Jan 27, 2025 6 min read

Understanding the Firewall Sandwich: A Robust Network Security Architecture

In today’s complex cyber threat landscape, organizations need more than just basic perimeter defenses to safeguard their networks. This is where the concept of a firewall sandwich comes into play. By layering multiple firewalls from different vendors, organizations can implement a defense-in-depth strategy that mitigates vulnerabilities, increases resilience, and provides enhanced protection. This method ensures that even as threats evolve, the network remains robust against various forms of attacks. In this blog, we’ll explore what a firewall sandwich is, why it’s effective, and how tools like pfSense and Firewalla can serve as practical use cases.

TL;DR

A firewall sandwich is a layered network security strategy that deploys multiple firewalls from different vendors to enhance security and resilience. By leveraging the strengths of diverse technologies, this approach mitigates vulnerabilities, ensures compliance, and optimizes performance. Tools like pfSense and Firewalla illustrate how an outer and inner firewall can work together to provide robust defense-in-depth.

What Is a Firewall Sandwich?

A firewall sandwich is a network architecture where multiple firewalls are deployed in a sequential, layered setup to bolster security. This approach leverages the strengths of different vendors’ technologies, ensuring that vulnerabilities in one system do not compromise the entire network. By diversifying the defensive tools in use, the architecture provides a more robust solution to network security.

Structure of a Firewall Sandwich

Outer Firewall (Perimeter Firewall):
- Protects the network from external threats.
- Focuses on general packet filtering, intrusion detection, and access control rules for incoming traffic.
- Often serves as the first line of defense, deflecting broad, low-level attacks before they penetrate deeper into the network.
Inner Firewall (Core Firewall):
- Positioned closer to sensitive internal systems.
- Provides deeper inspection of traffic, such as application-layer filtering and granular controls.
- Focuses on advanced threat detection, ensuring that sophisticated attacks do not bypass security measures.
Optional Middle Zone (DMZ):
- A Demilitarized Zone (DMZ) is often placed between the outer and inner firewalls. This segment hosts public-facing services, such as web servers and mail servers, isolating them from the internal network.
- Ensures that traffic flowing to and from these public services is thoroughly monitored and filtered, adding an extra layer of security.

By segmenting the network in this way, the firewall sandwich ensures that even if the perimeter firewall is compromised, the core firewall can still protect sensitive internal resources. This layered design minimizes the attack surface and delays attackers, increasing the likelihood of detection.

Why Use a Firewall Sandwich?

1. Defense-in-Depth:

Layering firewalls from different vendors ensures that vulnerabilities in one firewall are less likely to compromise the entire network. If an attacker exploits a weakness in one firewall, the other layers provide additional protection, making the network significantly harder to penetrate. This approach mirrors the security principle of redundancy: having multiple, independent layers of defense reduces the likelihood of a successful attack.

2. Layered Security:

Each firewall in the sandwich has a specific role:

The outer firewall handles broad filtering, blocking known malicious IPs, and managing large traffic volumes. This prevents resource-intensive traffic from overwhelming internal defenses.
The inner firewall focuses on stricter rules, deeper inspection, and advanced threat detection, ensuring that malicious activity does not reach critical systems.

3. Redundancy and Resilience:

Having multiple firewalls ensures continued protection even if one fails or is bypassed. This redundancy is particularly crucial for organizations that operate critical infrastructure or handle sensitive data. For example, if a perimeter firewall experiences downtime due to maintenance or an attack, the core firewall maintains the integrity of internal systems.

4. Performance Optimization:

The outer firewall reduces the load on the inner firewall by filtering out unwanted traffic, allowing the inner firewall to focus on deeper packet inspection. This layered approach minimizes latency and improves overall network performance. Additionally, the division of labor between firewalls ensures that resources are used efficiently.

5. Vendor-Specific Strengths:

Different vendors excel in various areas, such as malware detection, VPN support, or application-layer security. Using firewalls from different vendors leverages their unique capabilities. For example, one vendor may offer superior intrusion prevention systems (IPS), while another provides advanced analytics and reporting features. This diversity creates a more versatile and adaptable security solution.

6. Compliance with Regulations:

For organizations in regulated industries, a firewall sandwich can help meet strict compliance requirements like PCI DSS, HIPAA, or NERC CIP. The layered architecture provides the segmentation and monitoring necessary for audits and regulatory reporting. Additionally, having multiple firewalls aids in logging and reporting by providing detailed insights into traffic at different network levels.

7. Mitigating Zero-Day Vulnerabilities:

Zero-day vulnerabilities can pose significant risks to single-vendor solutions. A firewall sandwich mitigates this by diversifying the security landscape, ensuring that even if one firewall is compromised due to an unknown vulnerability, the others remain unaffected.

