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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:
ActorDescriptionExamples
AdministratorsManage infrastructure and policiesCluster operators, DevOps engineers
DevelopersDeploy and maintain applicationsCI/CD engineers, application owners
Robotic ClientsAutomated systems accessing the API serverMonitoring tools, pipelines
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:
MethodDescription
Static Password FileCSV listing username, password, UID, [optional groups]
Static Token FileCSV listing bearer token, username, UID, [optional groups]
Client CertificatesX.509 certificates for users
External Identity ServiceLDAP, OIDC, webhook token authentication
The image illustrates authentication mechanisms for "kube-apiserver," including static password files, static token files, certificates, and identity services.

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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

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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.

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Testing Password Authentication

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

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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

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Testing Token Authentication

curl -H "Authorization: Bearer KpjCVbI7cCEAHYPkByTizRb7gulcUc4B" \
     https://<api-server-ip>:6443/api/v1/namespaces
Storing usernames, passwords, or tokens in plain text is not recommended for production. Use secure vaults or external identity providers for sensitive environments.
The image contains a note with three bullet points about authentication mechanisms, volume mounting in kubeadm setup, and setting up role-based authorization for new users.

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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.

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