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Kestrel in ASP.NET Core

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Kestrel in ASP.NET Core

Kestrel is the cross-platform web server included and used by default in ASP.NET Core applications. It's lightweight, fast, and designed to handle both development and production environments.

What is Kestrel?

Kestrel is:

  • A web server that listens for HTTP requests and forwards them to your ASP.NET Core application.

  • Cross-platform and built on top of libuv (older versions) or Sockets (newer versions).

  • The default web server in ASP.NET Core since version 1.0.

ASP.NET Core Hosting Model with Kestrel










+-------------------+ | Browser | +--------+----------+ | v +------------+-------------+ | Kestrel Web Server | +------------+-------------+ | v +------------+-------------+ | Middleware Components | +------------+-------------+ | v +------------+-------------+ | Controllers / Endpoints | +--------------------------+ 









Kestrel vs IIS/Nginx/Apache







    
        

            Feature
            Kestrel
            IIS/Nginx/Apache
        

    
    
        

            Type
            Application Web Server
            Reverse Proxy / Full Web Server
        

        

            Cross-platform
            ✅ Yes
            ❌ IIS, ✅ Nginx/Apache
        

        

            Static file handling
            ✅ But basic
            ✅ Optimized
        

        

            TLS/HTTPS support
            ✅ Yes
            ✅ Yes
        

        

            Reverse Proxy Required?
            Optional (Prod recommended)
            Not needed (they are full servers)
        

        

            Use Case
            ASP.NET Core apps
            General purpose + ASP.NET Core
        

    










In production, it's recommended to use Kestrel behind a reverse proxy (like IIS or Nginx) for enhanced security, load balancing, and better handling of TLS, logs, and static files.






Configuring Kestrel



You can configure Kestrel in Program.cs (or Startup.cs in older versions):


Basic Example (ASP.NET Core 6+ / 7+):







csharp



var builder = WebApplication.CreateBuilder(args); 


// Configure Kestrel options


builder.WebHost.ConfigureKestrel(serverOptions => { serverOptions.Limits.MaxRequestBodySize = 10 * 1024;// 10 KB


serverOptions.ListenAnyIP(5000);// HTTPserverOptions.ListenLocalhost(5001, listenOptions =>


{ listenOptions.UseHttps(); 


// HTTPS});});


var app = builder.Build(); 


app.MapGet("/", () => "Hello World!"); 


app.Run(); 









HTTPS with Kestrel



Kestrel supports HTTPS with certificates. You can configure it in appsettings.json:







json







{ "Kestrel": { "Endpoints": { "Https": { "Url": "https://localhost:5001", "Certificate": { "Path": "certs/localhost.pfx", "Password":
"yourpassword" } } } } } 








Or configure it programmatically:







builder.WebHost.ConfigureKestrel(


options => { options.Listen(IPAddress.Any, 5001, listenOptions => { listenOptions.UseHttps("path/to/cert.pfx", "password"); }); }


); 









Performance



Kestrel is high-performance and asynchronous, designed to handle thousands of concurrent connections. Microsoft benchmarks show Kestrel to be among the fastest web servers available.




Features and Limits in Kestrel



You can set various limits:







csharp



options.Limits.MaxRequestBodySize = 10 * 1024; 


// Max body size


options.Limits.KeepAliveTimeout = TimeSpan.FromMinutes(2); 


options.Limits.RequestHeadersTimeout = TimeSpan.FromSeconds(30); 









When to Use Kestrel Directly


    
    
  • 
    
    
    For microservices or containerized applications in Docker/Kubernetes.
    
    
    • 
    
  • 
    
    
    When you're running apps behind a load balancer or API Gateway.
    
    
    • 
    
  • 
    
    
    For development and testing environments.
    
    
    • 





Summary







    
        

            Feature
            Kestrel
        

    
    
        

            Type
            Web server for ASP.NET Core
        

        

            Cross-platform
            ✅ Yes
        

        

            Default server
            ✅ Yes (since ASP.NET Core 1.0)
        

        

            Reverse proxy support
            ✅ Recommended in production
        

        

            HTTPS support
            ✅ Yes
        

        

            Performance
            🔥 High-performance, async I/O
        

    











                        

                               
                                                            
                        


                    



                

            

        

    






        
        
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