Access Clusters Using the Kubernetes API
This page shows how to access clusters using the Kubernetes API.
Before you begin
You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using Minikube, or you can use one of these Kubernetes playgrounds:
To check the version, enter kubectl version.
Accessing the cluster API
Accessing for the first time with kubectl
When accessing the Kubernetes API for the first time, use the
Kubernetes command-line tool, 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:
$ kubectl config view
Many of the examples provide an introduction to using kubectl. Complete documentation is found in the kubectl manual.
Directly accessing the REST API
kubectl handles locating and authenticating to the API server. If you want to directly access the REST API with an http client like
curl or wget, or a browser, there are multiple ways you can locate and authenticate against the API server:
- Run kubectl in proxy mode (recommended). This method is recommended, since it uses the stored apiserver location and verifies the identity of the API server using a self-signed cert. No man-in-the-middle (MITM) attack is possible using this method.
- Alternatively, you can provide the location and credentials directly to the http client. This works with client code that is confused by proxies. To protect against man in the middle attacks, you’ll need to import a root cert into your browser.
Using the Go or Python client libraries provides accessing kubectl in proxy mode.
Using kubectl proxy
The following command runs kubectl in a mode where it acts as a reverse proxy. It handles locating the API server and authenticating.
Run it like this:
$ kubectl proxy --port=8080 &
See kubectl proxy for more details.
Then you can explore the API with curl, wget, or a browser, like so:
$ curl http://localhost:8080/api/
{
"versions": [
"v1"
],
"serverAddressByClientCIDRs": [
{
"clientCIDR": "0.0.0.0/0",
"serverAddress": "10.0.1.149:443"
}
]
}
Without kubectl proxy
It is possible to avoid using kubectl proxy by passing an authentication token directly to the API server, like this:
$ APISERVER=$(kubectl config view | grep server | cut -f 2- -d ":" | tr -d " ")
$ TOKEN=$(kubectl describe secret $(kubectl get secrets | grep default | cut -f1 -d ' ') | grep -E '^token' | cut -f2 -d':' | tr -d '\t')
$ curl $APISERVER/api --header "Authorization: Bearer $TOKEN" --insecure
{
"kind": "APIVersions",
"versions": [
"v1"
],
"serverAddressByClientCIDRs": [
{
"clientCIDR": "0.0.0.0/0",
"serverAddress": "10.0.1.149:443"
}
]
}
The above example uses 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 API server does not require authentication; it may serve on localhost, or be protected by a firewall. There is not a standard for this. Configuring Access to the API describes how a cluster admin can configure this. Such approaches may conflict with future high-availability support.
Programmatic access to the API
Kubernetes officially supports client libraries for Go and Python.
Go client
- To get the library, run the following command:
go get k8s.io/client-go/<version number>/kubernetesSee 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/1.4/pkg/api/v1"is correct.
The Go client can use the same kubeconfig file as the kubectl CLI does to locate and authenticate to the API server. See this example:
import (
"fmt"
"k8s.io/client-go/1.4/kubernetes"
"k8s.io/client-go/1.4/pkg/api/v1"
"k8s.io/client-go/1.4/tools/clientcmd"
)
...
// uses the current context in kubeconfig
config, _ := clientcmd.BuildConfigFromFlags("", "path to kubeconfig")
// creates the clientset
clientset, _:= kubernetes.NewForConfig(config)
// access the API to list pods
pods, _:= clientset.CoreV1().Pods("").List(v1.ListOptions{})
fmt.Printf("There are %d pods in the cluster\n", len(pods.Items))
...
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 API server. See this example:
from kubernetes import client, config
config.load_kube_config()
v1=client.CoreV1Api()
print("Listing pods with their IPs:")
ret = v1.list_pod_for_all_namespaces(watch=False)
for i in ret.items:
print("%s\t%s\t%s" % (i.status.pod_ip, i.metadata.namespace, i.metadata.name))
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 API server are somewhat different.
The easiest way to use the Kubernetes API from a Pod is to use one of the official client libraries. These libraries can automatically discover the API server and authenticate.
While running in a Pod, the Kubernetes apiserver is accessible via a Service named
kubernetes in the default namespace. Therefore, Pods can use the
kubernetes.default hostname to query the API server. Official client libraries
do this automatically.
From within a Pod, the recommended way to authenticate to the API server is with a
service account credential. By default, 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 API server.
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 the Kubernetes API are:
-
Use one of the official client libraries as they handle API host discovery and authentication automatically. For Go client, the
rest.InClusterConfig()function assists with this. See an example here. -
If you would like to query the API without an official client library, you can run
kubectl proxyas the command of a new sidecar container in the Pod. This way,kubectl proxywill authenticate to the API and expose it on thelocalhostinterface of the Pod, so that other containers in the Pod can use it directly.
In each case, the service account credentials of the Pod are used to communicate securely with the API server.