What is cross-domain? Under what circumstances will cross-domain requests occur?

When it comes to web development, you may come across a common concept known as "cross-domain". This is an important security feature used to protect user privacy and data security. This article will take a deep dive into what cross-origin requests are, and under what circumstances cross-origin requests occur.

What is cross domain?

Cross-Origin (Cross-Origin) refers to that in a Web application, a document or script in one domain tries to request a resource in another domain, but the protocols, ports, or subdomains of the two domains are different. The browser's Same-Origin Policy (Same-Origin Policy) is an important security feature that restricts how a web page or script can interact with servers of different origins.

Specifically, the same-origin policy requires that the JavaScript script of a web page can only access resources from the same source, and cannot directly access resources from other sources. This is to prevent malicious websites from stealing user's data or performing malicious actions.

Under what circumstances will cross-domain requests occur?

Cross-origin requests usually occur in the following situations:

1. Different domain names: When the origin of the page is different from the origin of the requested resource.

2. Different subdomains: Even different subdomains are considered different origins. For example, treat as a different source subdomain.example.comthan .example.com

3. Different protocols: When a page uses the HTTPS protocol, the requested resource uses the HTTP protocol, or vice versa.

4. Different port: used when a page is used http://example.com:8080while the requested resource is used http://example.com:3000.

Cross-origin requests are restricted by the browser's same-origin policy. For example, a web page cannot request data directly from another domain via XMLHttpRequest or the Fetch API. This restriction is to ensure the security of users' data and prevent malicious websites from abusing cross-origin requests to access users' sensitive information.

In order to overcome cross-domain problems, web developers can use the CORS (Cross-Origin Resource Sharing) mechanism to set headers allowing cross-origin requests on the server side. By configuring the CORS header, the server can tell the browser which domains are allowed, thereby allowing specific cross-domain requests.

In summary, cross-origin is an important web security concept that helps protect users' data and privacy. Understanding when cross-origin requests occur and how to resolve cross-origin issues is fundamental knowledge that every web developer should master.