Dirty Trick to use CUSeeMe from behind a NAT

Provided by: Fernando J. Echevarrieta

What on earth is a NAT? Is my DSL router a NAT?

NAT stands for Network Address Translation. It's a kind of invention made in order to save IP addresses and let a whole network share only one IP address.

If you're not using an internal DSL modem, what you have got is really a DSL router. Some routers may work in two ways: as a bridge and as a router. The former case is more unusual, and, if you are reading this, you will likely use your DSL router as a router. In this case the router has a public IP address (provided by your ISP), or WAN (Wide Area Network) address, and, also, a private IP address, which is only routable in your LAN (Local Area Network, which may consist of your computer alone). All the computers in your LAN will have IP addresses from this private range, which is not routable by the rest of the Internet.

This is why your DSL router also must behave as a NAT, that means, when it receives an IP packet (called datagram) from your LAN, it translates its embedded origin IP address to its own WAN IP address. So every datagram originated in any computer in your LAN is delivered to the internet with the same origin IP address, the WAN IP of the DSL router. When a computer outside answers, the DSL router, the NAT, gets the packet and changes again the IP address (which now will be the destiny IP address) from the WAN IP of the DSL router, to the destination IP in the inner LAN. How can it be done? Because the NAT has taken down some extra data in a table, as the ports related to each session. By using these data, it can identify the real inner destination.

When the communication is originated by a computer outside your LAN, there's no way to identify the inner destination (route). So datagrams cannot be delivered. In other words, they are filtered as if you had a firewall in your DSL route (in fact, this is the way a firewall works). So, you can statically assign some incoming ports to a specific IP address inside your LAN, and only to that one.

So you must (look your DSL router docs) map the following ports:

TCP 7648 and 7649
TCP 1503
UDP 7648 and 7649
UDP 24032

to the computer in your LAN where you are going to run CuSeeMe

So why I cannot connect to some Refs when using a DSL NAT?

The answer is quite simple. CuSeeMe software takes the last IP address of the source computer. That means, as an example, that if your computer private address is "192.168.0.2", this address is include into the CuSeeMe protocol data which travels through Internet as the payload of an IP datagram. So your CuSeeMe data is (IP says origin is your WAN IP; CuSeeMe says origin is: 192.168.0.2). Everything goes on smoothly, and if you connect to aMeeting point 4.x Reflector, everything will be ok.

But if you try and connect to any other type of reflector, this datagram, because it crossed the NAT, which will change the IP origin address, sending something like: (IP says origin is DSL WAN IP; CuSeeMe says origin is: 192.168.0.2). The reflectors compare both the origin addresses, the one reported by IP and the one reported by CuSeeMe. If they don't match, you will be rejected.

Save me please! A realy dirty trick

THIS TRICK HAS ONLY BE TESTED IN WIN95 OSR2

After some research in Internet, I read about a kind of bug which consisted on CuSeeMe, getting the 1st IP address it found in the computer to be reported as the origin address. This was a problem because this was sometimes your modem or any other other NIC (Network Interface Controller) dynamically assigned IP address. Sometimes, this address was not released so, it reported an IP address no longer in use. One of the proposed solutions was to remove this unused NIC from Windows if it was no longer being used.

Well this bug can save you. You must create this situation. If you have a NIC you no longer use, as a modem, find it in Windows Registry and edit it in order to be assigned your DSL router WAN IP address. This interface will never be used so you won't have any routing problem. When CuSeeMe ask the system for an IP address, the system will report this one. But it won't be used for routing, but simply to be annotated inside the CuSeeMe data and to be compared by the Ref with the origin IP address which, after crossing the NAT will be the same!!!!

So let's work... AT YOUR OWN RISK!!!!!

1. Use regedit and Back up the registry.
2. Go to HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\NetTrans
   
3. Here you can see an entry "0000", "0001" and so on for each NIC installed in your computer (please notice you can have logical NICs)
   
4. Provided you don't have a remaining forgotten unused IP address, look for the 1st NIC with an IP address like "0.0.0.0" (maybe your modem)
   
5. You can assign more than one IP address to this NIC if you separate them by ",". So let's put "0.0.0.0,x.x.x.x" where "x.x.x.x" is your DSL router WAN IP address.
   
6. Modify also the IP Net Mask to be the same you have in your LAN.
   
7. Cross your fingers and restart the computer.

Comments? Send them to Hoople <hoople_ny@hotmail.com>