Allow certificates to contain subjects with allowed external names

Overview

Currently, secret-operator can only sign/request certificates with IP/DNS names based on these scopes:

• listener-volume (The external-name that is reported back by the Load Balancer controller[^1]).
• node (eg: 10.0.0.1 or node-1.local.domain. If I understand correctly, this becomes obsolete since listener-volume scope)
• pod (STS) (eg: pod-0.app-service.app-namespace.svc.cluster.local)
• service (FQDN of the named service, eg: app-service.app-namespace.svc.cluster.local

Customers often want to use an external[^2] DNS name, and in many cases use external-dns to automatically create DNS records for Services/Ingress/Gateway resources.

Common environment setups

In many environments (both Cloud and On-Prem):

• The kubelet clusterDomain is not usable outside of the cluster (especially
  when it is the default of cluster.local), and
• Customers often use names under a suffix separate from the kubelet
  clusterDomain. Often this is a shorter name (eg: app.team.example.com
  instead of app.app-service.app-namespace.cluster.domain).
• DNS is often auto-configured using External DNS, which is configured in these
  ways:
  • For Services, an annotation is place to map the DNS name to the Service
    (NodePort, or LoadBalancer) IP.
  • For Ingress/Gateway, there are dedicated fields for specifying the DNS name.
• Load balancers do TLS termination
  • In the case of internet-facing load balancers, this can be used to hide the
    subject names on the internal certificate which leak information such as the
    Kubernetes namespace and clusterDomain.

In some cases, they can work around this by using a Load Balancer that does L5 (TLS termination) or L7 (HTTP proxying), however with SNI checks becoming more prevalent these methods are becoming less effective when the external name used is not in the certificate used by the application.

This issue is about allowing external names to be added to certificates and a suggested approach follows...

[^1]: Often, this is a dynamic (and not user-friendly) name and is not intended to be used directly, but rather be the value of the DNS CNAME record of the intended external name.
[^2]: Whether that is on the internet, or just within their organization - it is external to the Kubernetes cluster.

Suggested approach

1. Extend the SecretClass to allow a list of suffixes (and minDepth/maxDepth number of dots) for additional names that can be added to the certificate.
2. Extend Listener with an externalName (or externalNames?) field.
   • If the listener-volume scope is used, and the suffix is permitted (within the minDepth/maxDepth bounds), then secret-operator will add that as a subject to the certificate request.
3. Extend Listener with a serviceAnnotations field.
   • We could use this opportunity to consistently name the fields in Listener and ListenerClass (either all fields to be passed to Service are prefixed with service, or we put everything under ServiceOverrides.

[!IMPORTANT]
Question: If the externalName set on the listener is not permitted in the certificate (or minDepth/maxDepth are out of bounds), where should an error event go? It could go on the Listener, but the listener doesn't concern itself too much with TLS details.

Example config:

Have a SecretClass which allows signing names under certain additional suffixes.

apiVersion: secrets.stackable.tech/v1alpha1
kind: SecretClass
metadata:
  name: org-pki
spec:
  backend:
    # Using autoTls as an example, but same applies for any supported backend
    autoTls:
      ca:
        secret:
          name: secret-provisioner-tls-ca
          namespace: default
        autoGenerate: true
      maxCertificateLifetime: 15d
  # 👇 New
  allowedSuffixes: # maybe should be additionalSuffixes, since it is on top of existing names
    - suffix: internal.example.com
      maxDepth: 0 # allow anything under internal.example.com, eg: a.b.c.d.internal.example.com

    # or
    - suffix: internal.example.com
      maxDepth: 1 # default, only allow one dot between the name and the suffix, eg: a.internal.example.com

    #  or
    - suffix: internal.example.com
      # Example: the organisation allows signing certs for $app.$team.internal.example.com
      # by requiring two dots between the name and the suffix
      # allow a.b.internal.example.com
      # disallow a.internal.example.com
      # disallow a.b.c.internal.example.com
      minDepth: 2
      maxDepth: 2
      # alternatively we could define depth and make it mutually exclusive to minDepth/maxDepth

