Transparent Firewall Validation: Impact on Nmap OS Detection

This page is intentionally written in English.

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Overview

This article presents a practical validation of how a transparent-mode firewall affects network scanning results using Nmap.

The goal is simple: verify whether a transparent security device behaves as a truly invisible layer, or whether it introduces measurable changes in network behavior.

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Test Approach

To ensure accuracy, the same target host and identical scan command were used under two conditions:

• Direct connection (no firewall)
• Transparent firewall inserted (FortiGate)

The test was conducted using a wired interface in both cases to eliminate variability caused by wireless conditions.

The following command was used consistently:

nmap -Pn -O -sV -T4 --osscan-guess 192.168.100.xxx

Host discovery was disabled to avoid delays caused by filtered ICMP responses.

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Key Observations

1. OS Detection Accuracy

Without the firewall, Nmap was able to identify the operating system with a high degree of confidence.

After inserting the transparent firewall, OS detection degraded to a guess-based result, with no exact match available.

This indicates that the firewall altered TCP/IP fingerprint characteristics used by Nmap.

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2. Port Behavior Changes

In the firewall-enabled scenario, additional ports appeared as tcpwrapped.

This behavior suggests that the firewall is influencing how services respond to scanning attempts, either through inspection or controlled response handling.

Even though the underlying services did not change, their observable behavior from the scanner’s perspective did.

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3. Scan Duration

The scan completed significantly faster in the direct connection scenario.

With the firewall in place, scan time increased noticeably. This is likely due to inspection overhead and retry behavior caused by filtered or modified responses.

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4. Latency Characteristics

Interestingly, reported latency decreased when the firewall was inserted.

This is not an actual performance improvement, but rather an artifact of how the firewall responds to probes, affecting measurement behavior.

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Analysis

This test was conducted under a key constraint: no changes were made to the existing network design.

The results clearly show that a transparent firewall is not completely invisible from a network analysis perspective.

While it does preserve connectivity and allows existing communication to continue without topology changes, it still modifies packet-level behavior in ways that are detectable by tools like Nmap.

Specifically:

• TCP/IP fingerprinting is altered
• Service exposure appears modified
• Scan performance characteristics change

These effects are not necessarily negative. In fact, they can be considered a form of passive obfuscation, making it more difficult for external scanners to accurately identify systems.

Security Implication

The results indicate that identifying the operating system from external scans becomes more difficult when a transparent firewall is introduced.

This reduction in fingerprinting accuracy does not eliminate vulnerabilities, but it does make automated reconnaissance and OS-based targeting less reliable.

In practice, this behavior can be interpreted as a form of passive obfuscation, increasing the effort required for external attackers to accurately profile the system.

While this test focuses on network-level behavior, encryption strength should be evaluated separately based on established international standards such as those defined by organizations like NIST and ENISA.
These observations also have direct implications for external reconnaissance and attack surface visibility.

In environments influenced by NIS2 and similar regulations, where funding may be available but implementation paths remain unclear, introducing a single transparent firewall can serve as a practical compliance entry point, requiring no redesign and only minimal physical insertion into the existing network.

This means that external attackers can no longer reliably determine the operating system, directly reducing the effectiveness of vulnerability-based targeting.

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Conclusion

Transparent-mode firewalls provide strong operational advantages, especially in environments where modifying network topology is undesirable.

However, this test demonstrates that “transparent” does not mean “undetectable.”

Even without breaking connectivity:

• OS detection accuracy can be reduced
• Port response behavior can change
• Inspection introduces measurable overhead

From a security perspective, this behavior may actually be beneficial, as it reduces the accuracy of external reconnaissance.

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Practical Notes

• All tests were performed in a controlled local environment
• MAC addresses and host identifiers have been masked
• Initial connectivity issues were caused by incorrect interface orientation and were resolved before testing

This validation reflects real-world deployment conditions and highlights the importance of understanding how security devices interact with network diagnostics.

→ Read our Network Architecture Philosophy (Why we avoid redesign)