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Network Security First-Step, Second Edition by Donald Stoddard, Thomas M. Thomas

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Firewall Operational Overview

Every long journey begins with the first step. Before delving too deeply into other areas of security appliance behavior, it is essential to understand how a firewall performs its magic.

Most firewalls (most, not all) rely on Stateful Packet Inspection (SPI) to keep track of all outbound packets and the responses these packets might generate. Keeping track of the hosts on the protected network that are generating outbound packets keeps rogue or unsolicited WAN packets from entering an external interface.

In other words, a firewall that uses SPI, as discussed in Chapter 5, “Overview of Security Technologies,” watches all traffic that originates from an inside host, tracks the conversation from that host to the desired ...

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