Two-factor SSH using oathtool on Ubuntu 18.04July 16, 2018
I’ve been using the Google Authenticator PAM Module for years. It works great, and is easy to set up. But, I generally try to stay away from all things Google, so I wanted to set up two-factor ssh using something else.
After a bit of googling (…), I found OATH Toolkit. Reading their documentation, it seems rather easy to set up. Also, this blog has a nice TL;DR of the setup. I’ve pretty much followed that, and added a few bits.
# Install oathtool. sudo apt install oathtool libpam-oath export HEX_SECRET=$(head -15 /dev/urandom | sha1sum | cut -b 1-30) oathtool --verbose --totp $HEX_SECRET --digits=8 # Type in the Base32-secret on your phone sudo touch /etc/users.oath sudo chmod 0600 /etc/users.oath # Running subshell so we can send output to file with sudo-permissions sudo /bin/bash -c "echo HOTP/T30 $USER - $HEX_SECRET \ >> /etc/users.oath" # Unset your secret unset HEX_SECRET
Setting up an access-list for two-factor
Now, I want to be able to define who I require two-factor for, and from where I require it. I have a couple of hosts that I trust, where I can log in from in case I lose my two-factor.
/etc/security/login_token.conf. My file has the following contents:
# Do not require two-factor from here: + : dennis : 184.108.40.206/24 # lolnope don't need two-factor at all + : lolnope : ALL # Demand two-factor from everywhere and everyone else - : ALL : ALL
man 5 access.conf for details on the format
First install the package.
sudo apt install libpam-oath
Add the following to the top of /etc/pam.d/sshd:
Exceptions from two-factor auth [success=1 default=ignore] pam_access.so accessfile=/etc/security/login_token.conf # Two-factor auth reqiored pam_oath.so usersfile=/etc/users.oath
Now all we need to do is to enable two-facor in
/etc/ssh/sshd_config. Now we can
ssh, and test it!
This is what it should look like when logging in:
$ # I use ssh-keys, do I need to auth without them $ ssh -o PubkeyAuthentication=no dennis@host One-time password (OATH) for `dennis': Password:
And, when logging in from one of the hosts I’ve defined in
/etc/security/login_token.conf, I’m not asked about the OTP!