Follow the below steps for the default installation of firewalld service in centos 7.

First: firewalld is pre-installed in centos 7. If firewalld is not installed, install it. To verify and install, issue the below commands.

rpm -qa firewalld
# expected output: firewalld-0.6.3-2.el7_7.2.noarch

# if not installed, install it
yum install firewalld

Second: firewalld mostly runs as a service. Check the service exists on the host.

ll /usr/lib/systemd/system | grep firewalld
# expected output: -rw-r--r-. 1 root root 674 Jan 30 2018 firewalld.service
# OR
ll /etc/systemd/system | grep firewalld

Third: Verify firewalld service is running and enabled.

If the service is running and it's not enabled, every restart will bring it down. Use the below command.

# check the status of the service (running and enabled)
systemctl status firewalld

# if the service is not running, start it
systemctl start firewalld

# if the service has exited, restart it(check for error if any)
systemctl restart firewalld

# if the service is not enabled, enable it
systemctl enable firewalld

first check status of firewalld status by below command :

sudo systemctl status firewalld

if you get output Active: active so run the following command :

sudo systemctl enable firewalld

but if you get output Active: inactive (dead) or Loaded: masked (/dev/null; bad) follow the below commands :

sudo systemctl unmask --now firewalld

the output should indicate that the symlink has been removed. then run the below command :

sudo systemctl enable firewalld

after enabling the firewall, start the firewalld service:

sudo systemctl start firewalld

when the system executes the command, there is no output. Therefore, it is wise to verify whether the firewall has been activated successfully.

check firewall status with:

sudo systemctl status firewalld

if you see the Active: active (running) message you can now do what you wanted to do.

First install and start firewalld service

sudo yum install -y firewalld
sudo systemctl start firewalld 

Then start adding your rules with firewall-cmd command. Use --permanent flag to keep changes after system reboot.

You may have to open ssh service if using remote SSH access.

Then finally update firewalld with the new configuration

sudo firewall-cmd --reload