The client user would carry an extra utility, which could be as simple as netcat or a modified ping program or as complicated as a full hash-generator, and use that before they attempted to connect to the machine in the usual way.
[citation needed] Defeating port knocking protection requires large-scale brute force attacks in order to discover even simple sequences.
The average case scenario requires approximately 141 trillion (655353 / 2) packets to determine a correct three-port number.
This technique, in combination with knock attempt-limiting, longer or more complex sequences and cryptographic hashes, makes successful port access attempts extremely difficult.
Instead of using a preconfigured static IP whitelist on the firewall, an authorised user situated anywhere in the world would be able to open any necessary port without assistance from the server administrator.
Implementation of the technique is straightforward, using at the bare minimum a shell script on the server and a Windows batch file or command line utility on the client.
A port knock system implemented on password-authenticated services, like SSH, sidesteps the issue of brute force password attacks on logins.
[7] In addition to mitigating brute force password attacks and the inevitable growth in logs associated with the process daemon, port knocking also protects against protocol vulnerability exploits.
Authorized users would continue to be served once they provide the correct knock sequence while random access attempts would be ignored.
Port knocking should only be viewed as part of an overall network defense strategy providing protection against random and targeted attacks, not as complete standalone solution.
At worst, systems such as port knocking introduce new security issues through poor implementation or expose ambivalent administration attitudes through situations such as risk compensation.
Once compromised, the log files on the device are a source of other valid knock sequences, revealing another point of failure.
In practice, port knocking must be combined with other forms of authentication that are not vulnerable to replay or man-in-the-middle attacks for the whole system to be effective.