how to upnp forward ports via two levels of routers

Question

i am using the java sbbi library to forward ports. but my router is conected to another router, so the external ip of the first router is a ip that was given by the second router dhcp. is there a way to forward the ports on the second router as well? here is my code:

InternetGatewayDevice[] devices=InternetGatewayDevice.getDevices( 1000 );
System.out.println("Found "+devices.length+" Devices");
InternetGatewayDevice dev = devices[ 0 ];
System.out.println(dev);
System.out.println( "External IP = " + dev.getExternalIPAddress() );

System.out.println( dev.addPortMapping( "Streamternet", "TCP", null, 1333,
                                        "192.168.0.105", 8888, 0 ) );

System.out.println( "waiting for connection" );
HTTPServer.main(null);

Answer 1

As far as i know it's not possible with your current setup because you cannot access your router's gateway directly.

There are 2 solutions i can think of.

1. Disable your router DHCP server so that he and you will all get your IPs from the second router and than you'll all have the same subnet and you could access each one of the routers.
2. Have another PC that is connected to the second router network and make him a server that listens to your commands and this server will execute the UPNP tasks for you. (This will probably need a manual setup of port forwarding first)

Hope i could help.

Answer 2

As @Danpe said in his answer, UPnP won't let you open ports when you're behind multiple NATs.

As I'm also working on a P2P software and from what I've gathered, here is what one can try to connect two peers:

1. Connect using TCP or UDP hole punching.
2. If above doesn't work, one of the routers on either side uses a Symmetric NAT. Try UPnP or NAT-PNP.

3. If above doesn't work, app is either behind a router that doesn't support UPnP, NAT-PNP... or you're behind multiple levels of NATs. Now you can either:

   1. Communicate with the user of the app that she needs to open ports on those routers. This is, as you've mentioned in comments, an unfortunate solution as it assumes advanced technical knowledge from the user. But it does seem to be what some companies actually do. See here for example.

   2. Use a relay server such as a one using TURN protocol through which users with closed ports will communicate. This relay server may be one or more of peers whose ports are open (as is the case with uTorrent), or it can be your server, or a combination of the two: if there are peers with open ports, use those, otherwise use your own server (this is the case with Skype).