Certificates

Introduction

Certificates play a critical role in Kubernetes security by enabling secure communication between various components of a Kubernetes cluster. They are primarily used for authenticating system components and services, ensuring data confidentiality, integrity, and authentication within the cluster.

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What Are Certificates in Kubernetes?

Certificates in Kubernetes are digital certificates used to secure communications between components, enable TLS encryption, and facilitate authentication. Certificates ensure the confidentiality, integrity, and authentication of data in transit within the Kubernetes cluster.

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Common Use Cases for Certificates in Kubernetes

1. Securing Kubernetes API Server Communication:
   • mTLS (Mutual TLS) between API server and Kubelets.
2. Intra-Cluster Communication:
   • Enable mTLS between pods and services.
3. Ingress TLS/SSL Certificates:
   • Secure external access through Ingress Controllers.
4. Authentication:
   • Generate X.509 certificates for user authentication.

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Types of Certificates in Kubernetes

1. Client Certificates

Required knowledge for the CKS certification.

Client certificates are essential for secure communication between Kubernetes components. They provide mutual authentication, enabling the API server and internal services to verify each other's identities.

Use Cases for Client Certificates

• System Component Authentication: Establish secure connections between API servers, kubelets, and other internal services.
• Service-to-Service Communication: Secure communication within the cluster for services that require API access.

Generating a Client Certificate

openssl req -new -newkey rsa:2048 -nodes -keyout user.key -out user.csr -subj "/CN=example-user"

openssl x509 -req -in user.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out user.crt -days 365

Key Differences Between Client and User Certificates

Certificate Type	Purpose	Example Use Case
Client Certificate	Component-to-component authentication	API Server to Kubelet communication
User Certificate	API access for external users	Developer access via kubectl

Cross-Reference

For a detailed guide on issuing a certificate for a Kubernetes user, refer to the Issuing a Certificate for a User.

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2. Server Certificates

Required knowledge for the CKS certification.

• Secures server endpoints with TLS encryption.
• Commonly used for the API server, Kubelets, and webhooks.

# Create a server certificate for the API server
openssl req -new -key apiserver.key -out apiserver.csr -subj "/CN=kube-apiserver"

# Sign with the Kubernetes Certificate Authority (CA)
openssl x509 -req -in apiserver.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out apiserver.crt -days 365

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3. TLS Certificates for Ingress Controllers

• Used to secure Ingress traffic using HTTPS.
• Automate with Cert-Manager and Let's Encrypt.

# Example: Issuing a TLS Certificate with Cert-Manager
apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: example-cert
  namespace: default
spec:
  secretName: example-cert-tls
  issuerRef:
    name: letsencrypt-issuer
    kind: ClusterIssuer
  dnsNames:
    - example.com
    - www.example.com

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4. Service Account Tokens vs. Certificates

Feature	Service Account Token	Certificate
Use Case	Pod authentication	User and server authentication
Renewal	Automatic	Requires manual or automated rotation
Security	Token-based JWT	Encrypted X.509 certificates

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Automating Certificate Management with Cert-Manager

Cert-Manager is a Kubernetes add-on that automates the creation, renewal, and management of certificates.

Installation

kubectl apply -f https://github.com/cert-manager/cert-manager/releases/download/v1.9.1/cert-manager.yaml

Create a Self-Signed Issuer

apiVersion: cert-manager.io/v1
kind: Issuer
metadata:
  name: selfsigned-issuer
  namespace: default
spec:
  selfSigned: {}

Requesting a Certificate

apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: selfsigned-cert
spec:
  secretName: selfsigned-cert-tls
  duration: 90d
  renewBefore: 30d
  commonName: example.com
  dnsNames:
    - example.com
  issuerRef:
    name: selfsigned-issuer
    kind: Issuer

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Best Practices for Managing Certificates

1. Automate Renewal:

   • Use Cert-Manager to avoid expired certificates.

2. Enforce TLS Everywhere:

   • Enable TLS for all internal and external communications.

3. Rotate Certificates Regularly:

   • Automate with cert-manager or kubeadm cert renew.

4. Monitor Certificate Expiry:

   • Set up alerts for expiring certificates using Prometheus or Grafana.

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Conclusion

Certificates are a core primitive in Kubernetes security, providing encryption and authentication for API server communications, Ingress traffic, and internal service interactions. Implementing certificates with tools like Cert-Manager ensures your cluster remains secure and compliant with best practices.

For more advanced scenarios, including mTLS setup between services or using Cert-Manager for dynamic certificate management, refer to the additional How-To Guides.