CORS

This guide provides instructions for configuring Cross-Origin Resource Sharing (CORS) on Envoy Gateway. CORS defines a way for client web applications that are loaded in one domain to interact with resources in a different domain.

Envoy Gateway introduces a new CRD called SecurityPolicy that allows the user to configure CORS. This instantiated resource can be linked to a Gateway, HTTPRoute or GRPCRoute resource.

Prerequisites

Follow the steps from the Quickstart guide to install Envoy Gateway and the example manifest. Before proceeding, you should be able to query the example backend using HTTP.

Configuration

When configuring CORS either an origin with a precise hostname can be configured or an hostname containing a wildcard prefix, allowing all subdomains of the specified hostname. In addition to that the entire origin (with or without specifying a scheme) can be a wildcard to allow all origins.

The below example defines a SecurityPolicy that allows CORS for all HTTP requests originating from www.foo.com.

cat <<EOF | kubectl apply -f -
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: SecurityPolicy
metadata:
  name: cors-example
spec:
  targetRef:
    group: gateway.networking.k8s.io
    kind: HTTPRoute
    name: backend
  cors:
    allowOrigins:
    - "http://*.foo.com"
    - "http://*.foo.com:80"
    allowMethods:
    - GET
    - POST
    allowHeaders:
    - "x-header-1"
    - "x-header-2"
    exposeHeaders:
    - "x-header-3"
    - "x-header-4"
EOF

Verify the SecurityPolicy configuration:

kubectl get securitypolicy/cors-example -o yaml

Testing

Ensure the GATEWAY_HOST environment variable from the Quickstart guide is set. If not, follow the Quickstart instructions to set the variable.

echo $GATEWAY_HOST

Verify that the CORS headers are present in the response of the OPTIONS request from http://www.foo.com:

curl -H "Origin: http://www.foo.com" \
  -H "Host: www.example.com" \
  -H "Access-Control-Request-Method: GET" \
  -X OPTIONS -v -s \
  http://$GATEWAY_HOST \
  1> /dev/null

You should see the below response, indicating that the request from http://www.foo.com is allowed:

< access-control-allow-origin: http://www.foo.com
< access-control-allow-methods: GET, POST
< access-control-allow-headers: x-header-1, x-header-2
< access-control-max-age: 86400
< access-control-expose-headers: x-header-3, x-header-4

If you try to send a request from http://www.bar.com, you should see the below response:

curl -H "Origin: http://www.bar.com" \
  -H "Host: www.example.com" \
  -H "Access-Control-Request-Method: GET" \
  -X OPTIONS -v -s \
  http://$GATEWAY_HOST \
  1> /dev/null

You won’t see any CORS headers in the response, indicating that the request from http://www.bar.com was not allowed.

If you try to send a request from http://www.foo.com:8080, you should also see similar response because the port number 8080 is not included in the allowed origins.

```shell
curl -H "Origin: http://www.foo.com:8080" \
  -H "Host: www.example.com" \
  -H "Access-Control-Request-Method: GET" \
  -X OPTIONS -v -s \
  http://$GATEWAY_HOST \
  1> /dev/null

Note:

• CORS specification requires that the browsers to send a preflight request to the server to ask if it’s allowed to access the limited resource in another domains. The browsers are supposed to follow the response from the server to determine whether to send the actual request or not. The CORS filter only response to the preflight requests according to its configuration. It won’t deny any requests. The browsers are responsible for enforcing the CORS policy.
• The targeted HTTPRoute or the HTTPRoutes that the targeted Gateway routes to must allow the OPTIONS method for the CORS filter to work. Otherwise, the OPTIONS request won’t match the routes and the CORS filter won’t be invoked.

Clean-Up

Follow the steps from the Quickstart guide to uninstall Envoy Gateway and the example manifest.

Delete the SecurityPolicy:

kubectl delete securitypolicy/cors-example

Next Steps

Checkout the Developer Guide to get involved in the project.