smallstep-certificates/autocert

Autocert

Autocert is a kubernetes add-on that automatically injects TLS/HTTPS certificates into your containers.

To request a certificate simply annotate your pods with a name. Certificates are issued by a private internal certificate authority that runs on your cluster and are mounted at /var/run/autocert.step.sm along with a corresponding private key and root certificate.

TLS (e.g., HTTPS) is the most widely deployed cryptographic protocol in the world. Mutual TLS (mTLS) provides end-to-end security for service-to-service communication and can replace complex VPNs to secure communication into, out of, and between kubernetes clusters. But to use mTLS you need certificates issued by your own certificate authority (CA).

Building and operating a CA, issuing certificates, and making sure they're renewed before they expire is tricky. Autocert does all of this for you.

Key Features

• A complete public key infrastructure that you control for your kubernetes clusters
• Certificate authority that's easy to initialize and install
• Automatic injection of certificates and keys in annotated containers
• Enable on a per-namespace basis
• Namespaced installation to restrict access to privileged CA and provisioner containers
• Ability to run subordinate to an existing public key infrastructure
• Supports federatation with other roots
• Short-lived certificates
• Automatic renewal
• Uses your own certificate authority -- you control who or what gets a certificate

Getting Started

These instructions will get autocert installed quickly on an existing kubernetes cluster.

Prerequisites

Make sure you've installed step version 0.8.3 or later:

$ step version
Smallstep CLI/0.8.3 (darwin/amd64)
Release Date: 2019-01-16 01:46 UTC

You'll also need kubectl and a kubernetes cluster running version 1.9 or later with webhook admission controllers enabled:

$ kubectl version --short
Client Version: v1.13.1
Server Version: v1.10.11
$ kubectl api-versions | grep "admissionregistration.k8s.io/v1beta1"
admissionregistration.k8s.io/v1beta1

We'll be creating a new kubernetes namespace and setting up some RBAC rules during installation. You'll need appropriate permissions in your cluster (e.g., you may need to be cluster-admin).

TODO: Check whether you have cluster permissions..? GKE instructions here if you don't have them.

In order to grant these permissions you may need to give yourself cluster-admin rights in your cluster. GKE, in particular, does not give the cluster owner these rights by default. You can give yourself cluster-admin rights by running:

kubectl create clusterrolebinding cluster-admin-binding \
    --clusterrole cluster-admin \
    --user $(gcloud config get-value account)

Install

To install step certificates and autocert in one step run:

$ kubectl run autocert-init -it --rm --image smallstep/autocert-init --restart Never

You may need to adjust the RBAC policies to run autocert-init:

$ kubectl create clusterrolebinding autocert-init-binding --clusterrole cluster-admin --user "system:serviceaccount:default:default"

Once autocert-init is complete you can delete this binding:

$ kubectl delete clusterrolebinding autocert-init-binding

You can also install manually.

Enable autocert

To enable autocert for a namespace the autocert.step.sm=enabled label (the autocert webhook will not affect namespaces for which it is not enabled). To enable autocert for the default namespace run:

$ kubectl label namespace default autocert.step.sm=enabled

To check which namespaces have autocert enabled run:

$ kubectl get namespace -L autocert.step.sm
NAME          STATUS   AGE   AUTOCERT.STEP.SM
default       Active   59m   enabled
...

Annotate pods

In addition to enabling autocert for a namespace, pods must be annotated with their name for certificates to be injected. The annotated name will appear as the common name and SAN in the issued certificate.

To trigger certificate injection pods must be annotated at creation time. You can do this in your deployment YAMLs:

$ cat <<EOF | kubectl apply -f -
apiVersion: apps/v1
kind: Deployment
metadata: {name: sleep}
spec:
  replicas: 1
  selector: {matchLabels: {app: sleep}}
  template:
    metadata:
      annotations:
        autocert.step.sm/name: sleep.default.svc.cluster.local
      labels: {app: sleep}
    spec:
      containers:
      - name: sleep
        image: alpine
        command: ["/bin/sleep", "86400"]
        imagePullPolicy: IfNotPresent
EOF

The autocert admission webhook will intercept this pod creation request and inject an init container and sidecar to manage certificate issuance and renewal, respectively.

