Error: listen EACCES: permission denied 127.0.0.1:8000

[DYH1319]

• Version: 0.4.0

• Target: Windows, dotnet 10.0

Steps to Reproduce:

1. When I minimized the startup console application, the following error occurred:

A JavaScript error occurred in the main process
Uncaught Exception:
Error: listen EACCES: permission denied 127.0.0.1:8000
at Server.setupListenHandle [as listen2] (node:net:1881:21)
at listenlnCluster (node:net:1946:12)
at GetAddrlnfoReqWrap.doListen [as callback] (node:net:2116:7)
at GetAddrlnfoReqWrap.onlookup [as oncomplete] (node:dns:111:8)

2. I used the Windows netsh command to obtain a range of ports that, for reasons unknown, have been excluded by Windows, resulting in these ports being unavailable for use:

3. The GetActiveTcpListeners() method within GetFreePort() cannot retrieve the aforementioned excluded ports.

// PortHelper.cs

public static int GetFreePort(int? defaultPost)
{
    var listeners = IPGlobalProperties.GetIPGlobalProperties().GetActiveTcpListeners().Select(e => e.Port).ToList();

    int port = defaultPost ?? 8000;

    while (true)
    {
        if (!listeners.Contains(port))
        {
            return port;
        }

        port += 2;
    }
}

[DYH1319]

This is a port occupancy issue caused by Hyper-V. I have resolved it by directly executing netsh int ipv4 set dynamicport tcp start=49152 num=16384. Is it possible to exclude ports occupied by Hyper-V in the Electron.NET source code?

[FlorianRappl]

Is it possible to exclude ports occupied by Hyper-V in the Electron.NET source code?

No, in general you can always have such an issue - if its not Hyper-V then maybe some other software. We cannot possibly know all software or anything that can run (if we'd go after this we should not open any ports).

I think that the given code is flawed anyway - there is a better mechanism to reliably open a port.

[DYH1319]

Thank you for your response.

The GetFreePort() code provided above is internal to ElectronNet, located at https://github.com/ElectronNET/Electron.NET/blob/main/src/ElectronNET.API/Runtime/Helpers/PortHelper.cs.
It attempts to obtain an available port to serve as the listening port for Electron's socket (line 272 of https://github.com/ElectronNET/Electron.NET/blob/main/src/ElectronNET.Host/main.js#L272).
The port used cannot be modified externally.

Perhaps I didn't say it clearly. The netsh interface ipv4 show excludedportrange protocol=tcp command displays TCP port ranges excluded by the Windows system - not by some software. When Windows hosts software like WSL or Docker Desktop, it may reserve certain ports at the operating system level.

However, the current GetFreePort() method only selects a free port not currently being listened to. I think it's also necessary to choose the port not excluded by the Windows operating system.

[FlorianRappl]

I think you misread my response.

What I've written is that we should change the GetFreePort to incl. only available ports (as written "reliably open a port", i.e., select a port that can be reserved and opened) - which means excl. OS occupied ports.

[DYH1319]

My apologies for my question. I misread your response.

I can submit a PR to fix this issue.

When the platform is Windows, use Process to execute netsh interface ipv4 show excludedportrange protocol=tcp, parse the returned results, and apply them as additional exclusion criteria.

This is the solution I've devised. Is there a better approach?

[FlorianRappl]

This is the solution I've devised. Is there a better approach?

Yes (potentially), we can be platform independent by following an approach reading the network interfaces, then just trying to open / use the port. If the port is blocked / refused, then try another one.

A good example is the get-port npm package.

[softworkz]

we can be platform independent by following an approach reading the network interfaces,

Why would that be required for being platform independent? Anyway, we don't need to deal with multi-homed network configurations. The simple way of port allocation will always open the port on local loopback.

Going through NIs can even be wrong when a port is opened specifically on local loopback only and we are probing a different NI, then the port would be available there - but not on local loopback where we need it.

[softworkz]

If we want to do it right, we would anyway need to find a PORT-PAIR rather than a single port, because the Electron side uses port+1.

[FlorianRappl]

Why would that be required for being platform independent?

I am not sure I understand your question. We need to be able to run the code on multiple platforms, i.e., it must be capable not only of Windows, but also on Linux and MacOS.

[softworkz]

We need to be able to run the code on multiple platforms,

I know.

I am not sure I understand your question

Rephrased:

Why would it be required on Linux or MacOS to "read the network interfaces" for opening a port?

[softworkz]

Anyway - that's not where the problem lies.

The real problem is that once you have opened a port and closed it, there's no guarantee that you will be able to open it again immediately afterwards.
The way to deal with those situations is to use setsockopt(SO_REUSEADDRESS). But we cannot set that, because the second opening of the port is done by SocketIOClient - which doesn't set this of course).

For those reasons, GetActiveTcpListeners() was the most suitable option, yet - admittedly - I wasn't even aware of those port reservations' existence. Nonetheless, that's where I would put the focus on for fixing (like reading them somehow and adding these to the exclusion list.

[softworkz]

Side note: For getting a single free port, you don't do iterations over port numbers. You specify '0' as port and then your socket gets a free port assigned which you can read out then.

[softworkz]

Why would it be required on Linux or MacOS to "read the network interfaces" for opening a port?

I need to go out, so the answer is: It's not needed to use a specific NI to open a port. If you don't do that, the port will be open on all NIs (except for Multicast - in that case it's opened on the default NI only and you need to explicitly bind it on other NIs if you want).
You CAN open a port on a specific NI, but you only do that when you want to open the port on that specific NI - ONLY.