2FA for RD Gateway - On Premises Solutions

[taro75]

In my environment, I have RD Gateway setup behind the firewall, and it does not have the Internet connection. In this case what are my best options to enable 2FA. I can facilitate SMTP from RD Gateway to mail server on corporate LAN and allow the required access from DMZ to corporate LAN but not to the Internet. For DUO is there any on-premises solutions?

[DuoKristina]

As Duo is a cloud service all our solutions require some ability to connect from the on-premises application to Duo's service. Many of our applications support use of an HTTP proxy for the outbound connection to Duo.