Port knocking allows clients to establish connections a server with no ports open. Firewalls are usually running on the network layer, so that it can only see IP addresses and its characteristics but not a user name and password, which can only be seen by the application layer. What that buys you is…. Hence the client must send the exact tcp flag that the server looks for. Users encrypt the remote connection to the server using the SSH protocol settings to improve the security of the communication between the server and the client in a public and insecure network.
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