istio

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