Document kubernetes.io/kube-apiserver-serving ClusterTrustBundle signer

content/en/docs/reference/access-authn-authz/certificate-signing-requests.md

@@ -206,15 +206,33 @@ Kubernetes provides built-in signers that each have a well-known `signerName`:
       of the `--cluster-signing-duration` option or, if specified, the `spec.expirationSeconds` field of the CSR object.
    1. CA bit allowed/disallowed - not allowed.
 
-The kube-controller-manager implements [control plane signing](#signer-control-plane) for each of the built in
-signers. Failures for all of these are only reported in kube-controller-manager logs.
+1. `kubernetes.io/kube-apiserver-serving`: signs certificates that can be used to verify kube-apiserver serving
+   certificates. Signing and approval are handled outside kube-controller-manager.
+    - {{< feature-state feature_gate_name="ClusterTrustBundle" >}}
+    1. Trust distribution: signed certificates are used by the kube-apiserver for TLS
+       server authentication. The CA bundle is distributed using a ClusterTrustBundle object
+       identifiable by the `kubernetes.io/kube-apiserver-serving` signer name.
+    1. Permitted subjects - "Subject" itself is deprecated for TLS server authentication by RFC2818. However,
+       it should still follow the same rules on DNS/IP {{< glossary_tooltip text="SANs" term_id="san" >}}
+       from the "Permitted x509 extensions" section below.
+    1. Permitted x509 extensions - honors subjectAltName and key usage extensions. At
+       least one DNS or IP subjectAltName must be present. The SAN DNS/IP of the certificates
+       must resolve/point to kube-apiserver's hostname/IP.
+    1. Permitted key usages - ["key encipherment", "digital signature", "server auth"] or ["digital signature", "server auth"].
+    1. Expiration/certificate lifetime - The recommended maximum lifetime is 30 days.
+    1. CA bit allowed/disallowed - not recommended by the Kubernetes project.
 
 {{< note >}}
 The `spec.expirationSeconds` field was added in Kubernetes v1.22. Earlier versions of Kubernetes do not honor this field.
 Kubernetes API servers prior to v1.22 will silently drop this field when the object is created.
 {{< /note >}}
 
-Distribution of trust happens out of band for these signers. Any trust outside of those described above are strictly
+The kube-controller-manager implements [control plane signing](#signer-control-plane) for each of the built in
+signers except for `kubernetes.io/kube-apiserver-serving`. Failures for all of these are only reported in kube-controller-manager logs.
+Signing of certificates in the trust domain of the `kubernetes.io/kube-apiserver-serving` signer is in full control of
+the cluster administrator(s).
+
+Any trust outside of the above described cases is strictly
 coincidental. For instance, some distributions may honor `kubernetes.io/legacy-unknown` as client certificates for the
 kube-apiserver, but this is not a standard.
 None of these usages are related to ServiceAccount token secrets `.data[ca.crt]` in any way. That CA bundle is only
@@ -623,4 +641,4 @@ kubectl config use-context myuser
 * For details of X.509 itself, refer to [RFC 5280](https://tools.ietf.org/html/rfc5280#section-3.1) section 3.1
 * For information on the syntax of PKCS#10 certificate signing requests, refer to [RFC 2986](https://tools.ietf.org/html/rfc2986)
 * Read about the ClusterTrustBundle API:
-  * {{< page-api-reference kind="ClusterTrustBundle" >}}
+  * {{< page-api-reference kind="ClusterTrustBundle" >}}

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

@@ -10,4 +10,13 @@ stages:
     defaultValue: false
     fromVersion: "1.27"
 ---
-Enable ClusterTrustBundle objects and kubelet integration.
+This feature gate exists in the Kubernetes API server and the controller manager.
+
+Used from the kube-apiserver, it enables ClusterTrustBundle support.
+
+In order to use the ClusterTrustBundle API in your cluster, you need to enable this feature gate
+and also [enable](/docs/tasks/administer-cluster/enable-disable-api/) the associated alpha API group
+using the `--runtime-config` command line argument to kube-apiserver.
+
+In the Kubernetes controller manager, it is used to control publishing of a ClusterTrustBundle
+for the `kubernetes.io/kube-apiserver-serving` signer.

content/en/docs/reference/glossary/san.md

@@ -0,0 +1,18 @@
+---
+title: Subject Alternative Name
+id: san
+date: 2024-11-25
+full_link: https://datatracker.ietf.org/doc/html/rfc4985
+short_description: >
+   An X.509 certificate extension to identify what hostname or IP address the certificate applies to.
+
+aka:
+tags:
+- security
+---
+Subject Alternative Name is an {{< glossary_tooltip text="X.509 certificate" term_id="certificate" >}}
+extension that allows identities to be bound to the subject of the certificate.
+
+<!--more-->
+The [standard](https://datatracker.ietf.org/doc/html/rfc4985) defines identities represented
+as an email address, a DNS name, an IP address or a Uniform Resource Identifier (URI).