This tutorial describes how to configure a PC with Ubuntu and two network cards (eth0 and eth1) in order to share an Internet connection through ethernet between two PCs and only one Internet connection. **This tutorial is also usefull if you want to:**

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1) Create a subnetwork to communicate several PCs with each other through ethernet.

2) Establish a DHCP service for other purposes.

3) Get a flexible communication with an embedded system through ethernet (and then SSH). For instance, a LeanxCam [1] or a TS-7800 Single Board Computer [2] without display output.

4) Create an alternative router to a DSL router Internet Service Provider (ISP), but using a standard PC. 

5) Create an advance or personalized firewall for a private subnetwork.

6) Share a wifi connection (//wlan0// or //ra0//) instead a wired connection (//eth0//). Here //eth0// must substituted with //wlan0// or //ra0// depending on your //ifconfig// commnad output.
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In this tutorial it is going to be configured a private sub-network with IP 192.168.6.0 address. Windows systems and ISPs usually take 192.168.1.0 private IP sub-network address to provide Internet access in the same way we are going to do. To avoid this, we will use 192.168.6.0 to make sure there will be no conflict.

The PC has two network/LAN cards: eth0 and eth1. The first card is connected to Internet and eth1 is connected to the other PC (or to a switch if you want to create a private sub-net with more PCs). 
There must be made three main steps:


=== 1 Configure the static address for eth1 ===

Go to /etc/network and edit "interfaces" with sudo (//sudo gedit /etc/network/interfaces//).
Here the eth0 configuration must not be touched. Just assign to eth1 an IP the static address 192.168.6.1.
The eth1 card is going to be the gateway/router of the PCs in sub-network 192.168.6.0.
The file should look something like this:

''auto lo\\
iface lo inet loopback\\
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iface eth0 inet dhcp\\
auto eth0\\
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iface eth1 inet static\\
address 192.168.6.1\\
netmask 255.255.255.0\\
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auto eth1''

Here //lo// is the loopback interface, //eth0// is connected to the DSL router of the ISP which uses also a DHCP service to provide access to Internet, and //eth1// is going to be connected to the other PC or our private sub-network.


=== 2 Setup the DHCP service ===

The DHCP service assigns IP address to our private sub-network in a dynamic way. It can be used a Network Address Translation (NAT) solution instead of DHCP, specially if we are connecting in //eth1// only one PC. However, DHCP gives us flexibility and scalability if we want to connect in the future more PCs using a switch.

First of all, the DHCP service must be installed in your Ubuntu PC using //Sypnatic// or //apt-get install// in a bash shell. cHere it is showed an example of a configuration of the DHCP service using GADMIN-DHCP which is a visual editor of the dhcp3 service of Ununtu.

{{:pantallazo-gadmin-dhcpd_0.4.4.png|}}

The sub-net network is 192.168.6.0 and IP range of addresses starts at 192.168.6.100 and finish with 192.168.6.200. This can be changed if you pleased. You just must renember that with this configuration, the first PC connected to //eth1// will have the 192.168.6.100 address.

The "Domain name servers" (DNS) must be the same of your ISP. Or you can put a universal DNS like 8.8.8.8 or just the gateway or router of this sub-net: 192.168.6.1.

If you don not want to use GADMIN-DHCP, you can add the next code (or similar changing the "domain-name-servers" IPs) at the end of the /etc/dhcp3/dhcpd.conf file. Use a text editor to do this (edit with sudo): 

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''subnet 192.168.6.0 netmask 255.255.255.0 {\\
    interface eth1;\\
    range 192.168.6.100 192.168.6.200;\\
    default-lease-time 6000;\\
    max-lease-time 7200;\\
    option subnet-mask 255.255.255.0;\\
    option routers 192.168.6.1;\\
    option domain-name-servers 150.214.186.69, 150.214.130.15;\\
    option time-offset -3600;\\
}''\\

Finally, the DHCP service must be started: type in a bash shell //sudo /etc/init.d/dhcpd3 restart//. Renember to start this service always you boot your Ubuntu. See below how can this be done, because is the same process to start the IPTables script.

 

=== 3 Provide external Internet traffic to sub-network PCs (eth1)  ===

Go to /etc/init.d/. Then with sudo and a text editor create the following script (//sudo gedit firewall_eth0_to_eth1//):

''#!/bin/bash\\
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# eth0 connected to Internet and eth1 providing dhcp service.\\
# Substitute eth0 with wlan0 or ra0 if this is the wifi Internet connection.\\
# iptables --flush\\
iptables --table nat --append POSTROUTING --out-interface eth0 -j MASQUERADE\\
iptables --append FORWARD --in-interface eth1 -j ACCEPT\\
echo 1 > /proc/sys/net/ipv4/ip_forward\\
''

Give execution permission to the script: //sudo chmod +x firewall_eth0_to_eth1//
Finally, if you want this script to be executed at startup, go to System Menu > Preferences > Startup Applications in the Ubuntu menu and add a new entry for this script. You can also use //update-rc.d// or //rcconf// commands in the bash shell, alternately. (I recommend //rcconf// for stooping or starting services al startup). 
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== References: ==

[1] http://leanxcam.origo.ethz.ch

[2] http://www.embeddedarm.com/products/board-detail.php?product=TS-7800