knockd

Start knockd system daemon

$ knockd [[-d|--daemon]]

$ knockd [[-c|--config]] [path/to/file].configuration

knockd [-d] [-i interface] [-f configfile] [-v] [-V] [-h]

-d
    Run knockd in daemon mode. Detach from the terminal and run in the background.

-i
    Listen on the specified network interface. If not specified, knockd attempts to auto-detect the interface. If auto-detection fails, it will listen on all interfaces.

-f
    Use the specified configuration file. Defaults to /etc/knockd.conf.

-v
    Verbose mode. Increases the verbosity level.

-V
    Display version information and exit.

-h
    Display help message and exit.

Knockd is a lightweight port knocking daemon for Linux. Port knocking is a method of authenticating connections to a server by requiring clients to attempt connections to a series of closed ports in a specific sequence. Knockd listens for these specific port sequences, and when the correct sequence is received, it can be configured to run arbitrary commands, typically to open firewall rules to allow the client access to specific services. This adds a layer of security by obscuring open ports and requiring a secret sequence for access.

Knockd enhances security by acting as a simple brute-force detection and prevention mechanism. Since the ports are closed by default, successful exploits are very rare.
The advantage of Knockd is its minimal resource consumption and easy configuration.

Knockd adds a layer of security, but it should not be considered a replacement for strong passwords and other standard security practices. It relies on the client being able to send packets to the server, and firewall rules may interfere with this. Port knocking relies on packet timing, which can be affected by network latency. It can be vulnerable to replay attacks and packet sniffing if not configured correctly.

The configuration file (/etc/knockd.conf) defines the port sequences and commands to be executed. Each 'sequence' defines a port knocking event and specifies ports, TCP/UDP, and the commands to execute on successful knock sequences.

Knockd was originally developed as a simple way to hide open ports and add a layer of obfuscation to network services. Its popularity grew due to its simplicity and low resource usage. It has been actively maintained and updated to address security concerns and improve functionality.

iptables(8), tcpdump(1)