🚀 Introduction

When hosting ASP.NET Core applications, two main web servers are usually mentioned: Kestrel and IIS (Internet Information Services). Both are important for running applications, but they are not the same. Many developers often wonder what Kestrel is, how it works, and how it differs from IIS. In this article, we will explore the topic with examples, making it accessible to even those who are not familiar with the difference.

⚡ What is Kestrel?

Kestrel is a lightweight, cross-platform, and open-source web server for ASP.NET Core that runs on Linux, Windows, and macOS. It is designed to be fast, scalable, and efficient, making it the preferred web server for all new ASP.NET Core applications.

Key Features of Kestrel:

Cross-Platform Support – Runs on Windows, Linux, and macOS, making it perfect for modern cloud and containerized environments.
Open-Source – Kestrel is fully open-source and actively maintained by Microsoft and the .NET community.
High Performance – Lightweight and optimized for speed, it can handle thousands of requests per second.
Default ASP.NET Core Server – Every new ASP.NET Core project automatically uses Kestrel.
Integration Options – Kestrel can run alone or behind a reverse proxy like IIS, Nginx, or Apache for added features.

Example: If you create a new ASP.NET Core project and run it with the dotnet run command, it will start on the Kestrel server by default and listen on http://localhost:5000.

🖥️ What is IIS?

IIS (Internet Information Services) is a Windows-specific web server developed and maintained only by Microsoft. Unlike Kestrel, IIS is not cross-platform and runs exclusively on Windows servers.

Key Features of IIS:

Windows-Centric – Deeply integrated with the Windows operating system.
Closed-Source – Unlike Kestrel, IIS is not open-source and is only developed by Microsoft.
Feature-Rich – Provides a wide range of tools such as SSL certificate management, authentication, application pools, detailed logging, and request filtering.
Enterprise Focused – Designed to host traditional ASP.NET applications, as well as other types of web apps.
Integration with Other Servers – Often used with Kestrel or other web servers to add extra layers like load balancing, reverse proxying, and SSL termination.

Example: A financial organization may use IIS to host their internal banking portal because it integrates directly with Windows authentication and enterprise security policies.

🔑 Key Differences Between Kestrel and IIS

Although both servers belong to the Microsoft ecosystem, they serve different purposes and have unique characteristics.

1. Platform Support

Kestrel: Cross-platform (Windows, Linux, macOS).
IIS: Windows-only.

2. Source Code

Kestrel: Fully open-source.
IIS: Closed-source, maintained only by Microsoft.

3. Performance

Kestrel: Lightweight and optimized for performance.
IIS: More resource-heavy but offers enterprise-level features.

4. Features

Kestrel: Simple, lightweight, best for ASP.NET Core apps.
IIS: Full-featured with security, logging, request filtering, authentication, and application pools.

5. Use Cases

Kestrel: Default server for ASP.NET Core, used in cloud, Docker, and Linux hosting.
IIS: Widely used in Windows enterprise environments and for hosting traditional ASP.NET apps.

6. Integration

Kestrel: Can run standalone or behind a reverse proxy like IIS, Nginx, or Apache.
IIS: Commonly used as a reverse proxy in front of Kestrel to combine Kestrel’s performance with IIS’s security and management features.

Example: A global e-commerce site may use Kestrel + Nginx on Linux in AWS, while a government agency may use Kestrel + IIS on Windows servers.

🌍 When to Use Kestrel vs IIS

Choosing between Kestrel and IIS depends on your application type, hosting environment, and business needs.

Use Kestrel Alone if:
- You are deploying to Linux, Docker, or cross-platform cloud environments.
- You need maximum performance with low overhead.
- You are hosting ASP.NET Core applications.
Use Kestrel with IIS if:
- You are in a Windows enterprise environment.
- You need features like SSL management, authentication, request filtering, and logging.
- You want IIS as a reverse proxy for security and load balancing.

✅ Summary

Kestrel is a lightweight, open-source, cross-platform web server for ASP.NET Core applications, while IIS is a Windows-only, closed-source, enterprise-grade web server. Kestrel is fast, scalable, and the default choice for new .NET apps, while IIS is powerful, feature-rich, and tailored for Windows servers. Many organizations use Kestrel + IIS together, combining Kestrel’s performance with IIS’s enterprise features. This hybrid approach ensures flexibility, security, and scalability in modern application hosting.