IP forwarding or sharing via /etc/NETWORK?

Hi to you all,

and here's my situation:

My main machine is connected to the internet via dhcp. I installed a second network interface card into this PC (A) and connected it via a RJ45 patch cable to a second PC (B).
My goal is to get B an internet connection using A.

So this is my main question:

If this is possible, can I do it with adjusting /etc/NETWORK and booting settings (in my case /etc/rc.local)?

(I'm using dyne:bolic 2.5.2, an older distro based on a 2.6.18 kernel. Why? I like it and I'm able to work fine with just a system cd and an usb flash drive.)

Due this is my first post, I did some quick searching but didn't found a comparable thread. If there is one, just post the link and please... do not hold this thread against me. ;)

Thank you in advance,
Gifti