What is CORS

CORS is a mechanism used in browsers to handle cross-origin resource access. When a web page attempts to request a resource from an origin, and the server of the resource is different from the origin where the web page is located, it will When it comes to cross-domain requests, CORS communicates by adding some specific field information in the HTTP request header to tell the server whether it supports cross-domain requests. In this way, the CORS mechanism allows web pages to be securely processed under restricted circumstances. Perform cross-domain resource access.

# Operating system for this tutorial: Windows 10 system, Dell G3 computer.

CORS (Cross-Origin Resource Sharing) is a mechanism used to handle cross-origin resource access in browsers. A cross-origin request occurs when a web page attempts to request a resource from a source (domain, protocol, or port) that is hosted on a different server than the origin of the web page.

By default, the browser's same-origin policy will restrict cross-domain requests, that is, only allow web pages to obtain data from the same domain name. This is to protect user security and privacy. But in some cases, we hope that web pages can make cross-domain requests and obtain resources under other domain names. In this case, we need to use CORS to solve cross-domain problems.

CORS communicates by adding some specific field information in the HTTP request header to tell the server whether it supports cross-domain requests. Specifically, when a web page sends a cross-domain request, the browser will automatically send a preflight request (OPTIONS request) to the server. This request contains some additional header information, such as Origin (identifying the source of the request), Access- Control-Request-Method (request method), Access-Control-Request-Headers (request headers), etc. After receiving the preflight request, the server decides whether to allow the cross-domain request based on the information in the request header. If the server confirms that it is allowed, it will add some field information to the response header, such as Access-Control-Allow-Origin (specify the allowed origin), Access-Control-Allow-Methods (specify the allowed request method), Access-Control- Allow-Headers (specify allowed request headers), etc.

In this way, the CORS mechanism enables web pages to safely access cross-domain resources under restricted circumstances. At the same time, the server can also perform fine-grained control to limit which sources, methods, and header information are allowed.

It should be noted that CORS only takes effect in the browser and does not involve inter-server communication in a non-browser environment. In addition, depending on the server's configuration and security policy, other cross-domain solution choices may be required, such as JSONP, proxy, etc.

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