Merge pull request #51414 from HirazawaUi/add-doc-4762

Add documentation for the HostnameOverride feature gate

Author: k8s-ci-robot

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

@@ -129,7 +129,7 @@ is an endpoint of a Service named `barista`, and the domain name for your cluste
 172-17-0-3.barista.cafe.svc.cluster.local
 ```
 
-### Pod's hostname and subdomain fields
+### Pod's hostname and subdomain fields {#pod-hostname-and-subdomain-field}
 
 Currently when a Pod is created, its hostname (as observed from within the Pod)
 is the Pod's `metadata.name` value.

content/en/docs/concepts/workloads/pods/pod-hostname.md

@@ -0,0 +1,122 @@
+---
+title: Pod Hostname
+content_type: concept
+weight: 85
+---
+
+<!-- overview -->
+
+This page explains how to set a Pod's hostname, 
+potential side effects after configuration, and the underlying mechanics.
+
+<!-- body -->
+
+## Default Pod Hostname
+
+When a Pod is created, its hostname (as observed from within the Pod) 
+is derived from the Pod's metadata.name value. 
+Both the hostname and its corresponding fully qualified domain name (FQDN) 
+are set to the metadata.name value (from the Pod's perspective)
+
+```yaml
+apiVersion: v1
+kind: Pod
+metadata:
+  name: busybox-1
+spec:
+  containers:
+  - image: busybox:1.28
+    command:
+      - sleep
+      - "3600"
+    name: busybox
+```
+The Pod created by this manifest will have its hostname and fully qualified domain name (FQDN) set to `busybox-1`.
+
+## Hostname with pod's hostname and subdomain fields
+The Pod spec includes an optional `hostname` field. 
+When set, this value takes precedence over the Pod's `metadata.name` as the 
+hostname (observed from within the Pod).
+For example, a Pod with spec.hostname set to `my-host` will have its hostname set to `my-host`.
+
+The Pod spec also includes an optional `subdomain` field, 
+indicating the Pod belongs to a subdomain within its namespace. 
+If a Pod has `spec.hostname` set to "foo" and spec.subdomain set 
+to "bar" in the namespace `my-namespace`, its hostname becomes `foo` and its 
+fully qualified domain name (FQDN) becomes 
+`foo.bar.my-namespace.svc.cluster-domain.example` (observed from within the Pod).
+
+When both hostname and subdomain are set, the cluster's DNS server will 
+create A and/or AAAA records based on these fields. 
+Refer to: [Pod's hostname and subdomain fields](/docs/concepts/services-networking/dns-pod-service/#pod-hostname-and-subdomain-field).
+
+## Hostname with pod's setHostnameAsFQDN fields
+
+{{< feature-state for_k8s_version="v1.22" state="stable" >}}
+
+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.busybox-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 both `setHostnameAsFQDN: true` and the subdomain field is set 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.
+
+The Pod's FQDN is constructed in the same manner as previously defined.
+It is composed of the Pod's `spec.hostname` (if specified) or `metadata.name` field,
+the `spec.subdomain`, the `namespace` name, and the cluster domain suffix.
+
+{{< 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".
+
+This means that when using this field, 
+you must ensure the combined length of the Pod's `metadata.name` (or `spec.hostname`) 
+and `spec.subdomain` fields results in an FQDN that does not exceed 64 characters.
+{{< /note >}}
+
+## Hostname with pod's hostnameOverride
+{{< feature-state feature_gate_name="HostnameOverride" >}}
+
+Setting a value for `hostnameOverride` in the Pod spec causes the kubelet 
+to unconditionally set both the Pod's hostname and fully qualified domain name (FQDN)
+to the `hostnameOverride` value. 
+
+The hostnameOverride field has a length limitation of 64 characters 
+and must adhere to the DNS subdomain names standard defined in [RFC 1123](https://datatracker.ietf.org/doc/html/rfc1123).
+
+Example:
+```yaml
+apiVersion: v1
+kind: Pod
+metadata:
+  name: busybox-2-busybox-example-domain
+spec:
+  hostnameOverride: busybox-2.busybox.example.domain
+  containers:
+  - image: busybox:1.28
+    command:
+      - sleep
+      - "3600"
+    name: busybox
+```
+{{< note >}}
+This only affects the hostname within the Pod; it does not affect the Pod's A or AAAA records in the cluster DNS server.
+{{< /note >}}
+
+If hostnameOverride is set alongside hostname and subdomain fields:
+* The hostname inside the Pod is overridden to the hostnameOverride value.
+* The Pod's A and/or AAAA records in the cluster DNS server are still generated based on the hostname and subdomain fields.
+
+Note: If `hostnameOverride` is set, you cannot simultaneously set the `hostNetwork` and `setHostnameAsFQDN` fields.
+The API server will explicitly reject any create request attempting this combination.
+
+For details on behavior when `hostnameOverride` is set in combination with 
+other fields (hostname, subdomain, setHostnameAsFQDN, hostNetwork), 
+see the table in the [KEP-4762 design details](https://github.com/kubernetes/enhancements/blob/master/keps/sig-network/4762-allow-arbitrary-fqdn-as-pod-hostname/README.md#design-details ).

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

@@ -0,0 +1,13 @@
+---
+title: HostnameOverride
+content_type: feature_gate
+_build:
+  list: never
+  render: false
+
+stages:
+  - stage: alpha
+    defaultValue: false
+    fromVersion: "1.34"
+---
+Allows setting any FQDN as the pod's hostname.