<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-kubernetes">
  <title>Kubernetes</title>
  <para>
    The NixOS Kubernetes module is a collective term for a handful of
    individual submodules implementing the Kubernetes cluster
    components.
  </para>
  <para>
    There are generally two ways of enabling Kubernetes on NixOS. One
    way is to enable and configure cluster components appropriately by
    hand:
  </para>
  <programlisting language="bash">
services.kubernetes = {
  apiserver.enable = true;
  controllerManager.enable = true;
  scheduler.enable = true;
  addonManager.enable = true;
  proxy.enable = true;
  flannel.enable = true;
};
</programlisting>
  <para>
    Another way is to assign cluster roles (&quot;master&quot; and/or
    &quot;node&quot;) to the host. This enables apiserver,
    controllerManager, scheduler, addonManager, kube-proxy and etcd:
  </para>
  <programlisting language="bash">
services.kubernetes.roles = [ &quot;master&quot; ];
</programlisting>
  <para>
    While this will enable the kubelet and kube-proxy only:
  </para>
  <programlisting language="bash">
services.kubernetes.roles = [ &quot;node&quot; ];
</programlisting>
  <para>
    Assigning both the master and node roles is usable if you want a
    single node Kubernetes cluster for dev or testing purposes:
  </para>
  <programlisting language="bash">
services.kubernetes.roles = [ &quot;master&quot; &quot;node&quot; ];
</programlisting>
  <para>
    Note: Assigning either role will also default both
    <xref linkend="opt-services.kubernetes.flannel.enable" /> and
    <xref linkend="opt-services.kubernetes.easyCerts" /> to true. This
    sets up flannel as CNI and activates automatic PKI bootstrapping.
  </para>
  <note>
    <para>
      As of NixOS 19.03, it is mandatory to configure:
      <xref linkend="opt-services.kubernetes.masterAddress" />. The
      masterAddress must be resolveable and routeable by all cluster
      nodes. In single node clusters, this can be set to
      <literal>localhost</literal>.
    </para>
  </note>
  <para>
    Role-based access control (RBAC) authorization mode is enabled by
    default. This means that anonymous requests to the apiserver secure
    port will expectedly cause a permission denied error. All cluster
    components must therefore be configured with x509 certificates for
    two-way tls communication. The x509 certificate subject section
    determines the roles and permissions granted by the apiserver to
    perform clusterwide or namespaced operations. See also:
    <link xlink:href="https://kubernetes.io/docs/reference/access-authn-authz/rbac/">
    Using RBAC Authorization</link>.
  </para>
  <para>
    The NixOS kubernetes module provides an option for automatic
    certificate bootstrapping and configuration,
    <xref linkend="opt-services.kubernetes.easyCerts" />. The PKI
    bootstrapping process involves setting up a certificate authority
    (CA) daemon (cfssl) on the kubernetes master node. cfssl generates a
    CA-cert for the cluster, and uses the CA-cert for signing
    subordinate certs issued to each of the cluster components.
    Subsequently, the certmgr daemon monitors active certificates and
    renews them when needed. For single node Kubernetes clusters,
    setting <xref linkend="opt-services.kubernetes.easyCerts" /> = true
    is sufficient and no further action is required. For joining extra
    node machines to an existing cluster on the other hand, establishing
    initial trust is mandatory.
  </para>
  <para>
    To add new nodes to the cluster: On any (non-master) cluster node
    where <xref linkend="opt-services.kubernetes.easyCerts" /> is
    enabled, the helper script
    <literal>nixos-kubernetes-node-join</literal> is available on PATH.
    Given a token on stdin, it will copy the token to the kubernetes
    secrets directory and restart the certmgr service. As requested
    certificates are issued, the script will restart kubernetes cluster
    components as needed for them to pick up new keypairs.
  </para>
  <note>
    <para>
      Multi-master (HA) clusters are not supported by the easyCerts
      module.
    </para>
  </note>
  <para>
    In order to interact with an RBAC-enabled cluster as an
    administrator, one needs to have cluster-admin privileges. By
    default, when easyCerts is enabled, a cluster-admin kubeconfig file
    is generated and linked into
    <literal>/etc/kubernetes/cluster-admin.kubeconfig</literal> as
    determined by
    <xref linkend="opt-services.kubernetes.pki.etcClusterAdminKubeconfig" />.
    <literal>export KUBECONFIG=/etc/kubernetes/cluster-admin.kubeconfig</literal>
    will make kubectl use this kubeconfig to access and authenticate the
    cluster. The cluster-admin kubeconfig references an auto-generated
    keypair owned by root. Thus, only root on the kubernetes master may
    obtain cluster-admin rights by means of this file.
  </para>
</chapter>
