Using UPnP for Programmatic Port Forwardings and NAT Traversal

Universal Plug-n-Play ("UPnP") is an attempt to extend the concept of ordinary plug-n-play, so that it applies to more than just your own machine: it applies to the whole network. For example, with ordinary plug-n-play, when a new peripheral is connected to your machine, it is automatically discovered and configured from your machine without access to the peripheral itself. UPnP extends this idea to the network: when a new network device is connected to the network, it can be automatically discovered over the network, and configured remotely from your machine over the network.

The idea is that a device can dynamically join a wired or wireless network, obtain an IP address, convey its capabilities, and learn about the presence and capabilities of other devices all over the network. UPnP envisions a future where all devices are networkable and controllable over the network, such as light switches, thermostats, toasters, automobiles, etc. More information can be found here.

Since 2002, most routers have UPnP capability. This allows you to solve one of the more vexing problems for users of network programs that must accept an incoming connection from the Internet. Examples of these programs include P2P file sharing programs, interactive games and gaming, video conferencing, and web or proxy servers. To allow others on the Internet to connect to these programs, it is necessary to configure the router to accept incoming connections and to route the connection to a local machine on the LAN behind the router. This process is called "port forwarding" or "NAT traversal" ("Network Address Translation").

For the ordinary user, this process can be daunting. Moreover, it's often not easy to explain how to configure a router for port forwarding, for the reason that the method differs for each different type of router by each different manufacturer.

UPnP works perfectly in this situation. With it, you can map a port-forwarding programmatically without user interaction.

This article describes a utility that discovers current port mappings on a UPnP-enabled router, and allows you to add/edit/delete mappings. The utility is conceptually broken into two pieces: an engine that performs the actual work, and the UI that uses the engine. This way, it should be possible for you to re-use the engine for your own purposes.