Today is the era of high penetration of digital services, HTTP protocol is the basis of Internet communication, carrying web page loading, API interaction, data synchronization and other core functions, but simple HTTP requests may also cause the server to reject the corresponding due to a variety of reasons. Denied access may present status codes such as 403 Forbidden, 404 Not Found, or 502 Bad Gateway. There are many reasons, such as a small configuration error on the client side, a deep-seated security policy error on the server side, or a hidden failure of the network transmission link. The following is a full link perspective check HTTP request rejected reasons, I hope to help you.

Common traps on the client side

As the initiator of the request, the configuration and behavior of the client directly determine the validity of the request. The first is missing or misformatted request headers. For example, when accessing an API that requires authentication, if a valid Bearer Token is not carried in the Authorization header, the server will directly return 401 Unauthorized. A more subtle problem occurs when the Content-Type header does not match the Request body: When the client declares Content-Type: application/json but sends data in XML format, the server may return 400 Bad requests due to parsing failure. In addition, outdated User-Agent strings can trigger interception mechanisms of security systems, especially in websites with strict anti-crawling policies, and the use of unconventional UA can result in requests being directly blocked.

HTTP method misuse is another high frequency problem. RESTful apis often strictly specify methods of resource manipulation, such as using a GET request to try to create a resource (which should be using POST), or sending a PUT/PATCH request to a read-only endpoint, which will trigger a 405 Method Not Allowed error. This kind of problem is especially prominent in the front end separation architecture, when the API documentation is not synchronized in time, front-end developers are prone to method confusion resulting in request invalidation.

URL construction errors cannot be ignored either. A classic scenario is when the path parameter is incorrectly encoded: if the URL contains Spaces or special characters such as' & ', '? '), no percentage encoding (such as converting a space to '%20') will cause the server to fail to parse the path. In addition, a misconfiguration of the Base URL - such as misassigning the production API address to the test environment - can send a request to an endpoint that does Not exist, naturally triggering a 404 Not Found response.

Server-side security and configuration barriers

As the recipient of the request, the server's security policy and configuration rules constitute the second line of defense. IP blacklists and geographical restrictions are common reasons for rejection: When Geo-Blocking is enabled on the server, IP segments from a specific country or region are automatically blocked, with 403 Forbidden returned. If an Intranet service incorrectly adds a public IP address to the firewall blacklist, legitimate users cannot access the service.

Requests are also rejected when authentication and permission control mechanisms fail. In the OAuth 2.0 process, if the Access Token expires or the Scope is insufficient, the server will return a 403 status code. A more complex situation arises in role-based access control (RBAC) systems: A user is authenticated, but a denial is triggered because the role is not assigned access to a specific resource. In this case, you can view the access logs of the server and often see detailed permission verification failure records.

Server resource limitations may also indirectly cause requests to be rejected. When the number of concurrent connections exceeds the Web server (such as Nginx or Apache) configuration

worker_connections

When the upper limit is reached, a connection cannot be established for new requests, as a connection timeout or 502 Bad Gateway error. In addition, if the request body size exceeds the 'client_max_body_size' set by the server (usually 1MB by default), Nginx will respond directly to 413 Payload Too Large, which is particularly common in file upload scenarios.

Miscalculation of security protection systems has become a new challenge in modern architectures. Web application firewalls (WAFs) can misinterpret legitimate requests as attacks due to overly strict rules. For example, JSON data containing '<script>' characters might be recognized as an XSS attack, triggering a WAF intercept and returning 406 Not Acceptable. Similarly, the Rate Limiting policy, while defending against DDoS attacks, may also temporarily block normal users due to high-frequency client retries, returning 429 Too Many Requests.

The rigor of character encoding and format verification can be a double-edged sword. For example, if the server forces the request parameter to be encoded in UTF-8, and the client sends GBK encoded Chinese characters, garbled characters appear when the parameter is parsed, resulting in service logic verification failures. At this point, the server may return a semantically ambiguous 400 error, rather than a clear coding error message, increasing the difficulty of troubleshooting.

To sum up, whether it is client to server, or application layer to transport layer, HTTP request rejection can not be regarded as a single dimension failure, but the result of multiple factors interweaving. Through systematic investigation combined with tool chain and rigorous reasoning, we can accurately locate the root cause of the problem.