Debian Router

I want create my debian home router:

|"dslmodem" wan ppp0|<-->|eth0<-->(eth1,eth2 - shaping router)|<---|"lan" comp1,comp2|

I've created some configurations:
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1. /etc/network/interfaces

# 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
auto eth0
iface eth0 inet static
address 192.168.1.3
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
gateway 192.168.1.1
# dns-nameservers 192.168.1.1

#allow-hotplug eth1
auto eth1
iface eth1 inet static
up /sbin/wondershaper eth1 320 320
down /sbin/wondershaper remove eth1
address 192.168.1.4
netmask 255.255.255.0
# network 192.168.1.0
# broadcast 192.168.1.255

#allow-hotplug eth2
auto eth2
iface eth2 inet static
up /sbin/wondershaper eth2 320 320
down /sbin/wondershaper remove eth2
address 192.168.1.5
netmask 255.255.255.0
# network 192.168.1.0
# broadcast 192.168.1.255

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2. /etc/init.d/iptables.sh

# !/bin/sh
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
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But it's not working maybe you know why, and help me.