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Implementation design for kubeadm

kubeadm init and kubeadm join together provides a nice user experience for creating a best-practice but bare Kubernetes cluster from scratch. However, it might not be obvious how kubeadm does that.

This document strives to explain the phases of work that happen under the hood. Also included is ComponentConfiguration API types for talking to kubeadm programmatically.

Note: Each and every one of the phases must be idempotent!

The scope of kubeadm

The scope of kubeadm init and kubeadm join is to provide a smooth user experience for the user while bootstrapping a best-practice cluster.

The cluster that kubeadm init and kubeadm join set up should be:

  • Secure:
    • It should adopt latest best-practices like:
      • enforcing RBAC
      • using the Node Authorizer
      • using secure communication between the control plane components
      • using secure communication between the API server and the kubelets
      • making it possible to lock-down the kubelet API
      • locking down access to the API system components like the kube-proxy and kube-dns
      • locking down what a Bootstrap Token can access
  • Easy to use:
    • The user should not have to run anything more than a couple of commands, including:
      • kubeadm init on the master
      • export KUBECONFIG=/etc/kubernetes/admin.conf
      • kubectl apply -f <network-of-choice.yaml>
      • kubeadm join --token <token> <master-ip>:<master-port>
      • The kubeadm join request to add a node should be automatically approved
  • Extendable:
    • It should for example not favor any network provider, instead configuring a network is out-of-scope
    • Should provide the possibility to use a config file for customizing various parameters

A note on constants / well-known values and paths

We have to draw the line somewhere about what should be configurable, what shouldn't, and what should be hard-coded in the binary.

We've decided to make the Kubernetes directory /etc/kubernetes a constant in the application, since it is clearly the given path in a majority of cases, and the most intuitive location. Having that path configurable would confuse readers of an on-top-of-kubeadm-implemented deployment solution.

This means we aim to standardize:

  • /etc/kubernetes/manifests as the path where kubelet should look for static Pod manifests
    • Temporarily when bootstrapping, these manifests are present:
      • etcd.yaml
      • kube-apiserver.yaml
      • kube-controller-manager.yaml
      • kube-scheduler.yaml
  • /etc/kubernetes/kubelet.conf as the path where the kubelet should store its credentials to the API server.
  • /etc/kubernetes/admin.conf as the path from where the admin can fetch his/her superuser credentials.
  • Names of certificates files:
    • ca.crt, ca.key (CA certificate)
    • apiserver.crt, apiserver.key (API server certificate)
    • apiserver-kubelet-client.crt, apiserver-kubelet-client.key (client certificate for the apiservers to connect to the kubelets securely)
    • sa.pub, sa.key (a private key for signing ServiceAccount )
    • front-proxy-ca.crt, front-proxy-ca.key (CA for the front proxy)
    • front-proxy-client.crt, front-proxy-client.key (client cert for the front proxy client)
  • Names of kubeconfig files:
    • admin.conf
    • kubelet.conf (bootstrap-kubelet.conf during TLS bootstrap)
    • controller-manager.conf
    • scheduler.conf

kubeadm init phases

kubeadm init internal workflow consists of a sequence of atomic work tasks to perform.

The kubeadm alpha phase command allows users to invoke individually each task, and ultimately offers a reusable and composable API/toolbox that can be used by other Kubernetes bootstrap tools / by any IT automation tool / by advanced user for creating custom clusters.

Preflight checks

kubeadm executes a set of preflight checks before starting the init, with the aim to verify preconditions and avoid common cluster startup problems. In any case the user can skip specific preflight checks (or eventually all preflight checks) with the --ignore-preflight-errors option.

