Share Ethernet Raspberry Pi

1. Set up interfaces
Your interfaces file could look something like this (we have prepared the eth0 with a static IP of 10.0.0.1 – this is your gateway for all clients).

# sudo vim /etc/network/interfaces

auto lo

iface lo inet loopback

# ETH0
iface eth0 inet static
address 10.0.0.1
netmask 255.255.255.0

# WLAN0 
allow-hotplug wlan0
auto wlan0
iface wlan0 inet dhcp
address 192.168.0.160
netmask 255.255.255.0
gateway 192.168.0.1
wpa-essid 
wpa-psk 

2. Install your DHCP server

# sudo apt-get install dnsmasq

# sudo vim /etc/dnsmasq.conf

interface=eth0
dhcp-range=10.0.0.2,10.0.0.9,255.255.255.0,24h

This will make dnsmasq listen to only eth0. It also sets the available IP range to 10.0.0.2 – 10.0.0.9, and the subnet mask to 255.255.255.0. Lease time will be set to 24 hours.

3. Enable packet forwarding in sysctl

sudo vim /etc/sysctl.conf

Uncomment this line:

net.ipv4.ip_forward=1

“Reload” the sysctl with

sysctl -p

5. Create a script that runs everytime the ‘Pi comes online

# sudo vim /etc/network/if-up.d/router.sh

Add this (make sure to change INET_ADDRESS to whatever you set for your wlan0 address)

#!/bin/sh
IPT=/sbin/iptables
LOCAL_IFACE=eth0
INET_IFACE=wlan0
INET_ADDRESS=192.168.1.98

# Flush the tables
$IPT -F INPUT
$IPT -F OUTPUT
$IPT -F FORWARD

$IPT -t nat -P PREROUTING ACCEPT
$IPT -t nat -P POSTROUTING ACCEPT
$IPT -t nat -P OUTPUT ACCEPT

# Allow forwarding packets:
$IPT -A FORWARD -p ALL -i $LOCAL_IFACE -j ACCEPT
$IPT -A FORWARD -i $INET_IFACE -m state --state ESTABLISHED,RELATED -j ACCEPT

# Packet masquerading
$IPT -t nat -A POSTROUTING -o $INET_IFACE -j SNAT --to-source $INET_ADDRESS

Make it executable

# chmod +x /etc/network/if-up.d/router.sh

Apply rules and export a restorable firewall configuration script

source /etc/network/if-up.d/router.sh
iptables-save > /home/pi/router.fw

Make it start on system boot

sudo vim /etc/rc.local

Add this right above the line with “exit 0″

/sbin/iptables-restore < /home/pi/router.fw

When the script has been run it should forward packets between the LAN and Wifi interfaces.

This entry was posted in Raspberry Pi on March 30, 2013 by Richard Lagerström.