Issuing an ACME certificate using HTTP validation

Todo

This guide needs rewriting to be clearer, splitting into sections and potentially rewriting altogether.

cert-manager can be used to obtain certificates from a CA using the ACME protocol. The ACME protocol supports various challenge mechanisms which are used to prove ownership of a domain so that a valid certificate can be issued for that domain.

One such challenge mechanism is the HTTP-01 challenge. With a HTTP-01 challenge, you prove ownership of a domain by ensuring that a particular file is present at the domain. It is assumed that you control the domain if you are able to publish the given file under a given path.

The following Issuer defines the necessary information to enable HTTP validation. You can read more about the Issuer resource in the Issuer reference docs.

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apiVersion: certmanager.k8s.io/v1alpha1
kind: Issuer
metadata:
  name: letsencrypt-staging
  namespace: default
spec:
  acme:
    # The ACME server URL
    server: https://acme-staging-v02.api.letsencrypt.org/directory
    # Email address used for ACME registration
    email: user@example.com
    # Name of a secret used to store the ACME account private key
    privateKeySecretRef:
      name: letsencrypt-staging
    # Enable the HTTP-01 challenge provider
    http01: {}

We have specified the ACME server URL for Let’s Encrypt’s staging environment. The staging environment will not issue trusted certificates but is used to ensure that the verification process is working properly before moving to production. Let’s Encrypt’s production environment imposes much stricter rate limits, so to reduce the chance of you hitting those limits it is highly recommended to start by using the staging environment. To move to production, simply create a new Issuer with the URL set to https://acme-v02.api.letsencrypt.org/directory.

The first stage of the ACME protocol is for the client to register with the ACME server. This phase includes generating an asymmetric key pair which is then associated with the email address specified in the Issuer. Make sure to change this email address to a valid one that you own. It is commonly used to send expiry notices when your certificates are coming up for renewal. The generated private key is stored in a Secret named letsencrypt-staging.

The presence of the http01 field simply enables the HTTP-01 challenge for this Issuer. No further configuration is necessary or currently possible.

Once we have created the above Issuer we can use it to obtain a certificate.

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apiVersion: certmanager.k8s.io/v1alpha1
kind: Certificate
metadata:
  name: example-com
  namespace: default
spec:
  secretName: example-com-tls
  issuerRef:
    name: letsencrypt-staging
  commonName: example.com
  dnsNames:
  - www.example.com
  acme:
    config:
    - http01:
        ingressClass: nginx
      domains:
      - example.com
    - http01:
        ingress: my-ingress
      domains:
      - www.example.com

The Certificate resource describes our desired certificate and the possible methods that can be used to obtain it. You can learn more about the Certificate resource in the reference docs. If the certificate is obtained successfully, the resulting key pair will be stored in a secret called example-com-tls in the same namespace as the Certificate.

The certificate will have a common name of example.com and the Subject Alternative Names (SANs) will be example.com and www.example.com.

In our Certificate we have referenced the letsencrypt-staging Issuer above. The Issuer must be in the same namespace as the Certificate. If you want to reference a ClusterIssuer, which is a cluster-scoped version of an Issuer, you must add kind: ClusterIssuer to the issuerRef stanza.

For more information on ClusterIssuers, read the ClusterIssuer reference docs.

The acme stanza defines the configuration for our ACME challenges. Here we have defined the configuration for our HTTP-01 challenges which will be used to verify domain ownership. To verify ownership of each domain mentioned in an http01 stanza, cert-manager will create a Pod, Service and Ingress that exposes an HTTP endpoint that satisfies the HTTP-01 challenge.

The fields ingress and ingressClass in the http01 stanza can be used to control how cert-manager interacts with Ingress resources:

  • If the ingress field is specified, then an Ingress resource with the same name in the same namespace as the Certificate must already exist and it will be modified only to add the appropriate rules to solve the challenge. This field is useful for the GCLB ingress controller, as well as a number of others, that assign a single public IP address for each ingress resource. Without manual intervention, creating a new ingress resource would cause any challenges to fail.
  • If the ingressClass field is specified, a new ingress resource with a randomly generated name will be created in order to solve the challenge. This new resource will have an annotation with key kubernetes.io/ingress.class and value set to the value of the ingressClass field. This works for the likes of the NGINX ingress controller.
  • If neither are specified, new ingress resources will be created with a randomly generated name, but they will not have the ingress class annotation set.
  • If both are specified, then the ingress field will take precedence.

Once domain ownership has been verified, any cert-manager affected resources will be cleaned up or deleted.

Note

It is your responsibilty to point each domain name at the correct IP address for your ingress controller.

After creating the above Certificate, we can check whether it has been obtained successfully using kubectl describe:

$ kubectl describe certificate example-com
Events:
  Type    Reason          Age      From          Message
  ----    ------          ----     ----          -------
  Normal  CreateOrder     57m      cert-manager  Created new ACME order, attempting validation...
  Normal  DomainVerified  55m      cert-manager  Domain "example.com" verified with "http-01" validation
  Normal  DomainVerified  55m      cert-manager  Domain "www.example.com" verified with "http-01" validation
  Normal  IssueCert       55m      cert-manager  Issuing certificate...
  Normal  CertObtained    55m      cert-manager  Obtained certificate from ACME server
  Normal  CertIssued      55m      cert-manager  Certificate issued successfully

You can also check whether issuance was successful with kubectl get secret example-com-tls -o yaml. You should see a base64 encoded signed TLS key pair.

Once our certificate has been obtained, cert-manager will periodically check its validity and attempt to renew it if it gets close to expiry. cert-manager considers certificates to be close to expiry when the ‘Not After’ field on the certificate is less than the current time plus 30 days.