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Document the communication paths between the master and node.
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| <!-- BEGIN MUNGE: UNVERSIONED_WARNING --> | ||
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| <!-- BEGIN STRIP_FOR_RELEASE --> | ||
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| <img src="http://kubernetes.io/img/warning.png" alt="WARNING" | ||
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| <h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2> | ||
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| If you are using a released version of Kubernetes, you should | ||
| refer to the docs that go with that version. | ||
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| Documentation for other releases can be found at | ||
| [releases.k8s.io](http://releases.k8s.io). | ||
| </strong> | ||
| -- | ||
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| <!-- END STRIP_FOR_RELEASE --> | ||
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| # Master <-> Node Communication | ||
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| **Table of Contents** | ||
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| - [Master <-> Node Communication](#master---node-communication) | ||
| - [Summary](#summary) | ||
| - [Cluster -> Master](#cluster---master) | ||
| - [Master -> Cluster](#master---cluster) | ||
| - [SSH Tunnels](#ssh-tunnels) | ||
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| <!-- END MUNGE: GENERATED_TOC --> | ||
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| ## Summary | ||
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| 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). | ||
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| ## Cluster -> Master | ||
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| 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](authentication.md) enabled. | ||
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| 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 GCE deployment, the client credentials | ||
| provided to the kubelet are in the form of a client certificate. 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. | ||
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| The master components communicate with the cluster apiserver over the | ||
| insecure (not encrypted or authenticated) port. This port is typically only | ||
| exposed on the localhost interface of the master machine, so that the master | ||
| components, all running on the same machine, can communicate with the | ||
| cluster apiserver. Over time, the master components will be migrated to use | ||
| the secure port with authentication and authorization (see | ||
| [#13598](https://github.com/kubernetes/kubernetes/issues/13598)). | ||
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| 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. | ||
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| ## Master -> Cluster | ||
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| 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. | ||
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| The connections from the apiserver to the kubelet are used for fetching logs | ||
| for pods, attaching (through kubectl) to running pods, and using the kubelet's | ||
| port-forwarding functionality. These connections terminate at the kubelet's | ||
| HTTPS endpoint, which is typically using a self-signed certificate, and | ||
| ignore the certificate presented by the kubelet (although you can override this | ||
| behavior by specifying the `--kubelet-certificate-authority`, | ||
| `--kubelet-client-certificate`, and `--kubelet-client-key` flags when starting | ||
| the cluster apiserver). By default, these connections **are not currently safe** | ||
| to run over untrusted and/or public networks as they are subject to | ||
| man-in-the-middle attacks. | ||
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| 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 by encrypted, it will not provide any guarentees of integrity. | ||
| These connections **are not currently safe** to run over untrusted and/or | ||
| public networks. | ||
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| ### SSH Tunnels | ||
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| [Google Container Engine](https://cloud.google.com/container-engine/docs/) uses | ||
| 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 private | ||
| GCE network in which the cluster is running. | ||
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| <!-- BEGIN MUNGE: GENERATED_ANALYTICS --> | ||
| []() | ||
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