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What is Port Knocking?


Cyberattackers employ an array of tactics to breach networks and compromise sensitive data while defenders strive to keep pace with changing attack tactics to develop impenetrable IT environments. "Attack surface" refers to the sum total of all possible points in a computer system, application, network, or organization that could be exploited by malicious actors to gain unauthorized access, disrupt operations, or compromise the security of the system.

Within the context of network security, these entry points are exposed services that malicious actors can interact with. A compromise of a network-exposed service grants an attacker unauthorized access to a network and can allow them to act on their primary goals of stealing and ransoming sensitive data or disabling critical functions. If attackers cannot gain access, they cannot achieve their ultimate goals.

The adage "you can't attack what you can't see" implies that the Cyber Kill Chain can be broken early by simply hiding services from attackers. On the other hand, “security through obscurity” has been criticized because secrets tend to be eventually exposed. 

In this article, we delve into a less well-known and not commonly deployed practical approach to hiding exposed services known as "port knocking" as well as uncover its weaknesses and most practical use cases.

What is Port Knocking?

Port knocking is a less commonly implemented yet intriguing approach to network security that adds an extra layer of defense by hiding an endpoint's exposed services until a specific "knock" is made. The earliest implementations of port knocking used predefined sequences of packets, while more modern implementations use cryptographic signatures known as Single Packet Authorization (SPA) that leverage the same type of public key infrastructure as TLS and other cryptography-based authentication schemes.

Only after the correct “knock” has been delivered, does the target system reveal that a service is active on the queried port. So, for example, if port knocking protected a remote database service on port 3306, the service would not respond to any connection attempts until the correct “knock” packets had been received. The underlying service’s presence would be otherwise undetectable - making a cyber attack against the service difficult. Think of it as a secret handshake that needs to be executed before the remote network service is exposed.

Practical Implementation of Port Knocking Defenses

Despite the fact that port knocking is not a commonly used defensive strategy, various tools have been developed to implement it:

  • Knockd: This open-source daemon is one of the pioneering implementations of port knocking. It monitors network activity for the specified sequence of knocks and triggers the opening of the requested port

  • fwknop: This tool, which stands for "FireWall KNock OPerator," takes port knocking a step further by integrating SPA techniques. It requires both a proper knock sequence and a valid encrypted SPA packet to access the service

  • pyknock: An open-source Python implementation of port knocking, pyknock provides the flexibility to define custom knock sequences and offers support for both TCP and UDP ports

Use Cases Where Port Knocking is Beneficial

Port knocking offers significant advantages in specific scenarios. For instance, it could prove highly valuable in the case of a zero-day vulnerability in an exposed network service that has not received a security update. The security of legacy systems and services past their end-of-life support cycle may harbor insecure protocol vulnerabilities insecure protocol vulnerabilities that can be mitigated through port knocking.

Moreover, for the most sensitive services, port knocking represents a sort of “holy grail” of authorization security, rendering the service undetectable to attackers and exclusively available to legitimate systems that can present the correct digital signature.

Limitations of Port Knocking

However, there are many attacks to which port knocking is vulnerable. Brute-force or man-in-the-middle attacks also threaten port-knocking authorization.  However, the use of a port-knocking solution that uses public key authentication makes these approaches ineffective.  Also, a port-knocking application may contain its own software vulnerabilities which theoretically makes it potentially vulnerable to misconfiguration vulnerabilities or zero-day exploits similar to the underlying service it is protecting.

Once attackers gain privileged access to an endpoint they obtain access to credentials such as private keys and password caches, so attacks against underlying infrastructure also make port knocking vulnerable to attacks that use stolen certificates.

Another drawback to port knocking is that it is yet another software application that needs to be understood, configured, and maintained which increases the burden on an IT team.  Failure to implement a port-knocking solution properly could lead to additional vulnerabilities or limit the availability of critical services if not functioning properly.


Port knocking presents a unique albeit less popular approach to enhancing network security by obscuring an endpoint’s exposed services until a specific sequence or digital signature is presented using a related concept known as single packet authentication (SPA). While port knocking is not a silver bullet for network security, with the aid of port knocking tools, organizations can implement this technique to add an extra layer of defense for their most sensitive network endpoints and services. 

While port knocking has its limitations it has not been counted out as a potentially beneficial technique for stealthily increasing network security posture.

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