To share an Internet connection with multiple computers on the LAN first look at this picture taken from Sharing Tips.

Notice you will require two NICs (Network Interface Cards) for the computer acting as a NAT (Network Address Translation) router. The first NIC will connect your Cable/DSL modem to your Linux box, and the second would connect to a HUB or another computer on your LAN.

On your Linux computer, you will need to set up the two NICs, that is, load the drivers/modules, configure the IP address if necessary.

First, enable IP forwarding. This can be done when compiling the kernel or through sysctl. To do so through sysctl type:

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

Now for those using Linux 2.0.x kernels or ipfwadm use:

/sbin/ipfwadm -F -p deny
/sbin/ipfwadm -F -a m -W eth0 -S 192.168.0.0/24 -D 0.0.0.0/0

For those using Linux 2.2.x kernels or ipchains use:

/sbin/ipchains -P forward DENY
/sbin/ipchains -A forward -i eth0 -s 192.168.0.0/24 -j MASQ

Additional information about IPFWADM or IPChains can be found here.

For those using 2.4.x or Netfilter/iptables use:

/sbin/iptables -P FORWARD DROP
/sbin/iptables -A POSTROUTING -t nat -o eth0 -j MASQUERADE
/sbin/iptables -A FORWARD -i eth0 -d 192.168.0.0/24 -j ACCEPT
/sbin/iptables -A FORWARD -o eth0 -s 192.168.0.0/24 -j ACCEPT

More information about Netfilter/IPTables can be found here.

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by howe81
last modified: 2002-06-12 11:09:25