Host rewrites

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"
  }
}

Set the host with a transformation

For an AgentgatewayBackend that uses a hostname, such as a static backend or an MCP target, agentgateway automatically sets the Host header to the hostname of the backend. This automatic host rewrite runs after transformations. If a transformation policy sets the :authority or Host header, the automatic host rewrite replaces that value, even though the policy status shows Attached. To keep the value from the transformation, turn off the automatic host rewrite for the route by setting hostRewrite.mode to None. This setting is also required when the transformation is in the spec.backend section of a policy that targets the AgentgatewayBackend.

Kubernetes Service backends do not get an automatic host rewrite by default, so a transformation can set their Host header without this setting. If you explicitly set hostRewrite.mode to Auto, agentgateway also rewrites the host for a Kubernetes Service backend, to the hostname of the Service. The URLRewrite filter in the previous sections also turns off the automatic host rewrite.

Create an AgentgatewayBackend that reaches the in-cluster httpbin app by its hostname.

kubectl apply -f- <<EOF
apiVersion: agentgateway.dev/v1alpha1
kind: AgentgatewayBackend
metadata:
  name: httpbin-static
  namespace: httpbin
spec:
  static:
    host: httpbin.httpbin.svc.cluster.local
    port: 8000
EOF

Create an HTTPRoute that matches traffic on the transform-host.example domain and forwards it to the AgentgatewayBackend.

kubectl apply -f- <<EOF
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: transform-host
  namespace: httpbin
spec:
  parentRefs:
  - name: agentgateway-proxy
    namespace: agentgateway-system
  hostnames:
    - transform-host.example
  rules:
    - backendRefs:
      - name: httpbin-static
        kind: AgentgatewayBackend
        group: agentgateway.dev
EOF

Create an AgentgatewayPolicy that sets the :authority header with a transformation and turns off the automatic host rewrite for the route.

kubectl apply -f- <<EOF
apiVersion: agentgateway.dev/v1alpha1
kind: AgentgatewayPolicy
metadata:
  name: transform-host
  namespace: httpbin
spec:
  targetRefs:
  - group: gateway.networking.k8s.io
    kind: HTTPRoute
    name: transform-host
  traffic:
    hostRewrite:
      mode: None
    transformation:
      request:
        set:
        - name: ":authority"
          value: '"transformed.example"'
EOF

Setting Description

spec.traffic.hostRewrite.mode Set to None so that the automatic host rewrite does not replace the :authority header from the transformation. If you leave this setting out, the backend gets the httpbin.httpbin.svc.cluster.local host. This setting applies to the whole route. Other hostname-based backends on the same route also lose the automatic host rewrite, so they get the host that the client sent, unless a transformation sets the host for them.

spec.traffic.transformation.request.set The :authority header to set on the request. The value is a CEL expression, so a fixed string needs quotes.

Setting	Description
`spec.traffic.hostRewrite.mode`	Set to `None` so that the automatic host rewrite does not replace the `:authority` header from the transformation. If you leave this setting out, the backend gets the `httpbin.httpbin.svc.cluster.local` host. This setting applies to the whole route. Other hostname-based backends on the same route also lose the automatic host rewrite, so they get the host that the client sent, unless a transformation sets the host for them.
`spec.traffic.transformation.request.set`	The `:authority` header to set on the request. The value is a CEL expression, so a fixed string needs quotes.

Send a request to the transform-host.example domain. Verify that the httpbin app receives the transformed.example host.

curl -s http://$INGRESS_GW_ADDRESS:80/headers -H "host: transform-host.example" | jq -c '.headers.Host'

Example output:

["transformed.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 set the host with a transformation, delete its resources.

kubectl delete AgentgatewayPolicy transform-host -n httpbin
kubectl delete httproute transform-host -n httpbin
kubectl delete AgentgatewayBackend httpbin-static -n httpbin

Path rewrites

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