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To enable a machine to act as a gateway for a network:

echo "1" > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -j MASQUERADE

Where 192.168.1.0/24 is the network which will use this machine as a gateway.

To make this permanent, on Debian/Ubuntu:

touch /etc/network/if-up.d/iptables
chmod 755 /etc/network/if-up.d/iptables

And add the following to the new file:

#!/bin/bash

echo "1" > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -j MASQUERADE

For Red Hat/Fedora:

iptables-save

and edit /etc/sysctl.conf, changing the line net.ipv4.ip_forward=0 to:

net.ipv4.ip_forward=1

Then running sysctl -p to make sysctl restart with the new values.

Good IP Tables Tutorial:

• http://iptables-tutorial.frozentux.net/iptables-tutorial.html
• http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch14_:_Linux_Firewalls_Using_iptables