certificates/autocert

Autocert

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

To get a certificate simply annotate your pods with a name. An X.509 (TLS/HTTPS) certificate is automatically created and mounted at /var/run/autocert.step.sm/ along with a corresponding private key and root certificate (everything you need for mTLS).

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Motivation

Autocert exists to make it easy to use mTLS (mutual TLS) to improve security within a cluster and to secure communication into, out of, and between kubernetes clusters.

TLS (and HTTPS, which is HTTP over TLS) provides authenticated encryption: an identity dialtone and end-to-end encryption for your workloads. It's like a secure line with caller ID. This has all sorts of benefits: better security, compliance, and easier auditability for starters. It makes workloads identity-aware, improving observability and enabling granular access control. Perhaps most compelling, mTLS lets you securely communicate with workloads running anywhere, not just inside kubernetes.

Unlike VPNs & SDNs, deploying and scaling mTLS is pretty easy. You're (hopefully) already using TLS, and your existing tools and standard libraries will provide most of what you need. If you know how to operate DNS and reverse proxies, you know how to operate mTLS infrastructure.

There's just one problem: 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.

Features

First and foremost, autocert is easy. You can get started in minutes.

Autocert uses step certificates to generate keys and issue certificates. This process is secure and automatic, all you have to do is install autocert and annotate your pods.

Features include:

• A fully featured private certificate authority (CA) for workloads running on kubernetes and elsewhere
• RFC5280 and CA/Browser Forum compliant certificates that work for TLS
• Namespaced installation into the step namespace so it's easy to lock down your CA
• Short-lived certificates with fully automated enrollment and renewal
• Private keys are never transmitted across the network and aren't stored in etcd

Because autocert is built on step certificates you can easily extend access to developers, endpoints, and workloads running outside your cluster, too.

Getting Started

⚠️ Warning: this project is in ALPHA. DON'T use it for anything mission critical. EXPECT breaking changes in minor revisions with little or no warning. PLEASE provide feedback.

Prerequisites

All you need to get started is kubectl and a cluster running kubernetes 1.9 or later with admission webhooks 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

Install

To install autocert run:

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

💥 installation complete.

You might want to check out what this command does before running it. You can also install autocert manually if that's your style.

Usage

Using autocert is also easy:

• Enable autocert for a namespace by labelling it with autocert.step.sm=enabled, then
• Inject certificates into containers by annotating pods with autocert.step.sm/name: <name>

Enable autocert (per namespace)

To enable autocert for a namespace it must be labelled autocert.step.sm=enabled.

To label 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 to get certificates

To get a certificate you need to tell autocert your workload's name using the autocert.step.sm/name annotation (this name will appear as the X.509 common name and SAN).

Let's deploy a simple mTLS server named hello-mtls.default.svc.cluster.local:

cat <<EOF | kubectl apply -f - 
apiVersion: apps/v1
kind: Deployment
metadata: {name: hello-mtls, labels: {app: hello-mtls}}
spec:
  replicas: 1
  selector: {matchLabels: {app: hello-mtls}}
  template:
    metadata:
      annotations:
        # AUTOCERT ANNOTATION HERE -v ###############################
        autocert.step.sm/name: hello-mtls.default.svc.cluster.local #
        # AUTOCERT ANNOTATION HERE -^ ###############################
      labels: {app: hello-mtls}
    spec:
      containers:
      - name: hello-mtls
        image: smallstep/hello-mtls-server-go:latest
EOF

In our new container we should find a certificate, private key, and root certificate mounted at /var/run/autocert.step.sm:

$ export HELLO_MTLS=$(kubectl get pods -l app=hello-mtls -o jsonpath={$.items[0].metadata.name})
$ kubectl exec -it $HELLO_MTLS -c hello-mtls -- ls /var/run/autocert.step.sm
root.crt  site.crt  site.key

We're done. Our container has a certificate, issued by our CA, which autocert will automatically renew.

✅ Certificates.

