Route using as backend a k8s service with no endpoints for GAMMA chaining

[baburciu]

We're considering a design in which both NGF and a Service Mesh Gateway API implementation would need to coexist, and we'd like the NGF-backed HTTPRoute to point to a K8s service with no endpoints. This is something that ingress-nginx Ingress Controller would handle in its service-upstream working mode and we'd like to learn if something similar could be achieved with NGF.
Design looks like this:

Our motivation is that we'd like to have Argo Rollouts manage a GAMMA-based HTTPRoute (for canary traffic split), which can be provided by a Service Mesh like Cilium, but we'd also want NGF to be the primary entry point to the cluster (fronted by cloud provider CDN and Load Balancer) since NGF has good OpenTelemetry collector integration (it generates the root span), while Cilium doesn't (given cilium/cilium#41259).

To reproduce such a scenario in kind I've put together this example, where the problem is also elaborated.

In a nutshell, due to the parent Service (used as backend for the NGF HTTPRoute) not having an endpoint assigned, NGF drops the traffic. Would there be a way to have data plane configuration just forward to the parent service ClusterIP socket, so that Cilium can take over?

Many thanks in advance for any inputs you may have.

[sjberman]

@ciarams87 Do you think an ExternalName service could be used to accomplish this? Kind of hacky, but it would just use the DNS name in the Service, and no endpoints. Though maybe that wouldn't be compatible with Cilium. (ExternalName support being released in 2.2)

Otherwise, we'd need to implement this, because I can't think of another way right now.