Change:
This is a Bug Report <!-- Thanks for filing an issue! Before submitting, please fill in the following information. --<!-- See for guidance on writing an actionable issue description. --<!--Required Information--Problem: I am not clearly able to differentiate the Cluster to Master. The topic states that its about Master and Node. What do you typically mean to say Cluster and Master in many places inside the doc. It should be Master and Node or Cluster and Node. There is something illogical to say Cluster and Master, since Master is part of the Cluster and we typically mean Cluster and Master interchangeably. So I would request that clarification is provided to the reader before getting into the topic. Proposed Solution: Page to Update: <!--Optional Information (remove the comment tags around information you would like to include)--<!--Kubernetes Version:--<!--Additional Information:--
that's a valid observation. I would reword: “Cluster to Master” → “Node to control plane” “Master to Cluster” → “Control plane to node” and make sure that the page makes it clear that it's common and typical to have several nodes (not just one). Note If anyone is picking this up, also consider renaming the page and adding a redirect. If you're not sure how to do those things you can ask for help using , or open a work-in-progress PR with your question in the comments. /help /language en /kind cleanup /priority important-longterm
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Discussion:
<ins> --- reviewers: - dchen1107 - roberthbailey - liggitt title: Control Plane-Node Communication content_template: templates/concept weight: 20 aliases: - master-node-communication --- {{% capture overview %}} This document catalogs the communication paths between the control plane (really the apiserver) and the Kubernetes cluster. The intent is to allow users to customize their installation to harden the network configuration such that the cluster can be run on an untrusted network (or on fully public IPs on a cloud provider). {{% /capture %}} {{% capture body %}} ## Node to Control Plane All communication paths from the nodes to the control plane terminate at the apiserver (none of the other master components are designed to expose remote services). In a typical deployment, the apiserver is configured to listen for remote connections on a secure HTTPS port (443) with one or more forms of client [authentication](/docs/reference/access-authn-authz/authentication/) enabled. One or more forms of [authorization](/docs/reference/access-authn-authz/authorization/) should be enabled, especially if [anonymous requests](/docs/reference/access-authn-authz/authentication/#anonymous-requests) or [service account tokens](/docs/reference/access-authn-authz/authentication/#service-account-tokens) are allowed. Nodes should be provisioned with the public root certificate for the cluster such that they can connect securely to the apiserver along with valid client credentials. For example, on a default GKE deployment, the client credentials provided to the kubelet are in the form of a client certificate. See [kubelet TLS bootstrapping](/docs/reference/command-line-tools-reference/kubelet-tls-bootstrapping/) for automated provisioning of kubelet client certificates. Pods that wish to connect to the apiserver can do so securely by leveraging a service account so that Kubernetes will automatically inject the public root certificate and a valid bearer token into the pod when it is instantiated. The `kubernetes` service (in all namespaces) is configured with a virtual IP address that is redirected (via kube-proxy) to the HTTPS endpoint on the apiserver. The control plane components also communicate with the cluster apiserver over the secure port. As a result, the default operating mode for connections from the nodes and pods running on the nodes to the control plane is secured by default and can run over untrusted and/or public networks. ## Control Plane to node There are two primary communication paths from the control plane (apiserver) to the nodes. The first is from the apiserver to the kubelet process which runs on each node in the cluster. The second is from the apiserver to any node, pod, or service through the apiserver's proxy functionality. ### apiserver to kubelet The connections from the apiserver to the kubelet are used for: * Fetching logs for pods. * Attaching (through kubectl) to running pods. * Providing the kubelet's port-forwarding functionality. These connections terminate at the kubelet's HTTPS endpoint. By default, the apiserver does not verify the kubelet's serving certificate, which makes the connection subject to man-in-the-middle attacks, and **unsafe** to run over untrusted and/or public networks. To verify this connection, use the `--kubelet-certificate-authority` flag to provide the apiserver with a root certificate bundle to use to verify the kubelet's serving certificate. If that is not possible, use [SSH tunneling](/docs/concepts/architecture/master-node-communication/#ssh-tunnels) between the apiserver and kubelet if required to avoid connecting over an untrusted or public network. Finally, [Kubelet authentication and/or authorization](/docs/admin/kubelet-authentication-authorization/) should be enabled to secure the kubelet API. ### apiserver to nodes, pods, and services The connections from the apiserver to a node, pod, or service default to plain HTTP connections and are therefore neither authenticated nor encrypted. They can be run over a secure HTTPS connection by prefixing `https:` to the node, pod, or service name in the API URL, but they will not validate the certificate provided by the HTTPS endpoint nor provide client credentials so while the connection will be encrypted, it will not provide any guarantees of integrity. These connections **are not currently safe** to run over untrusted and/or public networks. ### SSH tunnels Kubernetes supports SSH tunnels to protect the control plane to nodes communication paths. In this configuration, the apiserver initiates an SSH tunnel to each node in the cluster (connecting to the ssh server listening on port 22) and passes all traffic destined for a kubelet, node, pod, or service through the tunnel. This tunnel ensures that the traffic is not exposed outside of the network in which the nodes are running. SSH tunnels are currently deprecated so you shouldn't opt to use them unless you know what you are doing. The Konnectivity service is a replacement for this communication channel. ### Konnectivity service {{< feature-state for_k8s_version="v1.18" state="beta" >}} As a replacement to the SSH tunnels, the Konnectivity service provides TCP level proxy for the control plane to Cluster communication. The Konnectivity consists of two parts, the Konnectivity server and the Konnectivity agents, running in the control plane network and the nodes network respectively. The Konnectivity agents initiate connections to the Konnectivity server and maintain the connections. All control plane to nodes traffic then goes through these connections. See [Konnectivity Service Setup](/docs/tasks/setup-konnectivity/) on how to set it up in your cluster. </ins>

