I've installed the luci-app-upnpd package (miniupnpd). It looks like that the configuration file at /etc/config/upnpd has the internal_iface parameter configured as lan out of the box:
config upnpd 'config' option internal_iface 'lan'
As for the external_iface parameter, the script seems to automatically select the interface that has a default route as the external interface unless the external_iface parameter is explicitly defined in the configuration file.
Therefore, essentially, when the package luci-app-upnpd is installed, interface 'lan' is automatically assigned as the internal_iface and whichever interface has a default route is assigned as the external_iface for the purposes of the upnpd package.
It is possible to change the assignments of the interface_iface and external_iface parameters by manually configuring the following in the /etc/config/upnpd configuration file:
config upnpd 'config' option internal_iface 'interface_name' option external_iface 'interface_name'
So, it's all good, but why isn't there an option to define these parameters in LuCI Services > UPnP?
The reason I need to change the internal_iface parameter is that I want to create a VLAN for a device that needs to utilize UPnP and/or NAT-PMP so that it could request a port forwarding in the firewall. I don't want to expose my entire network to the UPnP vulnerabilities, and that's the reason I need the internal_iface parameter to be a subinterface of eth1 (eth1.255) with the name midi rather than lan. It would be nice to be able to do this in LuCI.
Where should I post a request for developers to add these two options to the LuCI upnpd module?