How to Prevent Denial of Service Attacks: 3 Tools to Test Vulnerability

9/29/23 8:00 AM  |  by RedLegg Blog

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Denial of Service (DoS) is one of the most common types of cyber attacks on a business's website, or application, designed to overwhelm its resources and make its services unavailable. 

So, what is a Denial of Service attack? 

AWS defines a Denial of Service (DoS) attack as "a malicious attempt to affect the availability of a targeted system, such as a website or application, to legitimate end users."

The attacker sends so many data packets or requests to the targeted website or app that it overloads the system and renders it unavailable for legitimate requests from real users.

A Distributed Denial of Service (DDoS) attack is similar, except that the attacker uses multiple sources to launch the requests in a synchronized effort to overload the target system. As the attack stems from multiple locations, it is more difficult to trace the source of the malicious requests and mitigate the damage.

The main types of DoS attacks are:

  • Volume-based attacks that use large amounts of traffic to overwhelm the system.
  • Application-layer attacks that flood the target system with malicious requests (measured as requests per second or RPS).
  • Network-layer attacks send data packets in large volumes to network infrastructure and infrastructure management. Smurf DDoS and SYN floods are examples of network-layer attacks.

Understanding Proactive Testing: How to Prevent Denial of Service Attacks

DoS attacks exploit weaknesses in networks, so having a proactive plan to enhance network security is critical.

DoS attacks usually target government websites, high-profile financial, banking, retail or media service companies, or other organizations with critical services accessed via websites or applications.

Cyber threat actors choose DoS attacks because they are fast, easy to implement, and less expensive to launch.

These attacks can cripple services, causing loss of revenue for commercial websites or apps and disruption of services for critical government or banking apps. DDoS attacks rarely lead to data leaks, but the loss of business and reputation is significant enough to warrant a proactive cybersecurity plan to mitigate DDoS Attacks.

For example, AWS reported mitigating a massive DDoS attack in February of 2020. At its peak, this attack saw incoming traffic at a rate of 2.3 terabits per second (Tbps).

A business can lose thousands of dollars a minute due to a DoS attack. As reported by Infosecurity Magazine, the average cost of a DDoS attack in the US is around $218k without factoring in any ransom costs.

Cloudflare's DDoS Trends Report for Q1 2023 showed that large-scale volumetric attacks continue to grow in size and frequency. In Q4 2022, attacks exceeding 100 Gbps increased by 67% quarter over quarter. In Q1 2023, the growth slowed slightly to just 6%, but it's still trending higher.

Clearly, it’s crucial to understand how to prevent a Denial of Service Attack and have a defined plan to mitigate its risks.

A point to note is that a DoS attack is successful only if the attacker gets enough time to send high volumes of data or traffic to the targeted system.

Therefore, the best way to mitigate an attack or minimize the damage is to have proactive measures in place to identify threats in the landscape and prevent such attacks—or, at the very least, to detect them early and control the effects as rapidly as possible. 

Vulnerability testing is the first line of defense for proactive mitigation and early detection of DDoS attacks.

Vulnerability testing tools work by simulating how a real-world attacker would target a system. The tools intentionally exploit an observed weakness to initiate an attack and fix the vulnerability—before a malicious attacker can do so.

The main benefit of such an intentional simulated attack is that your security team can pre-empt an actual attack, fortify your company's defenses, fix vulnerabilities in your network or apps, and prepare robust incident response strategies. 

Proactive vulnerability scanning as part of a defined cybersecurity strategy also fosters a culture of awareness of the existing threat landscape for your organization. It drives home the need for continuous monitoring of new and emerging cyber threats.

Tools to Prevent Denial of Service Vulnerability

We'll review 3 Denial of Service testing tools that simulate attacks to help mitigate the risks of full-fledged, real-world attacks.


Hping3, a Kali Linux open-source Hping3, is an invaluable tool for testing the robustness of your network and application layers. It can simulate a number of DoS attacks, such as SYN flood, TCP, UDP, and ICMP.

