Hi.

I have a slight problem. I live at a boarding school, and I'm going
home for the holidays. That means I'll be bringing home my computer,
and that's where the problems start. You see, my ISP only gives me one
IP, which is obviously not enough for both my computer and my
sister's. Back in the days when i used Windows XP, I solved
this problem by activating internet sharing on my computer and letting
my sister use me as a gateway. It worked fine, both of us could do what
we normally do on the Internet

However, since I now use Debian, I need to know how to do the same
with under Linux. Here's the network setup:
http://img143.exs.cx/img143/3264/layout4yg.png

Using my sister's computer to route the connection is not an
acceptable solution since she turns it off every night around 23:00.

How should i solve this? I have access to 3 NICs, my own and my
sister's computer, one cheapo ISP and copious amounts of cat 5e TP
cable.

Thank you for your time.

This is the route I would recommend and use myself;

Install a firewall, nat and masquerading using this howto . You need to decide whether you are going to use dhcp or static addressing. In this case it is probably easier to use static addressing. Once you have the firewall setup and running then just tell the xp box that the gateway is the ip address of the ethernet card that it is connected to on the debian box. You only need to use a x-over cable to connect the two. The gateway for the debian box will the be the router address if its broadband.
Your /etc/network/interfaces will look something like this;

auto lo
iface lo inet loopback

# This entry was created during the Debian installation
# (network, broadcast and gateway are optional)

auto eth1
iface eth1 inet static
address 10.0.0.10
netmask 255.0.0.0
gateway 10.0.0.2

auto eth0
iface eth0 inet static

address 192.168.0.2
netmask 255.255.255.0
gateway 192.168.0.5

Where, looking at the debian box 10.0.0.10 is the eth1 connected to the router, ip 10.0.0.2 is the router and 192.168.0.5 is eth0 connected to the xp box. Note that the connections must have different subnets.

Hello,
Thank you for this explanation. I tried it out, but now I still have a problem with connecting to the internet

I still have a problem though: Windows is still reporting "limited or no connectivity" I post below my copy of /etc/network/interfaces. Does anyone know what might be wrong?

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp

Duh! I should have saved the file, of course. *blushes*