<del> --- reviewers: - dchen1107 - roberthbailey - liggitt title: Master-Node Communication content_template: templates/concept weight: 20 --- {{% capture overview %}} This document catalogs the communication paths between the master (really the apiserver) and the Kubernetes cluster. The intent is to allow users to customize their installation to harden the network configuration such that the cluster can be run on an untrusted network (or on fully public IPs on a cloud provider). {{% /capture %}} {{% capture body %}} ## Cluster to Master All communication paths from the cluster to the master terminate at the apiserver (none of the other master components are designed to expose remote services). In a typical deployment, the apiserver is configured to listen for remote connections on a secure HTTPS port (443) with one or more forms of client [authentication](/docs/reference/access-authn-authz/authentication/) enabled. One or more forms of [authorization](/docs/reference/access-authn-authz/authorization/) should be enabled, especially if [anonymous requests](/docs/reference/access-authn-authz/authentication/#anonymous-requests) or [service account tokens](/docs/reference/access-authn-authz/authentication/#service-account-tokens) are allowed. Nodes should be provisioned with the public root certificate for the cluster such that they can connect securely to the apiserver along with valid client credentials. For example, on a default GKE deployment, the client credentials provided to the kubelet are in the form of a client certificate. See [kubelet TLS bootstrapping](/docs/reference/command-line-tools-reference/kubelet-tls-bootstrapping/) for automated provisioning of kubelet client certificates. Pods that wish to connect to the apiserver can do so securely by leveraging a service account so that Kubernetes will automatically inject the public root certificate and a valid bearer token into the pod when it is instantiated. The `kubernetes` service (in all namespaces) is configured with a virtual IP address that is redirected (via kube-proxy) to the HTTPS endpoint on the apiserver. The master components also communicate with the cluster apiserver over the secure port. As a result, the default operating mode for connections from the cluster (nodes and pods running on the nodes) to the master is secured by default and can run over untrusted and/or public networks. ## Master to Cluster There are two primary communication paths from the master (apiserver) to the cluster. The first is from the apiserver to the kubelet process which runs on each node in the cluster. The second is from the apiserver to any node, pod, or service through the apiserver's proxy functionality. ### apiserver to kubelet The connections from the apiserver to the kubelet are used for: * Fetching logs for pods. * Attaching (through kubectl) to running pods. * Providing the kubelet's port-forwarding functionality. These connections terminate at the kubelet's HTTPS endpoint. By default, the apiserver does not verify the kubelet's serving certificate, which makes the connection subject to man-in-the-middle attacks, and **unsafe** to run over untrusted and/or public networks. To verify this connection, use the `--kubelet-certificate-authority` flag to provide the apiserver with a root certificate bundle to use to verify the kubelet's serving certificate. If that is not possible, use [SSH tunneling](/docs/concepts/architecture/master-node-communication/#ssh-tunnels) between the apiserver and kubelet if required to avoid connecting over an untrusted or public network. Finally, [Kubelet authentication and/or authorization](/docs/admin/kubelet-authentication-authorization/) should be enabled to secure the kubelet API. ### apiserver to nodes, pods, and services The connections from the apiserver to a node, pod, or service default to plain HTTP connections and are therefore neither authenticated nor encrypted. They can be run over a secure HTTPS connection by prefixing `https:` to the node, pod, or service name in the API URL, but they will not validate the certificate provided by the HTTPS endpoint nor provide client credentials so while the connection will be encrypted, it will not provide any guarantees of integrity. These connections **are not currently safe** to run over untrusted and/or public networks. ### SSH Tunnels Kubernetes supports SSH tunnels to protect the Master → Cluster communication paths. In this configuration, the apiserver initiates an SSH tunnel to each node in the cluster (connecting to the ssh server listening on port 22) and passes all traffic destined for a kubelet, node, pod, or service through the tunnel. This tunnel ensures that the traffic is not exposed outside of the network in which the nodes are running. SSH tunnels are currently deprecated so you shouldn't opt to use them unless you know what you are doing. The Konnectivity service is a replacement for this communication channel. ### Konnectivity service {{< feature-state for_k8s_version="v1.18" state="beta" >}} As a replacement to the SSH tunnels, the Konnectivity service provides TCP level proxy for the Master → Cluster communication. The Konnectivity consists of two parts, the Konnectivity server and the Konnectivity agents, running in the Master network and the Cluster network respectively. The Konnectivity agents initiate connections to the Konnectivity server and maintain the connections. All Master → Cluster traffic then goes through these connections. See [Konnectivity Service Setup](/docs/tasks/setup-konnectivity/) on how to set it up in your cluster. {{% /capture %}} </del>