CORS

updated: 2022-04-20 04:24

CORS

CORS is a mechanism to allow code running in a browser (Javascript for example) make requests to a domain other then the one from where it originated.

Swift supports CORS requests to containers and objects.

CORS metadata is held on the container only. The values given apply to the container itself and all objects within it.

The supported headers are,

Metadata Use
X-Container-Meta-Access-Control-Allow-Origin Origins to be allowed to make Cross Origin Requests, space separated.
X-Container-Meta-Access-Control-Max-Age Max age for the Origin to hold the preflight results.
X-Container-Meta-Access-Control-Expose-Headers Headers exposed to the user agent (e.g. browser) in the actual request response. Space separated.

Before a browser issues an actual request it may issue a preflight request. The preflight request is an OPTIONS call to verify the Origin is allowed to make the request. The sequence of events are,

  • Browser makes OPTIONS request to Swift
  • Swift returns 200/401 to browser based on allowed origins
  • If 200, browser makes the “actual request” to Swift, i.e. PUT, POST, DELETE, HEAD, GET

When a browser receives a response to an actual request it only exposes those headers listed in the Access-Control-Expose-Headers header. By default Swift returns the following values for this header,

  • “simple response headers” as listed on http://www.w3.org/TR/cors/#simple-response-header
  • the headers etag, x-timestamp, x-trans-id, x-openstack-request-id
  • all metadata headers (X-Container-Meta-* for containers and X-Object-Meta-* for objects)
  • headers listed in X-Container-Meta-Access-Control-Expose-Headers

Note

An OPTIONS request to a symlink object will respond with the options for the symlink only, the request will not be redirected to the target object. Therefore, if the symlink’s target object is in another container with CORS settings, the response will not reflect the settings.

Sample Javascript

To see some CORS Javascript in action download the test CORS page (source below). Host it on a webserver and take note of the protocol and hostname (origin) you’ll be using to request the page, e.g. http://localhost.

Locate a container you’d like to query. Needless to say the Swift cluster hosting this container should have CORS support. Append the origin of the test page to the container’s X-Container-Meta-Access-Control-Allow-Origin header,:

curl -X POST -H 'X-Auth-Token: xxx' \
  -H 'X-Container-Meta-Access-Control-Allow-Origin: http://localhost' \
  http://192.168.56.3:8080/v1/AUTH_test/cont1

At this point the container is now accessible to CORS clients hosted on http://localhost. Open the test CORS page in your browser.

  1. Populate the Token field
  2. Populate the URL field with the URL of either a container or object
  3. Select the request method
  4. Hit Submit

Assuming the request succeeds you should see the response header and body. If something went wrong the response status will be 0.

Test CORS Page

A sample cross-site test page is located in the project source tree doc/source/test-cors.html.

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Test CORS</title>
  </head>
  <body>

    Token<br><input id="token" type="text" size="64"><br><br>

    Method<br>
    <select id="method">
        <option value="GET">GET</option>
        <option value="HEAD">HEAD</option>
        <option value="POST">POST</option>
        <option value="DELETE">DELETE</option>
        <option value="PUT">PUT</option>
    </select><br><br>

    URL (Container or Object)<br><input id="url" size="64" type="text"><br><br>

    <input id="submit" type="button" value="Submit" onclick="submit(); return false;">

    <pre id="response_headers"></pre>
    <p>
    <hr>
    <pre id="response_body"></pre>

    <script type="text/javascript">
      function submit() {
          var token = document.getElementById('token').value;
          var method = document.getElementById('method').value;
          var url = document.getElementById('url').value;

          document.getElementById('response_headers').textContent = null;
          document.getElementById('response_body').textContent = null;

          var request = new XMLHttpRequest();

          request.onreadystatechange = function (oEvent) {
              if (request.readyState == 4) {
                  responseHeaders = 'Status: ' + request.status;
                  responseHeaders = responseHeaders + '\nStatus Text: ' + request.statusText;
                  responseHeaders = responseHeaders + '\n\n' + request.getAllResponseHeaders();
                  document.getElementById('response_headers').textContent = responseHeaders;
                  document.getElementById('response_body').textContent = request.responseText;
              }
          }

          request.open(method, url);
          if (token != '') {
              // custom headers always trigger a pre-flight request
              request.setRequestHeader('X-Auth-Token', token);
          }
          request.send(null);
      }
    </script>

  </body>
</html>
updated: 2022-04-20 04:24
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