To make a connection between two NIC's (one being local intranet and the other being on the internet), you need to setup NAT by using iptables (linux firewall)

Run these commands to achieve NAT

eth0 is internet eth1 is local

yum install iptables

iptables --flush

iptables --table nat --flush

iptables --delete-chain

iptables --table nat --delete-chain

iptables --table nat --append POSTROUTING --out-interface eth0 -j MASQUERADE

iptables --append FORWARD --in-interface eth1 -j ACCEPT

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

service iptables restart

For some reason I had to type the path in front of iptables in CentOS 5.5

yum install iptables



/sbin/iptables --flush



/sbin/iptables --table nat --flush



/sbin/iptables --delete-chain



/sbin/iptables --table nat --delete-chain



/sbin/iptables --table nat --append POSTROUTING --out-interface eth0 -j MASQUERADE



/sbin/iptables --append FORWARD --in-interface eth1 -j ACCEPT



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



service iptables restart



/etc/init.d/iptables restart

Here is a better explanation

To accept all packets incoming on a particular interface, in this case the localhost interface:

iptables -A INPUT -i lo -j ACCEPT

Suppose we have 2 separate interfaces, eth1 which is our internal LAN connection and eth0 which is our external internet connection. We may want to allow all incoming packets on our internal LAN but still filter incoming packets on our external internet connection. We could do this as follows:

iptables -A INPUT -i lo -j ACCEPT


iptables -A INPUT -i eth1 -j ACCEPT

But be very careful - if we were to allow all packets for our external internet interface (for example, eth0 internet connection):

iptables -A INPUT -i eth0 -j ACCEPT

we would have effectively just disabled our firewall!