How to Accurately Test if a UDP Port is Open

How to Accurately Test if a UDP Port is Open

If you have ever tried to troubleshoot a connection on your InterServer VPS or Dedicated Server, you probably know that UDP (User Datagram Protocol) can be a bit of a headache. Unlike TCP, which uses a “handshake” to confirm a connection, UDP just sends data and hopes for the best. This makes traditional testing a little tricky.

You might have tried using nc -vz -u [IP] [PORT] and seen a message saying “Succeeded!”—only to find out the port is actually closed. In this guide, we will show you the foolproof way to test UDP ports by using two systems to confirm data is actually moving.

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The Problem with Basic UDP Scans

Most tools will report a UDP port as “open” simply because they didn’t receive an error message back. To be 100% sure your traffic is getting through, you need to see the data arrive on the destination server.

Method 1: The Two-Way Netcat Test

To do this right, you need access to both the source (the computer you are testing from) and the destination (your InterServer server).

1. On the Destination Server (The Listener)

First, tell your server to listen for incoming data on a specific port (we will use 6667 as an example). Run this command:

nc -luk x.x.x.x 6667

Note: Replace x.x.x.x with your server’s IP address. This is the destination.

2. On the Source System (The Sender)

From your local computer or a different remote system, send some data to that port:

nc -uv x.x.x.x 6667

3. Verify the Connection

Once connected, type a few characters like “XXXX” or “Hello” in the sender window and hit Enter. If the port is open and working, those characters will appear instantly on the destination server’s screen.

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Method 2: Using tcpdump to Watch Traffic

If you want to see exactly what is happening at the network level, tcpdump is your best friend. This tool captures the “packets” of data as they arrive.

Run this command on your destination server to watch for incoming UDP traffic on a specific port (e.g., 8002):

tcpdump -i any udp port 8002 -nn

When you send data from the source, you will see output like this:

20:10:17.563441 eno49 In IP y.y.y.y.60234 > x.x.x.x.8002: UDP, length 1

Understanding the Output:

• y.y.y.y (Source): This is the IP address of the computer sending the request (your home IP or another server).
• x.x.x.x (Destination): This is your InterServer server’s IP address that is receiving the data.

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How to Check if a Process is Already Using the Port

If your test fails, it might be because another program is already using that port. You can check this with the lsof command:

root@server:~# lsof -i :42021
COMMAND     PID USER   FD   TYPE    DEVICE SIZE/OFF NODE NAME
nc      2439862 root    3u  IPv4 229681656      0t0  UDP server.name:42021

In this example, we can see that nc (Netcat) is successfully holding the port open. If you see your application name listed there, your service is running and ready for traffic!

Quick Summary

Don’t trust a “succeeded” message from a simple UDP scan. Always verify by:

1. Setting up a listener on the destination.
2. Sending a string of text from the source.
3. Confirming the text appears on the other side.

If you’re still having trouble getting traffic through, feel free to reach out to the InterServer support team!