One important step to protect a system is using a firewall. Essentially, this is a network security system, acting like a security guard between internal and external networks. The firewall controls incoming and outgoing network traffic using security rules. The rules specify which connections you want to allow and the ports and zones through which the connection can be established.
All of the software featured in this article is released under an open source license. Each of the solutions offer a comparable set of features to many expensive commercial firewall solutions.
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