The IIS request processing process is divided into five main stages, each of which affects the performance and functions of the web application. 1. Receive requests and basic verification: IIS listens to ports and recognizes protocols, performs basic verification such as SSL/TLS handshake and IP restrictions. Common problems include firewall interception or certificate errors; 2. Authentication and authorization: supports multiple authentication methods and access control based on users or roles. 401 or 403 errors usually originate from improper configuration at this stage; 3. Request processing module intervention: multiple modules handle requests in sequence in integrated pipeline mode, such as URL rewriting, logging, content compression and output cache. Custom modules can be inserted into logic but may affect performance; 4. Handler mapping and execution: Generate response content based on extension or path matching handlers. Static files, ASPX pages, MVC routing and Web APIs have corresponding handlers; 5. Understanding these stages helps troubleshooting authentication failures, module loading exceptions, and Handler mapping errors.

When you use IIS (Internet Information Services) to process web requests, understanding how requests are processed step by step inside the server is critical for tuning, troubleshooting, and developing more efficient web applications. The request processing process of IIS is divided into multiple stages, each with specific functions and scalable points.

Let’s take a look at the role of these main stages and the possible impact they may have in practical applications.

1. Receive requests and basic verification

When the client initiates an HTTP request, IIS will first receive the request and make some preliminary judgments and settings:

• Listening port : IIS listens for configured ports (usually 80 or 443), and starts processing once a connection is in.
• Protocol Identification : Determine whether it is an HTTP or HTTPS request based on the request header. If it is HTTPS, an SSL/TLS handshake will be performed.
• Basic verification : Check whether basic rules such as IP address limit and bandwidth limit are enabled. If the rejection policy is matched, the error code will be returned directly.

At this stage, common problems include "inaccessibility" situations caused by firewall interception, SSL certificate errors, or incorrect binding address.

2. Authentication and Authorization

Next, IIS will try to determine who the user is and decide whether they have permission to access the target resource:

• Supports multiple authentication methods, such as anonymous authentication, Windows authentication, Basic authentication, OAuth, etc.
• If anonymous authentication is enabled, it will run using the identity of the application pool by default.
• Authorization rules determine whether to allow access to a page or directory based on the user or role.

If you encounter an error of "401 Unauthorized" or "403 Access Denied", it is usually a problem at this stage. Checking the <authentication></authentication> and <authorization></authorization> settings in web.config, or the application pool's identification account permissions is a good direction to check.

3. Integrated Pipeline

This is the most flexible and powerful part of the entire process. Integrated Pipeline was introduced since IIS 7, which unifies ASP.NET and IIS request processing in one process, rather than separate processing as before.

At this stage, various modules (Modules) will intervene in the request processing in order:

• The URL rewrite module can modify the request path
• Custom modules can record logs, perform permission control, add response headers, etc.
• The dynamic content compression module compresses the content before output
• The output cache module can skip subsequent processing and directly return cached results.

You can insert your own logic by registering a custom HttpModule or using the OWIN/Katana middleware. But it should be noted that too many modules or too long execution time will affect performance.

4. Handler mapping and execution

Finally, IIS will find the appropriate handler (Handler) based on the requested file extension or path to generate the response content:

• For static files (such as .html, .jpg), they are processed by StaticFileHandler
• For ASPX pages, leave it to PageHandlerFactory to create a page instance
• For MVC requests, the routing mechanism is triggered by UrlRoutingHandler
• The Web API or ASHX file also has its own Handler

If your application does not handle certain extensions or paths correctly, there may be problems with the handler mapping configuration. The <handlers></handlers> section can be viewed or modified in web.config.

Basically that's it. Understanding the various stages of IIS request processing can help you better troubleshoot problems, such as why a request does not come into your code, or why some middleware does not take effect. Although the overall process seems a bit complicated, in actual operation, many common problems focus on authentication, module loading and handler mapping.

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