Use Case: Combining pfSense and Firewalla

Outer Firewall: pfSense

pfSense is a widely used, open-source firewall that excels in perimeter defense. It provides features such as:

Packet filtering: Blocks malicious traffic at the network layer.
Intrusion detection and prevention (IDS/IPS): Detects and blocks known threats.
VPN support: Ensures secure remote connections.
Traffic shaping: Optimizes bandwidth usage by prioritizing critical traffic.
Customizable rulesets: Allows organizations to tailor security policies to their unique needs.

In a firewall sandwich, pfSense acts as the first line of defense, managing large volumes of traffic and filtering out known bad actors before they reach the core firewall. Its flexibility and cost-effectiveness make it an ideal choice for small to medium-sized organizations. Additionally, its extensive documentation and active community support make it accessible for IT teams of varying skill levels.

Inner Firewall: Firewalla

Firewalla is a compact, user-friendly firewall ideal for deeper traffic inspection closer to sensitive systems. Key features include:

Application-layer filtering: Identifies and blocks malicious traffic at the application level.
Parental controls: Useful for controlling access to specific websites or applications.
Behavior-based alerts: Notifies administrators of unusual activity in real time.
Device-level monitoring: Tracks individual devices on the network for suspicious behavior.
AI-driven analytics: Uses machine learning to detect patterns and anomalies that might indicate threats.

Firewalla, positioned as the inner firewall, provides granular control over the already filtered traffic, ensuring no malicious activity slips through. Its intuitive interface and behavior-based analytics make it a valuable addition to any network. Moreover, its small form factor and ease of deployment make it an excellent choice for both residential and enterprise use.

Example Deployment:

Deploy pfSense as the outer firewall to filter incoming traffic and block known threats.
Position Firewalla internally to inspect and monitor traffic that passes through pfSense, providing deeper application-layer security.
Optionally, create a DMZ between the two for hosting public-facing services, isolating them from internal networks.
Configure regular logs and alerts on both firewalls to monitor for anomalies, ensuring that security incidents are quickly identified and mitigated.
Regularly update both firewalls to ensure they are protected against the latest threats.

Benefits of the Firewall Sandwich Architecture

Enhanced Security: Multiple layers of firewalls significantly reduce the risk of successful attacks by ensuring that each layer compensates for the weaknesses of the others.
Cost-Effectiveness: Tools like pfSense (open-source) and Firewalla (affordable hardware) make this architecture accessible for small to medium-sized organizations without compromising on security.
Flexibility: The architecture can scale and adapt to changing security requirements, accommodating new devices, applications, and policies.
Regulatory Compliance: Helps organizations meet stringent industry standards by providing robust network segmentation and activity monitoring.
Improved Visibility: A layered setup offers more granular insights into traffic patterns and potential threats, enhancing the ability to respond proactively.
Proactive Threat Mitigation: By combining real-time monitoring with advanced analytics, the firewall sandwich can detect and respond to emerging threats before they cause significant damage.

Challenges and Considerations

Cost:

While tools like pfSense and Firewalla are cost-effective, deploying multiple firewalls can increase hardware, software, and maintenance costs. Organizations must carefully plan their budgets to ensure sustainable implementation. Additionally, scaling the architecture may require additional investments in training and resources.

Complexity:

Managing multiple firewalls requires careful configuration and monitoring to avoid misconfigurations that could introduce vulnerabilities. Administrators must be well-trained and knowledgeable about both vendors’ systems. Documenting policies and procedures is critical to ensure consistent management.

Performance Impact:

Improperly configured firewalls can cause latency due to redundant traffic filtering. Proper load balancing and optimization are critical to maintain performance while ensuring security. Regular performance audits can help identify bottlenecks and optimize settings.

Regular Maintenance:

Keeping firewalls updated and fine-tuning their configurations to address evolving threats is essential. Neglecting these tasks could compromise the effectiveness of the firewall sandwich. Regular training for IT staff is also important to keep them informed about new features and best practices.

Conclusion

A firewall sandwich is a powerful strategy for organizations looking to enhance their network security through a defense-in-depth approach. By leveraging tools like pfSense and Firewalla, organizations can build a robust architecture that combines perimeter defense, deep traffic inspection, and vendor-specific strengths. While the setup may introduce some complexity and cost, the benefits of improved security, resilience, and compliance far outweigh the challenges. Organizations of all sizes can use this architecture to better protect their sensitive data and systems from evolving threats. By embracing this multi-layered approach, businesses can stay ahead of adversaries and ensure long-term network integrity.

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