How does a perimeter firewall work?

A perimeter firewall is a network security device or software that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It acts as the first line of defense between an organization's internal network and external networks, such as the internet, by creating a "perimeter" around the internal systems.

How It Works:

1. Traffic Filtering: The firewall examines data packets (units of data sent over a network) and allows or blocks them based on rules like IP addresses, port numbers, protocols (e.g., TCP/UDP), or application types.
2. Stateful Inspection: Modern firewalls track the state of active connections and make decisions based on the context of the traffic (e.g., allowing response traffic for an outgoing request).
3. Network Address Translation (NAT): It can hide internal IP addresses from external networks, adding a layer of security.
4. Blocking Threats: It prevents unauthorized access, malware, and denial-of-service (DoS) attacks by filtering malicious traffic.

Example:

Imagine a company with an internal network containing employee computers and servers. The perimeter firewall sits between the company's network and the internet.

• Allowed Traffic: When an employee accesses a website (e.g., https://example.com), the firewall checks the request and allows outgoing traffic on port 443 (HTTPS). The response from the website is also permitted back in.
• Blocked Traffic: If an external hacker tries to scan the company’s internal servers on port 22 (SSH), the firewall blocks the attempt because such access is not permitted for external sources.

In cloud environments, Tencent Cloud provides Cloud Firewall (part of Tencent Cloud Network Security Services) to protect virtual private clouds (VPCs) and internet-facing resources. It offers similar perimeter defense capabilities, including traffic filtering, intrusion prevention, and DDoS protection.