Manage TLS Certificates in a Cluster
Kubernetes provides a certificates.k8s.io
API, which lets you provision TLS
certificates signed by a Certificate Authority (CA) that you control. These CA
and certificates can be used by your workloads to establish trust.
certificates.k8s.io
API uses a protocol that is similar to the ACME
draft.
Note: Certificates created using thecertificates.k8s.io
API are signed by a dedicated CA. It is possible to configure your cluster to use the cluster root CA for this purpose, but you should never rely on this. Do not assume that these certificates will validate against the cluster root CA.
- Before you begin
- Trusting TLS in a Cluster
- Requesting a Certificate
- Download and install CFSSL
- Create a Certificate Signing Request
- Create a Certificate Signing Request object to send to the Kubernetes API
- Get the Certificate Signing Request Approved
- Download the Certificate and Use It
- Approving Certificate Signing Requests
- A Word of Warning on the Approval Permission
- A Note to Cluster Administrators
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
.
Trusting TLS in a Cluster
Trusting the custom CA from an application running as a pod usually requires
some extra application configuration. You will need to add the CA certificate
bundle to the list of CA certificates that the TLS client or server trusts. For
example, you would do this with a golang TLS config by parsing the certificate
chain and adding the parsed certificates to the RootCAs
field in the
tls.Config
struct.
You can distribute the CA certificate as a ConfigMap that your pods have access to use.
Requesting a Certificate
The following section demonstrates how to create a TLS certificate for a Kubernetes service accessed through DNS.
Note: This tutorial uses CFSSL: Cloudflare’s PKI and TLS toolkit click here to know more.
Download and install CFSSL
The cfssl tools used in this example can be downloaded at https://pkg.cfssl.org/.
Create a Certificate Signing Request
Generate a private key and certificate signing request (or CSR) by running the following command:
cat <<EOF | cfssl genkey - | cfssljson -bare server
{
"hosts": [
"my-svc.my-namespace.svc.cluster.local",
"my-pod.my-namespace.pod.cluster.local",
"192.0.2.24",
"10.0.34.2"
],
"CN": "my-pod.my-namespace.pod.cluster.local",
"key": {
"algo": "ecdsa",
"size": 256
}
}
EOF
Where 192.0.2.24
is the service’s cluster IP,
my-svc.my-namespace.svc.cluster.local
is the service’s DNS name,
10.0.34.2
is the pod’s IP and my-pod.my-namespace.pod.cluster.local
is the pod’s DNS name. You should see the following output:
2017/03/21 06:48:17 [INFO] generate received request
2017/03/21 06:48:17 [INFO] received CSR
2017/03/21 06:48:17 [INFO] generating key: ecdsa-256
2017/03/21 06:48:17 [INFO] encoded CSR
This command generates two files; it generates server.csr
containing the PEM
encoded pkcs#10 certification request,
and server-key.pem
containing the PEM encoded key to the certificate that
is still to be created.
Create a Certificate Signing Request object to send to the Kubernetes API
Generate a CSR yaml blob and send it to the apiserver by running the following command:
cat <<EOF | kubectl apply -f -
apiVersion: certificates.k8s.io/v1beta1
kind: CertificateSigningRequest
metadata:
name: my-svc.my-namespace
spec:
request: $(cat server.csr | base64 | tr -d '\n')
usages:
- digital signature
- key encipherment
- server auth
EOF
Notice that the server.csr
file created in step 1 is base64 encoded
and stashed in the .spec.request
field. We are also requesting a
certificate with the “digital signature”, “key encipherment”, and “server
auth” key usages. We support all key usages and extended key usages listed
here
so you can request client certificates and other certificates using this
same API.
The CSR should now be visible from the API in a Pending state. You can see it by running:
kubectl describe csr my-svc.my-namespace
Name: my-svc.my-namespace
Labels: <none>
Annotations: <none>
CreationTimestamp: Tue, 21 Mar 2017 07:03:51 -0700
Requesting User: yourname@example.com
Status: Pending
Subject:
Common Name: my-svc.my-namespace.svc.cluster.local
Serial Number:
Subject Alternative Names:
DNS Names: my-svc.my-namespace.svc.cluster.local
IP Addresses: 192.0.2.24
10.0.34.2
Events: <none>
Get the Certificate Signing Request Approved
Approving the certificate signing request is either done by an automated approval process or on a one off basis by a cluster administrator. More information on what this involves is covered below.
Download the Certificate and Use It
Once the CSR is signed and approved you should see the following:
kubectl get csr
NAME AGE REQUESTOR CONDITION
my-svc.my-namespace 10m yourname@example.com Approved,Issued
You can download the issued certificate and save it to a server.crt
file
by running the following:
kubectl get csr my-svc.my-namespace -o jsonpath='{.status.certificate}' \
| base64 --decode > server.crt
Now you can use server.crt
and server-key.pem
as the keypair to start
your HTTPS server.
Approving Certificate Signing Requests
A Kubernetes administrator (with appropriate permissions) can manually approve
(or deny) Certificate Signing Requests by using the kubectl certificate
approve
and kubectl certificate deny
commands. However if you intend
to make heavy usage of this API, you might consider writing an automated
certificates controller.
Whether a machine or a human using kubectl as above, the role of the approver is to verify that the CSR satisfies two requirements:
- The subject of the CSR controls the private key used to sign the CSR. This addresses the threat of a third party masquerading as an authorized subject. In the above example, this step would be to verify that the pod controls the private key used to generate the CSR.
- The subject of the CSR is authorized to act in the requested context. This addresses the threat of an undesired subject joining the cluster. In the above example, this step would be to verify that the pod is allowed to participate in the requested service.
If and only if these two requirements are met, the approver should approve the CSR and otherwise should deny the CSR.
A Word of Warning on the Approval Permission
The ability to approve CSRs decides who trusts who within your environment. The ability to approve CSRs should not be granted broadly or lightly. The requirements of the challenge noted in the previous section and the repercussions of issuing a specific certificate should be fully understood before granting this permission.
A Note to Cluster Administrators
This tutorial assumes that a signer is setup to serve the certificates API. The
Kubernetes controller manager provides a default implementation of a signer. To
enable it, pass the --cluster-signing-cert-file
and
--cluster-signing-key-file
parameters to the controller manager with paths to
your Certificate Authority’s keypair.
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