Sharing a Wi-Fi Connection and more

[dolphin194]

I hope I make sense, but it doesn't feel like I am explaining it very well.

So here's a network map of what I'm trying to do:
Internet >> Modem >> Router >> Xubuntu >> Ubuntu Server

The Xubuntu machine is connected to the router through Wi-Fi. Now, I need to be able to share this Wi-Fi connection to the Ubuntu Server computer through a single, non-crossover ethernet cable.

From what I hear, you don't need a crossover cable as long as at least one computer is running Linux.

What I've tried:
I have tried enabling the ICS on my Xubuntu machine and connecting it via a single ethernet cable to my MacBook, but neither machine received an IP address. HOWEVER, I enabled ICS on my MacBook and shared it to the Xubuntu machine and it worked! Did I do something wrong on the Xubuntu machine, or is this just plain not supported?