Networking :: Ubuntu 10.10 As A Dumb Hub For Network Monitoring

I am currently stationed overseas in Japan, and I am happy to say that I have a 100 Mbit fiber line from my service provider... I am not so happy to say that they force me to use their "CTU", which is basically a Japanese router, which limits me from receiving external connections (such as would be required to host FTP, or administer my home machines via SSH or VNC). I have tried many methods of bypassing this piece of equipment, but so far none have worked (router setup for PPPoE, DHCP, Static IP, so on). I don't think the Japanese would mind me bypassing this device, as it's really there to 'protect' me, but there's such a language barrier that I can't figure out how to tell them what I want to do.

In order to troubleshoot the problem, I would like to setup a spare computer as a hub, dumbly (Is that a word? It is for this circumstance...) passing data from one device to the other, and allowing me to watch what is being passed via Wireshark. I am pretty linux savvy, but I'm completely useless with iptables.

Here is the final product I want:

After connecting eth0 to the fiber modem and eth1 to the CTU, I want the computer to duplicate eth0's distant end MAC to eth1 and vice versa (to simulate the computer not being on the network at all), then I just want the computer to pass any data coming in on eth0 to eth1, and any data coming in on eth1 to eth0. Finally, I need to be able to read the throughput with Wireshark, but I really think that will be very simple if I can get the rest of the setup complete.

One piece of info - there is no dhcp on this link of the network, and I have no way of knowing the MAC addresses of either end before connecting them.

As an added bonus, once I've captured the handshake between the CTU and modem, if someone knows how to retransmit those packets on demand (i.e. to replace the CTU with my computer), I would be quite happy to hear about it.