istio
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1apiVersion: release-notes/v2
2kind: bug-fix
3area: traffic-management
4issues:
5- 37331
6releaseNotes:
7- |
8**Improved** support for `ExternalName` services. See Upgrade Notes for more information
9upgradeNotes:
10- title: "Upcoming `ExternalName` support changes"
11content: |
12Below describes *upcoming* changes to `ExternalName`.
13In this release, there is no behavioral changes by default.
14However, you can explicitly opt-in to the new behavior early if desired, and prepare your environments for the upcoming change.
15
16Kubernetes `ExternalName` `Service`s allow users to create new DNS entries. For example, you can create an `example` service
17that points to `example.com`. This is implemented by a DNS `CNAME` redirect.
18
19In Istio, the implementation of `ExternalName`, historically, was substantially different. Each `ExternalName` represented its own
20service, and traffic matching the service was sent to the configured DNS name.
21
22This caused a few issues:
23* Ports are required in Istio, but not in Kubernetes. This can result in broken traffic if ports are not configured as Istio expects, despite them working without Istio.
24* Ports not declared as `HTTP` would match *all* traffic on that port, making it easy to accidentally send all traffic on a port to the wrong place.
25* Because the destination DNS name is treated as opaque, we cannot apply Istio policies to it as expected. For example, if I point
26an external name at another in-cluster Service (for example, `example.default.svc.cluster.local`), mTLS would not be used.
27
28`ExternalName` support has been revamped to fix these problems. `ExternalName`s are now simply treated as aliases.
29Wherever we would match `Host: <concrete service>` we additionally will match `Host: <external name service>`.
30Note that the primary implementation of `ExternalName` -- DNS -- is handled outside of Istio in the Kubernetes DNS implementation, and remains unchanged.
31
32If you are using `ExternalName` with Istio, please be advised of the following behavioral changes:
33* The `ports` field is no longer needed, matching Kubernetes behavior. If it is set, it will have no impact.
34* `VirtualServices` that match on an `ExternalName` service will generally no longer match. Instead, the match should be rewritten to the referenced service.
35* `DestinationRule` can no longer apply to `ExternalName` services. Instead, create rules where the `host` references service.
36
37These changes are off-by-default in this release, but will be on-by-default in the near future.
38To opt-in early, the `ENABLE_EXTERNAL_NAME_ALIAS=true` environment variable can be set.
39