How it works: Testers can craft the packages to be sent, control their attributes and control their speed at a microsecond interval. 

They can send out multiple packets at a very fast rate to the target system, which tries to respond to confirm the connection. But since Hping3 can mask the true identity of the sender or senders of these packers, the victim keeps repeatedly trying to confirm the connection request, using up all its resources. Since the attack comes from a fake address, it is difficult for the target system's admin to trace the source of the attack.

Hping3 can be leveraged for testing the following:

  • Firewall testing
  • Advanced port scanning
  • Network testing using different protocols, TOS, fragmentation
  • Manual path MTU discovery
  • Advanced traceroute, under all supported protocols
  • Remote OS fingerprinting
  • Remote uptime guessing
  • TCP/IP stacks auditing


HULK stands for HTTP Unbearable Load King. Created by security researcher Barry Shteiman, it can generate vast amounts of unique and obfuscated traffic to assess the vulnerabilities of your network and apps. It can bypass caching and hit the server’s direct resource pool to overload it. HULK is written in Python but has been ported to other languages like Golang.

How it works: 

HULK creates a unique, random value for each request being sent, obfuscating the source of the attack. The referrer that points at the request is masked and points to either the host itself or a pre-listed website. 

HULK uses a standard HTTP command to ask the server to maintain open connections or 'Keep-Alive' with a variable time window.

It randomizes and attaches custom parameters to each request, tricking the server into processing all the responses as being unique.

HULK also has a “safe” option to kill the process and control the attack in a lab setting. 

Some firewalls have specific settings to defend against HULK attacks, making this method a weaker option as time progresses and more vendors adopt these rules.


Goldeneye is a free and Open source tool available on GitHub, written in .NET Core.

Testers use it to send out massive amounts of HTTP traffic to a target web server or network to simulate a real-world DoS attack.

How it works:

It generates heavy botnet traffic by sending multiple requests to the target.

While Goldeneye is based on HULK, the main difference between them is that HULK simply sends packets to the server—but Goldeneye goes a step further to simulate numerous live connections to the server with Keep-Alive requests. It creates an illusion of 'active' users. By doing this, it tricks the target server into staying connected to the malicious senders and allows it to run out of resources for genuine requests.

Goldeneye is considered the ideal testing tool for stress tests on networks and applications.

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Considerations for Effective DoS Vulnerability Testing

These tools are invaluable for improving the security framework for your organization. But they must be used responsibly. 

Here are a few key considerations and best practices to follow during vulnerability testing:

Control the testing environment

Any testing tools need a controlled environment to ensure the test results are valid and reliable. Controlled environments minimize the damage from simulated attacks and provide ideal testing conditions to show how the systems react in a real-world attack.

Obtain appropriate permissions

These vulnerability testing tools are intrusive. Though they have 'safe' modes of operation so that a skilled tester can prevent any significant damage from the simulation, they still do bring down servers for a short while or render them unavailable. So, it's essential to obtain permission to conduct these tests and inform stakeholders. 

Set and follow ethical guidelines

Penetration testing is also called ethical hacking for a reason! After all, these tools are capable of launching attacks. Follow strict testing guidelines so the simulations do not result in real damages—whether monetary or reputational. Besides, the objective of the simulated attacks is to expose vulnerabilities, leading to a strengthening of the company's security posture. So, it is crucial to document all findings meticulously and provide recommendations and remediation measures.

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RedLegg: Strengthening Security Through Proactive DoS Vulnerability Testing

In a world where cyber attackers may be as sophisticated (if not more!) than professional, trained cybersecurity experts, safeguarding an organization's networks and digital assets is not a small task. 

At RedLegg, our vulnerability scanning and penetration testing methodology has been skillfully crafted by cybersecurity experts and is meticulously implemented by trained and experienced testers.

We also offer a full suite of security solutions, including advisory services, managed security services, network penetration testing, and mobile application assessments.

Get in touch with our security experts to understand how to prevent Denial of Service attacks using vulnerability testing tools.

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