So what exactly is a firewall? A firewall is a special computer system that sits
between your network and another network. It usually has two or more network
interfaces installed. One network interface is connected to the internal network and
the second interface is connected to the public network. This is shown in Figure 1-1.
14 Chapter 1: Introducing Linux
The job of a firewall is to analyze all of the network traffic that flows between
the internal networks and the external network, including inbound and outbound
traffic. The firewall is configured with rules that define what kind of traffic is allowed
through and what kind isn??™t. Any network traffic that violates the ruls is not allowed
through, as shown in Figure 1-2.
Linux can be customized and configured to operate as a firewall in your network.
Using netfilters and iptables, you can create rules that filter network traffic at the
packet level. Using Squid, you can create an application-level gateway on a Linux
system as well.
Now that you understand the various roles that a Linux system can fill on your
network, we need to shift gears and actually start working with the Linux user
interface. Let??™s do that next.
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