Hello mTLS

It's easy to deploy certificates using autocert, but it's up to you to use them correctly. To get you started, hello-mtls demonstrates the right way to use mTLS with various tools and languages (contributions welcome :). If you're a bit fuzzy on how mTLS works, the hello-mtls README is a great place to start.

To finish out this tutorial let's keep things simple and try curling the server we just deployed from inside and outside the cluster.

Connecting from inside the cluster

First, let's expose our workload to the rest of the cluster using a service:

kubectl expose deployment hello-mtls --port 443

Now let's deploy a client, with its own certificate, that curls our server in a loop:

cat <<EOF | kubectl apply -f -
apiVersion: apps/v1
kind: Deployment
metadata: {name: hello-mtls-client, labels: {app: hello-mtls-client}}
spec:
  replicas: 1
  selector: {matchLabels: {app: hello-mtls-client}}
  template:
    metadata:
      annotations:
        # AUTOCERT ANNOTATION HERE -v ######################################
        autocert.step.sm/name: hello-mtls-client.default.pod.cluster.local #
        # AUTOCERT ANNOTATION HERE -^ ######################################
      labels: {app: hello-mtls-client}
    spec:
      containers:
      - name: hello-mtls-client
        image: smallstep/hello-mtls-client-curl:latest
        env: [{name: HELLO_MTLS_URL, value: https://hello-mtls.default.svc.cluster.local}]
EOF

Note that the authority portion of the URL (the HELLO_MTLS_URL env var) matches the name of the server we're connecting to (both are hello-mtls.default.svc.cluster.local). That's required for standard HTTPS and can sometimes require some DNS trickery.

Once deployed we should start seeing the client log responses from the server saying hello:

$ export HELLO_MTLS_CLIENT=$(kubectl get pods -l app=hello-mtls-client -o jsonpath={$.items[0].metadata.name})
$ kubectl logs $HELLO_MTLS_CLIENT -c hello-mtls-client
Thu Feb  7 23:35:23 UTC 2019: Hello, hello-mtls-client.default.pod.cluster.local!
Thu Feb  7 23:35:28 UTC 2019: Hello, hello-mtls-client.default.pod.cluster.local!

For kicks, let's exec into this pod and try curling ourselves:

$ kubectl exec $HELLO_MTLS_CLIENT -c hello-mtls-client -- curl -sS \
       --cacert /var/run/autocert.step.sm/root.crt \
       --cert /var/run/autocert.step.sm/site.crt \
       --key /var/run/autocert.step.sm/site.key \
       https://hello-mtls.default.svc.cluster.local
Hello, hello-mtls-client.default.pod.cluster.local!

✅ mTLS inside cluster.

Connecting from outside the cluster

Connecting from outside the cluster is a bit more complicated. We need to handle DNS and obtain a certificate ourselves. These tasks were handled automatically inside the cluster by kubernetes and autocert, respectively.

That said, because our server uses mTLS only clients that have a certificate issued by our certificate authority will be allowed to connect. That means it can be safely and easily exposed directly to the public internet using a LoadBalancer service type:

kubectl expose deployment hello-mtls --name=hello-mtls-lb --port=443 --type=LoadBalancer

To connect we need a certificate. There are a couple different ways to get one, but for simplicity we'll just forward a port.

kubectl -n step port-forward $(kubectl -n step get pods -l app=ca -o jsonpath={$.items[0].metadata.name}) 4443:4443

In another window we'll use step to grab the root certificate, generate a key pair, and get a certificate.

To follow along you'll need to install step if you haven't already. You'll also need your admin password and CA fingerprint, which were output during installation (see here and here if you already lost them :).

