building a linux router

www.linuxrouter.org/

Although not recommended, you can use a ready-made firewall script that will cater for NAT and general security of the router. Alternative is to write a custom script manually.

Here is the script:
http://www.tux.org/~peterw/linux/bastille-firewall-scripts.tar.gz

/etc/sysconfig/network-scripts/
contains network interface settings for their appropriate names : eth0 through to eth4 in your case.

command used to alter these setting in the run time is either /sbin/ifconfig
or /sbin/ip
(use "ip link" and "ip addr" for interface setup)

/etc/ppp/
directory contains the config files for PPP links

pppoe command is what you need I guess for pppoe connection.

just setup this firewall/internet sharing script in a file called /etc/rc.d/rc.firewall

and call it from your /etc/rc.d/rc.local
(just add a line like this: /etc/rc.d/rc.firewall)

Per your configuration, this could help:

# I think you have DSL. if it's already starting, then
# delete the "adsl-start" line:
adsl-start

#Activate IP Forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward

ipt="/usr/local/sbin/iptables"
outside=ppp0
inside=eth1
other=eth0

$ipt -F
$ipt -t nat -F
$ipt -t mangle -F
$ipt -P INPUT DROP
$ipt -P OUTPUT DROP
$ipt -P FORWARD DROP

$ipt -A INPUT -i lo -j ACCEPT
$ipt -A INPUT -i $inside -j ACCEPT
$ipt -A OUTPUT -i $inside -j ACCEPT
$ipt -A FORWARD -i $inside -j ACCEPT

$ipt -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
#Accept at least traffic to ssh into this host.
$ipt -A INPUT -p tcp --dport 22 -j ACCEPT
#now the nat thing.
$ipt -t nat -A POSTROUTING -o $outside -j MASQUERADE

---
This script will not forward anything from eth0 to eth1.
as you asked to be, but forwards (it does not nat) from eth1 to eth0, and NAT from eth1 to ppp0, any ip they could have.

ON THE OTHER HAND:
you do not need a network card for each other computer you have. it's way simpler to have a HUB/SWITCH and connect your eth0 network card to it, and all other computers to the same HUB/SWITCH.

now, just setup the ip address of your eth0, and in all other computers setup that IP as the gateway to the internet.

it's that simple.

check this diagram:

internet---dslmodem---linuxbox---hub---othercomputers

you can see your linuxbox has two ends. one is the network card connected to the dsl modem, and the other is the card connected to the hub, which in turn connect all the computers.

a good network subnet if you do not know how to begin is
192.168.0.0/24, which means your linux box will be 192.168.0.1/255.255.255.0, and all other computers will follow with 192.168.0.2, .3, .4 etc. and same netmask.
all the other computers should have 192.168.0.1 as the default gateway, and your linux box should not have a gateway setup, as the dsl utility need to setup the default gateway dinamically.

Hope this helps

look to make my linux machine a router what i did, i first made all my lan cards up, obviously with an ip address each. this u can do with the help of running "#setup" command. or use the "#ifconfig" comman.
next u have to see the routing table of ur machine, with the help of "#route" command. set teh routing table according to ur needs.
see the man pages of "ifconfig" and "route". its easy, u can do it.
good luck.