Shorewall

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Shorewall (Shoreline Firewall) is a free software / open source firewall tool 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.

It is not an "application" per se since it does not run continuously itself, but rather configures rules in the kernel that allow and disallow traffic through the system. Shorewall is configured through a group of plain-text configuration files and does not have a graphical user interface, though a Webmin module is available separately. A monitoring utility packaged with Shorewall can be used to watch the status of the system as it operates and assist in testing.

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.

The plain-text configuration files are usually well-commented and easy to use, though Shorewall can be more difficult for new users to handle than other firewall systems with graphical front-ends.