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U2F Flags Command in ChromeOS: Configuring Second-Factor Authentication

The u2f_flags command in the ChromeOS developer shell (crosh) provides options to customize the configuration of the second-factor authentication (U2F) daemon. This command is useful for overriding device policy and enabling advanced features of U2F and G2F modes.

Command Syntax

u2f_flags <u2f | g2f>[, enable_global_key, verbose]

Options:

  • u2f: Enables the standard U2F mode, even if it is not set in the device policy.
  • g2f: Enables U2F mode with additional extensions.
  • enable_global_key: Allows the power button security key to function globally, making it usable outside a logged-in session for google.com as the relying party.
  • verbose: Increases the daemon logging verbosity for debugging, which can be viewed in /var/log/messages.

Examples

Enabling U2F Mode

Force enable the U2F standard mode regardless of the device policy:

u2f_flags u2f

Enabling G2F Mode with Verbose Logging

Activate G2F mode and increase logging verbosity:

u2f_flags g2f,verbose

Allowing Global Key Use

Enable the global key functionality along with U2F mode:

u2f_flags u2f,enable_global_key

Notes

  • The enable_global_key option is specific to devices with power button security keys and enables usage outside logged-in sessions for select services.
  • Logs generated with the verbose option can assist in diagnosing issues related to second-factor authentication.

Best Practices

  1. Testing Configuration:
  2. Always test the changes using a compatible U2F device after running the command.

  3. Check the system logs in /var/log/messages if issues arise.

  4. Verbose Logging for Debugging:

  5. Use the verbose option during troubleshooting to capture detailed logs of the U2F daemon's behavior.

  6. Combining Flags:

  7. Combine options for advanced configurations, such as enabling u2f mode with global key functionality and verbose logging.

Troubleshooting

Checking U2F Functionality

  1. Verify that the U2F hardware key is recognized by the system.
  2. Ensure that the desired flags are set correctly using the command.
  3. Review the logs in /var/log/messages for any error messages or warnings.

Resetting Configuration

To revert to the default device policy settings, reboot the system without setting the u2f_flags command.

By leveraging the u2f_flags command, you can fine-tune the behavior of second-factor authentication on your ChromeOS device, ensuring enhanced security and functionality tailored to your needs.