Debian :: Local DHCP Server With Two Ethernet Cards

I recently bought a used Server (FSC RX200 S4) and set it up running Debian 7. Being used to Ubuntu, that was no big deal. It works as it should out of the box. This magic computing machine has four Network cards (eth0 - eth3). Now, I want to set it up as a multi-purpose server. On the one hand, it shall provide a Apache2 webserve, a SQL Server, a File/Streaming Server and so on. I have no problems setting up these.

On the other hand, it should work as a DHCP server in my local lan with some PCs and a printer/scanner. Until now, my cheap router did that job including to connecting me to the internet. Since I can't really set up a proper firewall with that box, I want to use my server for that.

Thats's my desired setup I want to achieve:
[] Router provides internet connection to Server
--> cable goes into eth0
[] Server therefore has a internet connection
--> Some internal magic routes the internet from eth0 to eth1 (preferably as a iptables setup) and does the Firwall & DHCP Service
--> cable goes from eth1 to a switch
--> switch connects the PCs and printer
[] PCs have internet and I can use the printer

So far so good, here's what I've come up with. I installed dnsmasq and isc-dhcp-server. I got some config files for both. I got a script to set up my iptables

Iptables script:
Code: Select all#!/bin/sh
#
# rc.firewall
# firewall with iptables ... stronger version
#
FWVER=0.88s

echo -e "
Loading STRONGER rc.firewall - version $FWVER..

[CODE].....

When starting up the services, I don't see any errors. Still, it doesn't work. My Server loses its internet connection and the PCs connected to the network are unreachable.