Testing Kubernetes Certificate Expiration

Due to work, I needed to verify the procedure for renewing certificates on an expired Kubernetes cluster. Since I hadn’t done this before, I’m recording the process here.

Modify and Build kubeadm Source

Install go and git on the build environment, and add go’s binary path to PATH.
Download the Kubernetes source code. This time, I used v1.18.18.
git clone -b v1.18.18 https://github.com/kubernetes/kubernetes

Modify the following files to set the certificate validity to 10 minutes:

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diff --git a/cmd/kubeadm/app/constants/constants.go b/cmd/kubeadm/app/constants/constants.go
index b56891ca908..eed934280e7 100644
--- a/cmd/kubeadm/app/constants/constants.go
+++ b/cmd/kubeadm/app/constants/constants.go
@@ -46,7 +46,8 @@ const (
TempDirForKubeadm = "tmp"

// CertificateValidity defines the validity for all the signed certificates generated by kubeadm
- CertificateValidity = time.Hour * 24 * 365
+ //CertificateValidity = time.Hour * 24 * 365
+ CertificateValidity = time.Second * 600

// CACertAndKeyBaseName defines certificate authority base name
CACertAndKeyBaseName = "ca"
diff --git a/staging/src/k8s.io/client-go/util/cert/cert.go b/staging/src/k8s.io/client-go/util/cert/cert.go
index 9fd097af5e3..64e1dd90f43 100644
--- a/staging/src/k8s.io/client-go/util/cert/cert.go
+++ b/staging/src/k8s.io/client-go/util/cert/cert.go
@@ -35,7 +35,8 @@ import (
"k8s.io/client-go/util/keyutil"
)

-const duration365d = time.Hour * 24 * 365
+//const duration365d = time.Hour * 24 * 365
+const duration365d = time.Second * 600

// Config contains the basic fields required for creating a certificate
type Config struct {
@@ -93,7 +94,8 @@ func GenerateSelfSignedCertKey(host string, alternateIPs []net.IP, alternateDNS
// Certs/keys not existing in that directory are created.
func GenerateSelfSignedCertKeyWithFixtures(host string, alternateIPs []net.IP, alternateDNS []string, fixtureDirectory string) ([]byte, []byte, error) {
validFrom := time.Now().Add(-time.Hour) // valid an hour earlier to avoid flakes due to clock skew
- maxAge := time.Hour * 24 * 365 // one year self-signed certs
+ //maxAge := time.Hour * 24 * 365 // one year self-signed certs
+ maxAge := time.Second * 600 // one year self-signed certs

baseName := fmt.Sprintf("%s_%s_%s", host, strings.Join(ipsToStrings(alternateIPs), "-"), strings.Join(alternateDNS, "-"))
certFixturePath := path.Join(fixtureDirectory, baseName+".crt")

Build only kubeadm with the following command:
make WHAT=cmd/kubeadm GOFLAGS=-v
After the build completes, an _output folder will be created, and the kubeadm binary will be in the bin subdirectory. If you use this binary to deploy a cluster, the certificate validity will be 10 minutes.

Checking & Renewing Certificates

Build a 3-node cluster and confirm the certificates have about 4 minutes left:

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root@wenhan-adm-cp:~# ./kubeadm alpha certs check-expiration
[check-expiration] Reading configuration from the cluster...
[check-expiration] FYI: You can look at this config file with 'kubectl -n kube-system get cm kubeadm-config -oyaml'

CERTIFICATE                EXPIRES                  RESIDUAL TIME   CERTIFICATE AUTHORITY   EXTERNALLY MANAGED
admin.conf                 Jun 24, 2021 06:01 UTC   4m                                      no
apiserver                  Jun 24, 2021 06:01 UTC   4m              ca                      no
apiserver-etcd-client      Jun 24, 2021 06:01 UTC   4m              etcd-ca                 no
apiserver-kubelet-client   Jun 24, 2021 06:01 UTC   4m              ca                      no
controller-manager.conf    Jun 24, 2021 06:01 UTC   4m                                      no
etcd-healthcheck-client    Jun 24, 2021 06:01 UTC   4m              etcd-ca                 no
etcd-peer                  Jun 24, 2021 06:01 UTC   4m              etcd-ca                 no
etcd-server                Jun 24, 2021 06:01 UTC   4m              etcd-ca                 no
front-proxy-client         Jun 24, 2021 06:01 UTC   4m              front-proxy-ca          no
scheduler.conf             Jun 24, 2021 06:01 UTC   4m                                      no

CERTIFICATE AUTHORITY   EXPIRES                  RESIDUAL TIME   EXTERNALLY MANAGED
ca                      Jun 24, 2021 07:31 UTC   1h              no
etcd-ca                 Jun 24, 2021 07:31 UTC   1h              no
front-proxy-ca          Jun 24, 2021 07:31 UTC   1h              no

root@wenhan-adm-cp:~# kubectl get node
NAME             STATUS   ROLES    AGE     VERSION
wenhan-adm-cp    Ready    master   4m35s   v1.18.18
wenhan-adm-wk1   Ready    <none>   3m32s   v1.18.18
wenhan-adm-wk2   Ready    <none>   3m28s   v1.18.18

After expiration, kubectl get node fails:

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root@wenhan-adm-cp:~# ./kubeadm alpha certs check-expiration
[check-expiration] Reading configuration from the cluster...
[check-expiration] FYI: You can look at this config file with 'kubectl -n kube-system get cm kubeadm-config -oyaml'
[check-expiration] Error reading configuration from the Cluster. Falling back to default configuration

