Which type of attack is indicated by millions of half-open connections to port 443 from various source IPs?

The scenario describes millions of half-open connections to port 443, which is typically used for HTTPS traffic. This pattern is indicative of a Distributed Denial-of-Service (DDoS) attack, where an attacker attempts to overwhelm a target server by inundating it with traffic from multiple sources.

In this case, the half-open connections suggest that multiple devices are trying to establish a connection with the target server but are unable to complete the handshake process. This is characteristic of a DDoS attack, particularly a SYN flood attack, where the attacker exploits the TCP handshake process by sending a large number of SYN packets and not responding to the server's SYN-ACK responses. As a result, the server exhausts its available connections, leading to denial of service for legitimate users.

Understanding the nature and symptoms of DDoS attacks is crucial in cybersecurity, especially in recognizing the signs of potential network overloads and service disruptions that can impact organizations. Other options, such as man-in-the-middle attacks, MAC flooding, and domain hijacking, do not match the characteristics or behaviors exhibited in this particular situation.