Firewall configuration using iptables or firewalld

Configuring a firewall is crucial for securing your CentOS system by controlling incoming and outgoing network traffic. There are two main firewall management tools available in CentOS: iptables and firewalld. Here's an overview of how to configure a firewall using both tools:

1. iptables:

Installation:

• Ensure that iptables is installed on your CentOS system. If not, you can install it using the following command:

  sudo yum install iptables 

Basic Configuration:

• List the current firewall rules:

  sudo iptables -L 

• By default, the iptables ruleset is empty.

Adding Rules:

• Add rules to allow or deny traffic based on specific criteria (e.g., IP addresses, ports, protocols).
• For example, to allow incoming traffic on port 22 (SSH), use the following command:

  sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT 

• Similarly, you can add rules to allow or deny traffic for other services and ports.

Saving Rules:

• To save the iptables rules so they persist across reboots, use the following command:

  sudo service iptables save 

2. firewalld:

Installation:

• firewalld is installed by default on CentOS systems. If not, you can install it using the following command:

  sudo yum install firewalld 

Basic Configuration:

• Enable and start the firewalld service:

  sudo systemctl enable firewalld
  sudo systemctl start firewalld
  

• Check the status of the firewalld service:

  sudo systemctl status firewalld 

Adding Rules:

• Use the firewall-cmd command to add rules to the firewall.
• For example, to allow incoming traffic on port 80 (HTTP), use the following command:

  sudo firewall-cmd --zone=public --add-port=80/tcp --permanent 

• Similarly, you can add rules to allow or deny traffic for other services and ports.

Reloading Rules:

• After adding or modifying rules, reload the firewall to apply the changes:

  sudo firewall-cmd --reload 

Managing Zones:

• firewalld uses zones to group network interfaces and apply firewall rules to them.
• Use the firewall-cmd command to manage zones, such as adding or removing interfaces from zones.

Using GUI Tools:

• CentOS also provides GUI tools like firewall-config and firewall-applet for managing firewalld settings.