Locallhost:5000 - Did you mean localhost:5000? Flask & .NET

"Port 5000 is already in use"

This error occurs when another application is already using port 5000. Solutions:

Find and close the application using port 5000
Use a different port:
- Flask: app.run(port=5001)
- .NET Core: dotnet run --urls="http://localhost:5001"
- In appsettings.json: Change the URLs configuration
On Windows, find and kill the process: netstat -ano | findstr :5000 then taskkill /PID [PID] /F
On Linux/Mac: lsof -i :5000 then kill -9 [PID]
Note: On macOS Monterey and later, AirPlay uses port 5000 by default. You can disable this in System Preferences > Sharing

"Flask app only accessible from localhost"

By default, Flask only listens on localhost:

To make it accessible from other devices: app.run(host='0.0.0.0', port=5000)
Warning: This exposes your development server to your network, which may have security implications

"CORS issues with ASP.NET Core"

If you're experiencing CORS errors:

Configure CORS in your ASP.NET Core application:

services.AddCors(options =>
{
    options.AddPolicy("AllowAll",
        builder => builder
            .AllowAnyOrigin()
            .AllowAnyMethod()
            .AllowAnyHeader());
});

Then apply the policy: app.UseCors("AllowAll");
Note: For production, use more restrictive CORS policies

Did you mean to visit localhost:5000?

About Port 5000

Common Issues with Port 5000

"Port 5000 is already in use"

"Flask app only accessible from localhost"

"CORS issues with ASP.NET Core"

Useful Resources