What Is a Screened Host Firewall?

In today’s cyber security landscape, protecting sensitive data and critical infrastructure is more important than ever. One effective method organizations use to bolster their network defenses is the screened host firewall architecture. But what exactly is it, and how does it work? This article breaks it down in simple terms.

What Is a Screened Host Firewall?

A screened host firewall is a network security architecture that uses a combination of packet-filtering routers and a bastion host to create a secure zone for monitoring and controlling traffic between internal and external networks, such as the Internet.

It acts as a middle layer of protection, screening incoming and outgoing traffic based on pre-configured rules, and routing it through a secure, monitored host before allowing it into the internal network.

Key Components of a Screened Host Firewall

To understand how a screened host firewall works, it helps to know the key components involved:

• Packet-Filtering Router – This is the first line of defense. It examines incoming packets and blocks or allows them based on rules such as source/destination IP addresses, port numbers, and protocols.
• Bastion Host – The bastion host is a hardened, public-facing server placed on the perimeter of the network. It typically runs proxy services and has limited functionality to reduce vulnerabilities. All traffic from the Internet passes through this host for inspection and verification.
• Internal Network – The private part of the organization’s network that contains sensitive data, applications, and resources. It’s shielded from direct access by external users.

Screened Host Firewall vs. Screened Subnet Firewall

It’s important not to confuse a screened host firewall with a screened subnet (DMZ) firewall.

• Screened Host: Only one bastion host is exposed between the Internet and the internal network.
• Screened Subnet: A whole subnet (known as a DMZ) exists between two firewalls, often used for hosting multiple public-facing services, such as email servers, DNS, and web servers.

How Does It Work?

Here’s a simplified step-by-step flow:

• Incoming traffic first hits the packet-filtering router.
• The router allows only specific types of traffic, such as HTTP, SMTP, or FTP, to reach the bastion host.
• The bastion host evaluates the traffic, possibly using proxy services or application-level gateways.
• If the traffic passes inspection, it may be allowed to access the internal network.

Benefits of Using a Screened Host Firewall

• Layered Security: Offers multiple checkpoints before traffic enters the internal network.
• Reduced Attack Surface: The bastion host is the only device exposed to the Internet, limiting direct access to internal systems.
• Customizable Rules: Network administrators can fine-tune traffic flow policies to meet security requirements.
• Scalability: Suitable for small to medium-sized businesses looking to scale their cyber security posture.

Best Practices for Screened Host Firewalls

• Harden the Bastion Host: Regularly update and minimize the number of services.
• Review Firewall Rules Frequently: Ensure only necessary traffic is permitted.
• Enable Logging and Monitoring: Detect suspicious activity early.
• Use Strong Authentication: Secure access to the bastion host with multi-factor authentication (MFA).

Final Thoughts

A screened host firewall is a strategic and cost-effective way to enhance your network’s security. By combining packet filtering and a bastion host, it creates a robust defense line that monitors and restricts access to critical resources. Whether you’re a small business or an enterprise-level organization, implementing a screened host firewall can significantly reduce the risk of external attacks and unauthorized access.