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).
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 enabled. One or more forms of authorization should be enabled, especially if anonymous requests or 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 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.
kubernetes service (in all namespaces) is configured with a virtual IP
address that is redirected (via kube-proxy) to the HTTPS endpoint on the
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 between the apiserver and kubelet if required to avoid connecting over an untrusted or public network.
Finally, Kubelet authentication and/or 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
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.
- The version names contain beta (e.g. v2beta3).
- Code is well tested. Enabling the feature is considered safe. Enabled by default.
- Support for the overall feature will not be dropped, though details may change.
- The schema and/or semantics of objects may change in incompatible ways in a subsequent beta or stable release. When this happens, we will provide instructions for migrating to the next version. This may require deleting, editing, and re-creating API objects. The editing process may require some thought. This may require downtime for applications that rely on the feature.
- Recommended for only non-business-critical uses because of potential for incompatible changes in subsequent releases. If you have multiple clusters that can be upgraded independently, you may be able to relax this restriction.
- Please do try our beta features and give feedback on them! After they exit beta, it may not be practical for us to make more changes.
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 on how to set it up in your cluster.
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