a user with higher privileges. On the other hand, santitation is never bad if it negligible in terms of computations and you are doing it right 🙂

Another way of sending notifications is to use a Telegram bot. See below for an example output.

For this purpose, one might use e.g. telegram-send.