The story
It has been a while since I got my Yubikey from Harry Chen. Now that I'm back working on Gentoo, so it is time to explore functions of the Yubikey on Linux. Instead of just using the Yubikey as a OpenPGP card, its U2F function can serve more than simply logging you into Google. In this article, we'll show how to make the Yubikey a convenient and secure login token for Linux systems, enabling you to log into the system with a plug and a touch, and lock the session when the token is removed.
U2F capability for PAM
U2F can be used locally on Linux systems thanks to the pam_u2f module by Yubico. Emerge sys-auth/pam_u2f and add the active user (jsteward in my case) to the usb group for access to the device:
emerge -av sys-auth/pam_u2f
gpasswd -a jsteward usb
Note: remember to log out and then log back in to make the new group settings come into place.
Register the device
Make sure you've configured the appropriate kernel options (CONFIG_HIDRAW and CONFIG_USB_HIDDEV). Plug in the device and observe dmesg to see if the deivce is detected by the kernel correctly. We can then setup the authorization mappings, registers the device to the user that uses it.
mkdir -p ~/.config/Yubico
pamu2fcfg -ujsteward > ~/.config/Yubico/u2f_keys
Touch the device as the LED on it starts blinking. Verify that the device is properly registered by verifying the contents of ~/.config/Yubico/u2f_keys. To use multiple keys, register the device with different authorization file locations (i.e. u2f_keys) and then merge them into a single u2f_keys in the following format:
jsteward:<KeyHandle1>,<UserKey1>:<KeyHandle2>,<UserKey2>:...
Configure PAM module
We want to use our Yubikey for system authentications, which would include logins (DE or tty), session unlock (screensaver), sudo, and more. Place the following line in /etc/pam.d/system-auth:
auth sufficient pam_u2f.so cue
Just above the following existing line in the file:
auth required pam_unix.so try_first_pass likeauth nullok
Note: the cue parameter makes the module display a prompt "Please touch the device." when it is waiting for a response so that the user does not mistake it as the system has hung.
This enables you to authenticate with a Yubikey without the need of user passwords. To use the Yubikey as a 2FA tool, change sufficient to required. However, this will result in failed authentication when the Yubikey is not present (and touched in time).
Warning: system-auth affects all authentications, including remote ones. Think twice before using required for pam_u2f on a server.
Warning: messing up PAM configuration may result in being locked out of the system, which is only possible to fix with a LiveCD or init=/bin/bash.
Test the configuration by trying to log in or sudo. SDDM logins will be a little bit tricky due to the way it is designed: press Login without entering a correct password and touch the device.
Lock the session when Yubikey gets removed
This works with a simple udev rule. For an easy way of locking sessions, use elogind instead of the default consolekit (on Gentoo) as described in the Gentoo Wiki page for elogind. Write the following rule in /etc/udev/rules.d/20-yubikey.rules:
ACTION=="remove", ENV{ID_BUS}=="usb", ENV{ID_MODEL_ID}=="0407", ENV{ID_VENDOR_ID}=="1050", RUN+="/bin/loginctl lock-sessions"
Check your Yubikey's VID:PID pair and substitute 1050:0407 accordingly. Reload the udev rules:
sudo udevadm control --reload-rules
Unplug your Yubikey to see the rule in action.
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