Firewall management
Getting to Know firewalld
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Managing a firewall can be a hassle, but it's worse to manage a breach because you didn't have one.
Afirewall is an important part of a security strategy. However, it is only one component and not a security panacea for reasons that will become clear later in this article. A host-based firewall protects the local system just as a network firewall protects an entire network or part of a network, such as a DMZ.
On CentOS 7 and newer, Red Hat Enterprise Linux 7 and newer, and Fedora 18 and newer, the default firewall is firewalld (see the "Features" box for more information.) If you use a Red Hat-based distribution, then you probably have it already. If you use other distributions, firewalld is available via git and as a tarball [1]. Firewalld uses zones to define trust levels of network connections or interfaces. (Zones are an advanced topic not covered in this article; look for a future article covering firewalld zones).
Troubleshooting Firewalls
Sys admins of all skill levels have wasted countless hours troubleshooting a problem that ended up pointing to a firewall that has prevented remote access to a service. The term "remote" is important. Firewalls don't prevent access to local services; firewalls prevent access from remote systems across the network but not access from the local system itself. The point of a firewall is to deny everything from the outside except what you specifically allow in. Unfortunately, frustration with firewall rules often ends in the firewall being disabled by an otherwise well-meaning sys admin.
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