The built-in PowerShell cmdlet Test-NetConnection is a powerful tool for diagnosing network issues, but it has a significant limitation: it does not support the User Datagram Protocol (UDP). If you attempt to use it to verify a UDP port, you will find that there is no -Protocol UDP parameter available. By default, Test-NetConnection relies on TCP handshakes or ICMP echo requests (pings) to determine reachability.

To properly test UDP connectivity on Windows, you must look beyond the standard cmdlets. Because UDP is a connectionless protocol, verifying whether a port is "open" requires different methodologies than the three-way handshake used by TCP.

Why Test-NetConnection Fails for UDP Testing

The primary reason Test-NetConnection cannot verify UDP ports lies in the fundamental design of the protocol itself.

TCP (Transmission Control Protocol) is connection-oriented. When you run Test-NetConnection -Port 80, the tool initiates a SYN packet. If the remote server is listening, it responds with a SYN-ACK. This exchange confirms the port is open before any actual data is sent.

UDP (User Datagram Protocol) is connectionless. There is no initial handshake. You simply send a packet to a destination and hope it arrives. If a UDP port is open, the application might process the data and send nothing back, or it might respond with data. If the port is closed, the remote host's operating system might send back an ICMP "Destination Unreachable" message, but firewalls often drop these messages to prevent network mapping.

Consequently, a tool like Test-NetConnection has no reliable way to confirm a "successful" UDP connection without a specific application-layer response, which is why Microsoft focused the cmdlet on TCP and ICMP.

Custom PowerShell Script to Test UDP Ports

While there is no built-in cmdlet, PowerShell can leverage the underlying .NET Framework to send UDP packets. To test a UDP port, we must create a socket, send a payload, and wait for a response or a specific error.

The following script simulates a UDP test by sending a small byte array to a target IP and port.