Inject response headers

Documentation

The following example walks you through how to use an Inja template to extract a value from a request header and to add this value as a header to your responses.

Before you begin

Follow the Get started guide to install kgateway.
Follow the Sample app guide to create an API gateway proxy with an HTTP listener and deploy the httpbin sample app.

Get the external address of the gateway and save it in an environment variable.

export INGRESS_GW_ADDRESS=$(kubectl get svc -n kgateway-system http -o jsonpath="{.status.loadBalancer.ingress[0]['hostname','ip']}")
echo $INGRESS_GW_ADDRESS

kubectl port-forward deployment/http -n kgateway-system 8080:8080

Inject response headers

Create a RoutePolicy resource with your transformation rules. Make sure to create the RoutePolicy in the same namespace as the HTTPRoute resource. In the following example, you use the value from the x-solo-request request header and populate the value of that header into an x-solo-response response header.

kubectl apply -f- <<EOF
apiVersion: gateway.kgateway.dev/v1alpha1
kind: RoutePolicy
metadata:
  name: transformation
  namespace: httpbin
spec:
  transformation:
    response:
      set:
      - name: x-solo-response
        value: '{{ request_header("x-solo-request") }}' 
EOF

Update the HTTPRoute resource to apply the RoutePolicy to the httpbin route by using an extensionRef filter.

kubectl apply -f- <<EOF
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: httpbin
  namespace: httpbin
  labels:
    example: httpbin-route
spec:
  parentRefs:
    - name: http
      namespace: kgateway-system
  hostnames:
    - "www.example.com"
  rules:
    - backendRefs:
        - name: httpbin
          port: 8000
      filters:
      - type: ExtensionRef
        extensionRef:
          group: gateway.kgateway.dev
          kind: RoutePolicy
          name: transformation
EOF

Send a request to the httpbin app and include the x-solo-request request header.

curl -vik http://$INGRESS_GW_ADDRESS:8080/response-headers \
 -H "host: www.example.com:8080" \
 -H "x-solo-request: my custom request header"

curl -vik localhost:8080/response-headers \
-H "host: www.example.com" \
-H "x-solo-request: my custom request header"

In the example output, verify the following:

The x-solo-request request header is included in the request.
The request is successful and returns a 200 HTTP response code.
The x-solo-response response header is included in the response and has the same value as the x-solo-request request header.

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* Host <host-address>:8080 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:8080...
* Connected to <host-address> (::1) port 8080
> GET /response-headers HTTP/1.1
> Host: www.example.com
> User-Agent: curl/8.7.1
> Accept: */*
> x-solo-request: my custom request header
> 
* Request completely sent off
< HTTP/1.1 200 OK
HTTP/1.1 200 OK
< access-control-allow-credentials: true
access-control-allow-credentials: true
< access-control-allow-origin: *
access-control-allow-origin: *
< content-type: application/json; encoding=utf-8
content-type: application/json; encoding=utf-8
< date: Wed, 26 Jun 2024 02:54:48 GMT
date: Wed, 26 Jun 2024 02:54:48 GMT
< content-length: 3
content-length: 3
< x-envoy-upstream-service-time: 2
x-envoy-upstream-service-time: 2
< server: envoy
server: envoy
< x-envoy-decorator-operation: httpbin.httpbin.svc.cluster.local:8000/*
x-envoy-decorator-operation: httpbin.httpbin.svc.cluster.local:8000/*
< x-solo-response: my custom request header
x-solo-response: my custom request header

Cleanup

You can remove the resources that you created in this guide.

Delete the RoutePolicy resource.

kubectl delete RoutePolicy transformation -n httpbin

Remove the extensionRef filter from the HTTPRoute resource.

kubectl apply -f- <<EOF
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: httpbin
  namespace: httpbin
  labels:
    example: httpbin-route
spec:
  parentRefs:
    - name: http
      namespace: kgateway-system
  hostnames:
    - "www.example.com"
  rules:
    - backendRefs:
        - name: httpbin
          port: 8000
EOF

Decode base64 headers