Azure Key Vault Provider for Secrets Store CSI Driver

In the previous post, I talked about akv2k8s. akv2k8s is a Kubernetes controller that synchronizes secrets and certificates from Key Vault. Besides synchronizing to a regular secret, it can also inject secrets into pods.

Instead of akv2k8s, you can also use the secrets store CSI driver with the Azure Key Vault provider. As a CSI driver, its main purpose is to mount secrets and certificates as storage volumes. Next to that, it can also create regular Kubernetes secrets that can be used with an ingress controller or mounted as environment variables. That might be required if the application was not designed to read the secret from the file system.

In the previous post, I used akv2k8s to grab a certificate from Key Vault, create a Kubernetes secret and use that secret with nginx ingress controller:

certificate in Key Vault ------akv2aks periodic sync -----> Kubernetes secret ------> nginx ingress controller

Let’s briefly look at how to do this with the secrets store CSI driver.

Installation

Follow the guide to install the Helm chart with Helm v3:

helm repo add csi-secrets-store-provider-azure https://raw.githubusercontent.com/Azure/secrets-store-csi-driver-provider-azure/master/charts
helm install csi-secrets-store-provider-azure/csi-secrets-store-provider-azure --generate-name

This will install the components in the current Kubernetes namespace.

Easy no?

Syncing the certificate

Following the same example as with akv2aks, we need to point at the certificate in Key Vault, set the right permissions, and bring the certificate down to Kubernetes.

You will first need to decide how to access Key Vault. You can use the managed identity of your AKS cluster or be more granular and use pod identity. If you have setup AKS with a managed identity, that is the simplest solution. You just need to grab the clientId of the managed identity like so:

az aks show -g <resource group> -n <aks cluster name> --query identityProfile.kubeletidentity.clientId -o tsv

Next, create a file with the content below and apply it to your cluster in a namespace of your choosing.

apiVersion: secrets-store.csi.x-k8s.io/v1alpha1
kind: SecretProviderClass
metadata:
  name: azure-gebakv
  namespace: YOUR NAMESPACE
spec:
  provider: azure
  secretObjects:
  - secretName: nginx-cert
    type: kubernetes.io/tls
    data:
    - objectName: nginx
      key: tls.key
    - objectName: nginx
      key: tls.crt
  parameters:
    useVMManagedIdentity: "true"
    userAssignedIdentityID: "CLIENTID YOU OBTAINED ABOVE" 
    keyvaultName: "gebakv"         
    objects:  |
      array:
        - |
          objectName: nginx
          objectType: secret        
    tenantId: "ID OF YOUR AZURE AD TENANT"

Compared to the akv2k8s controller, the above configuration is a bit more complex. In the parameters section, in the objects array, you specify the name of the certificate in Key Vault and its object type. Yes, you saw that correctly, the objectType actually has to be secret for this to work.

The other settings are self-explanatory: we use the managed identity, set its clientId and in keyvaultName we set the short name of our Key Vault.

The settings in the parameters section are actually sufficient to mount the secret/certificate in a pod. With the secretObjects section though, we can also ask for the creation of regular Kubernetes secrets. Here, we ask for a secret of type kubernetes.io/tls with name nginx-cert to be created. You need to explicitly set both the tls.key and the tls.crt value and correctly reference the objectName in the array.

The akv2k8s controller is simpler to use as you only need to point it to your certificate in Key Vault (and specify it’s a certificate, not a secret) and set a secret name. There is no need to set the different values in the secret.

Using the secret

The advantage of the secrets store CSI driver is that the secret is only mounted/created when an application requires it. That also means we have to instruct our application to mount the secret explicitly. You do that via a volume as the example below illustrates (part of a deployment):

spec:
      containers:
      - name: realtimeapp
        image: gbaeke/fluxapp:1.0.2
        volumeMounts:
          - mountPath: "/mnt/secrets-store"
            name: secrets-store-inline
            readOnly: true
        env:
        - name: REDISHOST
          value: "redis:6379"
        resources:
          requests:
            cpu: 25m
            memory: 50Mi
          limits:
            cpu: 150m
            memory: 150Mi
        ports:
        - containerPort: 8080
      volumes:
      - name: secrets-store-inline
        csi:
          driver: secrets-store.csi.k8s.io
          readOnly: true
          volumeAttributes:
            secretProviderClass: "azure-gebakv"

In the above YAML, the following happens:

  • in volumes: we create a volume called secrets-store-inline and use the csi driver to mount the secrets we specified in the SecretProviderClass we created earlier (azure-gebakv)
  • in volumeMounts: we mount the volume on /mnt/secrets-store

Because we used secretObjects in our SecretProviderClass, this mount is accompanied by the creation of a regular Kubernetes secret as well.

When you remove the deployment, the Kubernetes secret will be removed instead of lingering behind for all to see.

