Authentication

Welcome to this lesson on authenticating access to a Kubernetes cluster. A cluster consists of multiple physical or virtual nodes and several internal components working together to run your workloads. It’s crucial to secure management access by verifying identities for anyone or anything interacting with the API server.

Actors interacting with the cluster:

Kubernetes relies on external identity sources (files, certificates, identity services like LDAP or OIDC) for human user authentication, while it internally manages service accounts. All requests pass through the kube-apiserver, which authenticates before authorizing.

Supported authentication methods:

1. Static Password File

The simplest approach uses a CSV file with one line per user:

password123,user1,u0001,group1
password123,user2,u0002,group1
password123,user3,u0003,group2
password123,user4,u0004,group2
password123,user5,u0005,group2

Configuring the API Server

Choose your setup:

1. Systemd unit (/etc/systemd/system/kube-apiserver.service):

   ExecStart=/usr/local/bin/kube-apiserver \
     --advertise-address=${INTERNAL_IP} \
     --allow-privileged=true \
     --authorization-mode=Node,RBAC \
     --bind-address=0.0.0.0 \
     --etcd-servers=https://127.0.0.1:2379 \
     --service-cluster-ip-range=10.32.0.0/24 \
     --service-node-port-range=30000-32767 \
     --runtime-config=api/all \
     --enable-swagger-ui=true \
     --event-ttl=1h \
     --v=2 \
     --basic-auth-file=/path/to/user-details.csv
   

2. kubeadm (edit /etc/kubernetes/manifests/kube-apiserver.yaml under spec.containers.command):

   apiVersion: v1
   kind: Pod
   metadata:
     name: kube-apiserver
     namespace: kube-system
   spec:
     containers:
     - name: kube-apiserver
       image: k8s.gcr.io/kube-apiserver-amd64:v1.11.3
       command:
       - kube-apiserver
       - --authorization-mode=Node,RBAC
       - --allow-privileged=true
       - --advertise-address=172.17.0.107
       - --etcd-servers=https://127.0.0.1:2379
       - --service-cluster-ip-range=10.32.0.0/24
       - --service-node-port-range=30000-32767
       - --runtime-config=api/all
       - --enable-bootstrap-token-auth=true
       - --enable-swagger-ui=true
       - --event-ttl=1h
       - --v=2
       - --basic-auth-file=/path/to/user-details.csv
   

After saving changes, the API server will restart automatically (kubeadm) or after reloading your systemd unit.

Testing Password Authentication

curl -u user1:password123 https://<api-server-ip>:6443/api/v1/nodes

2. Static Token File

Bearer tokens offer another static method. Create a CSV containing tokens:

KpjCVbI7cCEAHYPkByTizRb7gulcUc4B,user0,u0010,group1
rJjncHmvtXHc6MlWQddhtvNyyhgTdxSC,user1,u0011,group1
mjoOFTEiFOKL9toikaRNtt59ePtczZSq,user2,u0012,group2
PG41IXhs7QjqWkmBkvG9gICloYuQzI,user3,u0013,group2

Add to your API server flags:

--token-auth-file=/path/to/user-token-details.csv

Testing Token Authentication

curl -H "Authorization: Bearer KpjCVbI7cCEAHYPkByTizRb7gulcUc4B" \
     https://<api-server-ip>:6443/api/v1/namespaces

Security Considerations

• For kubeadm clusters, mount your credential files into the API server Pod via a volume.
• Protect files with restrictive filesystem permissions (chmod 600).
• After authenticating users, configure Role-Based Access Control (RBAC) to grant least-privilege permissions.

Next, we’ll explore certificate-based authentication and how Kubernetes components use TLS certificates to secure communication.