Accessing Clusters

This topic discusses multiple ways to interact with clusters.

Accessing for the first time with kubectl

When accessing the Kubernetes API for the first time, we suggest using the Kubernetes CLI, kubectl.

To access a cluster, you need to know the location of the cluster and have credentials to access it. Typically, this is automatically set-up when you work through a Getting started guide, or someone else setup the cluster and provided you with credentials and a location.

Check the location and credentials that kubectl knows about with this command:

  1. kubectl config view

Many of the examples provide an introduction to using kubectl and complete documentation is found in the kubectl manual.

Directly accessing the REST API

Kubectl handles locating and authenticating to the apiserver. If you want to directly access the REST API with an http client like curl or wget, or a browser, there are several ways to locate and authenticate:

  • Run kubectl in proxy mode.
    • Recommended approach.
    • Uses stored apiserver location.
    • Verifies identity of apiserver using self-signed cert. No MITM possible.
    • Authenticates to apiserver.
    • In future, may do intelligent client-side load-balancing and failover.
  • Provide the location and credentials directly to the http client.
    • Alternate approach.
    • Works with some types of client code that are confused by using a proxy.
    • Need to import a root cert into your browser to protect against MITM.

Using kubectl proxy

The following command runs kubectl in a mode where it acts as a reverse proxy. It handles locating the apiserver and authenticating. Run it like this:

  1. kubectl proxy --port=8080

See kubectl proxy for more details.

Then you can explore the API with curl, wget, or a browser, replacing localhost with [::1] for IPv6, like so:

  1. curl http://localhost:8080/api/

The output is similar to this:

  1. {
  2.   "kind": "APIVersions",
  3.   "versions": [
  4.     "v1"
  5.   ],
  6.   "serverAddressByClientCIDRs": [
  7.     {
  8.       "clientCIDR": "0.0.0.0/0",
  9.       "serverAddress": "10.0.1.149:443"
  10.     }
  11.   ]
  12. }

Without kubectl proxy

Use kubectl describe secret... to get the token for the default service account with grep/cut:

  1. APISERVER=$(kubectl config view --minify | grep server | cut -f 2- -d ":" | tr -d " ")
  2. SECRET_NAME=$(kubectl get secrets | grep ^default | cut -f1 -d ' ')
  3. TOKEN=$(kubectl describe secret $SECRET_NAME | grep -E '^token' | cut -f2 -d':' | tr -d " ")
  5. curl $APISERVER/api --header "Authorization: Bearer $TOKEN" --insecure

The output is similar to this:

  1. {
  2.   "kind": "APIVersions",
  3.   "versions": [
  4.     "v1"
  5.   ],
  6.   "serverAddressByClientCIDRs": [
  7.     {
  8.       "clientCIDR": "0.0.0.0/0",
  9.       "serverAddress": "10.0.1.149:443"
  10.     }
  11.   ]
  12. }

Using jsonpath:

  1. APISERVER=$(kubectl config view --minify -o jsonpath='{.clusters[0].cluster.server}')
  2. SECRET_NAME=$(kubectl get serviceaccount default -o jsonpath='{.secrets[0].name}')
  3. TOKEN=$(kubectl get secret $SECRET_NAME -o jsonpath='{.data.token}' | base64 --decode)
  5. curl $APISERVER/api --header "Authorization: Bearer $TOKEN" --insecure

The output is similar to this:

  1. {
  2.   "kind": "APIVersions",
  3.   "versions": [
  4.     "v1"
  5.   ],
  6.   "serverAddressByClientCIDRs": [
  7.     {
  8.       "clientCIDR": "0.0.0.0/0",
  9.       "serverAddress": "10.0.1.149:443"
  10.     }
  11.   ]
  12. }

The above examples use the --insecure flag. This leaves it subject to MITM attacks. When kubectl accesses the cluster it uses a stored root certificate and client certificates to access the server. (These are installed in the ~/.kube directory). Since cluster certificates are typically self-signed, it may take special configuration to get your http client to use root certificate.

On some clusters, the apiserver does not require authentication; it may serve on localhost, or be protected by a firewall. There is not a standard for this. Controlling Access to the API describes how a cluster admin can configure this.

