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).

Note: this project is in ALPHA. DON'T use it for anything mission critical. EXPECT breaking changes in minor revisions with little or not warning. PLEASE provide feedback:

TODO: Twitter, Slack, Issues (tagged with #autocert / special template)...

Table of Contents

• Features
• Motivation
• Getting Started
  • Prerequisites
  • Installation
  • Enabling autocert
  • Annotating pods
• Examples
  • Mutual TLS
  • Getting a certificate locally
• How it works
• Uninstalling
• Questions

Features

Autocert uses step certificates to generate keys and issue certificates from your own internal certificate authority. This process is secure and automatic, all you have to do is install autocert and annotate your pods. Features include:

• A complete public key infrastructure (PKI) for your kubernetes clusters
  • A fully featured internal certificate authority (CA) that you control so you can use mTLS to control access to services
  • Ability to run subnordinate to or federated with an existing PKI
  • CA and PKI artifacts are installed in their own namespace (step) for easy access control
• Modern certificate best practices
  • Automated certificate management (auto enrollment and renewal)
  • Short-lived certificates
  • Private keys are never transmitted across the network (and aren't stored in etcd)
  • RFC5280 and CA/Browser Forum compliant certificates that work with browsers and other standard TLS implementations
• Easily enable/disable per-namespace using labels
• Builds on step certificates so you can also issue certificates to servers, people, and code running in a different cluster and outside of kubernetes

Motivation

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 to secure internal services 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.

Getting Started

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

Prerequisites

You'll 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

For manual installation you'll also need to install step version 0.8.3 or later.

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

Install

To install step certificates and autocert in one step run:

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

The init script will end by printing:

• The admin provisioner password (also used to encrypt the CA's key material)
• The autocert provisioner password (used by the mutating webhook)
• Your CA's root certificate fingerprint (used to bootstrap secure communication)

Feel free to store these some place safe. The passwords are also stored as secrets in the step namespace.

🤔 Tip: If you lose your root certificate fingerprint you can calculate it again by running:

$ export CA_POD=$(kubectl -n step get pods -l app=ca \
    -o jsonpath={$.items[0].metadata.name})
$ kubectl -n step exec -it $CA_POD -- step certificate fingerprint /home/step/.step/certs/root_ca.crt

🤯 Note: You may need to adjust your 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

Feel free to check out what the autocert-init container does if you're curious. You can also install manually.

Enable autocert

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

For autocert to inject a certificate pods must use the autocert.step.sm/name annotation to specify their name. The value of this annotation will appear as the name in the issued certificate (the X.509 common name and SAN).

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

Once the pod has started we can check that our certificate, private key, and root certificate have been properly mounted in our container at /var/run/autocert.step.sm.

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

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

$ kubectl exec -it $SLEEP_POD -c autocert-renewer -- step \
    certificate inspect /var/run/autocert.step.sm/site.crt
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 38872628668914277126045555806003435350 (0x1d3e9890a42ae5861b8a6cb51aa29756)
    Signature Algorithm: ECDSA-SHA256
        Issuer: CN=Autocert Intermediate CA
        Validity
            Not Before: Jan 19 01:59:06 2019 UTC
            Not After : Jan 20 01:59:06 2019 UTC
        Subject: CN=sleep.default.svc.cluster.local
        Subject Public Key Info:
            Public Key Algorithm: ECDSA
                Public-Key: (256 bit)
                X:
                    e9:f7:f6:04:c5:b5:af:c7:ff:95:19:69:09:74:57:
                    31:a9:24:a7:31:d8:e4:f1:2a:0e:8c:89:fa:b5:aa:
                    fa:d9
                Y:
                    26:bc:6c:0f:ad:57:6e:75:ea:8e:d5:ca:bf:b0:c9:
                    43:61:dc:42:8a:ef:42:79:17:b7:02:8a:07:2e:58:
                    4c:50
                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:
                BE:3E:92:68:7D:82:61:91:93:C2:E0:DF:77:1F:CD:EF:36:2D:8E:41
            X509v3 Authority Key Identifier:
                keyid:69:BA:E5:9C:6D:66:39:B3:3E:8B:28:85:26:75:34:A6:91:07:F6:4E
            X509v3 Subject Alternative Name:
                DNS:sleep.default.svc.cluster.local
            X509v3 Step Provisioner:
                Type: JWK
                Name: autocert
                CredentialID: 7OOZUAEgixopdF_Yk7wMtkHv-op6p8FqSfEk3B6nry0

