Inject response headers

Use an Inja template to extract a value from a request header and add it as a header to your responses.

Before you begin

  1. Follow the Get started guide to install kgateway.

  2. Follow the Sample app guide to create a gateway proxy with an HTTP listener and deploy the httpbin sample app.

  3. 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

  1. Create a TrafficPolicy resource with the folloing transformation rules:

    • x-gateway-response: Use the value from the x-gateway-request request header and populate the value of that header into an x-gateway-response response header.
    • x-podname: Retrieve the value of the POD_NAME environment variable and add the value to the x-podname response header. Because the transformation is processed in the gateway proxy, these environment variables refer to the variables that are set on the proxy. You can view supported environment variables when you run kubectl get deployment http -n kgateway-system -o yaml and look at the spec.containers.env` section.
    • x-season: Adds a static string value of summer to the x-season response header.
    • x-response-raw: Adds a static string values of hello with all escape characters intact.
    • x-replace: Replaces the pattern-to-replace text in the baz header with a random string.
    kubectl apply -f- <<EOF
apiVersion: gateway.kgateway.dev/v1alpha1
kind: TrafficPolicy
metadata:
  name: transformation
  namespace: httpbin
spec:
  targetRefs:
  - group: gateway.networking.k8s.io
    kind: HTTPRoute
    name: httpbin
  transformation:
    response:
      set:
      - name: x-gateway-response
        value: '{{ request_header("x-gateway-request") }}' 
      - name: x-podname
        value: '{{ env("POD_NAME") }}'
      - name: x-season
        value: 'summer'
      - name: x-response-raw
        value: '{{ raw_string("hello") }}'
      - name: x-replace
        value: '{{ replace_with_random(request_header("baz"), "pattern-to-replace") }}'
EOF
    kubectl apply -f- <<EOF
apiVersion: gateway.kgateway.dev/v1alpha1
kind: TrafficPolicy
metadata:
  name: transformation
  namespace: httpbin
spec:
  targetRefs:
  - group: gateway.networking.k8s.io
    kind: HTTPRoute
    name: httpbin
  transformation:     
    response:
      set:
      - name: x-gateway-response
        value: 'request.headers["x-gateway-request"]'
      - name: x-podname
        value: 'request.headers["x-pod-name"]'
      - name: x-season
        value: '"summer"'
      - name: x-response-raw
        value: '"hello"'
      - name: x-replace
        value: 'request.headers["baz"].replace("pattern-to-replace", string(random()))'
EOF

  2. Send a request to the httpbin app and include the x-gateway-request and baz request headers. Verify that you get back a 200 HTTP response code and that the following response headers are included:

    • x-podname that is set to the name of the gateway proxy pod.
    • x-season that is set to summer.
    • x-gateway-response that is set to the value of the x-gateway-request request header.
    • x-response-raw that is set to hello.
    • x-replace that is set to a random string.
    curl -vi http://$INGRESS_GW_ADDRESS:8080/response-headers \
 -H "host: www.example.com:8080" \
 -H "x-gateway-request: my custom request header" \
 -H "baz: pattern-to-replace"
    curl -vi localhost:8080/response-headers \
-H "host: www.example.com" \
-H "x-gateway-request: my custom request header" \
-H "baz: pattern-to-replace"

    Example output:

    ...
* 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
< 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-envoy-upstream-service-time: 1
< x-podname: http-85d5775587-tkxmt
x-podname: http-85d5775587-tkxmt
< x-replace: zljPMhO86gJCFc69jZ0+kQ
x-replace: zljPMhO86gJCFc69jZ0+kQ
< x-response-raw: hello
x-response-raw: hello
< x-gateway-response: my custom request header
x-gateway-response: my custom request header
< x-season: summer
x-season: summer

Cleanup

You can remove the resources that you created in this guide. 
kubectl delete TrafficPolicy transformation -n httpbin
Templating languageDecode base64 headers