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What I have discovered is that there are a number of ports on my Windows 10 box which (1) are not in use by any process and (2) I cannot listen on.
I discovered this problem trying to run a node server which used port 3000. I found a number of questions on this topic. This one is typical: Node.js Port 3000 already in use but it actually isn't?
All the respondents of this question and similar questions all suggest using "netstat -ano" to find the process which is using the port and killing it.
What I have found is that there are large number of ports blocked which are not tied to processes. This is not related to AV or firewall. I turned off the firewall and I have only Windows Defender AV.
I wrote a program to listen on the ports between 3000 and 5000 inclusive on 127.0.0.1.
int port = 3000;
while(port <= 5001)
{
try
{
ListenOnPort(port);
++port;
}
catch (Exception ex)
{
Console.WriteLine($"Listen on {port} failed: {ex.Message}");
++port;
}
}
Where ListenOnPort is...
private static void ListenOnPort(int v)
{
var uri = new UriBuilder("http", "127.0.0.1", v);
HttpListener listener = new HttpListener();
listener.Prefixes.Add(uri.Uri.ToString());
Console.WriteLine($"Listening on {v}");
listener.TimeoutManager.IdleConnection = new TimeSpan(0, 0, 1);
listener.Start();
var task = listener.GetContextAsync();
if(task.Wait(new TimeSpan(0,0,1)))
{
HttpListenerResponse response = task.Result.Response;
// Construct a response.
string responseString = "<HTML><BODY> Hello world!</BODY></HTML>";
byte[] buffer = System.Text.Encoding.UTF8.GetBytes(responseString);
// Get a response stream and write the response to it.
response.ContentLength64 = buffer.Length;
System.IO.Stream output = response.OutputStream;
output.Write(buffer, 0, buffer.Length);
// You must close the output stream.
output.Close();
}
listener.Stop();
}
The program produced output similar to this...
Listening on 3000
Listen on 3000 failed: The process cannot access the file because it is being used by another process
Listening on 3001
Listen on 3001 failed: The process cannot access the file because it is being used by another process
Listening on 3002
Listen on 3002 failed: The process cannot access the file because it is being used by another process
Listening on 3003
Listen on 3003 failed: The process cannot access the file because it is being used by another process
Listening on 3004
Listen on 3004 failed: The process cannot access the file because it is being used by another process
Listening on 3005
Listen on 3005 failed: The process cannot access the file because it is being used by another process
Listening on 3006
Listening on 3007
Listening on 3008
Listening on 3009
Listening on 3010
What I discovered is that between the ranges of 3000 and 5000, there are 624 ports which are blocked. Meanwhile "netstat -ano" shows that there are exactly 5 ports in use in that range. So what is blocking the 619 other ports?
567
Right...
While looking for something else I found the answer (at least the source of the problem). The reason I cannot connect to these ports is because they are all part of excluded port ranges on windows. To see the excluded ports use...
$ netsh int ipv4 show excludedportrange tcp
And there, magically, is a list of all the ports I cannot connect to. These excluded port ranges apparently originate from HyperV and Docker, both of which I have installed. There is apparently a way to get the ports back...not easy since it involves uninstalling Docker and HyperV, then reserving the port ranges for yourself and then reinstalling HyperV and Docker. Not worth it. Now that I simply know how to find the ports I cannot use, I will simply not use them!
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