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.

   Cloud Provider LoadBalancerPort-forward for local testing

   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

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.
   Cloud Provider LoadBalancerPort-forward for local testing

   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

kubectl delete TrafficPolicy transformation -n httpbin