# GEP 1323: Response Header Filter * Issue: [#1323](https://github.com/kubernetes-sigs/gateway-api/issues/1323) * Status: Standard > **Note**: This GEP is exempt from the [Probationary Period][expprob] rules of > our GEP overview as it existed before those rules did, and so it has been > explicitly grandfathered in. [expprob]:https://gateway-api.sigs.k8s.io/geps/overview/#probationary-period ## TLDR Similar to how we have `RequestHeaderModifier` in `HTTPRouteFilter`, which lets users modify request headers before the request is forwarded to a backend (or a group of backends), it’d be helpful to have a `ResponseHeaderModifier` field which would let users modify response headers before they are returned to the client. ## Goals * Provide a way to modify HTTP response headers in a `HTTPRoute`. * Reuse existing types as much as possible to reduce boilerplate code. ## Non Goals * Provide a way to modify other parts of a HTTP response like status code. * Add fields specifically for standard headers such as `Cookie`. ## Introduction Currently, the `HTTPRouteFilter` API provides a way for request headers to be modified through the `RequestHeaderModifier` field of type `HTTPRequestHeaderModifier`. But, a similar API to modify response headers does not exist. This proposal intends to introduce a new field in `HTTPRouteFilter` named `ResponseHeaderModifier`. ## API We could introduce a new API named `HTTPResponseHeaderModifier` which would look exactly like the existing `HTTPRequestHeaderModifier` API. But since HTTP headers have the same semantics for both requests and responses, it makes more sense to rename `HTTPRequestHeaderModifier` to `HTTPHeaderModifier` and use this for both `RequestHeaderModifier` and `ResponseHeaderModifier`. ```golang // HTTPHeaderModifier defines a filter that modifies the headers of a HTTP // request or response. type HTTPHeaderModifier struct { // Set overwrites the request with the given header (name, value) // before the action. // +optional // +listType=map // +listMapKey=name // +kubebuilder:validation:MaxItems=16 Set []HTTPHeader `json:"set,omitempty"` // Add adds the given header(s) (name, value) to the request // before the action. It appends to any existing values associated // with the header name. // +optional // +listType=map // +listMapKey=name // +kubebuilder:validation:MaxItems=16 Add []HTTPHeader `json:"add,omitempty"` // Remove the given header(s) from the HTTP request before the action. The // value of Remove is a list of HTTP header names. Note that the header // names are case-insensitive (see // https://datatracker.ietf.org/doc/html/rfc2616#section-4.2). // +optional // +kubebuilder:validation:MaxItems=16 Remove []string `json:"remove,omitempty"` } ``` Given the fact that this functionality is offered by only a few projects that currently implement Gateway API when using their own traffic routing CRDs, it’s better to support `ResponseHeaderModifier` as an _Extended_ feature, unlike `RequestHeaderModifier` which is a _Core_ feature. This will also not increase the difficulty of implementing Gateway API for any future ingress or service mesh. This feature can be further extended via [Policy Attachment](../reference/policy-attachment.md). The mechanism and use cases of this may be explored in a future GEP. ## Usage Adding support for this unlocks a lot of real world use cases. Let’s review a couple of them: * A team has a frontend web app, along with two different versions of their backends exposed as Kubernetes services. If, the frontend needs to know which backend it’s talking to, this can be easily achieved without any modifications to the application code. ```yaml apiVersion: gateway.networking.k8s.io/v1beta1 kind: HTTPRoute metadata: name: http-response-header spec: hostnames: - response.header.example rules: - backendRefs: - name: example-svc-beta weight: 50 port: 80 # set a custom header for all responses being sent from the beta build of the backend server. filters: - type: ResponseHeaderModifier responseHeaderModifier: add: name: build value: beta - name: example-svc-stable weight: 50 port: 80 ``` * Cookies can be automatically injected into the response of services. This can enable services to identify users that were redirected to a certain backend. ```yaml apiVersion: gateway.networking.k8s.io/v1beta1 kind: HTTPRoute metadata: name: http-response-header spec: hostnames: - response.header.example rules: # match against any requests that has the cookie set due to the below rule - matches: - headers: type: Exact name: Cookie value: user=insider backendRefs: - name: foo-svc port: 8080 - filters: - type: ResponseHeaderModifier # set cookies for all requests being forwarded to this service responseHeaderModifier: set: name: Set-Cookie value: user=insider backendRefs: - name: example-svc weight: 1 port: 80 ``` > Note: Some projects like Envoy support interpolating a few predefined [variables into header values](https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_conn_man/headers#custom-request-response-headers). Similar functionality might be supported by other implementations but its unlikely to be portable and thus has been excluded from the API for the time being. ## Prior Art A few projects that implement Gateway API already have support for similar functionality (in their custom CRDs), like: * Istio’s `VirtualService`: ```yaml apiVersion: networking.istio.io/v1alpha3 kind: VirtualService metadata: name: reviews-route spec: hosts: - reviews.prod.svc.cluster.local http: - headers: request: set: test: "true" route: - destination: host: reviews.prod.svc.cluster.local subset: v2 weight: 25 - destination: host: reviews.prod.svc.cluster.local subset: v1 headers: response: remove: - foo weight: 75 ``` * Contour’s `HTTPProxy`: ```yaml apiVersion: projectcontour.io/v1 kind: HTTPProxy metadata: name: basic spec: virtualhost: fqdn: foo-basic.bar.com routes: - conditions: - prefix: / services: - name: s1 port: 80 responseHeadersPolicy: set: name: test value: true ``` * Ingress NGINX: ```yaml apiVersion: networking.k8s.io/v1 kind: Ingress metadata: name: nginx-headers annotations: nginx.ingress.kubernetes.io/configuration-snippet: | add_header ingress nginx; spec: ingressClassName: nginx rules: - host: custom.configuration.com http: paths: - path: / pathType: Prefix backend: service: name: http-svc port: number: 80 ``` ## External Links * [Contour `HeaderValue`](https://projectcontour.io/docs/v1.22.0/config/api/#projectcontour.io/v1.HeaderValue) * [Istio `Headers`](https://istio.io/latest/docs/reference/config/networking/virtual-service/#Headers) * [Ingress NGINX Configuration Snippets](https://kubernetes.github.io/ingress-nginx/examples/customization/configuration-snippets/) * [NGINX `add_header` directive](https://nginx.org/en/docs/http/ngx_http_headers_module.html#add_header)