How to get the active WAN interface from Ubus in Lua?

For Developers
1

Is there a preferred way for a Lua script to get the Ubus interface name (i.e. network.interface.????) of the interface that's currently providing the default route?

Ultimately, I'm looking for a way to reliably get the WAN-side IP address regardless of which interface it is attached to.

I know how to get the IP addresses of an interface from Ubus, but I can't figure out how to get routing information from Ubus to know which interface is currently providing WAN connectivity. If OpenWRT is terminating a PPPoE or VPN connection, the actual WAN interface will be the PPPoE/VPN interface and not the one named 'wan.'

2

Isn't the best way to lookup the route with the ip tool? Include one or more optional parameters if you use policy based routing.

ip route get [ ROUTE_GET_FLAGS ] ADDRESS
                            [ from ADDRESS iif STRING ]
                            [ oif STRING ] [ tos TOS ]
                            [ mark NUMBER ] [ vrf NAME ]
                            [ uid NUMBER ] [ ipproto PROTOCOL ]
                            [ sport NUMBER ] [ dport NUMBER ]
3

I was hoping for an easier way to do this.

Getting the output into ip route get into a usable form in Lua would seem to require a lot of glue logic. OpenWRT's Lua interpreter does not include io.popen, so simply getting the output of a shell command into a Lua script is awkward. Ip route get also returns a device name, which cannot be readily mapped to the ubus interface namespace which exposes addressing information.

These problems are solvable, but at the cost of maybe 100-200 lines of code, which is a level of effort I'd rather not undertake if there is an easier way.

4

really?

local sys  = require "luci.sys"
local defroute
defroute = luci.sys.exec("ip route | grep '^default' 2>/dev/null")
if defroute then
	io.write(defroute)
end