Why might a UDP scan take longer to complete than other types of scans?

A UDP scan might take longer to complete compared to other types of scans primarily due to the nature of how UDP (User Datagram Protocol) operates. UDP is a connectionless protocol, meaning it does not require a handshake or acknowledgment for packets sent to the target. While this characteristic allows for faster transmission of data in some cases, it introduces challenges for scanning because many UDP packets will simply result in no response if the target port is closed or filtered.

When a scanner sends a UDP packet to a target, it may wait for a timeout period to determine if the packet is acknowledged. If the port is closed, the scanner might receive an "ICMP Port Unreachable" message, but if the port is open, there is no response. Consequently, the lack of responses, particularly with closed ports leading to timeouts, makes UDP scans inherently slower since the scanning tool needs to wait for the timeout cycles to complete to conclude the status of each port.

Thus, even though it can scan a large number of ports quickly, the efficiency is hampered by the lack of required replies, resulting in extended scan times. This aspect of UDP scanning is what contributes to it taking longer to complete than TCP scans, which typically have a clearer acknowledgment mechanism.