the information to create traffic replay attacks to repeat prior port knock sequences. Port knocking is used as part of a defense in depth strategy. Even if the attacker were to successfully gain port access, other port security mechanisms are still in place, along with the assigned service authentication mechanisms on the opened ports. 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. Overhead on both the server and client in terms of traffic, CPU and memory consumption is minimal. Port knock daemons are not complex to code; any type of vulnerability within the code is obvious and auditable. A port knock system implemented on ports such as the SSH sidesteps the issue of brute force password attacks on logins. In the case of SSH, the SSH daemon is not activated without the correct port knock, and the attack is filtered by the TCP/IP stack rather than using SSH authentication resources. To the attacker, the daemon is inaccessible until the correct port knock is supplied. Security considerations Port knocking is a flexible, customisable system add-in. If the administrator chooses to link a knock sequence to an activity such as running a shell script, other changes such as implementing additional firewall rules to open ports for specific IP addresses can easily be incorporated into the script. Simultaneous sessions are easily accommodated. By using strategies like dynamic length and pool of length can reduce the probability of hacking knock sequences to near zero. 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. If an exploit were discovered that could compromise a daemon in its default configuration, using port knocking on the listening port reduces the possibility of compromise until the software or process is updated. 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. Network security professionals have largely ignored port knocking as a solution in the past since early implementations relied solely on providing the correct port combinations to achieve access. Modern port knock systems