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