    Signature Algorithm: ECDSA-SHA256
         30:45:02:20:6c:79:31:69:11:65:88:48:fc:a0:a0:f4:8e:bd:
         81:62:83:6a:d7:66:fa:9c:d0:43:1e:15:69:3a:3c:e0:8e:2b:
         02:21:00:c2:4a:51:85:25:4f:c1:68:de:07:50:53:8c:36:b3:
         2c:a3:56:d1:1d:11:3d:aa:77:d1:2e:1e:54:75:1d:f3:0d

Examples

With autocert issuing and rotating certificates we can start using mTLS between services. The examples/hello-mtls directory demonstrates the right way to do mTLS in several languages (contributions welcome :). Let's deploy one.

Mutual TLS

Build and deploy the hello-mtls server for golang:

$ cd examples/hello-mtls/go
$ docker build -f Dockerfile.server -t hello-mtls-server-go .
$ kubectl apply -f hello-mtls.server.yaml

Build and deploy the hello-mtls client for golang:

$ docker build -f Dockerfile.client -t hello-mtls-client-go .
$ kubectl apply -f hello-mtls.client.yaml

Check that the client is connecting and working as expected:

$ export HELLO_MTLS=$(kubectl get pods -l app=hello-mtls-client \
    -o jsonpath={$.items[0].metadata.name})
$ kubectl logs $HELLO_MTLS -c hello-mtls-client -f
2019-01-25T01:36:57Z: Hello, hello-mtls-client.default.pod.cluster.local!
2019-01-25T01:37:02Z: Hello, hello-mtls-client.default.pod.cluster.local!
2019-01-25T01:37:07Z: Hello, hello-mtls-client.default.pod.cluster.local!
...

We can also exec into the sleep container we deployed earlier

$ kubectl exec -it $SLEEP_POD -c sleep -- sh

install curl, and hit hello-mtls from there:

sleep# apk add curl
sleep# curl --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, sleep.default.svc.cluster.local!

🤯 A few ways things that can go sideways:

If we don't provide a client certificate for authentication the request will fail because we haven't authenticated ourselves to the server:

sleep# curl --cacert /var/run/autocert.step.sm/root.crt \
            https://hello-mtls.default.svc.cluster.local
curl: (35) error:1401E412:SSL routines:CONNECT_CR_FINISHED:sslv3 alert bad certificate

curl will also balk if we don't tell it to trust our root.crt, this time because it can't validate the server's certificate:

sleep# curl https://hello-mtls.default.svc.cluster.local
curl: (60) SSL certificate problem: unable to get local issuer certificate
More details here: https://curl.haxx.se/docs/sslcerts.html

curl failed to verify the legitimacy of the server and therefore could not
establish a secure connection to it. To learn more about this situation and
how to fix it, please visit the web page mentioned above.

You'll get similar errors from other tools, libraries, and applications if they're not properly configured to use the autocert certificates and keys. Minimally, for (non-mutual) TLS:

• Clients must be configured to trust the autocert root certificate (/var/run/autocert.step.sm/root.crt) to authenticate a server
• Servers must be configured to use the key and certificate issued by autocert (/var/run/autocert.step.sm/site.crt and /var/run/autocert.step.sm/site.key) to authenticate to a client

If you're doing mTLS the inverse is also true: the server must trust the root certificate to authenticate client, and the client must be configured to use the autocert-issued key and certificate. In other words, for mTLS both the client and server should be configured to use autocert's root.crt, site.crt, and site.key. With curl this is done using the --cacert, --cert, and --key flags, respectively.

Exposing services using mTLS

With properly configured mTLS, services can be safely exposed directly to the public internet: only clients that have a certificate issued by the internal certificate authority will be allowed to connect. To demonstrate let's expose our hello-mtls service.

If you need a refresher, here's a rough approximation of how an mTLS handshake works:

A few things to note:

• It's the signing of random numbers that proves we're talking to the right remote. It's the digital equivalent of asking someone to send you a photo of them with today's newspaper.
• The client and server need to have prior knowledge of the root certificate(s) used for signing other certificates.
• The client and server need to be configured to use the correct certificate and private key (the certificate must have been issued by a CA with a trusted root certificate)

Exposing hello-mtls

Because hello-mtls does proper mTLS itself we can expose it simply using a service with type LoadBalancer.

