A firewall is a secure and trusted machine that sits between a private
network and a public network.
The firewall machine is configured with a set of rules that determine
which network traffic will be allowed to pass and which will be blocked
or refused. In some large organizations, you may even find a firewall
located inside their corporate network to segregate sensitive areas
of the organization from other employees. Many cases of computer
crime occur from within an organization, not just from outside.
Firewalls can be constructed in quite a variety of ways. The most
sophisticated arrangement involves a number of separate machines and
is known as a perimeter network. Two machines act
as "filters" called chokes to allow only certain types of network
traffic to pass, and between these chokes reside network servers such as a
mail gateway or a World Wide Web proxy server. This configuration can
be very safe and easily allows quite a great range of control over who
can connect both from the inside to the outside, and from the outside
to the inside. This sort of configuration might be used by large
Typically though, firewalls are single machines that serve all of these
functions. These are a little less secure, because if there is some
weakness in the firewall machine itself that allows people to gain access
to it, the whole network security has been breached. Nevertheless,
these types of firewalls are cheaper and easier to manage than the more
sophisticated arrangement just described.
Figure 9-1 illustrates the two most
common firewall configurations.
Figure 9-1. The two major classes of firewall design
The Linux kernel provides a range of built-in features that allow it
to function quite nicely as an IP firewall. The network implementation
includes code to do IP filtering in a number of different ways, and
provides a mechanism to quite accurately configure what sort of rules
you'd like to put in place. The Linux firewall is flexible enough to
make it very useful in either of the configurations illustrated in
Figure 9-1. Linux firewall
software provides two other useful features that we'll discuss in
separate chapters: IP Accounting (Chapter 10) and IP masquerade (Chapter 11).