In the world of IT security, there are solutions that, while technically correct, eventually become more of a historical curiosity than a practical tool. Port knocking, a technique that in its time aroused both admiration for its ingenuity and controversy regarding its actual effectiveness, falls into this category.
Origin and principle of operation
Port knocking was created as a response to the problem of excessive exposure of network services. The classic approach assumed that if we wanted to provide a service (e.g. SSH), we simply had to open the appropriate port in the firewall. However, this meant that the port was visible to anyone who scanned the network.
The port knocking mechanism introduced a completely new philosophy. Instead of a permanently open port, the service remains invisible until the client performs a specific sequence of connection attempts on closed ports. Only after “knocking” the right combination does the firewall temporarily unblock access.
In practice, it looks like this:
- All ports are closed by default – the network scanner will not detect any open services.
- The client sends TCP/UDP packets to successive, pre-defined ports (e.g. 1001, then 2002, then 3003).
- The port knocking daemon (e.g. knockd) analyzes the firewall logs and if the sequence is correct, temporarily opens the destination port (e.g. 22 for SSH).
- After a set time (usually a few dozen seconds), the port disappears from view again.
Practical implementation
Let’s look at a typical configuration on a Linux system. This requires installing the knockd package and properly configuring the iptables firewall. Example configuration file /etc/knockd.conf:
[options]
logfile = /var/log/knockd.log
interface = eth0
[openSSH]
sequence = 1001,2002,3003
seq_timeout = 15
command = /sbin/iptables -A INPUT -s %IP% -p tcp –dport 22 -j ACCEPT
tcpflags = syn
[closeSSH]
sequence = 3003,2002,1001
seq_timeout = 15
command = /sbin/iptables -D INPUT -s %IP% -p tcp –dport 22 -j ACCEPT
tcpflags = syn
In this configuration:
- The sequence 1001→2002→3003 opens the SSH port for the IP address that made the “knocking”
- The reverse sequence 3003→2002→1001 closes access
- Each “knock” must be performed within 15 seconds
The client can perform the sequence using any network tool, e.g. nmap:
nmap -Pn –host-timeout 100ms -T4 -p 1001,2002,3003 192.168.1.100
Security analysis
Port knocking introduces several interesting security features:
- Service concealment – port scanning will not detect the protected service, since the target port is closed by default.
- Time-limited availability – even after the sequence is correctly executed, the port is only available for a short period.
- No direct exposure – there is no permanent entry point for the attack.
However, this technique also has serious limitations:
- No real authentication – the port sequence acts like a password, which if captured, can be easily repeated.
- Problems with NAT – in networks with address translation, the implementation becomes much more complicated.
- Dependence on firewall logs – the mechanism is dependent on tracking entries in firewall logs, which is a rather non-standard approach that can cause performance problems in larger networks.
Modern alternatives
In the current reality, port knocking has rather historical value. Modern alternatives offer much better security:
- Single Packet Authorization (SPA) – an extension of the port knocking concept, where instead of a sequence of packets, a single, encrypted packet (e.g. fwknop) is used.
- VPN (WireGuard/OpenVPN) – creates a secure tunnel through which all services are available, eliminating the need to open ports.
- Zero Trust Network Access – advanced authentication and authorization systems before granting access to any resources.
Conclusions
Port knocking is an interesting example of a creative approach to the problem of network security. While this technique works and can be useful in specific scenarios, its practical application in modern environments is very limited.
It is worth knowing and understanding, as it illustrates an important security principle – sometimes the best protection is simply not being visible. However, in an age of advanced authentication and encryption systems, port knocking remains more of a curiosity for enthusiasts than a practical tool in the security administrator’s arsenal.
Is there still a place for such solutions in your infrastructure? It depends on your specific needs, but in most cases, modern alternatives offer a much better balance of security to implementation complexity.