$ export CA_POD=$(kubectl -n step get pods -l app=ca -o jsonpath={$.items[0].metadata.name})
$ step ca root root.crt --ca-url https://127.0.0.1:4443 --fingerprint <fingerprint>
$ step ca certificate mike mike.crt mike.key --ca-url https://127.0.0.1:4443 --root root.crt
✔ Key ID: H4vH5VfvaMro0yrk-UIkkeCoPFqEfjF6vg0GHFdhVyM (admin)
✔ Please enter the password to decrypt the provisioner key: 0QOC9xcq56R1aEyLHPzBqN18Z3WfGZ01
✔ CA: https://127.0.0.1:4443/1.0/sign
✔ Certificate: mike.crt
✔ Private Key: mike.key

Now we can simply curl the service:

If you're using minikube or docker for mac the load balancer's "IP" might be localhost, which won't work. In that case, simply export HELLO_MTLS_IP=127.0.0.1 and try again.

$ export HELLO_MTLS_IP=$(kubectl get svc hello-mtls-lb -ojsonpath={$.status.loadBalancer.ingress[0].ip})
$ curl --resolve hello-mtls.default.svc.cluster.local:443:$HELLO_MTLS_IP \
       --cacert root.crt \
       --cert mike.crt \
       --key mike.key \
       https://hello-mtls.default.svc.cluster.local
Hello, mike!

Note that we're using --resolve to tell curl to override DNS and resolve the name in our workload's certificate to its public IP address. In a real production infrastructure you could configure DNS manually, or you could propagate DNS to workloads outside kubernetes using something like ExternalDNS.

✅ mTLS outside cluster.

Cleanup & uninstall

To clean up after running through the tutorial remove the hello-mtls and hello-mtls-client deployments and services:

kubectl delete deployment hello-mtls
kubectl delete deployment hello-mtls-client
kubectl delete service hello-mtls
kubectl delete service hello-mtls-lb

See the runbook for instructions on uninstalling autocert.

How it works

Architecture

Autocert is an admission webhook that intercepts and patches pod creation requests with some YAML to inject an init container and sidecar that handle obtaining and renewing certificates, respectively.

Enrollment & renewal

It integrates with step certificates and uses the one-time token bootstrap protocol from that project to mutually authenticate a new pod with your certificate authority, and obtain a certificate.

Tokens are generated by the admission webhook and transmitted to the injected init container via a kubernetes secret. The init container uses the one-time token to obtain a certificate. A sidecar is also installed to renew certificates before they expire. Renewal simply uses mTLS with the CA.

Further Reading

• We tweet @smallsteplabs
• Read our blog
• Check out the runbook
• Check out step CLI

Questions

Wait, so any pod can get a certificate with any identity? How is that secure?

1. Don't give people kubectl access to your production clusters
2. Use a deploy pipeline based on git artifacts
3. Enforce code review on those git artifacts

If that doesn't work for you, or if you have a better idea, we'd love to hear! Please open an issue!

Why do I have to tell you the name to put in a certificate? Why can't you automatically bind service names?

Mostly because monitoring the API server to figure out which services are associated with which workloads is complicated and somewhat magical. And it might not be what you want.

That said, we're not totally opposed to this idea. If anyone has strong feels and a good design please open an issue.

Doesn't kubernetes already ship with a certificate authority?

Yes, it uses a bunch of CAs for different sorts of control plane communication. Technically, kubernetes doesn't come with a CA. It has integration points that allow you to use any CA (e.g., Kubernetes the hard way uses CFSSL. You could use step certificates, which autocert is based on, instead.

In any case, these CAs are meant for control plane communication. You could use them for your service-to-service data plane, but it's probably not a good idea.

What permissions does autocert require in my cluster and why?

Autocert needs permission to create and delete secrets cluster-wide. You can check out our RBAC config here. These permissions are needed in order to transmit one-time tokens to workloads using secrets, and to clean up afterwards. We'd love to scope these permissions down further. If anyone has any ideas please open an issue.

Why does autocert create secrets?

The autocert admission webhook needs to securely transmit one-time bootstrap tokens to containers. This could be accomplished without using secrets. The webhook returns a JSONPatch response that's applied to the pod spec. This response could patch the literal token value into our init container's environment.

Unfortunately, the kubernetes API server does not authenticate itself to admission webhooks by default, and configuring it to do so requires passing a custom config file at apiserver startup. This isn't an option for everyone (e.g., on GKE) so we opted not to rely on it.

