What is preflight options request
Cross-Origin Resource Sharing CORS is a mechanism that uses additional HTTP headers to let a user agent gain permission to access selected resources from a server on a different origin domain than the site currently in use. A user agent makes a cross-origin HTTP request when it requests a resource from a different domain, protocol, or port than the one from which the current document originated.
An example of a cross-origin request: A HTML page served from http: Many pages on the web today load resources like CSS stylesheets, images, and scripts from separate domains, such as content delivery networks CDNs. The CORS mechanism supports secure cross-domain requests and data transfers between browsers and web servers. More specifically, this article is for web administrators, server developers, and front-end developers.
Modern browsers handle the client-side components of cross-origin sharing, including headers and policy enforcement. But this new standard means servers have to handle new request and response headers. Another article for server developers discussing cross-origin sharing from a server perspective with PHP code snippets is supplementary reading.
This cross-origin sharing standard is used to enable cross-site HTTP requests for:. The Cross-Origin Resource Sharing standard works by adding new HTTP headers that allow servers to describe the set of origins that are permitted to read that information using a web browser. Servers can also notify clients whether "credentials" including Cookies and HTTP Authentication data should be sent with requests.
Here, we present three scenarios that illustrate how Cross-Origin Resource Sharing works. All of these examples use the XMLHttpRequest object, which can be used to make cross-site invocations in any supporting browser.
Let us look at what the browser will send to the what is preflight options request in this case, and let's see how the server responds:. Lines 1 - 10 are headers sent. Lines 13 - 22 show the HTTP response from the server on domain http: In response, the server sends back an Access-Control-Allow-Origin header, shown above in line The use of the Origin header and of Access-Control-Allow-Origin show the access control protocol in its simplest use.
In this case, the server responds with a Access-Control-Allow-Origin: If the resource owners at http: Note that now, no domain other than http: The Access-Control-Allow-Origin header should contain the value that was sent in the request's Origin header. Cross-site requests are preflighted like this since they may have implications to user data. What is preflight options request particular, a request is preflighted if any of the following conditions is true:.
The server now has an opportunity to determine whether it wishes to accept a request under these circumstances. In particular, let's look at lines Note that this header is similar to the Allow response header, but used strictly within the context of access control. Finally, Access-Control-Max-Age gives the value in seconds for how long the response to the preflight request can be cached for without sending another preflight request. In this case, seconds is what is preflight options request hours.
If a redirect occurs for a preflighted request, most current browsers will report an error message such as the following. The request was redirected to 'https: The CORS protocol originally required that behavior but was subsquently changed to no longer require it. However, most browsers have not what is preflight options request implemented the change and still exhibit the behavior that was originally required.
So until browsers catch up with the spec, you may be able to work around this limitation by doing one or both of the following:. In this example, content originally loaded from http: Line 7 shows the flag on XMLHttpRequest that has to be set in order to make the invocation with What is preflight options request, namely the withCredentials boolean value.
By default, the invocation is made without Cookies. Since this is a simple GET request, it is not preflighted, but the browser will reject any response that does not have the Access-Control-Allow-Credentials: Although line 11 contains the Cookie destined for the content on http: But it does not fail: Because the value of the Access-Control-Allow-Origin header is " http: Note that the Set-Cookie response header in the example above also sets a further cookie.
In case of failure, an exception—depending on the API used—is raised. Note that cookies set in CORS responses are subject to normal third-party cookie policies.
In the example above, the page is loaded from foo. This section lists the HTTP response headers that servers send back for access control requests as defined by what is preflight options request Cross-Origin Resource Sharing specification. The previous section gives an overview of these in action. A returned resource may have one Access-Control-Allow-Origin header, with the following syntax:. The origin parameter specifies a URI that may access the resource. The browser must enforce this.
The Access-Control-Expose-Headers header lets a server whitelist headers that browsers are allowed to access. For an example of a preflight request, see the above examples. The delta-seconds parameter indicates the number of seconds the results can be cached.
