How to view firewall status in centos

The state of the CentOS firewall can be viewed through the sudo firewall-cmd --state command, returning to running or not running. For more detailed information, you can use sudo firewall-cmd --list-all to view, including configured areas, services, ports, etc. If firewall-cmd does not solve the problem, you can use sudo iptables -L -n to view iptables rules. Be sure to make a backup before modifying the firewall configuration to ensure server security.

CentOS Firewall Status: More complex than you think

Many newbies will ask, what do you think of the state of CentOS's firewall? Just one line of commands to get it done? Of course, but that's just the tip of the iceberg. To truly understand the state of the firewall, you need to have a deep understanding of the mechanism behind it in order to be at ease when encountering problems. After reading this article, you can not only view the status, but also better control the security of your server.

First of all, we have to be clear that CentOS's firewall usually refers to firewalld, not iptables (although iptables is the underlying layer of firewalld). iptables is a low-level tool that is more flexible and complex, while firewalld is easier to use, providing a more friendly interface on top of iptables.

To view the status of firewalld, the easiest command is:

<code class="bash">sudo firewall-cmd --state</code>

This returns running or not running . But this just tells you whether the service is running is far from enough. You may need to know which ports are open and which services are allowed to pass through the firewall.

More comprehensive information can be used:

<code class="bash">sudo firewall-cmd --list-all</code>

This outputs all configured zones (zone), enabled/disabled services, open ports, and more. If you study the output results carefully, you will find many details, such as what areas such as public , internal , and external represent, and what are their differences. This is about your security strategy and needs to be understood seriously.

For example, you might see output like this:

<code>public (active) target: default icmp-block-inversion: no interfaces: eth0 sources: services: ssh ports: masquerade: no forward-ports: source-ports: icmp-blocks: rich rules:</code>

This shows that the public area is active, the default policy is default (usually rejected), allowing SSH services to pass, and the eth0 interface belongs to this area. Only by understanding these configurations can you have a clearer understanding of your server security.

However, relying solely on firewall-cmd is sometimes not enough. If you encounter problems, such as a port is clearly open but still inaccessible, you may need to check the iptables rules more deeply. At this time, you can use:

<code class="bash">sudo iptables -L -n</code>

This command will display the rules of iptables, which is more underlying and more complex than firewall-cmd . But when firewall-cmd doesn't solve your problem, it can provide more detailed information. Remember, the order of rules of iptables is very important. Only by understanding the matching order of rules can we better debug problems.

Finally, I would like to remind you that if an error occurs in the firewall configuration, the server may not be able to access normally. Therefore, before modifying the firewall configuration, you must make a backup or operate in the test environment. Security is an eternal theme, and a deep understanding of firewalls is the first step to ensuring server security. Don't underestimate any details, every configuration may affect your server security. Only by practicing and thinking more can you become a true security expert.

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