Master <-> Node Communication
Summary
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 -> 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.
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.
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).
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 -> 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.
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.
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 guarantees of integrity.
These connections are not currently safe to run over untrusted and/or
public networks.
SSH Tunnels
Google Container Engine 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.
Kubelet TLS Bootstrap
Kubernetes 1.4 introduces an experimental API for requesting certificates from a cluster-level Certificate Authority (CA). The first supported use of this API is the provisioning of TLS client certificates for kubelets. The proposal can be found here and progress on the feature is being tracked as feature #43.
apiserver configuration
You must provide a token file which specifies at least one “bootstrap token” assigned to a kubelet boostrap-specific group. This group will later be used in the controller-manager configuration to scope approvals in the default approval controller. As this feature matures, you should ensure tokens are bound to an RBAC policy which limits requests using the bootstrap token to only be able to make requests related to certificate provisioning. When RBAC policy is in place, scoping the tokens to a group will allow great flexibility (e.g. you could disable a particular bootstrap group’s access when you are done provisioning the nodes).
Token auth file
Tokens are arbitrary but should represent at least 128 bits of entropy derived from a secure random number generator (such as /dev/urandom on most modern systems). There are multiple ways you can generate a token. For example:
head -c 16 /dev/urandom | od -An -t x | tr -d ' '
will generate tokens that look like 02b50b05283e98dd0fd71db496ef01e8
The token file will look like the following example, where the first three values can be anything and the quoted group name should be as depicted:
02b50b05283e98dd0fd71db496ef01e8,kubelet-bootstrap,10001,"system:kubelet-bootstrap"
Add the --token-auth-file=FILENAME flag to the apiserver command to enable the token file.
See docs at http://kubernetes.io/docs/admin/authentication/#static-token-file for further details.
controller-manager configuration
The API for requesting certificates adds a certificate-issuing control loop to the KCM. This takes the form of a cfssl local signer using assets on disk. Currently, all certificates issued have one year validity and a default set of key usages.
Signing assets
You must provide a Certificate Authority in order to provide the cryptographic materials necessary to issue certificates.
This CA should be trusted by the apiserver for authentication with the --client-ca-file=SOMEFILE flag. The management
of the CA is beyond the scope of this document but it is recommended that you generate a dedicated CA for Kubernetes.
Both certificate and key are assumed to be PEM-encoded.
The new controller-manager flags are:
--cluster-signing-cert-file="/etc/path/to/kubernetes/ca/ca.crt" --cluster-signing-key-file="/etc/path/to/kubernetes/ca/ca.key"
Auto-approval
To ease deployment and testing, the alpha version of the certificate request API includes a flag to approve all certificate requests made by users in a certain group. The intended use of this is to whitelist only the group corresponding to the bootstrap token in the token file above. Use of this flag circumvents makes the “approval” process described below and is not recommended for production use.
The flag is:
--insecure-experimental-approve-all-kubelet-csrs-for-group="system:kubelet-bootstrap"
kubelet configuration
To use request a client cert from the certificate request API, the kubelet needs a path to a kubeconfig file that contains the
bootstrap auth token. If the file specified by --kubeconfig does not exist, the bootstrap kubeconfig is used to request a
client certificate from the API server. On success, a kubeconfig file referencing the generated key and obtained certificate
is written to the path specified by --kubeconfig. The certificate and key file will be stored in the directory pointed
by --cert-dir. The new flag is:
--experimental-bootstrap-kubeconfig="/path/to/bootstrap/kubeconfig"
kubectl approval
The signing controller does not immediately sign all certificate requests. Instead, it waits until they have been flagged with an
“Approved” status by an appropriately-privileged user. This is intended to eventually be an automated process handled by an external
approval controller, but for the alpha version of the API it can be done manually by a cluster administrator using kubectl.
An administrator can list CSRs with kubectl get csr, describe one in detail with kubectl describe <name>. There are
currently no direct approve/deny commands so an approver will need to update
the Status field directly. A rough example of how to do this in bash which should only be used until the porcelain merges is available
at https://github.com/gtank/csrctl.