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?
