ZyWALL 5/35/70 Series User’s Guide

CH A P T E R 10

Firewalls

This chapter gives some background information on firewalls and introduces the ZyWALL firewall.

10.1 Firewall Overview

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.

10.2 Types of Firewalls

There are three main types of firewalls:

1Packet Filtering Firewalls

2Application-level Firewalls

3Stateful Inspection Firewalls

10.2.1Packet Filtering Firewalls

Packet filtering firewalls restrict access based on the source/destination computer network address of a packet and the type of application.

10.2.2 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:

Chapter 10 Firewalls

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