Firewalld is a complete firewall solution available by default on CentOS 7 servers. In this guide, we will cover how to set up a firewall for your server and show you the basics of managing the firewall with the firewall-cmd administrative tool (if you'd rather use iptables with CentOS, follow this guide).
Basic Concepts in FirewalldBefore we begin talking about how to actually use the firewall-cmd utility to manage your firewall configuration, we should get familiar with a few basic concepts that the tool introduces.
The firewalld daemon manages groups of rules using entities called "zones". Zones are basically sets of rules dictating what traffic should be allowed depending on the level of trust you have in the networks your computer is connected to. Network interfaces are assigned a zone to dictate the behavior that the firewall should allow.
For computers that might move between networks frequently (like laptops), this kind of flexibility provides a good method of changing your rules depending on your environment. You may have strict rules in place prohibiting most traffic when operating on a public WiFi network, while allowing more relaxed restrictions when connected to your home network. For a server, these zones are not as immediately important because the network environment rarely, if ever, changes.
Regardless of how dymaic your network environment may be, it is still useful to be familiar with the general idea behind each of the pre-defined zones for firewalld. In order from least trusted to most trusted, the pre-defined zones within firewalld are:
drop: The lowest level of trust. All incoming connections are dropped without reply and only outgoing connections are possible.
block: Similar to the above, but instead of simply dropping connections, incoming requests are rejected with an icmp-host-prohibited or icmp6-adm-prohibited message.
public: Represents public, untrusted networks. You don't trust other computers but may allow selected incoming connections on a case-by-case basis.
external: External networks in the event that you are using the firewall as your gateway. It is configured for NAT masquerading so that your internal network remains private but reachable.
internal: The other side of the external zone, used for the internal portion of a gateway. The computers are fairly trustworthy and some additional services are available.
dmz: Used for computers located in a DMZ (isolated computers that will not have access to the rest of your network). Only certain incoming connections are allowed.
work: Used for work machines. Trust most of the computers in the network. A few more services might be allowed.
home: A home environment. It generally implies that you trust most of the other computers and that a few more services will be accepted.
trusted: Trust all of the machines in the network. The most open of the available options and should be used sparingly.
To use the firewall, we can create rules and alter the properties of our zones and then assign our network interfaces to whichever zones are most appropriate.