What does it mean if no response is received from a UDP port?

• A. The port may be open or filtered.
• B. The port is closed
• C. The host is unreachable
• D. The port is in use by another application

Answer: (A)

In UDP, there is no connection setup or guaranteed reply. When you send a probe to a UDP port, not getting any response is common and doesn’t prove the port’s state. The lack of a reply can happen if a service is listening but doesn’t respond to your probe, or if a firewall or network device is filtering and dropping the traffic. The usual signal that a port is closed would be an ICMP Port Unreachable reply, but that depends on the network path allowing such messages. Therefore, no response from a UDP port most directly means the port may be open or filtered.