Introduction
Currently RHEL6 has severe limitations impacting the flexibility of the two-step login process.
Instructions
The only supported authentication methods are Duo Push or the use of an SSH public key.
If your Duo-registered device is broken or unavailable, the only method to authenticate is through use of an SSH public key.
Unfortunately, passcodes cannot be entered due to the need in RHEL6 to use auto-push, preventing the use of Duo Mobile Passcodes, SMS Passcodes, Hardware Tokens, Temporary Bypass Codes, and Phone Callbacks.