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cert-manager has the concept of Certificates that define a desired X.509 certificate which will be renewed and kept up to date. A Certificate is a namespaced resource that references an Issuer or ClusterIssuer that determine what will be honoring the certificate request.

When a Certificate is created, a corresponding CertificateRequest resource is created by cert-manager containing the encoded X.509 certificate request, Issuer reference, and other options based upon the specification of the Certificate resource.

Here is one such example of a Certificate resource.

kind: Certificate
name: acme-crt
secretName: acme-crt-secret
name: letsencrypt-prod
# We can reference ClusterIssuers by changing the kind here.
# The default value is Issuer (i.e. a locally namespaced Issuer)
kind: Issuer

This Certificate will tell cert-manager to attempt to use the Issuer named letsencrypt-prod to obtain a certificate key pair for the and domains. If successful, resulting TLS key and certificate will be stored in a secret named acme-crt-secret, with keys of tls.key, and tls.crt respectively. This secret will live in the same namespace as the Certificate resource.

Additionally, if the Certificate Authority is known, the corresponding CA certificate will be stored in the secret with key ca.crt. For example, with the ACME issuer, the CA is not known and ca.crt will not exist in acme-crt-secret.

When a certificate is issued by intermediates of the CA and the Issuer knows the intermediates, the content of tls.crt will be a resulting certificate followed by a certificate chain. The certificate chain doesn't include a root CA certificate, as it is stored in ca.crt.

This format is used as it allows TLS implementations to validate the leaf certificate as long as the root CA is already trusted. During the TLS handshake, peers construct a full trust chain by checking the issuer of each certificate until they end up at a root in their trust store. If the intermediates aren't sent along with the leaf, there's no way to know that the issuing CA was signed by the root, and the certificate won't be trusted.

The dnsNames field specifies a list of Subject Alternative Names to be associated with the certificate.

The referenced Issuer must exist in the same namespace as the Certificate. A Certificate can alternatively reference a ClusterIssuer which is non-namespaced and so can be referenced from any namespace.

You can read more on how to configure your Certificate resources here.