How to Open and Manage Linux Firewall Ports on CentOS 6/7

Using firewalld on CentOS 7

Check firewall status: firewall-cmd --state If not running, start it: systemctl start firewalld.service Add a permanent port rule (example MySQL 3306): firewall-cmd --zone=public --add-port=3306/tcp --permanent Restart firewalld to apply changes: systemctl restart firewalld.service Reload the configuration: firewall-cmd --reload List opened ports: firewall-cmd --list-ports Remove a port (e.g., 8080):

firewall-cmd --zone=public --remove-port=8080/tcp --permanent

then restart and reload.

Temporarily stop the firewall: systemctl stop firewalld.service Disable firewall permanently:

systemctl disable firewalld.service

Modifying iptables on CentOS 6

Edit /etc/sysconfig/iptables with a text editor (e.g., vi).

Restart iptables service:

/etc/init.d/iptables restart

Important notes

Opening a port only permits traffic; a service must be listening on that port for remote connections to succeed. Verify listening processes with netstat -lnpt | grep <em>PORT</em>. On CentOS 7, install net-tools if netstat is unavailable: yum install -y net-tools.