DNS Validation

Issuing an ACME certificate using DNS 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 DNS01. With a DNS01 challenge, you prove ownership of a domain by proving you control its DNS records. This is done by creating a TXT record with specific content that proves you have control of the domains DNS records.

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

apiVersion: cert-manager.io/v1alpha2
kind: Issuer
metadata:
  name: letsencrypt-staging
  namespace: default
spec:
  acme:
    server: https://acme-staging-v02.api.letsencrypt.org/directory
    email: user@example.com

    # Name of a secret used to store the ACME account private key
    privateKeySecretRef:
      name: letsencrypt-staging

    # ACME DNS-01 provider configurations
    solvers:
    # An empty 'selector' means that this solver matches all domains
    - selector: {}
      dns01:
        clouddns:
          # The ID of the GCP project
          # reference: https://docs.cert-manager.io/en/latest/tasks/issuers/setup-acme/dns01/google.html
          project: $PROJECT_ID
          # This is the secret used to access the service account
          serviceAccountSecretRef:
            name: clouddns-dns01-solver-svc-acct
            key: key.json

    # We only use cloudflare to solve challenges for example.org.
    # Alternative options such as 'matchLabels' and 'dnsZones' can be specified
    # as part of a solver's selector too.
    - selector:
        dnsNames:
        - example.org
      dns01:
        cloudflare:
          email: my-cloudflare-acc@example.com
          # !! Remember to create a k8s secret before
          # kubectl create secret generic cloudflare-api-key-secret
          apiKeySecretRef:
            name: cloudflare-api-key-secret
            key: api-key

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 dns01 stanza contains a list of DNS01 providers that can be used to solve DNS challenges. Our Issuer defines two providers. This gives us a choice of which one to use when obtaining certificates.

More information about the DNS provider configuration, including a list of supported providers, can be found in the DNS01 reference docs.

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

apiVersion: cert-manager.io/v1alpha2
kind: Certificate
metadata:
  name: example-com
  namespace: default
spec:
  secretName: example-com-tls
  issuerRef:
    name: letsencrypt-staging
  dnsNames:
  - '*.example.com'
  - example.com
  - example.org

The Certificate resource describes our desired certificate and the possible methods that can be used to obtain it. You can obtain certificates for wildcard domains just like any other. Make sure to wrap wildcard domains with asterisks in your YAML resources, to avoid formatting issues. If you specify both example.com and *.example.com on the same Certificate, it will take slightly longer to perform validation as each domain will have to be validated one after the other. You can learn more about the Certificate resource in the 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, example.com and example.org.

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 issuer concepts.

The acme stanza defines the configuration for our ACME challenges. Here we have defined the configuration for our DNS challenges which will be used to verify domain ownership. For each domain mentioned in a dns01 stanza, cert-manager will use the provider’s credentials from the referenced Issuer to create a TXT record called _acme-challenge. This record will then be verified by the ACME server in order to issue the certificate. Once domain ownership has been verified, any cert-manager affected records will be cleaned up.

Note: It is your responsibility to ensure the selected provider is authoritative for your domain.

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 "dns-01" validation
  Normal  DomainVerified  55m      cert-manager  Domain "example.com" verified with "dns-01" validation
  Normal  DomainVerified  55m      cert-manager  Domain "example.org" verified with "dns-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.