Update HostFilteringOptions based on K8s Ingresses

[pinkfloydx33]

When using the Kubernetes controller, is there a way to have it reflect the hosts it detects on Ingress resources into the HostFilteringOptions used by the HostFilteringMiddleware?

We currently (without the controller) set the AllowedHosts based on a pre-determined list of host names. We are looking at moving to the Ingress Controller and would ideally like the HostFilteringOptions to be populated/updated in response to changes in Ingress/routes. That would play nicely with something like external-dns (which we also use) and allow us to have the downstream changes reflected in YARP without requiring us to update it as well.

At quick glance it doesn't look like there's anything built in that does this. I assume it would require a custom IConfigureOptions<HostFilteringOptions> as well as an IOptionsChangeTokenSource<HostFilteringOptions> for the options monitor to reflect any updates.

I have an idea how to implement the former by injecting IProxyStateLookup, looping over GetRoutes and extracting/parsing all the Host matches (sans Port), but I'm honestly not sure how to implement the IOptionsChangeTokenSource<> to react to/trigger based on updates to the routes.

Any ideas?

Thinking about this a bit more... I don't suppose this actually needs to be limited to the Kubernetes case. It feels like something that can be useful all around, opt-in of course

[adityamandaleeka]

Triage: this seems like it should be doable once #1734 is merged.

Essentially you would need to listen to the config change and look for hosts, then trigger the IOptionsMonitor which will update the HostFilteringMiddleware (assuming that's what you want).

[pinkfloydx33]

Right. I guess I was mainly trying to figure out what I'd use for the change token source. I guess I'd use a cancellation change token source and trigger it from the config notifier. Seems doable. Cool