During an Xmas scan what indicates a port is closed?

A 'No return response' from an Xmas scan typically indicates a filtered port, where a firewall or other security device is silently dropping the probe packet, rather than a definitively closed port.

An Xmas scan sends TCP packets with the FIN, URG, and PSH flags set. According to RFC 793 (TCP specification), if a port on the target system is closed, the system should respond with a TCP RST (Reset) packet, indicating that the connection cannot be established as no application is listening on that port.

An ACK packet is part of the TCP handshake and data transfer process, usually sent in response to received data or a SYN-ACK, but not as the response from a closed port to an Xmas scan.

A SYN packet is the initial connection request packet in the TCP three-way handshake and is sent *to* a port to initiate a connection, not returned *from* a closed port in response to an Xmas scan.