# istio_disable-host-header-fallback.yaml
apiVersion: release-notes/v2
kind: bug-fix
area: telemetry
releaseNotes:
- |
  **Updated** Prometheus telemetry behavior for inbound traffic to disable host header fallback by default. This will
  prevent traffic coming from out-of-mesh locations from potential polluting the `destination_service` dimension in
  metrics with junk data (and exploding metrics cardinality). With this change, it is possible that users relying on
  host headers for labeling the destination service for inbound traffic from out-of-mesh workloads will see that traffic
  labeled as `unknown`. The behavior can be restored by modifying Istio configuration to remove the `disable_host_header_fallback: true`
  configuration.
upgradeNotes:
  - title: Host header fallback disabled by default for Prometheus metrics for *all* inbound traffic.
    content: |
      Host header fallback for determining values for Prometheus `destination_service` labels has been disabled for all incoming traffic.
      Previously, this was disabled *only* for traffic arriving at Gateways. If you are relying on host header fallback behavior to properly
      label the `destination_service` in Prometheus metrics for traffic originating from out-of-mesh workloads, then you will need to update the telemetry
      configuration to enable host header fallback.