Duo provides simple two-factor authentication as a service via:
1. Phone callback
2. SMS-delivered one-time passcode
3. Duo mobile app to generate one-time passcode
4. Duo mobile app for smartphone push authentication
5. Duo hardware token to generate one-time passcode
This package allows an admin (or ordinary user) to quickly add Duo
authentication to any UNIX login without setting up secondary user
accounts, directory synchronization, servers, or hardware.