What is a firewall?

Hardware or software component that controls incoming and outgoing network traffic based on predefined security rules. It acts as a barrier between trusted and untrusted networks.

Main functions:

• Traffic filtering: Allowing or blocking data packets by IP addresses, ports, protocols.
• Access control: Restricting access to resources.
• Logging: Recording information about network traffic for analysis and auditing.
• Intrusion prevention: Some firewalls have IDPS (Intrusion Detection and Prevention System) features.

Examples of software firewalls in Linux:

• iptables
• firewalld

Example of an iptables rule to allow incoming traffic on port 22 (SSH):

# Allow incoming connections on port 22
iptables -A INPUT -p tcp --dport 22 -j ACCEPT