Since our webhook can't authenticate callers, including bootstrap tokens in patch responses would be dangerous. By using secrets an attacker can still trick autocert into generating superflous bootstrap tokens, but they'd also need read access to cluster secrets to do anything with them.

Hopefully this story will improve with time.

Why not use kubernetes service accounts instead of bootstrap tokens?

Great idea! This should be pretty easy to add. However, existing service accounts are somewhat broken for this use case. The upcoming TokenRequest API should fix most of these issues.

TODO: Link to issue for people who want this.

Too. many. containers. Why do you need to install an init container and sidecar?

We don't. It's just easier for you. Your containers can generate key pairs, exchange them for certificates, and manage renewals themselves. This is pretty easy if you install step in your containers, or integrate with our golang SDK. To support this we'd need to add the option to inject a bootstrap token without injecting these containers.

TODO: Link to issue for people who want this.

That said, the init container and sidecar are both super lightweight.

Why are keys and certificates managed via volume mounts? Why not use a secret or some custom resource?

Because, by default, kubernetes secrets are stored in plaintext in etcd and might even be transmitted unencrypted across the network. Even if secrets were properly encrypted, transmitting a private key across the network violates PKI best practices. Key pairs should always be generated where they're used, and private keys should never be known by anyone but their owners.

That said, there are use cases where a certificate mounted in a secret resource is desirable (e.g., for use with a kubernetes Ingress). We may add support for this in the future. However, we think the current method is easier and a better default.

TODO: Link to issue for people who want this.

Why not use kubernetes CSR resources for this?

It's harder and less secure. If any good and simple design exists for securely automating CSR approval using this resource we'd love to see it!

How is this different than cert-manager

Cert-manager is a great project. But it's design is focused on managing Web PKI certificates issued by Let's Encrypt's public certificate authority. These certificates are useful for TLS ingress from web browsers. Autocert is different. It's purpose-built to manage certificates issued by your own private CA to support the use of mTLS for internal communication (e.g., service-to-service).

What sorts of keys are issued and how often are certificates rotated?

Autocert builds on step certificates which issues ECDSA certificates using the P256 curve with ECDSA-SHA256 signatures by default. If this is all Greek to you, rest assured these are safe, sane, and modern defaults that are suitable for the vast majority of environments.

What crypto library is under the hood?

https://golang.org/pkg/crypto/

Building

This project is based on four docker containers. They use multi-stage builds so all you need in order to build them is docker.

Caveat: the controller container uses dep and dep init isn't run during the build. You'll need to run dep init in the controller/ subdirectory prior to building, and you'll need to run dep ensure -update if you change any dependencies.

Building autocert-controller (the admission webhook):

cd controller
docker build -t smallstep/autocert-controller:latest .

Building autocert-bootstrapper (the init container that generates a key pair and exchanges a bootstrap token for a certificate):

cd bootstrapper
docker build -t smallstep/autocert-bootstrapper:latest .

Building autocert-renewer (the sidecar that renews certificates):

cd renewer
docker build -t smallstep/autocert-renewer:latest .

Building autocert-init (the install script):

cd init
docker build -t smallstep/autocert-init:latest .

If you build your own containers you'll probably need to install manually. You'll also need to adjust which images are deployed in the deployment yaml.

Contributing

If you have improvements to autocert, send us your pull requests! For those just getting started, GitHub has a howto. A team member will review your pull requests, provide feedback, and merge your changes. In order to accept contributions we do need you to sign our contributor license agreement.

If you want to contribute but you're not sure where to start, take a look at the issues with the "good first issue" label. These are issues that we believe are particularly well suited for outside contributions, often because we probably won't get to them right now. If you decide to start on an issue, leave a comment so that other people know that you're working on it. If you want to help out, but not alone, use the issue comment thread to coordinate.

If you've identified a bug or have ideas for improving autocert that you don't have time to implement, we'd love to hear about them. Please open an issue to report a bug or suggest an enhancement!

License

Copyright 2019 Smallstep Labs

Licensed under the Apache License, Version 2.0