Host rewrites

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Replace the host header value before forwarding a request to a backend service.

Replace the host header value before forwarding a request to a backend service by using the URLRewrite filter.

For more information, see the Kubernetes Gateway API documentation.

Before you begin

Set up an agentgateway proxy.
Install the httpbin sample app.

In-cluster service host rewrites

Create an HTTPRoute resource for the httpbin app that uses the URLRewrite filter to rewrite the hostname of the request. In this example, all incoming requests on the rewrite.example domain are rewritten to the www.example.com host.

kubectl apply -f- <<EOF
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: httpbin-rewrite
  namespace: httpbin
spec:
  parentRefs:
  - name: agentgateway-proxy
    namespace: agentgateway-system
  hostnames:
    - rewrite.example
  rules:
    - matches:
        - path:
            type: PathPrefix
            value: /
      filters:
        - type: URLRewrite
          urlRewrite:
            hostname: "www.example.com"
      backendRefs:
        - name: httpbin
          port: 8000
EOF

Setting	Description
`spec.parentRefs`	The name and namespace of the Gateway that serves this HTTPRoute. In this example, you use the `agentgateway-proxy` gateway that was created as part of the get started guide.
`spec.rules.filters.type`	The type of filter that you want to apply to incoming requests. In this example, the `URLRewrite` filter is used.
`spec.rules.filters.urlRewrite.hostname`	The hostname that you want to rewrite requests to.
`spec.rules.backendRefs`	The backend destination you want to forward traffic to. In this example, all traffic is forwarded to the httpbin app that you set up as part of the get started guide.

Send a request to the httpbin app on the rewrite.example domain. Verify that you get back a 200 HTTP response code and that you see the Host: www.example.com header in your response.
Note
The following request returns a 200 HTTP response code, because you set up an HTTPRoute for the httpbin app on the www.example.com domain as part of the getting started guide. If you chose a different domain for your example, make sure that you have an HTTPRoute that can be reached under the host you want to rewrite to.
curl -vi http://$INGRESS_GW_ADDRESS:80/headers -H "host: rewrite.example:80"
curl -vi localhost:8080/headers -H "host: rewrite.example"
Example output:
```
...
{
 "headers": {
   "Accept": [
     "*/*"
   ],
   "Host": [
     "www.example.com"
   ],
   "User-Agent": [
     "curl/8.7.1"
   ]
 }
}
```

External service host rewrites

Create an AgentgatewayBackend that represents your external service. The following example creates an AgentgatewayBackend for the httpbin.org domain.

kubectl apply -f- <<EOF
apiVersion: agentgateway.dev/v1alpha1
kind: AgentgatewayBackend
metadata:
  name: httpbin
  namespace: httpbin
spec:
  static:
    host: httpbin.org
    port: 80
EOF

Create an HTTPRoute resource that matches incoming traffic on the external-rewrite.example domain and forwards traffic to the AgentgatewayBackend that you created. Because the AgentgatewayBackend expects a different domain, you use the URLRewrite filter to rewrite the hostname from external-rewrite.example to httpbin.org.

kubectl apply -f- <<EOF
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: backend-rewrite
  namespace: httpbin
spec:
  parentRefs:
  - name: agentgateway-proxy
    namespace: agentgateway-system
  hostnames:
    - external-rewrite.example
  rules:
     - filters:
       - type: URLRewrite
         urlRewrite:
           hostname: "httpbin.org"
       backendRefs:
       - name: httpbin
         kind: AgentgatewayBackend
         group: agentgateway.dev
EOF

Send a request to the external-rewrite.example domain. Verify that you get back a 200 HTTP response code and that you see the Host: httpbin.org header in your response.

curl -vi http://$INGRESS_GW_ADDRESS:80/headers -H "host: external-rewrite.example:80"

Example output:

* Request completely sent off
< HTTP/1.1 200 OK
HTTP/1.1 200 OK
< content-type: application/json
content-type: application/json
< content-length: 268
content-length: 268
< server: envoy
server: envoy
< access-control-allow-origin: *
access-control-allow-origin: *
< access-control-allow-credentials: true
access-control-allow-credentials: true
< x-envoy-upstream-service-time: 2416
x-envoy-upstream-service-time: 2416
< 

{
  "headers": {
    "Accept": "*/*", 
    "Host": "httpbin.org", 
    "User-Agent": "curl/8.7.1"
  }
}

Backend-specific host rewrites

When a rule splits traffic between backends that expect different hosts, rewrite the host for one backend only. To do so, add the URLRewrite filter to the backendRefs entry of that backend instead of to the rule. Requests to the other backends in the rule keep their original host.

Create an HTTPRoute that splits traffic on the backend-host-rewrite.example domain between the in-cluster httpbin app and the httpbin.org AgentgatewayBackend that you created in the external service section. Only the requests to httpbin.org are rewritten to the httpbin.org host.

kubectl apply -f- <<EOF
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: backend-host-rewrite
  namespace: httpbin
spec:
  parentRefs:
  - name: agentgateway-proxy
    namespace: agentgateway-system
  hostnames:
    - backend-host-rewrite.example
  rules:
    - backendRefs:
        - name: httpbin
          port: 8000
          weight: 50
        - name: httpbin
          kind: AgentgatewayBackend
          group: agentgateway.dev
          weight: 50
          filters:
            - type: URLRewrite
              urlRewrite:
                hostname: "httpbin.org"
EOF

Setting	Description
`spec.rules.backendRefs.weight`	The share of requests that each backend gets. In this example, each backend gets half of the requests.
`spec.rules.backendRefs.filters.type`	The filter that applies only to the requests that the gateway forwards to this backend. In this example, the `URLRewrite` filter is used. The in-cluster httpbin backend has no filter, so its requests keep the `backend-host-rewrite.example` host.
`spec.rules.backendRefs.filters.urlRewrite.hostname`	The hostname that you want to rewrite to for the requests that the gateway forwards to this backend.

Send six requests to the backend-host-rewrite.example domain, and print the Host header that each backend received. Verify that the requests to the in-cluster httpbin app keep the backend-host-rewrite.example host, and that the requests to httpbin.org have the httpbin.org host.
for i in $(seq 1 6); do curl -s http://$INGRESS_GW_ADDRESS:80/headers -H "host: backend-host-rewrite.example" | jq -c '.headers.Host' done
for i in $(seq 1 6); do curl -s localhost:8080/headers -H "host: backend-host-rewrite.example" | jq -c '.headers.Host' done
Example output: The in-cluster httpbin app returns the host as a list, and httpbin.org returns it as a string. Because the gateway splits the traffic by weight, the order of your output differs.
```
["backend-host-rewrite.example"]
"httpbin.org"
"httpbin.org"
["backend-host-rewrite.example"]
"httpbin.org"
["backend-host-rewrite.example"]
```

Cleanup

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

kubectl delete httproute httpbin-rewrite -n httpbin
kubectl delete httproute backend-rewrite -n httpbin
kubectl delete AgentgatewayBackend httpbin -n httpbin

If you created the backend-specific host rewrite, delete its HTTPRoute.

kubectl delete httproute backend-host-rewrite -n httpbin

Path rewrites

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