    - suffix: internal.example.net

Have a ListenerClass ready for making public facing AWS NLBs with TLS termination

apiVersion: listeners.stackable.tech/v1alpha1
kind: ListenerClass
metadata:
  name: aws-ec2-nlb-public
spec:
  # I think it would make more sense to make a serviceOverrides key instead of prefixing some with "service" and missing it in others (eg: loadBalancerClass)
  serviceType: LoadBalancer
  # https://docs.aws.amazon.com/eks/latest/userguide/auto-configure-nlb.html#_sample_service
  loadBalancerClass: eks.amazonaws.com/nlb
  loadBalancerAllocateNodePorts: false
  preferredAddressType: HostnameConservative
  serviceExternalTrafficPolicy: Local
  serviceAnotations:
    service.beta.kubernetes.io/aws-load-balancer-scheme: internet-facing
    service.beta.kubernetes.io/aws-load-balancer-nlb-target-type: ip # Forward directly through node to Pod IP instead of an L3 hop/NAT through the node.
    service.beta.kubernetes.io/aws-load-balancer-attributes: >-
      proxy_protocol_v2.enabled=true

Have a NifiCluster, specifying an externalName (to be used in the resultant Listener) and service annotations to enable TLS temination on the Load Balancer, and a hostname to be configured by External DNS.

apiVersion: nifi.stackable.tech/v1alpha1
kind: NifiCluster
metadata:
  name: simple-nifi
spec:
  clusterConfig:
    tls:
      serverSecretClass: org-pki
  nodes:
    roleConfig:
      # I think this should be moved under listenerOverrides as className...
      listenerClass: aws-nlb-tls
      # 👇 New
      listenerOverrides:
        className: aws-nlb-tls # moved from roleConfig.listenerClass
        externalName: app.internal.example.com
        serviceAnnotations:
          external-dns.alpha.kubernetes.io/hostname: app.internal.example.com
          service.beta.kubernetes.io/aws-load-balancer-ssl-cert: arn:aws:acm:eu-central-1:123456789012:certificate/4e12c4fe-eed9-48db-98d8-820b6b50ace4
          service.beta.kubernetes.io/aws-load-balancer-ssl-ports: "8443"

Based on the configurations above, the objects below should be created by the NiFi Operator:

# This is the Listener produced by NifiCluster
kind: Listener
metadata:
  name: the-nifi-listener
spec:
  className: aws-nlb-tls-public
  # 👇 New
  externalName: app.internal.example.com # this came from the NifiCluster overrides
  # 👇 Not yet available, see (see: https://github.com/stackabletech/listener-operator/issues/331)
  serviceAnnotations:
    external-dns.alpha.kubernetes.io/hostname: app.internal.example.com
    service.beta.kubernetes.io/aws-load-balancer-ssl-cert: arn:aws:acm:eu-central-1:123456789012:certificate/4e12c4fe-eed9-48db-98d8-820b6b50ace4
    service.beta.kubernetes.io/aws-load-balancer-ssl-ports: "8443"

# This is the Pod produced by the Deployment produced by the NifiCluster
apiVersion: v1
kind: Pod
metadata:
  name: nifi-0
spec:
  volumes:
    - name: tls
      ephemeral:
        volumeClaimTemplate:
          metadata:
            annotations:
              secrets.stackable.tech/class: org-pki
              secrets.stackable.tech/scope: pod,service=nifi,listener-volume=the-nifi-listener

Based on the configurations above, the object below should be created by the Listener Operator:

apiVersion: v1
kind: Service
metadata:
  name: nifi-listener
  annotations:
    external-dns.alpha.kubernetes.io/hostname: app.internal.example.com
    service.beta.kubernetes.io/aws-load-balancer-attributes: proxy_protocol_v2.enabled=true
    service.beta.kubernetes.io/aws-load-balancer-nlb-target-type: ip
    service.beta.kubernetes.io/aws-load-balancer-scheme: internet-facing
    service.beta.kubernetes.io/aws-load-balancer-ssl-cert: arn:aws:acm:eu-central-1:123456789012:certificate/4e12c4fe-eed9-48db-98d8-820b6b50ace4
    service.beta.kubernetes.io/aws-load-balancer-ssl-ports: "8443"
spec:
  type: LoadBalancer
  loadBalancerClass: eks.amazonaws.com/nlb
  loadBalancerAllocateNodePorts: false
  externalTrafficPolicy: Local
  ports: ...