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.
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
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
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
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.
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
ip6tables configuration files to the
Construct your Iptables Firewall Rules
Next, you need to construct your
iptables firewall rules by modifying the
/etc/sysconfig/ip6tables files. These files hold the rules that will be read and applied when we start the
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/ip6tables files to implement the policies for your
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:
If you have the graphical UI installed, you can launch it by typing:
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
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
ip6tables services so that they will start automatically at boot:
sudo systemctl enable iptables sudo systemctl enable ip6tables
This should complete your firewall transition.
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.