Which response from a server indicates an open port?

A server indicating an open port will respond with a SYN/ACK. This response is part of the TCP three-way handshake process, which is used to establish a connection between a client and a server. When a client attempts to initiate a connection to a server's open port, it sends a SYN packet to request a connection. If the port is open and the server is willing to accept the connection, it replies with a SYN/ACK packet, indicating that it has acknowledged the SYN request and is also sending a synchronization signal back to the client.

This combination of SYN (synchronize) and ACK (acknowledgment) demonstrates that both parties are set to communicate, confirming that the port is indeed open and the connection can proceed. The subsequent final step of the handshake would involve the client responding with an ACK packet back to the server to complete the connection setup.

This process is essential in TCP communications, during which the ability to check port status is fundamental for security assessments, as open ports can be potential entry points for unauthorized access if not properly secured.