$ kubectl expose deployment hello-mtls --name=hello-mtls-lb --port=443 --target-port=443 --type=LoadBalancer
service/loadbalancer exposed
$ kubectl get svc hello-mtls-lb
NAME            TYPE           CLUSTER-IP    EXTERNAL-IP       PORT(S)   AGE
hello-mtls-lb   LoadBalancer   10.0.65.118   104.198.149.140   443/TCP    5m

Obtaining a certificate locally

To connect to hello-mtls from outside kubernetes we need a certificate issued by our internal CA. Since autocert is built on step certificates we can securely issue certificates to users, devices, and workloads running in other environments.

🤯 Note: To follow along you'll need step installed locally.

First, port-forward from localhost to the step-ca pod:

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

Now we can use step to securely grab the CA's root certificate and obtain a certificate. You'll be prompted to select a provisioner and enter the correct password to continue:

# Get root certificate fingerprint
$ export FINGERPRINT="$(kubectl -n step exec -it $CA_POD -- \
    step certificate fingerprint /home/step/.step/certs/root_ca.crt | tr -d '[:space:]')"

# Fetch and verify root certificate
$ step ca root root.crt \
    --ca-url https://127.0.0.1:4443 \
    --fingerprint $FINGERPRINT

# Get a certificate locally
$ step ca certificate snarf.local.dev snarf.crt snarf.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: snarf.crt
✔ Private Key: snarf.key
$ step ca certificate snarf.local.dev snarf.crt snarf.key

We can inspect our newly minted certificate and verify that it's been issued by autocert and includes the right common name:

$ step certificate inspect --format json snarf.crt | jq '{issuer,subject}'
{
  "issuer": {
    "common_name": [
      "Autocert Intermediate CA"
    ]
  },
  "subject": {
    "common_name": [
      "snarf.local.dev"
    ]
  }
}

🤔 Tip: If you want someone (or something) to have a certificate with a particular name, but don't want to give them the ability to provision arbitrary certificates, you can generate a bootstrap token for them:

$ step ca token snarf.local.dev \
    --ca-url https://127.0.0.1:4443 \
    --root root.crt
eyJhbG...

They can use the token to obtain a certificate (once):

$ step ca certificate snarf.local.dev snarf.crt snarf.key --token "eyJhbG..."

Actually, this is exactly what the autocert mutating webhook is doing for your pods! Read how it works for more info.

Connecting to hello-mtls

We're ready to securely connect to hello-mtls.

$ export HELLO_MTLS_IP=$(kubectl get svc hello-mtls-lb -ojsonpath={$.status.loadBalancer.ingress...?})
$ export HELLO_MTLS_IP="127.0.0.1"
$ curl --resolve hello-mtls.default.svc.cluster.local:443:$HELLO_MTLS_IP \
       --cacert root.crt \
       --cert snarf.crt \
       --key snarf.key \
       https://hello-mtls.default.svc.cluster.local
Hello, snarf.local.dev!

🎉

🤯 Note: HTTPS clients check that the name in the server's cerificate match the authority portion of the URL (e.g., https://smallstep.com/ must present a certificate with the name smallstep.com). (See RFC2818.)

Our hello-mtls service's certificate binds the name hello-mtls.default.svc.cluster.local so we must connect to it using that name. If we use a different authority we'll get an error:

$ curl --cacert root.crt \
       --cert snarf.crt \
       --key snarf.key \
       https://127.0.0.1
curl: (51) SSL: no alternative certificate subject name matches target host name '127.0.0.1'

In a real production environment you'd address this by either:

• using a properly registered domain name and configuring DNS either globally (e.g., using ExternalDNS), or
• using internal names and configuring DNS locally in each environment (e.g., using an ExternalName service)

In any case, hello-mtls.default.svc.cluster.local must resolve to the right IP.

You could use /etc/hosts. Since we're testing with curl it's even easier to use the --resolve flag to override resolution for a single request.

How it works

Architecture

Autocert consists of a webhook admission controllers that injects one init container and one sidecar container to handle obtaining a certificate for the first time and renewing a certificate, respectively.

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

Enrollment & renewal

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

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.

Why not use kubernetes service accounts instead of bootstrap tokens?

Why does the mutating webhook have to create secrets / need cluster role bindings?

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

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.

How is this different than a service mesh?

Certificate management is a necessary building block for any service mesh that uses mutual TLS for authenticated encryption (e.g., istio, linkerd, consul connect). Typically, service mesh systems will provide their own certificate management solution. However, these systems

What about DaemonSets, ReplicaSets, StatefulSets, and all the other things that might need certificates?

...?

Building

...

Contributing

...

License

Copyright 2019 Smallstep Labs

Licensed under the Apache License, Version 2.0