Zyxel-communications 10 Manual do Utilizador descarregar pdf (Página 173)

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101

ZyWALL 10~100 Series Internet Security Gateway

Firewalls 13-1

Chapter 13

Firewalls

This chapter gives some background information on firewalls and explains how to get started with

the ZyWALL firewall.

13.1 What Is a Firewall?

Originally, the term firewall referred to a construction technique designed to prevent the spread of fire from

one room to another. The networking term “firewall” is a system or group of systems that enforces an access-

control policy between two networks. It may also be defined as a mechanism used to protect a trusted

network from an untrusted network. Of course, firewalls cannot solve every security problem. A firewall is

one of the mechanisms used to establish a network security perimeter in support of a network security policy.

It should never be the only mechanism or method employed. For a firewall to guard effectively, you must

design and deploy it appropriately. This requires integrating the firewall into a broad information-security

policy. In addition, specific policies must be implemented within the firewall itself.

13.2 Types of Firewalls

There are three main types of firewalls:

1. Packet Filtering Firewalls

2. Application-level Firewalls

3. Stateful Inspection Firewalls

13.2.1 Packet Filtering Firewalls

Packet filtering firewalls restrict access based on the source/destination computer network address of a

packet and the type of application.

13.2.2 Application-level Firewalls

Application-level firewalls restrict access by serving as proxies for external servers. Since they use programs

written for specific Internet services, such as HTTP, FTP and telnet, they can evaluate network packets for

valid application-specific data. Application-level gateways have a number of general advantages over the

default mode of permitting application traffic directly to internal hosts:

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