This post is intended to help some of the people on the Chat forums.
To use Linux as a router you need two nics. we will call them ethX and ethN, where X is external and N is internal - yours will be called eth0 and eth1. if you have a PPP connection like ADSL just ignore the bits about ethX.
ethN should be physically cabled into th LAN and have a sensible IP address - off a nonroutable range such as 192.168.10.1 (255.255.255.0). Cable this up and set all other pcs on the lan to have similar ips (192.168.10.2) and the same subnet. Make sure you can ping some other the other computers and they can ping you.
Now ethX is more complicated. Unplug ethN and plug in ethX - this should be cabled into some internet connectivity device. a cable modem or ADSL modem. Unless you have purchased a static IP from your ISP set this device to obtain all its inforation (IP address etc) from DHCP.
Activate the NIC and check that you can surf the net. try pinging www.google.com.
(if you use a ppp connection then check you can dial out and surf the web).
At this point we know that both our nics are fully functional and the two halves of the router work. ie. we can connect to the pcs on the lan - and we can talk to the internet. now to connect these things together.
Now we are going to use firestarter instead of the standard iptables - as its pretty much the same thing and firestarter has a pretty GUI.
Download and install firestarter - http://firestarter.sourceforge.net/
Now you need to disable iptables - in fedora i select system settings \ server settings \ services from the main menu. then remove the tick next to iptables and click save - now it won't run on boot. and with iptables selected click stop - to terminate the daemon.
your distro may have other methods of starting and stopping daemons.
Run firestarter - from command line or from the GUI.
If the wizard hasn't launched then start it.
Follow the wizard through and finish.
2. choose extX or PPP0 click forward
3. Enable NAT - choose extN - auto detect settings click forward
4. the rest of the defaults should be fine.
the firewall should now work. (click start)
using your favourite text editor check that the file /etc/sysctl.conf contains the line
net.ipv4.ip_forward=1 and that it is not commented out. This will activate ip routing - allows data to be sent across your NICs. Most distros seem to have this on by default but just in case....
Now using the service manager (or however you like) set the firestarter daemon to start on boot (Tick it & Save).
And you're done.
Now on each of your other clients set the default gateway address = ip addess of ethN e.g. 192.168.10.1. and set the dns server addresses = the ones from your isp.
Or set dns server address = ethN and install BIND on your system. The default BIND configuration will work out of the box and provides a caching only nameserver setup.
If you have any questions please post them here. But can you make sure that you have tried to isolate the problem and post details of your problem solving quest.