Internet connection sharing

Firestarter has the ability to share the firewall host's Internet connection among all the computers on your local network. This is done through a technique called Network Address Translation, or NAT. To the outside world the cluster of machines will look like a single machine with a single IP address.
For connection sharing to work you need to have two or more network devices in your firewall. If the local network is set up correctly, enabling connection sharing is as easy as enabling the option in either the firewall wizard or the Firestarter preferences.

The physical setup and network device settings

Sharing a connection with a local network

The procedure for setting up a network using connection sharing is essentially the same whether you have only two computers or a more complex network with hubs or switches connecting multiple computers. For this example we will be assuming that the Internet connected device on the firewall is an Ethernet card, but a modem or ISDN will work too.
The Firewall/gateway machine connected to the Internet will need two network cards and the clients need one each.
The first network card in the firewall, the external interface, will be the one physically connected to the Internet. This card is usually automatically configured with DHCP. The second network card in the firewall, the internal interface, will be connected to the client machines via either a crossover cable if the connection goes directly to another computer, or regular cable if you have a hub or switch.

Sharing a connection with a single computer

The internal interface of the firewall needs to be statically configured. There are many ways to configure a network interface depending on the distribution you use. Fedora and Red Hat Linux ship with a simple command line tool called netconfig and a more sophisticated graphical tool called system-config-network. system-config-network works better with multiple network cards in the same machine, so we recommend you try it. Other distributions include their own configuration tools, for example in SuSE you would use the Yast program.
No matter how you decide to configure the network cards, these are settings you should enter:
For the external device (usually eth0):

• Enable dynamic IP configuration (DHCP)
• That's it. You're done, don't touch this card further.

The internal device (usually eth1):

• Disable dynamic IP configuration
• IP address: 192.168.0.1
• Netmask: 255.255.255.0
• Default gateway (IP):

Any changes you make will take effect after a reboot, or more elegantly after a restart of the network services (run "/etc/init.d/network restart" as root in most distributions).

Configuring the clients

There are two ways to configure the clients. The more elegant and in the long run easier way is to run a DHCP service on the firewall. A DHCP server distributes the network settings such the IP address, the default gateway, nameservers, etc. at run time to the each client. The alternative to using a DHCP server is to configure every client manually.
Using the DHCP service is as easy as simply enabling it in Firestarter. For more information about the service and how to configure it, refer to the section on configuring the DHCP server.
When using DHCP, the clients need only be configured to use dynamic IP configuration. No other settings need to be changed.

Configuring the clients manually

If you do not wish to use the DHCP service, configure the network devices of the clients to use the following settings:

• Disable dynamic IP configuration
• IP address: 192.168.0.2 to 192.168.0.254, with each client using an unique IP
• Netmask: 255.255.255.0
• Default gateway (IP): 192.168.0.1
• Primary nameserver: Set this to the same nameserver as used on the firewall. You can see the correct setting in the /etc/resolv.conf file on the firewall.

Restart the network service and you're done.

Testing the Setup

The computers should now be connected and the hardware level configuration complete. To test that everything is ok, try pinging the gateway from the client and vice versa.
Enter the following at the firewall machine console, to test that the gateway can reach the client:

[bash]$ ping 192.168.0.2
PING 192.168.0.2 (192.168.0.2) from 192.168.0.1 : 56(84) bytes of data.
64 bytes from 192.168.0.2: icmp_seq=1 ttl=255 time=1.37 ms
64 bytes from 192.168.0.2: icmp_seq=2 ttl=255 time=0.635 ms
64 bytes from 192.168.0.2: icmp_seq=3 ttl=255 time=0.638 ms

--- 192.168.0.2 ping statistics ---
3 packets transmitted, 3 received, 0% loss, time 2010ms
rtt min/avg/max/mdev = 0.635/0.882/1.375/0.349 ms
[bash]$

In case of DHCP, the IP's might be randomly assigned
If it is not working you know that the problem lies with the hardware or network configuration. It is common to get the default gateway setting wrong, so double check it.
At this point:

• The firewall machine should be able to reach the Internet
• The clients and firewall should be able to ping each other
• The clients should be able to reach the Internet if the Internet connection sharing option is enabled in Firestarter.

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