tulemüür

Tulemüür, Estonian for firewall, is a system or device that monitors and controls incoming and outgoing network traffic between trusted internal networks and untrusted external networks. Its primary purpose is to enforce security policies by allowing or blocking traffic according to predefined rules.

A firewall can operate at different layers and use various techniques. Stateless packet filtering examines individual packets and applies simple rules. Stateful inspection tracks active connections and makes decisions based on context. Deep packet inspection analyzes the content of packets beyond header information. Some firewalls act as proxies, mediating communication between hosts, while many integrate Network Address Translation (NAT) to conceal internal addresses. Modern firewalls often provide logging, alerting, and integration with other security tools.

Common types include network firewalls, which sit at the network perimeter (hardware, virtual, or cloud-based) and protect entire networks; host-based firewalls, installed on individual devices to control their traffic; and next-generation firewalls, which combine traditional filtering with application awareness, user identity, and intrusion prevention capabilities.

Deployment considerations include placement between trusted internal networks and the internet, between network segments, or in a demilitarized zone (DMZ). Administrators typically configure a default-deny policy and maintain rulesets that reflect organizational security requirements. Firewalls are often used in conjunction with VPNs, intrusion detection systems, and other protective measures to form a multi-layered defense.

Limitations include the need for correct configuration to avoid gaps, potential performance impacts, and the challenge of inspecting encrypted traffic. Firewalls complement other security controls but do not by themselves guarantee complete protection.