Mastering Nmap Firewall Evasion Techniques: A Detailed Guide

In the world of cybersecurity, bypassing security measures like firewalls is often essential for both penetration testers and attackers. Firewalls are designed to prevent unauthorized access to a network, but with the right tools and techniques, you can evade these defenses to assess vulnerabilities. Nmap, one of the most powerful and popular network scanning tools, offers several advanced methods to evade firewalls and detect weaknesses that are otherwise hidden.

In this article, we will explore multiple firewall evasion techniques using Nmap, showing how you can use these methods to identify security loopholes, evade detection, and unlock access to hidden services. Mastering these techniques not only helps in ethical hacking but also increases your chances of finding bugs in bug bounty programs, which can lead to substantial rewards.

Nmap, short for Network Mapper, is a free and open-source tool used for network discovery and security auditing. One of its most impressive features is the ability to evade firewalls, IDS (Intrusion Detection Systems), and IPS (Intrusion Prevention Systems). When properly configured, firewalls block unauthorized scans and probes, but using Nmap’s evasion techniques, you can:

These evasion techniques are crucial for ethical hackers and penetration testers looking to identify and report misconfigurations in a target network.

Here are the most effective Nmap firewall evasion techniques, complete with explanations and examples of how to implement them.

1. Fragmented Packets (-f)

Firewalls often inspect packet sizes to detect suspicious activity. Fragmented packets break down larger data packets into smaller pieces, making it more difficult for firewalls to detect or analyze them. By using fragmented packets, you can slip past the firewall’s filters unnoticed.

Command:

How it Works: This command breaks the scan traffic into smaller fragments. The firewall will only see partial data segments, which may not trigger detection. This technique works well against firewalls that filter based on packet size or content patterns.

Use Case: If a firewall is configured to block large packets but allows smaller ones through, fragmented packets can bypass the filtering rules and give you access to the target network.

2. Source Port Manipulation ( — source-port)

Firewalls typically block suspicious traffic based on source ports. However, some firewalls trust traffic originating from certain well-known ports, such as DNS (port 53) or HTTP (port 80). By specifying a trusted source port, you can trick the firewall into letting your scan through.

Command:

How it Works: In this example, you’re telling Nmap to send packets from port 53, typically used by DNS. Many firewalls allow DNS traffic, so your scan may bypass the security rules.

Use Case: This technique works best when the target firewall is configured to allow traffic from specific ports that are essential to network services, like DNS or HTTP.

3. Decoy Scanning (-D)

Decoy scanning involves sending fake packets from multiple IP addresses to confuse the firewall or IDS/IPS. By sending traffic from decoys, you can disguise your real IP address, making it harder for the firewall to track the true source of the scan.

Command:

How it Works: Nmap generates 10 random IP addresses that act as decoys, along with your real IP address. The target’s firewall or IDS/IPS sees packets coming from multiple sources, making it hard to distinguish the real attacker.

Use Case: This technique is ideal when you don’t want your actual IP address to be logged by the target’s security system. The more decoys you use, the harder it becomes for the target system to detect your real identity.

4. Idle Scan (-sI)

The Idle Scan is one of the most stealthy scanning techniques in Nmap. This method allows you to perform a scan without sending any packets directly from your machine. Instead, Nmap uses a “zombie” machine to interact with the target. The target’s firewall or IDS logs the zombie’s IP address, while your machine remains completely hidden.

Command:

How it Works: You need to find a zombie host that has an idle system and predictable IP ID sequencing. Once found, Nmap sends probes to the target through the zombie, and the responses are analyzed to infer open ports without revealing your identity.

Use Case: This is ideal for extremely stealthy scans where you want to remain completely anonymous and avoid any detection by firewalls or IDS systems.

5. TCP ACK Scan (-sA)

Firewalls often block incoming traffic unless a legitimate connection has already been established. A TCP ACK scan can be used to determine whether a firewall is filtering traffic or simply blocking connection requests.

Command:

How it Works: This scan sends TCP ACK packets to the target. Firewalls that only track connection states will allow the ACK packets through, even though no connection was established. The response from the target can indicate whether the ports are filtered or unfiltered.

Use Case: This technique helps identify which ports are actively filtered by the firewall and which are open for further testing or exploitation.

6. MAC Address Spoofing ( — spoof-mac)

Some networks grant access to certain MAC addresses or have whitelisted specific devices. Spoofing the MAC address of a trusted device can allow you to bypass the firewall and access the target network.

Command:

How it Works: This command spoofs the MAC address of your machine to appear as if it is coming from a trusted or whitelisted device. The firewall may grant access based on this spoofed MAC address.

Use Case: This is useful when you know the MAC address of a device that is trusted by the firewall. By impersonating that device, you can bypass MAC address filtering rules.

Now that you’re familiar with the powerful evasion techniques in Nmap, let’s discuss how to leverage these skills in bug bounty hunting. Many organizations run bug bounty programs where they reward ethical hackers for finding and reporting vulnerabilities, including those involving firewalls.

1. Find Misconfigurations

Firewalls can be complex to configure correctly. Misconfigurations such as allowing unnecessary open ports or improperly trusting traffic from certain sources are common. Using Nmap’s evasion techniques, you can discover misconfigurations that expose networks to attacks.

2. Discover Hidden Services

Many firewalls block public access to sensitive services. However, with evasion techniques like fragmented packets or source port manipulation, you can discover services that are hidden behind firewalls, increasing the scope of your findings.

3. Submit Detailed Reports

Once you find a vulnerability using Nmap’s evasion techniques, write a thorough report. Include evidence of the vulnerability, such as screenshots and detailed descriptions of the steps you took. Bug bounty platforms value clear, well-documented submissions.

4. Reap the Rewards

Many bug bounty programs offer significant payouts for network vulnerabilities, especially those involving firewall weaknesses. Mastering these Nmap techniques can lead to high-severity bug discoveries, which in turn, can lead to substantial rewards.