For example if user a goes to googlecom the proxy

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For example, if user A goes to, the proxy actually sends the request to and retrieves the web page. When user B initiates a request to connect to, the proxy sends the information it has already retrieved for user A. The proxy has algorithms to ensure that current data remains in the cache. The effect is that the page is returned much faster to the user than having to get it from again. A proxy can also be configured to block access to certain web sites and filter certain port traffic to protect the internal network. It is important to note that there are two types of solutions that are called Proxy servers. One is an application that is loaded on a PC-based server. The second is a feature incorporated into a firewall. The application that is loaded on a PC server is not considered a solution for providing security on a network, but does provide the acceleration benefits outlined in this document. When a proxy server is to be used for security purposes, it should be a feature incorporated into a hardened firewall solution. Application Gateways Application gateways are a variation of a proxy server and functions as follows: The internal client first establishes a connection with the application gateway. The application gateway determines if the connection should be allowed or not and then establishes a connection with the destination computer. All communications go through two connections: 1.) client to application gateway and 2.) application gateway to destination. The application gateway monitors all traffic against its rules before deciding whether or not to forward it. As with the other proxy server types, the application gateway is the only address seen by the outside world so the internal network is protected. Firewall Rules Design When developing the rules for the firewall, it is important to keep the following guidelines in mind. Start with a totally locked-down configuration, where nothing is permitted through the firewall. Then open only the minimum ports necessary for the application to function. It will be necessary to thoroughly identify the data flow requirements from all zones. Software suppliers can usually provide the port and protocol information about their applications. If they cannot, a network sniffer application can be used to identify the ports and protocols used. When using the DMZ, it is necessary to continue the lockdown philosophy. Inexperienced firewall ruleset designers will get a false sense of security with the DMZ and allow too many ports to be opened. It is necessary to keep in mind what risk is presented if the server in the DMZ is compromised. Process Network Security - Firewall Configuration and Policies Rev. 0.2 Page 6 Copyright 2004, Invensys Systems, Inc. All Rights Reserved.
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Christopher Reinemann
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