Firewall and Caching Technology Overview

In generic terms, a proxy server is a type of firewall that helps an organization securely communicate with an untrusted network, typically the Internet. The word “proxy” simply implies that the server runs one or more applications on behalf of services running on computers hidden behind it. Not all types of firewalls are proxy servers-some are essentially routers that filter packets and translate IP addresses based on a set of rules, but without actually processing the data contained within those packets.

A proxy server may act as a firewall, a caching server, or both.

Firewall Features

In order of increasing security, firewalls commonly use filtering routers, circuit-level proxies, application-level proxies, or combinations thereof to provide secure barriers between trusted and untrusted networks.

Packet filters. Static packet filters simply inspect the IP addresses and port numbers of traffic passing through the firewall and either route or drop the packet depending on rules defined by an administrator. Firewalls supporting dynamic filters can open and close ports “on the fly” based on the type of initial connection request and the port numbers that the client and remote server negotiated. In this way, packets based on protocols that do not use fixed port numbers, such as Remote Procedure Calls (RPCs), can be permitted passage without opening up the entire range of possible port numbers. Some firewalls use a technique termed “stateful inspection” that uses even more intelligence in tracking the progress of a connection and looking for unexpected changes of state that might indicate a hacker attack. Microsoft Proxy Server 2.0 supports dynamic filters, but not stateful inspection. ISA Server 2000 adds support for a form of stateful inspection, but not for all protocols.