$ kubectl get pods -l app=sleep \
    -o=custom-columns=NAME:.metadata.name,CONTAINERS:.spec.containers[*].name,INIT_CONTAINERS:.spec.initContainers[*].name
NAME                    CONTAINERS               INIT_CONTAINERS
sleep-f996bd578-tzwvm   sleep,autocert-renewer   autocert-bootstrapper

Certificates are mounted in containers at /var/run/autocert.step.sm. We can inspect this directory to make sure everything worked correctly:

$ kubectl exec -it sleep-f996bd578-nch7c -c sleep -- ls -lias /var/run/autocert.step.sm
total 20
1593393      4 drwxrwxrwx    2 root     root          4096 Jan 17 21:27 .
1339651      4 drwxr-xr-x    1 root     root          4096 Jan 17 21:27 ..
1593451      4 -rw-------    1 root     root           574 Jan 17 21:27 root.crt
1593442      4 -rw-r--r--    1 root     root          1352 Jan 17 21:41 site.crt
1593443      4 -rw-r--r--    1 root     root           227 Jan 17 21:27 site.key

The autocert-renewer sidecar also installs the step CLI tool, which we can use to inspect the issued certificate:

$ kubectl exec -it sleep-f996bd578-nch7c -c autocert-renewer -- step certificate inspect /var/run/autocert.step.sm/site.crt
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 46935033335539540860078000614852612373 (0x234f5bce23705f015a8377ab1cfd5115)
    Signature Algorithm: ECDSA-SHA256
        Issuer: CN=Autocert Intermediate CA
        Validity
            Not Before: Jan 17 21:41:04 2019 UTC
            Not After : Jan 17 21:46:14 2019 UTC
        Subject: CN=sleep.default.svc.cluster.local
        Subject Public Key Info:
            Public Key Algorithm: ECDSA
                Public-Key: (256 bit)
                X:
                    31:aa:a1:7f:c8:b4:c6:da:90:fc:b8:3a:e9:cc:48:
                    f9:89:b9:5d:d7:a4:63:80:76:9f:21:6d:e5:88:4c:
                    a8:e4
                Y:
                    ed:21:38:57:cd:3f:32:71:6f:ca:81:34:b0:4a:bd:
                    a3:c4:8d:d1:87:bc:2c:4c:42:79:e5:35:49:38:3f:
                    b7:c8
                Curve: P-256
        X509v3 extensions:
            X509v3 Key Usage: critical
                Digital Signature, Key Encipherment
            X509v3 Extended Key Usage:
                TLS Web Server Authentication, TLS Web Client Authentication
            X509v3 Subject Key Identifier:
                43:0E:0A:50:30:A5:5B:AF:22:AC:28:49:26:53:2A:B4:D4:20:E0:E0
            X509v3 Authority Key Identifier:
                keyid:61:45:1E:E4:95:4C:0A:6B:37:4C:43:41:FD:54:2E:8E:5E:A2:24:EF
            X509v3 Subject Alternative Name:
                DNS:sleep.default.svc.cluster.local

    Signature Algorithm: ECDSA-SHA256
         30:44:02:20:0c:c5:ab:0d:22:17:a2:04:9f:ff:5f:b1:c0:a5:
         8b:94:88:e0:40:66:e1:19:e9:34:2f:67:74:12:4f:bb:51:8b:
         02:20:01:7e:0d:44:ce:b2:92:41:d5:78:0d:02:5a:68:05:7c:
         c2:a9:81:28:71:5c:95:6d:56:51:49:e0:37:b7:09:87

Test your installation

To test your installation you can install the hello-mtls demo app.

• Install app, which uses mTLS and responds "hello, identity"
• Do a kubectl run of step-cli then get a certificate using step and curl hello-mtls from within the cluster
• Port forward from localhost to get a certificate then curl with --resolve

Further reading

• Link to ExternalDNS example
• Link to multiple cluster with Service type ExternalDNS so they can communicate

Uninstall

• Delete the sleep deployment (if you created it)
• Remove labels (show how to find labelled namespaces)
• Remove annotations (show how to find any annotated pods)
• Remove secrets (show how to find labelled secrets)
• Delete step namespace

Questions

How is this different than cert-manager

Doesn't kubernetes already ship with a certificate authority?

Yes, but it's designed for use by the kubernetes control plane rather than by your data plane services. You could use the kubernetes CA to issue certificates for data plane communication, but it's probably not a good idea.

Why not use kubernetes CSR resources for this?

It's harder and less secure.

What are autocert certificates good for?

Autocert certificates let you secure your data plane (service-to-service) communication using mutual TLS (mTLS). Services and proxies can limit access to clients that also have a certificate issued by your certificate authority (CA). Servers can identify which client is connecting improving visibility and enabling granular access control.

Once certificates are issued you can use mTLS to secure communication in to, out of, and between kubernetes clusters. Services can use mTLS to only allow connections from clients that have their own certificate issued from your CA.

It's like your own Let's Encrypt, but you control who gets a certificate.