196 lines
6.8 KiB
ReStructuredText
196 lines
6.8 KiB
ReStructuredText
.. _cross-project:
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=============================
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Cross-origin resource sharing
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=============================
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.. note::
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This is a new feature in OpenStack Liberty.
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OpenStack supports :term:`Cross-Origin Resource Sharing (CORS)`, a W3C
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specification defining a contract by which the single-origin policy of a user
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agent (usually a browser) may be relaxed. It permits the javascript engine
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to access an API that does not reside on the same domain, protocol, or port.
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This feature is most useful to organizations which maintain one or more
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custom user interfaces for OpenStack, as it permits those interfaces to access
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the services directly, rather than requiring an intermediate proxy server. It
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can, however, also be misused by malicious actors; please review the
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security advisory below for more information.
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Enabling CORS with configuration
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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In most cases, CORS support is built directly into the service itself. To
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enable it, simply follow the configuration options exposed in the default
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configuration file, or add it yourself according to the pattern below.
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.. code-block:: ini
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[cors]
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allowed_origin = https://first_ui.example.com
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max_age = 3600
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allow_methods = GET,POST,PUT,DELETE
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allow_headers = Content-Type,Cache-Control,Content-Language,Expires,Last-Modified,Pragma,X-Custom-Header
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expose_headers = Content-Type,Cache-Control,Content-Language,Expires,Last-Modified,Pragma,X-Custom-Header
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Additional origins can be explicitly added. To express this in
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your configuration file, first begin with a ``[cors]`` group as above,
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into which you place your default configuration values. Then, add as many
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additional configuration groups as necessary, naming them
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``[cors.{something}]`` (each name must be unique). The purpose of the
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suffix to ``cors.`` is legibility, we recommend using a reasonable
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human-readable string:
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.. code-block:: ini
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[cors.ironic_webclient]
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# CORS Configuration for a hypothetical ironic webclient, which overrides
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# authentication
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allowed_origin = https://ironic.example.com:443
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allow_credentials = True
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[cors.horizon]
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# CORS Configuration for horizon, which uses global options.
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allowed_origin = https://horizon.example.com:443
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[cors.wildcard]
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# CORS Configuration for the CORS specified domain wildcard, which only
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# permits HTTP GET requests.
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allowed_origin = *
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allow_methods = GET
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For more information about CORS configuration,
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see `cross-origin resource sharing
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<https://docs.openstack.org/ocata/config-reference/common-configurations/cors.html>`_
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in OpenStack Configuration Reference.
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Enabling CORS with PasteDeploy
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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CORS can also be configured using PasteDeploy. First of all, ensure that
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OpenStack's ``oslo_middleware`` package (version 2.4.0 or later) is
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available in the Python environment that is running the service. Then,
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add the following configuration block to your ``paste.ini`` file.
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.. code-block:: ini
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[filter:cors]
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paste.filter_factory = oslo_middleware.cors:filter_factory
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allowed_origin = https://website.example.com:443
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max_age = 3600
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allow_methods = GET,POST,PUT,DELETE
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allow_headers = Content-Type,Cache-Control,Content-Language,Expires,Last-Modified,Pragma,X-Custom-Header
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expose_headers = Content-Type,Cache-Control,Content-Language,Expires,Last-Modified,Pragma,X-Custom-Header
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.. note::
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To add an additional domain in oslo_middleware v2.4.0, add
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another filter. In v3.0.0 and after, you may add multiple domains
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in the above ``allowed_origin`` field, separated by commas.
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Security concerns
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~~~~~~~~~~~~~~~~~
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CORS specifies a wildcard character ``*``, which permits access to all user
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agents, regardless of domain, protocol, or host. While there are valid use
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cases for this approach, it also permits a malicious actor to create a
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convincing facsimile of a user interface, and trick users into revealing
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authentication credentials. Please carefully evaluate your use case and the
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relevant documentation for any risk to your organization.
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.. note::
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The CORS specification does not support using this wildcard as
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a part of a URI. Setting ``allowed_origin`` to ``*`` would work, while
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``*.openstack.org`` would not.
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Troubleshooting
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~~~~~~~~~~~~~~~
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CORS is very easy to get wrong, as even one incorrect property will violate
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the prescribed contract. Here are some steps you can take to troubleshoot
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your configuration.
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Check the service log
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---------------------
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The CORS middleware used by OpenStack provides verbose debug logging that
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should reveal most configuration problems. Here are some example log
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messages, and how to resolve them.
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Problem
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-------
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``CORS request from origin 'http://example.com' not permitted.``
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Solution
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--------
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A request was received from the origin ``http://example.com``, however this
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origin was not found in the permitted list. The cause may be a superfluous
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port notation (ports 80 and 443 do not need to be specified). To correct,
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ensure that the configuration property for this host is identical to the
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host indicated in the log message.
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Problem
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-------
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``Request method 'DELETE' not in permitted list: GET,PUT,POST``
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Solution
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--------
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A user agent has requested permission to perform a DELETE request, however
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the CORS configuration for the domain does not permit this. To correct, add
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this method to the ``allow_methods`` configuration property.
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Problem
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-------
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``Request header 'X-Custom-Header' not in permitted list: X-Other-Header``
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Solution
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--------
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A request was received with the header ``X-Custom-Header``, which is not
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permitted. Add this header to the ``allow_headers`` configuration
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property.
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Open your browser's console log
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-------------------------------
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Most browsers provide helpful debug output when a CORS request is rejected.
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Usually this happens when a request was successful, but the return headers on
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the response do not permit access to a property which the browser is trying
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to access.
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Manually construct a CORS request
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---------------------------------
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By using ``curl`` or a similar tool, you can trigger a CORS response with a
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properly constructed HTTP request. An example request and response might look
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like this.
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Request example:
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.. code-block:: console
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$ curl -I -X OPTIONS https://api.example.com/api -H "Origin: https://ui.example.com"
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Response example:
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.. code-block:: console
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HTTP/1.1 204 No Content
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Content-Length: 0
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Access-Control-Allow-Origin: https://ui.example.com
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Access-Control-Allow-Methods: GET,POST,PUT,DELETE
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Access-Control-Expose-Headers: origin,authorization,accept,x-total,x-limit,x-marker,x-client,content-type
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Access-Control-Allow-Headers: origin,authorization,accept,x-total,x-limit,x-marker,x-client,content-type
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Access-Control-Max-Age: 3600
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If the service does not return any access control headers, check the service
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log, such as ``/var/log/upstart/ironic-api.log`` for an indication on what
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went wrong.
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