Dotnet | Kestrel

 In ASP.NET Core, Kestrel is the default cross-platform web server used to run your application.

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What is Kestrel?

Kestrel is:

• A lightweight, high-performance web server
• Built into ASP.NET Core
• Responsible for handling HTTP requests and responses

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Simple Explanation

When a user opens your website:

Browser → Kestrel → ASP.NET Core App → Response → Browser

Kestrel is the engine that listens to requests and sends responses.

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Key Features

1. Cross-platform

Runs on:

• Windows
• Linux
• macOS

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2. High Performance

• Built for speed (used in production systems)
• Handles thousands of requests efficiently

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3. Supports HTTP protocols

• HTTP/1.1
• HTTP/2
• HTTP/3 (modern apps)

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4. Async & non-blocking

• Designed for scalable apps

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Where is Kestrel Used?

By default in every ASP.NET Core app:

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.Run();

✔ This automatically starts Kestrel

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Kestrel vs IIS

Feature	Kestrel	IIS (Internet Information Services)
Type	Lightweight web server	Full-featured web server
Platform	Cross-platform	Windows only
Usage	Default in ASP.NET Core	Often used as reverse proxy

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Production Setup

Common architecture:

Client → IIS / Nginx → Kestrel → App

• Nginx or IIS acts as:
• Reverse proxy
• Load balancer
• Kestrel handles app logic

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Why not expose Kestrel directly?

Kestrel alone:

• Doesn’t handle advanced security features
• No built-in load balancing
• Limited edge features

So we use:

• IIS (Windows)
• Nginx (Linux)

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Load balancing at IIS/Nginx

Distributing incoming requests across multiple app instances

Client → Nginx / IIS → App1 (Kestrel)
                     → App2 (Kestrel)
                     → App3 (Kestrel)

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How Nginx Does It (Simple Example)

upstream myapp {
    server localhost:5000;
    server localhost:5001;
}

server {
    location / {
        proxy_pass http://myapp;
    }
}

Nginx or  IIS:

• Receives request
• Sends it to one of many Kestrel instances

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Analogy

• Kestrel = Engine of a car 
• IIS/Nginx = Driver + security + traffic control 

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Key Takeaway

Kestrel is:

• The core web server in ASP.NET Core
• Fast, lightweight, and cross-platform
• Usually sits behind a reverse proxy in production