Of course, the pods in my deployment do not need the mounted volume. It was not immediately clear to me how to avoid the mount but still create the Kubernetes secret (not exactly the point of a CSI driver 😀). On the other hand, there is a way to have the secret created as part of ingress controller creation. That approach is more useful in this case because we want our ingress controller to use the certificate. More information can be found here. In short, it roughly works as follows:

  • instead of creating and mounting a volume in your application pod, a volume should be created and mounted on the ingress controller
  • to do so, you modify the deployment of your ingress controller (e.g. ingress-nginx) with extraVolumes: and extraVolumeMounts: sections; depending on the ingress controller you use, other settings might be required

Be aware that you need to enable auto rotation of secrets manually and that it is an alpha feature at this point (December 2020). The akv2k8s controller does that for you out of the box.

Conclusion

Both the akv2k8s controller and the Secrets Store CSI driver (for Azure) can be used to achieve the same objective: syncing secrets, keys and certificates from Key Vault to AKS. In my experience, the akv2k8s controller is easier to use. The big advantage of the Secrets Store CSI driver is that it is a broader solution (not just for AKS) and supports multiple secret stores. Next to Azure Key Vault, it also supports Hashicorp’s Vault for example. My recommendation: for Azure Key Vault and AKS, keep it simple and try akv2k8s first!

Certificates with Azure Key Vault and Nginx Ingress Controller

Let’s face it. If you deploy web applications and APIs of any sort, you need certificates. If you have been long enough in IT, there’s just no escape! In this article, we will take a look at getting a certificate from Azure Key Vault to Azure Kubernetes service. Next, we will use that certificate with Nginx Ingress Controller and check what happens when the certificate gets renewed.

If you are more into videos, check out the video below from my channel:

Video from https://youtube.com/geertbaeke

Prerequisites

What do you need to following along?

  • Azure subscription: see https://azure.microsoft.com/en-us/free/
  • Azure Key Vault: see the quickstart to create it with the Azure Portal
  • Azure Kubernetes Services (AKS): see the quickstart to deploy it via the portal
  • Azure CLI: see the installation options
  • Kubectl: the Kubernetes administration tool; check the installation instructions here; use a package manager such as brew of choco to easily install it
  • Helm: required to install Helm charts; use a package manager such as brew of choco to install it; use v3 and higher

When AKS is up and running and you have authenticated with the Azure CLI using az login, get the credentials to AKS with:

az aks get-credentials -n <clustername> -g <resourcegroup>

We can now proceed to install nginx ingress controller.

Installing nginx ingress controller

Use the Helm chart to install nginx. First add the repo:

helm repo add https://kubernetes.github.io/ingress-nginx
helm repo update

Now install the chart:

helm install my-release ingress-nginx/ingress-nginx

More information can be found here: https://kubernetes.github.io/ingress-nginx/deploy/. The Helm chart will result in an nginx pod on your cluster. It will use a Kubernetes service exposed via an Azure Public Load Balancer. Later, we will publish an application on our cluster via this endpoint. We will do that by creating a resource of kind Ingress.

The procedure below works equally well with an ingress controller on an internal IP address and potentially, internal DNS names and certificates. We just happen to use an external IP address and a self-signed certificate here.

Installing the akv2k8s controller

To sync a Key Vault certificate to Kubernetes, we need some extra software. You will often come across the secrets store CSI driver, which has a provider for Azure Key Vault. Although this works well and is probably the way forward in the future, I often use another solution that is just a bit easier to use: the Azure Key Vault to Kubernetes controller. Check out the documentation over at https://akv2k8s.io.

The controller can be configured to sync a certificate in Azure Key Vault to a secret of type kubernetes.io/tls. Normally, you would create such a secret with the following command:

kubectl create secret tls my-tls-secret --cert=path/to/cert/file --key=path/to/key/file

Indeed, you would need the certificate and private key files to create such a secret. The akv2k8s controller does that work for you, grabbing the certificate and private key from Key Vault. Do note that what we are doing here is creating a regular Kubernetes secret. Such a secret contains the certificate and key in base64 encoded format. Anyone with the proper access rights on your cluster can easily decode the secret and use it as they please. Check out the following document about the risks of regular secrets in Kubernetes.

To install the controller, see https://akv2k8s.io/installation/installing-with-helm.

Creating the certificate in Key Vault

There are many ways to generate certificates and store them in Key Vault. In general, you should automate as much as possible especially when it comes to renewing the certificate. However, this post focuses on getting a certificate to Kubernetes. That is the reason why we will generate a self-signed certificate in Key Vault.

In your Key Vault, navigate to Certificates and click Generate/Import:

Certificates in Key Vault

In Create a certificate, fill in the blanks. If you want to use a real domain, make sure you specify it in the DNS Names. I used test.baeke.info with a validity of 12 months. The content type can either be PKCS #12 or PEM. The akv2k8s controller can handle both formats.

