Joseph Herlant
version 1.0.0, 2014-07-08 : Initial version

First, remember that on RHEL, iptables config file is /etc/sysconfig/iptables and the service logs by default to /var/log/messages.

For this example, our internal network is and we are setting our server as a gateway with eth1 on local network and eth0 that has access to the internet.

# First allow ip forwarding at kernel level
sysctl -w net.ipv4.ip_forward=1
# Then set your iptables rules
iptables -t nat -I POSTROUTING -o eth0 -s -j MASQUERADE
iptables -I FORWARD 1 -i eth1 -o eth0 -j ACCEPT
iptables -I FORWARD 2 -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
# Finally save and restart service
service iptables save
service iptables restart
# Now check your rules
iptables -L -n -v
iptables --table nat -L -n -v
You will need to add the line net.ipv4.ip_forward = 1 or change it to that value in /etc/sysctl.conf to be sure the change will survive next reboot!

Now you have to put this host’s eth1 IP as a gateway in your /etc/sysconfig/network file if not already done (if there’s already a gateway, replace it!):

echo "GATEWAY=" >> /etc/sysconfig/network

Now you can start to try and ping servers outside your network.