HOWTO: Set up a Gateway for Internet Connection Sharing

by Gavin Henry

Introduction:

One of the most frequently questions I see on mailing lists, chat channels and newsgroups is "How do I set up Internet Connection sharing with Linux?" Well read on.

Configuration:

I will presume you already have an internet connection with a modem. Things that are needed:

• A machine that is going to be the Gateway.
• A modem for this machine and a network card for it.
• A switch/hub for other client machines to connect to the Gateway.
• A network card and cables for each client machine.
• Iptables.

All of the configuration will be on the command line, so open up a terminal and become root by typing su -

edit /etc/sysconfig/networking/devices/ifcfg-eth0 and make it look like this:

DEVICE=eth0
ONBOOT=yes
BOOTPROTO=none
NETMASK=255.255.255.0
USERCTL=yes
PEERDNS=no
GATEWAY=ppp0
TYPE=Ethernet
IPADDR=192.168.100.1

Then restart networking:

service network restart

Check that iptables is installed by typing as root:

rpm -q iptables

If not

apt-get install iptables

Then:

lsmod | grep iptable_nat

If this comes up blank, you need to load the right module:

modprobe iptable_nat

Now (if you have not issued "su -" and only "su", you will need to add /sbin/ to most of these commands, i.e. /sbin/iptables and /sbin/service):

iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE

Save this by:

service iptables save

Enable ip forwarding by typing:

echo "1" > /proc/sys/net/ipv4/ip_forward

Edit /etc/sysctl.conf changing net.ipv4.ip_forward = 0 to 1 to keep this enabled at reboot. You make the system reread /etc/sysctl.conf by typing:

sysctl -p

to check you haven't made a typo.

You can also setup a DHCP server to allow your clients whether it be Windows® or Linux clients to be given a IP address with the Gateway details included. Download DHCP by:

apt-get install dhcp

and then as root, make /etc/dhcpd.conf look like:

ddns-update-style interim;

default-lease-time              600;
max-lease-time                  7200;

subnet 192.168.100.0 netmask 255.255.255.0 {
  option routers 192.168.100.1;
  option subnet-mask 255.255.255.0;
  option domain-name-servers ns1.example.com, ns2.example.com;
  range 192.168.100.2 192.168.100.20;
} 

And:

service dhcpd start

Now if DHCP is enabled on all your clients, then that is all you need to do. Use the setup command as root to checkout your network settings on your Fedora clients.

Well, that's it for now. For any comments or corrections, please e-mail me.