What are BlackHole and Passthrough clusters? Understanding, controlling and securing your external service access is one of the key benefits that you get from a service mesh like Istio. From a security and operations point of view, it is critical to monitor what external service traffic is getting blocked as they might surface possible misconfigurations or a security vulnerability if an application is attempting to communicate with a service that it should not be allowed to.
Similarly, if you currently have a policy of allowing any external service access, it is beneficial to monitor the traffic so you can incrementally add explicit Istio configuration to allow access and better security your cluster. In either case, having visibility into this traffic via telemetry is quite helpful as it enables you to create alerts and dashboards, and better reason about your security posture. This was a highly requested feature by production users of Istio and we are excited that the support for this was added in release 1.3.
Source: istio.io