W0624 06:01:22.231290   13746 configset.go:202] WARNING: kubeadm cannot validate component configs for API groups [kubelet.config.k8s.io kubeproxy.config.k8s.io]
CERTIFICATE                EXPIRES                  RESIDUAL TIME   CERTIFICATE AUTHORITY   EXTERNALLY MANAGED
admin.conf                 Jun 24, 2021 06:01 UTC   <invalid>                               no
apiserver                  Jun 24, 2021 06:01 UTC   <invalid>       ca                      no
apiserver-etcd-client      Jun 24, 2021 06:01 UTC   <invalid>       etcd-ca                 no
apiserver-kubelet-client   Jun 24, 2021 06:01 UTC   <invalid>       ca                      no
controller-manager.conf    Jun 24, 2021 06:01 UTC   <invalid>                               no
etcd-healthcheck-client    Jun 24, 2021 06:01 UTC   <invalid>       etcd-ca                 no
etcd-peer                  Jun 24, 2021 06:01 UTC   <invalid>       etcd-ca                 no
etcd-server                Jun 24, 2021 06:01 UTC   <invalid>       etcd-ca                 no
front-proxy-client         Jun 24, 2021 06:01 UTC   <invalid>       front-proxy-ca          no
scheduler.conf             Jun 24, 2021 06:01 UTC   <invalid>                               no

CERTIFICATE AUTHORITY   EXPIRES                  RESIDUAL TIME   EXTERNALLY MANAGED
ca                      Jun 24, 2021 07:31 UTC   1h              no
etcd-ca                 Jun 24, 2021 07:31 UTC   1h              no
front-proxy-ca          Jun 24, 2021 07:31 UTC   1h              no

root@wenhan-adm-cp:~# kubectl get node
Unable to connect to the server: x509: certificate has expired or is not yet valid

Renew the certificates and confirm the new expiration:

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root@wenhan-adm-cp:~# ./kubeadm alpha certs renew all --config=kubeadm.yaml
W0624 06:04:00.447612   2860 configset.go:202] WARNING: kubeadm cannot validate component configs for API groups [kubelet.config.k8s.io kubeproxy.config.k8s.io]
certificate embedded in the kubeconfig file for the admin to use and for kubeadm itself renewed
certificate for serving the Kubernetes API renewed
certificate the apiserver uses to access etcd renewed
certificate for the API server to connect to kubelet renewed
certificate embedded in the kubeconfig file for the controller manager to use renewed
certificate for liveness probes to healthcheck etcd renewed
certificate for etcd nodes to communicate with each other renewed
certificate for serving etcd renewed
certificate for the front proxy client renewed
certificate embedded in the kubeconfig file for the scheduler manager to use renewed

After renewal, the expiration of each certificate is updated:

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root@wenhan-adm-cp:~# ./kubeadm alpha certs check-expiration
[check-expiration] Reading configuration from the cluster...
[check-expiration] FYI: You can look at this config file with 'kubectl -n kube-system get cm kubeadm-config -oyaml'
[check-expiration] Error reading configuration from the Cluster. Falling back to default configuration

W0624 06:04:21.176808   3230 configset.go:202] WARNING: kubeadm cannot validate component configs for API groups [kubelet.config.k8s.io kubeproxy.config.k8s.io]
CERTIFICATE         EXPIRES          RESIDUAL TIME  CERTIFICATE AUTHORITY  EXTERNALLY MANAGED
admin.conf         Jun 24, 2021 06:14 UTC  9m                    no
apiserver          Jun 24, 2021 06:14 UTC  9m        ca            no
apiserver-etcd-client    Jun 24, 2021 06:14 UTC  9m        etcd-ca         no
apiserver-kubelet-client  Jun 24, 2021 06:14 UTC  9m        ca            no
controller-manager.conf   Jun 24, 2021 06:14 UTC  9m                    no
etcd-healthcheck-client   Jun 24, 2021 06:14 UTC  9m        etcd-ca         no
etcd-peer          Jun 24, 2021 06:14 UTC  9m        etcd-ca         no
etcd-server         Jun 24, 2021 06:14 UTC  9m        etcd-ca         no
front-proxy-client     Jun 24, 2021 06:14 UTC  9m        front-proxy-ca      no
scheduler.conf       Jun 24, 2021 06:14 UTC  9m                    no

CERTIFICATE AUTHORITY  EXPIRES          RESIDUAL TIME  EXTERNALLY MANAGED
ca            Jun 24, 2021 07:31 UTC  1h        no
etcd-ca         Jun 24, 2021 07:31 UTC  1h        no
front-proxy-ca      Jun 24, 2021 07:31 UTC  1h        no

After renewing the certificates, restart the control plane:

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root@wenhan-adm-cp:~# reboot

This completes the certificate renewal.

After rebooting, you can access the cluster with the new authentication files:

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root@wenhan-adm-cp:~# cp -i /etc/kubernetes/admin.conf $HOME/.kube/config
cp: overwrite '/root/.kube/config'? yes

root@wenhan-adm-cp:~# kubectl get node
NAME       STATUS  ROLES   AGE  VERSION
wenhan-adm-cp   Ready   master  14m  v1.18.18
wenhan-adm-wk1  Ready   <none>  13m  v1.18.18
wenhan-adm-wk2  Ready   <none>  13m  v1.18.18

Modify and Build kubeadm Source#

Checking & Renewing Certificates#

Modify and Build kubeadm Source

Checking & Renewing Certificates