When a request is made to an IIS (Internet Information Services) server, the following steps generally outline the flow of the request:
Client Sends a Request: A client, such as a web browser, sends a request to the IIS server. The request includes the URL, HTTP method (e.g., GET, POST), headers, and any data or parameters associated with the request.
HTTP.sys: The request is initially received by the HTTP.sys component, which is a kernel-mode driver responsible for handling HTTP requests on Windows servers. HTTP.sys acts as a listener and intercepts incoming requests.
Request Routing: HTTP.sys determines the destination of the request based on the configuration of the server. This can include factors such as the domain, port number, and virtual directories configured in IIS.
IIS Request Processing: Once the destination is determined, the request is passed to the appropriate IIS application pool. An application pool represents a group of websites or applications that share the same configuration and runtime characteristics.
Modules and Handlers: In IIS, various modules and handlers process the request. Modules can perform a wide range of tasks, such as URL rewriting, authentication, compression, caching, and more. Handlers are responsible for executing code to generate the response, such as handling ASP.NET requests.
Authentication and Authorization: If authentication is required, IIS checks the client's credentials based on the configured authentication methods (e.g., Windows Authentication, Forms Authentication). If the client is authorized to access the requested resource, the processing continues; otherwise, an appropriate HTTP status code (e.g., 401 Unauthorized) is returned.
Request Execution: The request is passed to the appropriate handler for execution. For example, if the request is for an ASP.NET page, it is processed by the ASP.NET runtime, which compiles and executes the page's code.
Application Code Execution: If the request involves executing application-specific code, such as server-side scripts or APIs, the relevant code is executed, and any necessary data processing or interaction with databases or other systems takes place.
Response Generation: As the request is processed, the server generates a response based on the requested resource and any server-side code execution. The response includes headers, content, and other relevant information.
Response Sent to Client: The fully generated response is sent back to the client over the network. It includes an HTTP status code, headers, and the response content, which can be HTML, JSON, files, or any other type of data.
It's important to note that the specific flow and steps involved can vary depending on the configuration of IIS, the modules and handlers installed, and the nature of the requested resource (static file, dynamic content, etc.). Additionally, IIS can be extended and customized through various mechanisms, allowing developers to modify the request flow to meet specific requirements.