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NAT Cracker


This essay does not describe an existing computer program, just one that should exist. This essay is about a suggested student project in Java programming. This essay gives a rough overview of how it might work. I have no source, object, specifications, file layouts or anything else useful to implementing this project. Everything I have prepared to help you is right here.

This project outline is not like the artificial, tidy little problems you are spoon-fed in school, when all the facts you need are included, nothing extraneous is mentioned, the answer is fully specified, along with hints to nudge you toward a single expected canonical solution. This project is much more like the real world of messy problems where it is up to you to fully the define the end point, or a series of ever more difficult versions of this project and research the information yourself to solve them.

Everything I have to say to help you with this project is written below. I am not prepared to help you implement it; or give you any additional materials. I have too many other projects of my own.

Though I am a programmer by profession, I don’t do people’s homework for them. That just robs them of an education.

You have my full permission to implement this project in any way you please and to keep all the profits from your endeavour.

Please do not email me about this project without reading the disclaimer above.

This project is about autoconfiguring a NAT (Network Address Translation) router. This a quite a difficult problem, but if you solve it, programmers will wave palm fronds over you and sing hosannas. The world desperately needs this program.

The Problem

Consider a program like BitTorrent. To work properly it needs somebody to adjust the router/firewall to permit incoming packets on some free port and have them directed to the current machine. The system administrator may be too busy. The owner of a home LAN (Local Area Network) has no clue how and further they probably threw the router manual away. It would be great if the program could set this up for itself.

It turns out that many routers support a complicated protocol called UPnP (Universal Plug & Play) to let them do that. You sent the router SOAP-XML messages.

Because UPnP is often turned off by default as a security measure, at the very least somebody has to manually turn it back on.

To configure a firewall, you need fixed IP (Internet Protocol) addresses so you can talk about particular machines. This means you must give up the convenience of DHCP (Dynamic Host Configuration Protocol).

The project

Part 1 of the project is to create a Java class that will communicate with a UPnP-enabled NAT router and enable it to be configured to: I am not too clear on this, but there appear to be several configuring protocols UPnP-NAT(PC (Personal Computer)), NAT-PMP(Mac) and SOHO. And of course many routers are not configurable, which brings us to part 2 of the project.

For routers that are not configurable, or for routers whose UPnP is turned off, you write a wizard that works like this:

  1. The program specifies what configuration change it wants to make by calling a method of your class. This request is router-independent.
  2. Your program uses tables and screenshots from modem firmware to generate a recipe to let a naïve user make the necessary change. BitTorrent does this quite well for its own purposes.
This second part requires access to at least the manuals of hundreds of different routers, if not the routers themselves. Perhaps you need to make it easy for people who own a router with reasonable competence to provide the necessary information. You might get copies of these online from the major router manufacturers:

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