  • [warning] If the Kubernetes version to use (passed with the --kubernetes-version flag) is at least one minor version higher than the kubeadm CLI version.
  • Kubernetes system requirements:
    • if running on linux:
      • [error] if not Kernel 3.10+ or 4+ with specific KernelSpec.
      • [error] if required cgroups subsystem aren't in set up.
    • if using docker:
      • [error/warning] if Docker endpoint does not exist or does not work, if docker version >17.03. Note: starting from 1.9, kubeadm provides better support for CRI-generic functionality; in that case, docker specific controls are skipped or replaced by similar controls for crictl
  • [error] if user is not root.
  • [error] if hostname is not a valid DNS subdomain; [warning] if the host name cannot be reached via network lookup.
  • [error] if kubelet version is lower that the minimum kubelet version supported by kubeadm (current minor -1).
  • [error] if kubelet version is at least one minor higher than the required controlplane version (unsupported version skew).
  • [warning] if kubelet service does not exist, if it is disabled.
  • [warning] if firewalld is active.
  • [error] if API.BindPort or ports 10250/10251/10252 are used.
  • [Error] if /etc/kubernetes/manifest folder already exists and it is not empty.
  • [Error] if /proc/sys/net/bridge/bridge-nf-call-iptables file does not exist/does not contain 1.
  • [Error] if swap is on.
  • [Error] if "ip", "iptables", "mount", "nsenter" commands are not present in the command path.
  • [warning] if "ebtables", "ethtool", "socat", "tc", "touch", "crictl" commands are not present in the command path.
  • [warning] if extra arg flags for API server, controller manager, scheduler contains some invalid options.
  • [warning] if connection to https://API.AdvertiseAddress:API.BindPort goes thought proxy.
  • [warning] if connection to services subnet goes thought proxy (only first address checked).
  • [warning] if connection to Pods subnet goes thought proxy (only first address checked).
  • If using docker:
    • [warning/error] if Docker service does not exist, if it is disabled, if it is not active.
  • If using other cri engine:
    • [error] if crictl socket does not answer.
  • If external etcd is provided:
    • [Error] if etcd version less than 3.0.14.
    • [Error] if certificates or keys are specified, but not provided.
  • If external etcd is NOT provided:
    • [Error] if ports 2379 is used.
    • [Error] if Etcd.DataDir folder already exists and it is not empty.
  • If authorization mode is ABAC, [Error] if abac_policy.json does not exist.
  • If authorization mode is WebHook, [Error] if webhook_authz.conf does not exist.
  • [Error] if advertise address is ipv6 and /proc/sys/net/bridge/bridge-nf-call-ip6tables does not exist/does not contain 1.

Please note that:

  1. Preflight checks can be invoked individually with the kubeadm alpha phase preflight command.

Generate the necessary certificates

kubeadm generates certificate and private key pairs for different purposes. Certificates are stored by default in /etc/kubernetes/pki. This directory is configurable.

There should be:

  • A CA certificate (ca.crt) with its private key (ca.key).
  • An API server certificate (apiserver.crt) using ca.crt as the CA with its private key (apiserver.key). The certificate should:
    • Be a serving server certificate (x509.ExtKeyUsageServerAuth).
    • Contains altnames for:
      • The Kubernetes service's internal clusterIP (the first address in the services CIDR, e.g. 10.96.0.1 if service subnet is 10.96.0.0/12).
      • Kubernetes DNS names (e.g. kubernetes.default.svc.cluster.local if --service-dns-domain is cluster.local, kubernetes.default.svc, kubernetes.default, kubernetes).
      • The node-name.
      • The --apiserver-advertise-address.
      • Optional extra altnames specified by the user.
  • A client certificate for the API server to connect to the kubelets securely (apiserver-kubelet-client.crt) using ca.crt as the CA with its private key (apiserver-kubelet-client.key). The certificate should:
    • Be a client certificate (x509.ExtKeyUsageClientAuth).
    • Be in the system:masters organization.
  • A private key for signing ServiceAccount Tokens (sa.key) along with its public key (sa.pub).
  • A CA for the front proxy (front-proxy-ca.crt) with its key (front-proxy-ca.key).
  • A client cert for the front proxy client with its key (front-proxy-client.crt and front-proxy-client.key) generate using front-proxy-ca.crt as the CA.

Please note that:

  1. If a given certificate and private key pair both exist, and its content is evaluated compliant with the above specs, the existing files will be used and the generation phase for the given certificate skipped. This means the user can, for example, copy an existing CA to /etc/kubernetes/pki/ca.{crt,key} , and then then kubeadm will use those files for signing the rest of the certs.
  2. Only for the CA, it is possible to provide the ca.crt file but not the ca.key file, if all other certificates and kubeconfig files already are in place kubeadm recognize this condition and activates the ExternalCA , which also implies the csrsignercontroller in controller-manager won't be started.
  3. If kubeadm is running in "ExternalCA" mode; all the certificates must be provided as well, because kubeadm cannot generate them by itself.
  4. In case of kubeadm executed in the --dry-run mode, certificates files are written in a temporary folder.
  5. Certificate generation can be invoked individually with the kubeadm alpha phase certs all command.

