== Part 3: Introduction to Firewalls: iptables ==

Essentially, Iptables is a list of rules. Each rule is placed into a particular chain and when data is sent into, out of or through a PC the data is checked against these rules. If the data matches a particular rule, it then must “jump” to a condition. Simple conditions include ACCEPT, DROP and LOG but there are also more complex conditions that can be applied and there is even the option to create your own conditions.

'''FORWARD''' – LINUX PC's are often used as routers or gateways for other PC's. This means that data may have to be passed through this LINUX box, but the data is not intended for the LINUX PC nor is it being sent by the LINUX PC. Even though the data will go into this PC and it will exit this PC, the INPUT and OUTPUT chains do not apply here. Because the PC is acting as a router it does not actually send or receive data, it FORWARDS data from one machine to another. When this process happens, the data is checked against the FORWARD chain to see if it is allowed through. If it is the router will forward the data to it's destination. If not, the packet is dropped.}}

# As root on the CentOS host enter the following commands at the prompt:

#* <code>iptables -F</code> (This flushes out or clears all of your rules from the chains)

{{Admon/note | iptables Service |When your iptables service starts or at boot time it has to load the rules from the file <code>/etc/sysconfig/iptables</code>.}}

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