When a port scanner sends a SYN to a server, what are two possible responses that can be received?

When a port scanner sends a SYN packet to a server, it's typically attempting to initiate a TCP connection to determine if a particular port is open or closed. The two relevant responses that can be received in this scenario are SYN/ACK or RST/ACK.

Receiving a SYN/ACK indicates that the port is open and that the server is willing to establish a connection. This response is part of the three-way handshake that occurs when establishing a TCP connection, with the server acknowledging the SYN from the client (the port scanner in this case) and responding with a SYN to indicate it is ready to proceed with the connection.

On the other hand, if the port is closed, the server responds with an RST/ACK packet. This occurs because the server needs to reset the connection attempt, effectively indicating that there is no service listening on that port. The RST response signals to the port scanner that it cannot proceed with the connection as the port is not accepting connections.

Therefore, the two possible responses of SYN/ACK (indicating an open port) and RST/ACK (indicating a closed port) directly correlate to the scenarios a port scanner would encounter during its scanning activity on a server.