Generate kubeconfig files for the master components

There should be:

  • A kubeconfig file for kubeadm to use itself and the admin, /etc/kubernetes/admin.conf:
    • The "admin" here is defined as kubeadm itself and the actual person(s) that is administering the cluster and want to control the cluster:
      • With this file, the admin has full control (root) over the cluster.
    • Inside this file, a client certificate is generated from the ca.crt and ca.key. The client cert should:
      • Be a client certificate (x509.ExtKeyUsageClientAuth).
      • Be in the system:masters organization.
      • Include a CN, but that can be anything. kubeadm uses the kubernetes-admin CN.
  • A kubeconfig file for kubelet to use, /etc/kubernetes/kubelet.conf:
    • Inside this file, a client certificate is generated from the ca.crt and ca.key. The client cert should:
      • Be a client certificate (x509.ExtKeyUsageClientAuth).
      • Be in the system:nodes organization.
      • Have the CN system:node:<hostname-lowercased>.
  • A kubeconfig file for controller-manager, /etc/kubernetes/controller-manager.conf:
    • Inside this file, a client certificate is generated from the ca.crt and ca.key. The client cert should:
      • Be a client certificate (x509.ExtKeyUsageClientAuth).
      • Have the CN system:kube-controller-manager.
  • A kubeconfig file for scheduler, /etc/kubernetes/scheduler.conf:
    • Inside this file, a client certificate is generated from the ca.crt and ca.key. The client cert should:
      • Be a client certificate (x509.ExtKeyUsageClientAuth).
      • Have the CN system:kube-scheduler.

Please note that:

  1. ca.crt is embedded in all the kubeconfig files.
  2. If a given kubeconfig file exists, and its content is evaluated compliant with the above specs, the existing file will be used and the generation phase for the given kubeconfig skipped.
  3. If kubeadm is running in ExternalCA mode, all the required kubeconfig must be provided by the user as well, because kubeadm cannot generate any of them by itself.
  4. In case of kubeadm executed in the --dry-run mode, kubeconfig files are written in a temporary folder.
  5. Kubeconfig files generation can be invoked individually with the kubeadm alpha phase kubeconfig all command.

Generate static Pod manifests for control plane

Common properties for the control plane components:

  • All static Pods are deployed on kube-system namespace.
  • All static Pods get tier:control-plane and component:{component-name} labels.
  • All static Pods get scheduler.alpha.kubernetes.io/critical-pod annotation. Note. this will be moved over to the proper solution of using Pod Priority and Preemption when ready.
  • hostNetwork: true is set on all static Pods to allow control plane startup before a network is configured; accordingly:
    • The address that the controller-manager and the scheduler use to refer the API server is 127.0.0.1.
    • If using a local etcd server, etcd-servers address will be set to 127.0.0.1:2379.
  • Leader election is enabled for both the controller-manager and the scheduler.
  • Controller-manager and the scheduler will reference kubeconfig files with their respective, unique identities.
  • All static Pods get any extra flags specified by the user.
  • All static Pods get any extra extra Volumes specified by the user (Host path).

Please note that:

  1. All the images, for the --kubernetes-version/current architecture, will be pulled from gcr.io/google_containers; In case an alternative image repository or CI image repository is specified this one will be used; In case a specific container image should be used for all control plane components, this one will be used.
  2. In case of kubeadm executed in the --dry-run mode, static Pods files are written in a temporary folder.
  3. Static Pod manifest generation for master components can be invoked individually with the kubeadm alpha phase controlplane all command.

API server

The API server needs to know this in particular:

  • The apiserver-advertise-address and apiserver-bind-port to bind to (if not provided, those value defaults to the IP address of the default interface and port 6443).
  • The service-cluster-ip-range to use for services.
  • The etcd-servers address and related TLS settings etcd-cafile, etcd-certfile, etcd-keyfile if required; if an external etcd server won't be provided, a local etcd will be used (via host network).
  • If a cloud provider is specified, the corresponding --cloud-provider is configured, together with the --cloud-config path if such file exists. Note: this is experimental, alpha and will be removed in a future version
  • If kubeadm is invoked with --feature-gates=HighAvailability, the flag --endpoint-reconciler-type=lease is set, thus enabling automatic reconciliation of endpoints for the internal API server VIP.
  • If kubeadm is invoked with --feature-gates=DynamicKubeletConfig, the corresponding feature on kube-apiserver is activated with --feature-gates=DynamicKubeletConfig=true flag.

