Introduction

Like most other Linux distributions, CentOS 7 uses the netfilter framework inside the Linux kernel in order to access packets that flow through the network stack. This provides the necessary interface to inspect and manipulate packets in order to implement a firewall system.
Most distributions use the iptables firewall, which uses the netfilter hooks to enforce firewall rules. CentOS 7 comes with an alternative service called firewalld which fulfills this same purpose.
While firewalld is a very capable firewall solution with great features, it may be easier for some users to stick with iptables if they are comfortable with its syntax and happy with its behavior and performance. The iptables command is actually used by firewalld itself, but the iptables service is not installed on CentOS 7 by default. In this guide, we’ll demonstrate how to install the iptables service on CentOS 7 and migrate your firewall from firewalld to iptables (check out this guide if you’d like to learn how to use FirewallD instead).

Save your Current Firewall Rules (Optional)

Before making the switch to iptables as your server’s firewall solution, it is a good idea to save the current rules that firewalld is enforcing. We mentioned above that the firewalld daemon actually leverages the iptables command to speak to the netfilter kernel hooks. Because of this, we can dump the current rules using the iptables command.
Dump the current set of rules to standard output and to a file in your home directory called firewalld_iptables_rules by typing:

sudo iptables -S | tee ~/firewalld_iptables_rules

Do the same with ip6tables:

sudo ip6tables -S | tee ~/firewalld_ip6tables_rules

Depending on the firewalld zones that were active, the services that were enabled, and the rules that were passed from firewall-cmd directly to iptables, the dumped rule set might be quite extensive.
The firewalld service implements its firewall policies using normal iptables rules.It accomplishes this by building a management framework using iptables chains. Most of the rules you are likely to see will be used to create these management chains and direct the flow of traffic in and out of these structures.
The firewall rules you end up moving over to your iptables service will not need to recreate the management framework that firewalld relies on. Because of this, the rule set you end up implementing will likely be much simpler. We are saving the entire set here in order to keep as much raw data intact as possible.
You can see some of the more essential lines to get an idea of the policy you’ll have to recreate by typing something like this:

grep 'ACCEPT|DROP|QUEUE|RETURN|REJECT|LOG' ~/firewalld_iptables_rules

This will mostly display the rules that result in a final decision. Rules that only jump to user-created chains will not be shown.

Download and Install the Iptables Service

To begin your server’s transition, you need to download and install the iptables-service package from the CentOS repositories.
Download and install the service files by typing:

sudo yum install iptables-services

This will download and install the systemd scripts used to manage the iptables service. It will also write some default iptables and ip6tables configuration files to the /etc/sysconfig directory.

Construct your Iptables Firewall Rules

Next, you need to construct your iptables firewall rules by modifying the /etc/sysconfig/iptables and /etc/sysconfig/ip6tables files. These files hold the rules that will be read and applied when we start the iptables service.
How you construct your firewall rules depends on whether the system-config-firewall process is installed and being used to manage these files. Check the top of the /etc/sysconfig/iptables file to see whether it recommends against manual editing or not:

sudo head -2 /etc/sysconfig/iptables

If the output looks like this, feel free to manually edit the /etc/sysconfig/iptables and /etc/sysconfig/ip6tables files to implement the policies for your iptables firewall:

output# sample configuration for iptables service
# you can edit this manually or use system-config-firewall

Open and edit the files with sudo privileges to add your rules:

sudo nano /etc/sysconfig/iptables
sudo nano /etc/sysconfig/ip6tables

After you’ve made your rules, you can test your IPv4 and IPv6 rules using these commands:

sudo sh -c 'iptables-restore -t < /etc/sysconfig/iptables'
sudo sh -c 'ip6tables-restore -t < /etc/sysconfig/ip6tables'

If, on the other hand, the output from examining the /etc/sysconfig/iptables file looks like this, you should not manually edit the file:

output# Firewall configuration written by system-config-firewall
# Manual customization of this file is not recommended.

This means that the system-config-firewall management tool is installed and being used to manage this file. Any manual changes will be overwritten by the tool. If you see this, you should make changes to your firewall using one of the associated tools. For the text UI, type:

sudo system-config-firewall-tui

If you have the graphical UI installed, you can launch it by typing:

sudo system-config-firewall

If you need some help learning about iptables rules and syntax, the following guides may be helpful even though they are mainly targeted at Ubuntu systems:

How To Set Up a Firewall Using Iptables on Ubuntu 14.04
Iptables Essentials: Common Firewall Rules and Commands
How To Implement a Basic Firewall Template with Iptables on Ubuntu 14.04

Stop the FirewallD Service and Start the Iptables Service

Next, we need to stop the current firewalld firewall and bring up our iptables services. We will use the && construct to start the new firewall services as soon as the firewalld service successfully shuts down:

sudo systemctl stop firewalld && sudo systemctl start iptables; sudo systemctl start ip6tables

You can verify that firewalld is not running by typing:

sudo firewall-cmd --state

You can also see that the rules you set up in the /etc/sysconfig directory have been loaded and applied by typing:

sudo iptables -S
sudo ip6tables -S

At this point, the iptables and ip6tables services are active for the current session. However, currently, the firewalld service is still the one that will start automatically when the server reboots.
This is best time to test your firewall policies to make sure that you have the level of access that you need, because you can restart the server to revert to your old firewall if there are any issues.

Disable the FirewallD Service and Enable the Iptables Services

After testing your firewall rules to ensure that your policy is correctly being enforced, you can go ahead and disable the firewalld service by typing:

sudo systemctl disable firewalld

This will prevent the service from starting automatically at boot. Since the firewalld service should not be started manually while the iptables services are running either, you can take an extra step by masking the service. This will prevent the firewalld service from being started manually as well:

sudo systemctl mask firewalld

Now, you can enable your iptables and ip6tables services so that they will start automatically at boot:

sudo systemctl enable iptables
sudo systemctl enable ip6tables

This should complete your firewall transition.

Conclusion

Implementing a firewall is an essential step towards keeping your servers secure. While firewalld is a great firewall solution, sometimes using the most familiar tool or using the same systems across more diverse infrastructure makes the most sense.