UNIX for Advanced & Expert Users

I have a PC with KUBUNTU installed on it and with 2NIC's on it (two PCI network 100Mbit cards). I want to use it as a server packet router and firewall between two computers with windows installed on them, each of this computer being connected to one different card on the KUBUNTU server. The computers are connected to the network cards using a switch.
I tried to configure the server to route the packets from one IP address rank to another and with different masks, but I hadn't managed to do this. Instead I did the following:
1. I've set net.ipv4.ip_forward = 1 in sysctl.conf
2. I configured eth0 as follows:
ifconfig eth0 192.168.2.1 netmask 255.255.255.0 up
3. I configured eth1 as follows:
ifconfig eth1 192.168.1.2 netmask 255.255.255.0 up

After this I managed to ping say a computer with Windows with 192.168.1.5 from a computer connected to another card with the IP address 192.168.2.5.

(But initially I wanted to ping say a computer with the IP with 81.180.75.70(mask 255.255.255.192) from a computer with the IP of 192.168.2.5(mask 255.255.255.0) -> I didn't managed to do that)

NEXT, I want to configure the server to act as a firewall. Let say he will allow ping from one PC connected to the server to another one but not viceversa. At the same time I want that ping(or say packets) from the PCs will reach the server and viceversa. (So finally to deny ping from one PC to another, but another will be able to ping the one whose packets will be rejected).

I tried to do something like this:

iptables -A FORWARD -s 0/0 -i eth0 -d 192.168.1.5 -o eth1 -j REJECT

but this isn't working.
If I do this, on one PC ping will result in Request timed out,
and on another will result in some specific reject message. If I
remove this by typing iptables -F FORWARD, everything goes back
to normal and ping is allowed.

WHAT'S THE PROBLEM? CAN SOMEONE EXPLAIN STEP BY STEP WHAT I NEED TO DO?
THANKS IN ADVANCE TO EVERYONE WHO'LL TRY TO HELP ME!

PLEASE DON'T ASK WHY I NEED THAT AND WHY THIS VERSION OF LINUX(UBUNTU)
BECAUSE THE TASK IS SUPPOSED TO BE RESOLVED WITH THIS CONFIGURATION
AND NO MORE!

The problem is that your rule blocks both the ping request AND ping replies to 192.168.1.5. You might need to use the conntrack module:

Code:

iptables -A FORWARD -s 0/0 -i eth0 -d 192.168.1.5 -o eth1 -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -s 0/0 -i eth0 -d 192.168.1.5 -o eth1 -j REJECT