Access-Control-Allow-Origin 标头如何工作?

Access-Control-Allow-Origin 是一个 CORS(跨源资源共享)标头.

当站点 A 尝试从站点 B 获取内容时，站点 B 可以发送一个 Access-Control-Allow-Origin 响应标头来告诉浏览器该页面的内容可被某些来源访问.(来源是一个域，加上方案和端口号.)默认情况下，站点B 的页面任何其他来源都无法访问；使用 Access-Control-Allow-Origin 标头为特定请求源的跨域访问打开了一扇门.

When Site A tries to fetch content from Site B, Site B can send an Access-Control-Allow-Origin response header to tell the browser that the content of this page is accessible to certain origins. (An origin is a domain, plus a scheme and port number.) By default, Site B's pages are not accessible to any other origin; using the Access-Control-Allow-Origin header opens a door for cross-origin access by specific requesting origins.

对于站点 B 想让站点 A 访问的每个资源/页面，站点 B 应为其页面提供响应标头:

For each resource/page that Site B wants to make accessible to Site A, Site B should serve its pages with the response header:

Access-Control-Allow-Origin: http://siteA.com

现代浏览器不会完全阻止跨域请求.如果站点 A 从站点 B 请求页面，浏览器实际上会在网络级别获取请求的页面，并检查响应标头是否将站点 A 列为允许的请求者域.如果站点 B 没有表明允许站点 A 访问此页面，浏览器将触发 XMLHttpRequest 的 error 事件并拒绝对请求的 JavaScript 代码的响应数据.

Modern browsers will not block cross-domain requests outright. If Site A requests a page from Site B, the browser will actually fetch the requested page on the network level and check if the response headers list Site A as a permitted requester domain. If Site B has not indicated that Site A is allowed to access this page, the browser will trigger the XMLHttpRequest's error event and deny the response data to the requesting JavaScript code.

在网络级别发生的事情可能比上面解释的稍微复杂.如果请求是 "non-simple"请求，浏览器首先发送一个无数据的预检"OPTIONS请求，以验证服务器是否会接受请求.当其中一个(或两者):

What happens on the network level can be slightly more complex than explained above. If the request is a "non-simple" request, the browser first sends a data-less "preflight" OPTIONS request, to verify that the server will accept the request. A request is non-simple when either (or both):

• 使用 GET 或 POST 以外的 HTTP 动词(例如 PUT、DELETE)
• 使用非简单的请求头；唯一的简单请求标头是:
  • 接受
  • 接受语言
  • 内容语言
  • Content-Type(只有在其值为 application/x-www-form-urlencoded、multipart/form-data 时才简单>, 或 text/plain)
  • using an HTTP verb other than GET or POST (e.g. PUT, DELETE)
  • using non-simple request headers; the only simple requests headers are:
    • Accept
    • Accept-Language
    • Content-Language
    • Content-Type (this is only simple when its value is application/x-www-form-urlencoded, multipart/form-data, or text/plain)

    如果服务器使用适当的响应标头(Access-Control-Allow-Headers 对于非简单标头，Access-Control-Allow-Methods)响应 OPTIONS 预检对于非简单动词)匹配非简单动词和/或非简单标头，然后浏览器发送实际请求.

    If the server responds to the OPTIONS preflight with appropriate response headers (Access-Control-Allow-Headers for non-simple headers, Access-Control-Allow-Methods for non-simple verbs) that match the non-simple verb and/or non-simple headers, then the browser sends the actual request.

    假设站点A想发送一个/somePage的PUT请求，带有一个非简单的Content-Type值application/json，浏览器会首先发送一个预检请求:

    Supposing that Site A wants to send a PUT request for /somePage, with a non-simple Content-Type value of application/json, the browser would first send a preflight request:

    OPTIONS /somePage HTTP/1.1
    Origin: http://siteA.com
    Access-Control-Request-Method: PUT
    Access-Control-Request-Headers: Content-Type
    

    注意Access-Control-Request-Method和Access-Control-Request-Headers是浏览器自动添加的；您不需要添加它们.此 OPTIONS 预检获得成功的响应标头:

    Note that Access-Control-Request-Method and Access-Control-Request-Headers are added by the browser automatically; you do not need to add them. This OPTIONS preflight gets the successful response headers:

    Access-Control-Allow-Origin: http://siteA.com
    Access-Control-Allow-Methods: GET, POST, PUT
    Access-Control-Allow-Headers: Content-Type
    

    发送实际请求时(预检完成后)，行为与处理简单请求的方式相同.换句话说，预检成功的非简单请求被视为与简单请求相同(即，服务器仍必须再次发送 Access-Control-Allow-Origin 以获得实际响应).

    When sending the actual request (after preflight is done), the behavior is identical to how a simple request is handled. In other words, a non-simple request whose preflight is successful is treated the same as a simple request (i.e., the server must still send Access-Control-Allow-Origin again for the actual response).

    浏览器发送实际请求:

    PUT /somePage HTTP/1.1
    Origin: http://siteA.com
    Content-Type: application/json
    
    { "myRequestContent": "JSON is so great" }
    

    服务器发回一个Access-Control-Allow-Origin，就像一个简单的请求一样:

    And the server sends back an Access-Control-Allow-Origin, just as it would for a simple request:

    Access-Control-Allow-Origin: http://siteA.com
    

    有关非简单请求的更多信息，请参阅了解基于 CORS 的 XMLHttpRequest.

    See Understanding XMLHttpRequest over CORS for a little more information about non-simple requests.