FOREWORD
When hearing the term firewall, most people think of a product that inspects network traffic at the network and transport layers of the OSI Reference Model and makes pass or filter decisions. In terms of products, dozens of firewall types exist. They are differentiated by the data source they inspect (e.g., network traffic, host processes, or system calls) and the depth to which they inspect those sources. Almost any device that inspects communication and decides whether to pass or filter it could be considered a firewall product.
Marcus Ranum, inventor of the proxy firewall and the implementer of the first commercial firewall product, offered a definition of the term firewall in the mid-1990s when he said, "A firewall is the implementation ...
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