gluon-core: make sure dns traffic leaves the correct interface when there is ipv6 wan uplink

package/gluon-core/luasrc/lib/gluon/upgrade/820-dns-config

@@ -16,6 +16,13 @@ uci:delete('dhcp', dnsmasq, 'cachesize')
 
 uci:delete('firewall', 'client_dns')
 if dns.servers then
+	localipv6 = uci:get("network", "loopback", "ip6addr")
+	-- below is an ugly hack that makes sure the dns upstream traffic is leaving the correct network interface.
+	appended = {}
+	for i,s in ipairs(dns.servers) do
+		table.insert(appended, s .. "@" .. string.sub(localipv6,0, string.find(localipv6,"/")-1 ) )
+	end
+	uci:set('dhcp', dnsmasq, 'server', appended )
 	-- allow inbound traffic for dns from client zone
 	uci:section('firewall', 'rule', 'client_dns', {
 		src = 'local_client',