NAT Cracker

This essay is about a suggested student project in Java programming. This essay gives a rough overview of how it might work. It does not describe an actual complete program. I have no source, object, specifications, file layouts or anything else useful to implementing this project. Everything I have to say to help you with this project is written below. I am not prepared to help you implement it; I have too many other projects of my own.

I do contract work for a living, which could include writing a program such as this. However, 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 any way you please.

This project is about autoconfiguring a NAT 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 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 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 addresses so you can talk about particular machines. This means you must give up the convenience of DHCP.

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:

Allow incoming traffic on a given port/protocol.
Allow outgoing traffic on a given port/protocol.
Direct incoming traffic on a given port to a particular machine.
Find free ports on the router and on various machines on the LAN.
Allow you to do a full firewall configuration, purely symbolically with machine names, which your software translates to the current IPs for the not too bright firewall.

I am not too clear on this, but there appear to be several configuring protocols UPnP-NAT(PC), 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:

The program specifies what configuration change it wants to make by calling a method of your class. This request is router-independent.
Your program uses tables and screenshots from modem firmware to generate a recipe to let a naive 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 on-line from the major router manufacturers:

BitTorrent
firewall
NAT
router

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