Variously: SMS; the "call my landline and speak the code to me" options some systems have; 2FA app on an android tablet; special purpose just-shows-a-number don

Variously: SMS; the "call my landline and speak the code to me" options some systems have; 2FA app on an android tablet; special purpose just-shows-a-number dongle. None of this is great, though. I believe you can run 2-factor-auth clients on a Linux box somehow, but I've never investigated that.