Merge pull request #21210 from javidiaz/hostnamefqdn

Documentation for new feature setHostnameAsFQDN

content/en/docs/concepts/services-networking/dns-pod-service.md

@@ -157,6 +157,18 @@ pointing to the Pod's IP address. Also, Pod needs to become ready in order to ha
 record unless `publishNotReadyAddresses=True` is set on the Service.
 {{< /note >}}
 
+### Pod's setHostnameAsFQDN field {#pod-sethostnameasfqdn-field}
+
+When a Pod is configured to have fully qualified domain name (FQDN), its hostname is the short hostname. For example, if you have a Pod with the fully qualified domain name `busybox-1.default-subdomain.my-namespace.svc.cluster-domain.example`, then by default the `hostname` command inside that Pod returns `busybox-1` and  the `hostname --fqdn` command returns the FQDN.
+
+When you set `setHostnameAsFQDN: true` in the Pod spec, the kubelet writes the Pod's FQDN into the hostname for that Pod's namespace. In this case, both `hostname` and `hostname --fqdn` return the Pod's FQDN.
+
+{{< note >}}
+In Linux, the hostname field of the kernel (the `nodename` field of `struct utsname`) is limited to 64 characters.
+
+If a Pod enables this feature and its FQDN is longer than 64 character, it will fail to start. The Pod will remain in `Pending` status (`ContainerCreating` as seen by `kubectl`) generating error events, such as Failed to construct FQDN from pod hostname and cluster domain, FQDN `long-FDQN` is too long (64 characters is the max, 70 characters requested). One way of improving user experience for this scenario is to create an [admission webhook controller](/docs/reference/access-authn-authz/extensible-admission-controllers/#admission-webhooks) to control FQDN size when users create top level objects, for example, Deployment.
+{{< /note >}}
+
 ### Pod's DNS Policy
 
 DNS policies can be set on a per-pod basis. Currently Kubernetes supports the

content/en/docs/reference/command-line-tools-reference/feature-gates.md

@@ -136,6 +136,7 @@ different Kubernetes components.
 | `ServerSideApply` | `true` | Beta | 1.16 | |
 | `ServiceNodeExclusion` | `false` | Alpha | 1.8 | |
 | `ServiceTopology` | `false` | Alpha | 1.17 | |
+| `SetHostnameAsFQDN` | `false` | Alpha | 1.19 | |
 | `StartupProbe` | `false` | Alpha | 1.16 | 1.17 |
 | `StartupProbe` | `true` | Beta | 1.18 | |
 | `StorageVersionHash` | `false` | Alpha | 1.14 | 1.14 |
@@ -485,6 +486,7 @@ Each feature gate is designed for enabling/disabling a specific feature:
 - `ServiceNodeExclusion`: Enable the exclusion of nodes from load balancers created by a cloud provider.
   A node is eligible for exclusion if labelled with "`alpha.service-controller.kubernetes.io/exclude-balancer`" key or `node.kubernetes.io/exclude-from-external-load-balancers`.
 - `ServiceTopology`: Enable service to route traffic based upon the Node topology of the cluster. See [ServiceTopology](https://kubernetes.io/docs/concepts/services-networking/service-topology/) for more details.
+- `SetHostnameAsFQDN`: Enable the ability of setting Fully Qualified Domain Name(FQDN) as hostname of pod. See [Pod's `setHostnameAsFQDN` field](/docs/concepts/services-networking/dns-pod-service/#pod-sethostnameasfqdn-field).
 - `StartupProbe`: Enable the [startup](/docs/concepts/workloads/pods/pod-lifecycle/#when-should-you-use-a-startup-probe) probe in the kubelet.
 - `StorageObjectInUseProtection`: Postpone the deletion of PersistentVolume or
   PersistentVolumeClaim objects if they are still being used.