What ICMP message indicates a closed UDP port?

• A. Time Exceeded
• B. Echo Request
• C. Destination Unreachable (Port Unreachable)
• D. Redirect

Answer: (C)

When UDP is sent to a port and there’s no service listening on that port, the network stack typically returns an ICMP error to the sender to indicate the packet can’t be delivered to a higher-layer process. The specific ICMP message used for this situation is Destination Unreachable with the code indicating Port Unreachable. That combination tells you that the destination host received the UDP datagram but there isn’t any process listening on the destination port.

This is different from the other options: Time Exceeded is used when a packet’s TTL expires or there’s a fragmentation timing issue, not about port state. Echo Request is the ping mechanism and isn’t about port availability. Redirect is about routing and telling a sender to send packets via a different gateway.

Note that some networks or firewalls may block ICMP messages, so you might not always see this reply even if the UDP port is closed.