Description:

The documentation for HTTPHeaderFilter contains this statement:

Configuration to set or add multiple values for a header must use RFC 7230 header value formatting, separating each value with a comma.

My expectation that adding a header via Envoy Gateway's route filters with a comma-delimited value would add the header in the same way that specifying the header multiple times with curl would do.

Repro steps:

1. Deploy the quickstart example

2. Apply the following HTTPRoute:

   apiVersion: gateway.networking.k8s.io/v1
   kind: HTTPRoute
   metadata:
     name: backend
     namespace: default
   spec:
     hostnames:
     - www.example.com
     parentRefs:
     - group: gateway.networking.k8s.io
       kind: Gateway
       name: eg
     rules:
     - backendRefs:
       - group: ""
         kind: Service
         name: backend
         port: 3000
         weight: 1
       filters:
       - requestHeaderModifier:
           add:
           - name: add-header
             value: foo,bar
         type: RequestHeaderModifier
       matches:
       - path:
           type: PathPrefix
           value: /

3. Run the following curl command:

   curl http://${ENVOY_GATEWAY_IP}/headers -H 'Host: www.example.com' -H 'Test: one' -H 'Test: two'

   Compare the header Add-Header and the header Test in the output:

   {
    "path": "/headers",
    "host": "www.example.com",
    "method": "GET",
    "proto": "HTTP/1.1",
    "headers": {
     "Accept": [
      "*/*"
     ],
     "Add-Header": [
      "foo,bar"
     ],
     "Test": [
      "one",
      "two"
     ],
    },
    "namespace": "default",
    "ingress": "",
    "service": "",
    "pod": "backend-96f75bbf-778l6"
   }