How is firewall configuration performed in the Linux operating system?

In Linux, firewall configuration is usually carried out using utilities that manage packet filtering rules. The main tools are:

• iptables — a classic tool for setting up packet filtering in the Linux kernel.
• nftables — a modern replacement for iptables, more flexible and efficient.
• firewalld — a service with dynamic firewall management, using nftables or iptables under the hood.

Example of basic configuration with iptables:

# Allow incoming SSH connections
sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT

# Block all other incoming connections
sudo iptables -P INPUT DROP

# Allow outgoing connections
sudo iptables -P OUTPUT ACCEPT

To save rules permanently, special utilities or configuration files are used, as iptables rules are reset after reboot.

With firewalld, configuration is simpler:

# Allow SSH
sudo firewall-cmd --permanent --add-service=ssh

# Reload firewalld to apply changes
sudo firewall-cmd --reload

Thus, firewall configuration in Linux depends on the chosen tool and security goals.