YubiKey/PAM

This page goes over Article description::authenticating with [[PAM using a YubiKey.]]

Emerge
The main component is a PAM module called. The package also contains a tool called which is used to generate the required authorization mappings.

Kernel
Support for raw USB HID devices is required in the kernel for the YubiKey to function.

plugdev
For a non-root user to be able to access the YubiKey, they need to be a member of the group. To check if the current user is in the group, run:

If is not listed, add the user to the group by running:

The user needs to log out and log back in for the group membership to take effect.

Creating an authorization mapping
In order to authenticate with PAM using, a key token needs to be mapped to a user. By default, these mappings are stored in. To create a mapping, insert the YubiKey and run:

Touch the YubiKey when it starts blinking.

Mapping additional keys
To map additional keys to a user, remove the currently inserted YubiKey (if any) and insert the next one. Then run:

Touch the YubiKey when it starts blinking. Repeat for any remaining YubiKeys.

Configuring PAM
Global system authentication is configured through. Taking a backup of the current PAM configuration will make it easy to revert changes if needed.

Requiring a password and YubiKey
To require both a password and a YubiKey to authenticate with PAM, modify the file to include the following:

means PAM will skip over one module if the current one succeeds. In this case it will jump to the module if the correct password is given. means the module will succeed if the authenticating user doesn't have an authorization mapping. Without this, any users that don't have a mapping configured will be locked out. means the user is prompted to touch the YubiKey during authentication. Without this, no prompt is given.

Requiring a password or YubiKey
To require either a password or a YubiKey to authenticate with PAM (but preferring the YubiKey), modify the file to include the following:

means PAM will consider the authentication to be successful if this module succeeds. Otherwise it goes to the next module. is not included because otherwise will succeed for users without a mapping configured. This would result in successful authentication without prompting for a password.

Troubleshooting
If no user is able to authenticate after completing the above, then a broken PAM configuration is the likely culprit. Even if no active root login is available, the system can still be fixed and authentication mechanisms restored by either live booting or booting into single-user mode.

Fixing PAM through live boot
First, completely power off the machine. Insert the bootable medium and boot from it through the machine's firmware boot menu. There are no universal instructions since this process can vary greatly from machine to machine, so consult the relevant documentation if unfamiliar with how to do this.

Open up a root shell when booted, locate the block device corresponding to your root filesystem, and mount it (making sure to specify any required mount options):

Next, either restore a backup PAM configuration or manually edit to undo any changes. To non-destructively undo changes, comment out the necessary entries by prepending a  and add any new entries if needed.

Once done, commit the changes to disk, unmount your root filesystem, and reboot:

Authentication should be fully restored.

Fixing PAM through single-user mode
To enter single-user mode first reboot the machine. When the GRUB menu appears, press to bring up the menu entry editor. Any edits made in here are temporary and do not edit the on-disk GRUB configuration.

Locate the line which loads the kernel and append  to it. The actual content and number of kernel command line arguments is likely to differ from system to system, but the end result should look similar to the following:

Press to boot using the present command list.

Once the prompt appears, the root filesystem will need to be re-mounted as read/write:

Only specifying  will instruct  to read the entries in  to find the correct block device and to apply the mount options specified therein.

Next, either restore a backup PAM configuration or manually edit to undo any changes. To non-destructively undo changes, comment out the necessary entries by prepending a  and add any new entries if needed.

Once done, commit the changes to disk, re-mount the root filesystem as read-only, and exit:

This will not be a clean exit and the kernel will with the message. This is fine because all the filesystem changes were manually -ed.

Finally, reboot the system. Authentication should be fully restored.

Supported devices
The following tables list all the YubiKey devices and their U2F support as stated on the Yubico website.

External resources

 * [//www.man7.org/linux/man-pages/man5/pam.conf.5.html pam.conf(5)], the page describing PAM configuration files.