Shorewall (more appropriately the Shoreline Firewall) is an open source firewall tool for Linux that builds upon the Netfilter (iptables/ipchains) system built into the Linux kernel, making it easier to manage more complex configuration schemes.
Using an analogy understandable to programmers: Shorewall is to iptables, what C is to assembly language. It provides a higher level of abstraction for describing rules using text files.
Shorewall is mainly used in network installations (as opposed to a personal computer firewall), since most of its strength lies in its ability to work with “zones”, such as the DMZ or a ‘net’ zone. Each zone would then have different rules, making it easy to have for example relaxed rules on the company intranet, yet clamp down on traffic coming in from the internet.
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