What is Kestrel and how does it differ from IIS? (ASP.NET)

Kestrel is a lightweight, cross-platform, and open-source web server for ASP.NET Core. It is included and enabled by default in ASP.NET Core. Kestrel is supported on all platforms and versions supported by .NET Core.

In the Program class, the ConfigureWebHostDefaults() method configures Kestrel as the web server for the ASP.NET Core application.

public class Program{
   public static void Main(string[] args){
      CreateHostBuilder(args).Build().Run();
   }

   public static IHostBuilder CreateHostBuilder(string[] args) =>
      Host.CreateDefaultBuilder(args)
         .ConfigureWebHostDefaults(webBuilder =>{
            webBuilder.UseStartup<Startup>();
         });
}

Though Kestrel can serve an ASP.NET Core application on its own, Microsoft recommends using it along with a reverse proxy such as Apache, IIS, or Nginx for better performance, security, and reliability.

The main difference between IIS and Kestrel is that Kestrel is a cross-platform server. It runs on Linux, Windows, and Mac, whereas IIS is Windows-specific.

Another essential difference between the two is that Kestrel is fully open-source, whereas IIS is closed-source and developed and maintained only by Microsoft.

IIS is an old, albeit powerful software. With Kestrel, Microsoft started with cross-platform and high performance as explicit design goals. Since the Kestrel codebase started from scratch, it allowed developers to ignore the legacy/compatibility issues and focus on speed and efficiency.

However, Kestrel doesn’t provide all the rich functionality of a full-fledged web server such as IIS, Nginx, or Apache. Hence, we typically use it as an application server, with one of the above servers acting as a reverse proxy.