Example for creating a naive TCP port scanner.
Port scanning is a way to detect if there is a server that listen to a port at a given address.
The current examples are checking TCP connections. The reason for scanning TCP is simple - TCP have a mechanism to know if a port is open or not. That mechanism is implemented using SYNchronize and called 3 way handshake.
When reading network specifications, there are at least two characters involved and I'm not going to stop doing that, so meet Alice and Bob :-)
Alice wishes to contact with Bob, on a given channel (port), so Alice sends SYN.
Bob, if wishes, answer Alice with SYN-ACK.
Then Alice sends ACK, and there is a handshake between the two, and got the
name of 3 way handshake
3 way handshake exists for TCP but not for UDP. UDP does not sends a handshake and if the server answers with the proper payload, then you know that the server is alive. If the server does not sends any answer, there is no way to know if there is an access for the server or not.
In order to know if a port is open, there is a need to open a connection for :-)
However, the current implementation that I've created is to first resolve IP from
DNS records, then using timeout for a connection.
When a connection that is initiated does not have the proper 3 way handshake, it
might not return for a long time (almost forever), so the timeout will kick in
and we "know" that the connection is closed.
When an open request is back, something is filtering the request (firewall?).
The "naive" way, is guessing the meaning of each type of "error" and translate it to the status of a request.
The aim of this repo is to teach, so I have choosen the "The Unlicense" license.
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