The Resurgence of Port Knocking: A Stealthy Access Control Technique

Source Code:

In the world of cybersecurity, the constant battle between attackers and defenders has led to the evolution of innovative techniques to safeguard network infrastructures. Among these, “Port Knocking” has emerged as a unique and increasingly popular method for enhancing network security. Originally conceived as an obscure concept, port knocking has experienced a significant rise in popularity recently due to its effectiveness in adding an additional layer of defense against potential threats. In this article, we will explore what port knocking is, how it works, and why it has become a sought-after access control technique in modern cybersecurity practices.

What is Port Knocking?

Port knocking is a security technique used to control access to network services by sequentially sending specific connection attempts to a series of closed ports on a server. The process typically involves a client sending connection requests to predetermined ports in a specific order, and only if the correct sequence is followed, will the server dynamically open a previously closed port for the client to access. This method essentially creates a “secret knock” to gain entry to the server’s services, making it significantly more challenging for potential attackers to identify and exploit open ports.

How Port Knocking Works

The port knocking process can be broken down into several key steps:

  1. The Request: A client initiates the port knocking sequence by sending connection requests to the server’s closed ports in a predefined order. The sequence can be a combination of TCP or UDP packets, depending on the implementation.
  2. The Trigger: Once the server detects the correct sequence of connection attempts, it recognizes the “knock” pattern. In response, the server dynamically opens the desired port for a limited time window, granting access to the client.
  3. Access Granted: With the specified port opened, the client can now establish a connection to the server and access the desired service.
  4. Timeout and Closure: After a set duration or upon the successful connection of the client, the port is automatically closed again, leaving the server’s ports in their original closed state.

Here is an example how you might generate the sequence of knocks. You could also use something like TOTP to generate the sequence.

func main() {
	for i := 0; i < numberOfKnocks; i++ {
		ports = append(ports, randomPort())
		go createUdpServer(ports[i], &wg, resultChan)
  	fmt.Printf("Knocks sequence: %v\n", ports)

Why the Resurgence in Popularity?

Port knocking has experienced a surge in popularity recently due to several factors:

  1. Increased Security: Traditional firewalls and access control lists (ACLs) are not infallible, and attackers have become adept at exploiting open ports. Port knocking introduces an additional layer of security, making it harder for attackers to discover and target open ports.
  2. Stealthy Access: By keeping ports closed until a specific sequence is triggered, port knocking provides a stealthy way to access services. Attackers won’t easily detect open ports, reducing the attack surface and providing an added element of surprise.
  3. Low Overhead: Port knocking can be implemented without the need for additional software or complex configurations. This simplicity makes it an attractive option for both small and large-scale deployments.
  4. Customizability: Port knocking sequences can be tailored to individual needs, allowing administrators to define intricate patterns that suit their specific security requirements.
  5. Complementing Other Security Measures: Port knocking is often used in conjunction with other security measures like VPNs or two-factor authentication (2FA), enhancing overall network security.

We wrote a prototype application that shows off the capability of this called “knock”. Essentially 3 random ports will be generated. If those 3 ports are correctly connected to or “knocked”. A webserver gets activated. Here is the full source code.

As the cybersecurity landscape continues to evolve, innovative techniques like port knocking have gained renewed attention and popularity. By providing an additional layer of security, stealthy access control, and customizability, port knocking proves to be a valuable asset in the fight against cyber threats. Its ability to complement existing security measures and its low overhead have made it an appealing option for organizations seeking to safeguard their network infrastructures. As the need for robust cybersecurity solutions continues to grow, port knocking will undoubtedly remain an essential part of the defender’s toolkit.

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We are in an active state of fighting invisible war whether we know it or not. This is what fuels me to want to help organizations understand their objectives and protect them

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