The Access-Control-Allow-Credentials header Indicates whether or not the response to the request can be exposed when the credentials flag is true. When used as part of a response to a preflight request, this indicates whether or not the actual request can be made using credentials. Note that simple GET requests are not preflighted, and so if a request is made for a resource with credentials, if this header is not returned with the resource, the response is ignored by the browser and not returned to web content.
Credentialed requests are discussed above. The Access-Control-Allow-Methods header specifies the method or methods allowed when accessing the resource. This is used in response to a preflight request. The conditions under which a request is preflighted are discussed above. An example of a preflight request is given aboveincluding an example which sends this header to the browser.
Note that these headers are set for you when making invocations to servers. Developers using cross-site XMLHttpRequest capability do not have to set any cross-origin sharing request headers programmatically. The Origin header indicates the origin what is preflight options request the cross-site access request or preflight request. It what is preflight options request not include any path information, but what is preflight options request the server name.
Note that in any access control request, the Origin header is always sent. The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https: Get the latest and greatest from MDN delivered straight to your inbox.
Please check your inbox or your spam filter for an email from us. These are the same kinds of cross-site requests that web content can already issue, and no response data is released to the requester unless the server sends an appropriate header. Therefore, sites that prevent cross-site request forgery have nothing new to fear from HTTP access control. The origin can be the empty string; this is useful, for what is preflight options request, if the source is a data URL.
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The server will consider the request's Origin and either allow or disallow the request. If the server allows the request, then it will respond with the requested resource and an Access-Control-Allow-Origin header in the response.
This header will indicate to the client which client origins will be allowed to access the resource. Assuming that the Access-Control-Allow-Origin header matches the request's Originthe browser will allow the request.
On the other hand, if Access-Control-Allow-Origin is missing in the response or if it doesn't match the request's Originthe browser will disallow the request. For example, suppose that client code served from foo. The Origin header tells the server that the client code originated from http: So it checks its same-origin policies and determines that it can serve the request.
The response might look like this:. The Access-Control-Allow-Origin indicates that what is preflight options request http: This is considered an unsafe practice, however, except in special cases where an API is completely public and is expected to be consumed by any client. If a request may have implications on what is preflight options request data, a simple request is insufficient.
Instead, a what is preflight options request CORS request is sent in advance of the actual request to ensure that the actual request is safe to send. Also, if the request contains any custom headers, then a preflight request is required. This approach is arguably safer, because it doesn't assume that a service adheres to HTTP method semantics i. For example, suppose that a client served from foo. If the server allows the original request, then it will respond to the preflight request like this:.
The response might look like this: Wed, 20 Nov Pre-flight requests If a request may have implications on user data, a simple request is insufficient.
If the server allows the original request, then it will respond to the preflight request like this:
Cross-origin resource sharing CORS is a mechanism that allows restricted resources e. What is preflight options request defines a way in which a browser and server can interact to determine whether or not it is safe to allow the cross-origin request.
Although some validation and what is preflight options request can be performed by the server, it is generally the browser's responsibility to support these headers and honor the restrictions they impose. Servers can also notify clients whether "credentials" including Cookies and HTTP Authentication data should be sent with requests. This is generally not appropriate when using the same-origin what is preflight options request policy.
When a CORS-compatible browser attempts to make a cross-origin request:. A wildcard same-origin policy is appropriate when a page or API response is considered completely public content and it is intended to be accessible to everyone, including any code on any site. For example, a freely-available web font on a public hosting service like Google Fonts.
A wildcard same-origin policy is also widely what is preflight options request appropriately used in the object-capability modelwhere pages have unguessable URLs and are meant to be accessible to anyone who knows the secret. CORS allows the external web service to authorise the web application to use its services and does not control external services accessed by the web application.
For the latter, Content Security Policy should be used connect-src directive When performing certain types of cross-domain Ajax requests, modern browsers that support CORS will insert an extra "preflight" request to determine whether they have permission to perform the action. CORS is supported by most modern web browsers.
From Wikipedia, the free encyclopedia. Retrieved 4 April Web specifications support in Opera Presto 2. Retrieved 17 August Cross Origin Resource Sharing".