Firewalls. Intruders often attempt to gain access to networked systems by pretending to initiate connections from trusted hosts. They squash the emissions of the genuine host using a denial-of-service attack and then attempt to connect to a target system using the address of the genuine host. To counter these address-spoofing attacks and enforce limitations on authorized connections into the organization�s network, it is necessary to filter all incoming and outgoing network traffic.
A firewall is a collection of hardware and software designed to examine a stream of network traffic and service requests. Its purpose is to eliminate from the stream those packets or requests that fail to meet the security criteria established by the organization. A simple firewall may consist of a filtering router, configured to discard packets that arrive from unauthorized addresses or that represent attempts to connect to unauthorized service ports. More sophisticated implementations may include bastion hosts, on which proxy mechanisms operate on behalf of services. These mechanisms authenticate requests, verify their form and content, and relay approved service requests to the appropriate service hosts. Because firewalls are typically the first line of defense against intruders, their configuration must be carefully implemented and tested before connections are established between internal networks and the Internet.
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