New self-signed certificate

After clicking Create and refreshing the list a few times, you should see the certificate listed:

mycert lis in the list

Note: in what follows, I will use the nginx certificate in the list; it was created in the same way although it is valid for 24 months

Access Policy

The akv2k8s controller needs access to your Key Vault to retrieve the certificate. It used the service principal or managed identity of the cluster to do so. My cluster was setup with managed identity. You can retrieve the identity with the Azure CLI:

az aks show -n <clustername> -g <resourcegroup> | jq .identityProfile.kubeletidentity.objectId -r

jq is a tool to parse JSON content. We use it here to retrieve the objectId of the managed identity. Once you have the objectId, you can grant it the required access rights:

az keyvault set-policy --name <KeyVault> --object-id  <objectId> --certificate-permissions get

The above Azure CLI command gives the objectId of our managed identity access to retrieve certificates from the specified Key Vault. You can use the short name of the Key Vault in –name.

Syncing the certificate

With the controller installed and granted sufficient access rights, we can now instruct it to sync the certificate. We do so with the following YAML:

apiVersion: spv.no/v1
kind: AzureKeyVaultSecret
metadata:
  name: cert-sync
  namespace: certsync
spec:
  vault:
    name: gebakv
    object:
      name: nginx
      type: certificate
  output:
    secret:
      name: nginx-cert
      type: kubernetes.io/tls

Note that all the resources I deploy from now are in the certsync namespace. The above YAML is pretty clear: it syncs the nginx certificate in Key Vault to a Kubernetes secret called nginx-cert. The type of the secret is kubernetes.io/tls. After synchronization, it will appear in the namespace:

NAME                  TYPE                                  DATA   AGE
nginx-cert            kubernetes.io/tls                     2      19s

On my system, I have installed the krew view-cert plugin. The command kubectl view-cert in the namespace certsync results in the following output (it enumerates all certs as a JSON array but there is only one):

[
    {
        "SecretName": "nginx-cert",
        "Namespace": "certsync",
        "Version": 3,
        "SerialNumber": "15fd15ed11384d31a0a21f96f5e457c6",
        "Issuer": "CN=test.baeke.info",
        "Validity": {
            "NotBefore": "2020-12-05T14:09:53Z",
            "NotAfter": "2022-12-05T14:19:53Z"
        },
        "Subject": "CN=test.baeke.info",
        "IsCA": false
    }
]

When I check the serial number in Key Vault, it matches with the serial number above. The certificate is valid for two years.

Using the secret with nginx-ingress

In the certsync namespace, I installed a simple app that uses a service called realtime. We will expose that service on the Internet via the nginx ingress controller (version v0.41.2; image k8s.gcr.io/ingress-nginx/controller). We use the following Ingress definition:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: testingress
  namespace: certsync
  annotations:
    kubernetes.io/ingress.class: nginx
spec:
  tls:
  - hosts:
    - test.baeke.info
    secretName: nginx-cert
  rules:
  - host: test.baeke.info
    http:
      paths:
      - path: /
        backend:
          serviceName: realtime
          servicePort: 80

Important: my Kubernetes version is 1.18.8 so the above definition is still valid; for 1.19, check the docs

The above creates an ingress for test.baeke.info and requires tls with the certificate in the nginx-cert secret. After a while, you will see the address and ports the ingress uses. Use kubectl get ingress to check:

NAME          CLASS    HOSTS             ADDRESS       PORTS     AGE
testingress   <none>   test.baeke.info   20.73.37.74   80, 443   41s

At https://test.baeke.info, the following certificate is offered:

Self-signed certificate offered by nginx ingress for test.baeke.info

Note: you need to ensure the FQDN (test.baeke.info here) resolves to the IP of the ingress; on my cluster this is done automatically by external dns. Note that the certificate is valid for two years.

Renewing the certificate

While the renewal process can be configured to be automatic, we will configure a new certificate from Azure Key Vault. Just navigate to your certificate and click New Version:

Creating a new version of the certificate

In the screen that follows, you can adjust the settings of the new certificate. I changed the lifetime back to 12 months. When you save your changes, the akv2k8s controller will pick up the change and modify the certificate in the Kubernetes secret. It will not delete and create a new secret. With kubectl view-cert, I now get the following output:

[
    {
        "SecretName": "nginx-cert",
        "Namespace": "certsync",
        "Version": 3,
        "SerialNumber": "27f95965e2644e0a58a878bc8a86f7d",
        "Issuer": "CN=test.baeke.info",
        "Validity": {
            "NotBefore": "2020-12-07T09:05:27Z",
            "NotAfter": "2021-12-07T09:15:27Z"
        },
        "Subject": "CN=test.baeke.info",
        "IsCA": false
    }
]

The serial number has changed. You can also see that the validity period has changed to 12 months.

What about our ingress?

Nginx ingress controller is smart enough to detect the changed certificate and offer it to clients. I used SHIFT-F5 to refresh the page and ingore cached content. Here is the offered certificate:

New certificate with 12 month lifetime

Conclusion

When you work with certificates in Kubernetes, always automate as much as possible. You can do that with a solution such as cert-manager that can request certificates dynamically (e.g. from Let’s Encrypt). In many other cases though, there are other certificate management practices in place that might prevent you from using a tool like cert-manager. In that case, try to get the certificates into a system like Key Vault and create your automation from there.