Listen interface dropdown

[PeeZee]

I'm not sure where to post this since it relates to multiple packages, so I'll put it in "Catch All".

Could somebody tell me how to make services like openntpd or miniupnp listen on the openvpn interface ?

I have a setup at work where I use 2 pfsense boxes to create a site-to-site VPN tunnel over a 3km wireless link to our data center.
All internet traffic from our office is then tunneled through the openvpn tunnel, over to the pfsense box in the datacenter which does NAT for the packets going onto the internet.

For services like miniupnp or openntpd to work for my office clients the service would need to listen for requests on the openvpn interface, but the dropdown only shows the real interfaces and not the virtual ones.

Thanks !

PeeZee