When investigating a SYN Flood DOS attack, what condition indicates a successful attack?

A successful SYN Flood Denial of Service (DoS) attack is indicated by a large number of SYN packets appearing on a network without the corresponding reply packets. In a SYN Flood attack, the attacker sends a barrage of SYN packets, which are used to initiate a TCP connection, to the target server. However, due to the nature of this attack, the server is overwhelmed with these half-open connections and cannot respond adequately—leading to incomplete connection establishment processes.

The absence of the corresponding SYN-ACK reply packets signifies that the target server is unable to handle the requests effectively, which is a hallmark of a SYN Flood attack. When many SYN requests flood the server but responses do not get sent back, it indicates that the system is likely under attack or is being flooded with connection requests beyond its capacity.

Other options may seem plausible, but they do not specifically indicate a successful SYN Flood attack. For instance, a sudden drop in network performance may be a symptom of various problems not limited to SYN Flooding, and a high number of completed TCP connections typically suggests the opposite situation where the server is processing requests normally. Similarly, a large number of HTTP requests does not indicate a SYN Flood attack as it pertains to a different protocol and attack vector.