Programmatic access to the API

Kubernetes officially supports Go and Python client libraries.

Go client

  • To get the library, run the following command: go get k8s.io/client-go@kubernetes-<kubernetes-version-number>, see INSTALL.md for detailed installation instructions. See https://github.com/kubernetes/client-go to see which versions are supported.
  • Write an application atop of the client-go clients. Note that client-go defines its own API objects, so if needed, please import API definitions from client-go rather than from the main repository, e.g., import "k8s.io/client-go/kubernetes" is correct.

The Go client can use the same kubeconfig file as the kubectl CLI does to locate and authenticate to the apiserver. See this example.

If the application is deployed as a Pod in the cluster, please refer to the next section.

Python client

To use Python client, run the following command: pip install kubernetes. See Python Client Library page for more installation options.

The Python client can use the same kubeconfig file as the kubectl CLI does to locate and authenticate to the apiserver. See this example.

Other languages

There are client libraries for accessing the API from other languages. See documentation for other libraries for how they authenticate.

Accessing the API from a Pod

When accessing the API from a pod, locating and authenticating to the apiserver are somewhat different.

The recommended way to locate the apiserver within the pod is with the kubernetes.default.svc DNS name, which resolves to a Service IP which in turn will be routed to an apiserver.

The recommended way to authenticate to the apiserver is with a service account credential. By kube-system, a pod is associated with a service account, and a credential (token) for that service account is placed into the filesystem tree of each container in that pod, at /var/run/secrets/kubernetes.io/serviceaccount/token.

If available, a certificate bundle is placed into the filesystem tree of each container at /var/run/secrets/kubernetes.io/serviceaccount/ca.crt, and should be used to verify the serving certificate of the apiserver.

Finally, the default namespace to be used for namespaced API operations is placed in a file at /var/run/secrets/kubernetes.io/serviceaccount/namespace in each container.

From within a pod the recommended ways to connect to API are:

  • Run kubectl proxy in a sidecar container in the pod, or as a background process within the container. This proxies the Kubernetes API to the localhost interface of the pod, so that other processes in any container of the pod can access it.
  • Use the Go client library, and create a client using the rest.InClusterConfig() and kubernetes.NewForConfig() functions. They handle locating and authenticating to the apiserver. example

In each case, the credentials of the pod are used to communicate securely with the apiserver.

Accessing services running on the cluster

The previous section describes how to connect to the Kubernetes API server. For information about connecting to other services running on a Kubernetes cluster, see Access Cluster Services.

Requesting redirects

The redirect capabilities have been deprecated and removed. Please use a proxy (see below) instead.

So Many Proxies

There are several different proxies you may encounter when using Kubernetes:

  1. The kubectl proxy:

    • runs on a user’s desktop or in a pod
    • proxies from a localhost address to the Kubernetes apiserver
    • client to proxy uses HTTP
    • proxy to apiserver uses HTTPS
    • locates apiserver
    • adds authentication headers

  2. The apiserver proxy:

    • is a bastion built into the apiserver
    • connects a user outside of the cluster to cluster IPs which otherwise might not be reachable
    • runs in the apiserver processes
    • client to proxy uses HTTPS (or http if apiserver so configured)
    • proxy to target may use HTTP or HTTPS as chosen by proxy using available information
    • can be used to reach a Node, Pod, or Service
    • does load balancing when used to reach a Service

  3. The kube proxy:

    • runs on each node
    • proxies UDP and TCP
    • does not understand HTTP
    • provides load balancing
    • is only used to reach services

  4. A Proxy/Load-balancer in front of apiserver(s):

    • existence and implementation varies from cluster to cluster (e.g. nginx)
    • sits between all clients and one or more apiservers
    • acts as load balancer if there are several apiservers.

  5. Cloud Load Balancers on external services:

    • are provided by some cloud providers (e.g. AWS ELB, Google Cloud Load Balancer)
    • are created automatically when the Kubernetes service has type LoadBalancer
    • use UDP/TCP only
    • implementation varies by cloud provider.

Kubernetes users will typically not need to worry about anything other than the first two types. The cluster admin will typically ensure that the latter types are setup correctly.