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Enabling Cross-Origin Resource Sharing

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Cross-origin resource sharing (CORS) defines a way for client web applications that are loaded in one domain to interact with resources in a different domain. With CORS support in Amazon S3, you can build rich client-side web applications with Amazon S3 and selectively allow cross-origin access to your Amazon S3 resources.

Cross-Origin Resource Sharing: Examples

Cross-origin resource sharing enables several use cases. For example, suppose you are hosting a website in an Amazon S3 bucket named website as described in Hosting a Static Website on Amazon S3. Your users load the website endpoint http://website.s3-website-us-east-1.amazonaws.com. Now you want to use JavaScript on the web pages stored that are stored in this bucket to be able to make authenticated GET and PUT requests against the same bucket by using the Amazon S3's API endpoint for the bucket, website.s3.amazonaws.com. A browser would normally block JavaScript from allowing those requests, but with CORS, you can configure your bucket to explicitly enable cross-origin requests from website.s3-website-us-east-1.amazonaws.com.

As another example, suppose you want to host a web font from your S3 bucket. Again, browsers require a CORS check (also referred as a preflight check) for loading web fonts, so you would configure the bucket that is hosting the web font to allow any origin to make these requests.

How Do I Enable CORS On My Bucket?

To configure your bucket to allow cross-origin requests, you create a CORS configuration, an XML document with rules that identify the origins that you will allow to access your bucket, the operations (HTTP methods) will support for each origin, and other operation-specific information. You can add up to 100 rules to the configuration. You add the XML document as the cors subresource to the bucket.

For example, the following cors configuration on a bucket has two rules, which are specified as CORSRule elements:

The first rule allows cross-origin PUT, POST, and DELETE requests from the https://www.example.com origin. The rule also allows all headers in a preflight OPTIONS request through the Access-Control-Request-Headers header. In response to any preflight OPTIONS request, Amazon S3 will return any requested headers.
The second rule allows cross-origin GET requests from all origins. The '*' wildcard character refers to all origins.

<CORSConfiguration>
 <CORSRule>
   <AllowedOrigin>http://www.example.com</AllowedOrigin>

   <AllowedMethod>PUT</AllowedMethod>
   <AllowedMethod>POST</AllowedMethod>
   <AllowedMethod>DELETE</AllowedMethod>

   <AllowedHeader>*</AllowedHeader>
 </CORSRule>
 <CORSRule>
   <AllowedOrigin>*</AllowedOrigin>
   <AllowedMethod>GET</AllowedMethod>
 </CORSRule>
</CORSConfiguration>

The CORS configuration also allows optional configuration parameters, as shown in the following CORS configuration. In this example, the following CORS configuration allows cross-origin PUT and POST requests from the http://www.example.com origin.

<CORSConfiguration>
 <CORSRule>
   <AllowedOrigin>http://www.example.com</AllowedOrigin>
   <AllowedMethod>PUT</AllowedMethod>
   <AllowedMethod>POST</AllowedMethod>
   <AllowedMethod>DELETE</AllowedMethod>
   <AllowedHeader>*</AllowedHeader>
  <MaxAgeSeconds>3000</MaxAgeSeconds>
  <ExposeHeader>x-amz-server-side-encryption</ExposeHeader>
  <ExposeHeader>x-amz-request-id</ExposeHeader>
  <ExposeHeader>x-amz-id-2</ExposeHeader>
 </CORSRule>
</CORSConfiguration>

The CORSRule element in the preceding configuration includes the following optional elements:

MaxAgeSeconds—Specifies the amount of time in seconds (in this example, 3000) that the browser will cache an Amazon S3 response to a preflight OPTIONS request for the specified resource. By caching the response, the browser does not have to send preflight requests to Amazon S3 if the original request is to be repeated.
ExposeHeader—Identifies the response headers (in this example, x-amz-server-side-encryption, x-amz-request-id, and x-amz-id-2) that customers will be able to access from their applications (for example, from a JavaScript XMLHttpRequest object).

AllowedMethod Element

In the CORS configuration, you can specify the following values for the AllowedMethod element.

GET
PUT
POST
DELETE
HEAD

AllowedOrigin Element

In the AllowedOrigin element you specify the origins that you want to allow cross-domain requests from, for example, http://www.example.com. The origin string can contain at most one * wildcard character, such as http://*.example.com. You can optionally specify * as the origin to enable all the origins to send cross-origin requests. You can also specify https to enable only secure origins.

AllowedHeader Element

The AllowedHeader element specifies which headers are allowed in a preflight request through the Access-Control-Request-Headers header. Each header name in the Access-Control-Request-Headers header must match a corresponding entry in the rule. Amazon S3 will send only the allowed headers in a response that were requested. For a sample list of headers that can be used in requests to Amazon S3, go to Common Request Headers in the Amazon Simple Storage Service API Reference guide.

Each AllowedHeader string in the rule can contain at most one * wildcard character. For example, <AllowedHeader>x-amz-*</AllowedHeader> will enable all Amazon-specific headers.

ExposeHeader Element

Each ExposeHeader element identifies a header in the response that you want customers to be able to access from their applications (for example, from a JavasSript XMLHttpRequest object). For a list of common Amazon S3 response headers, go to Common Response Headers in the Amazon Simple Storage Service API Reference guide.

MaxAgeSeconds Element

The MaxAgeSeconds element specifies the time in seconds that your browser can cache the response for a preflight request as identified by the resource, the HTTP method, and the origin.

How Does Amazon S3 Evaluate the CORS Configuration On a Bucket?

When Amazon S3 receives a preflight request from a browser, it evaluates the CORS configuration for the bucket and uses the first CORSRule rule that matches the incoming browser request to enable a cross-origin request. For a rule to match, the following conditions must be met:

The request's Origin header must match an AllowedOrigin element.
The request method (for example, GET or PUT) or the Access-Control-Request-Method header in case of a preflight OPTIONS request must be one of the AllowedMethod elements.
Every header listed in the request's Access-Control-Request-Headers header on the preflight request must match an AllowedHeader element.

Note

The ACLs and policies continue to apply when you enable CORS on the bucket.

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