Other flags that are set unconditionally:

  • --insecure-port=0 to avoid insecure connections to the API server.
  • --enable-bootstrap-token-auth=true to enable the BootstrapTokenAuthenticator authentication module.
  • --allow-privileged to true (used e.g. by kube proxy).
  • --client-ca-file to ca.crt.
  • --tls-cert-file to apiserver.crt.
  • --tls-private-key-file to apiserver.key.
  • --kubelet-client-certificate to apiserver-kubelet-client.crt.
  • --kubelet-client-key to apiserver-kubelet-client.key.
  • --service-account-key-file to sa.pub.
  • --requestheader-client-ca-file to front-proxy-ca.crt.
  • --admission-control to:
  • --kubelet-preferred-address-types to InternalIP,ExternalIP,Hostname; this makes kubectl logs and other apiserver -> kubelet communication work in environments where the hostnames of the nodes aren't resolvable.
  • requestheader-client-ca-file tofront-proxy-ca.crt, proxy-client-cert-file to front-proxy-client.crt, proxy-client-key-file to front-proxy-client.key , and--requestheader-username-headers=X-Remote-User, --requestheader-group-headers=X-Remote-Group, --requestheader-extra-headers-prefix=X-Remote-Extra-, --requestheader-allowed-names=front-proxy-client so the front proxy (API Aggregation) communication is secure.

Controller manager

The controller-manager needs to know this in particular:

  • If kubeadm is invoked specifying a --pod-network-cidr, the Subnet manager feature required for some CNI network plugins is enabled by setting --allocate-node-cidrs=true, --cluster-cidr and --node-cidr-mask-size flags are set.
  • If a cloud provider is specified, the corresponding --cloud-provider is specified, together with the --cloud-config path if such configuration file exists. Note: this is experimental, alpha and will be removed in a future version

Other flags that are set unconditionally:

  • Enables all the default controllers plus BootstrapSigner and TokenCleaner controllers for TLS bootstrap.
  • --root-ca-file to ca.crt.
  • --cluster-signing-cert-file to ca.crt, if External CA mode is disabled, otherwise to "".
  • --cluster-signing-key-file to ca.key, if External CA mode is disabled, otherwise to "".
  • --service-account-private-key-file to sa.key.
  • --use-service-account-credentials to true.

Scheduler

Kubeadm doesn't set any special scheduler flags.

Generate static Pod manifest for local etcd

If the user specified an external etcd this step will be skipped, otherwise a static manifest file will be generated for creating a local etcd instance running in a Pod with following attributes:

  • listen on localhost:2379 and use HostNetwork=true.
  • make a hostPath mount out from the dataDir to the host's filesystem.
  • Any extra flags specified by the user.

Please note that:

  1. The etcd image will be pulled from gcr.io/google_containers. In case an alternative image repository is specified this one will be used; In case an alternative image name is specified, this one will be used.
  2. in case of kubeadm executed in the --dry-run mode, the etcd static Pod manifest is written in a temporary folder.
  3. Static Pod manifest generation for local etcd can be invoked individually with the kubeadm alpha phase etcd local command.

(optional and alpha in v1.9) Write init kubelet configuration

If kubeadm is invoked with --feature-gates=DynamicKubeletConfig, it writes the kubelet init configuration into /var/lib/kubelet/config/init/kubelet file.

The init configuration is used for starting the kubelet on this specific node, providing an alternative for the kubelet drop-in file; such configuration will be replaced by the kubelet base configuration as described in following steps. See online doc for additional info.

Please note that:

  1. to make dynamic kubelet configuration work, flag --dynamic-config-dir=/var/lib/kubelet/config/dynamic should be specified in /etc/systemd/system/kubelet.service.d/10-kubeadm.conf.
  2. kubelet init configuration can be changed by using kubeadm MasterConfiguration file (.kubeletConfiguration.baseConfig).

Wait for the control plane to come up

This is a critical moment in time for kubeadm clusters. kubeadm waits until localhost:6443/healthz returns ok, however in order to detect deadlock conditions, kubeadm fails fast if localhost:10255/healthz (kubelet liveness) or localhost:10255/healthz/syncloop (kubelet readiness) don't return ok, respectively after 40 and 60 second.

kubeadm relies on the kubelet to pull the control plane images and run them properly as static Pods. But there are (as we've seen) a lot of things that can go wrong. Most of them are network/resolv.conf/proxy related.

After the control plane is up, kubeadm completes a couple of tasks described in following paragraphs.

(optional and alpha in v1.9) Write base kubelet configuration

If kubeadm is invoked with --feature-gates=DynamicKubeletConfig:

  1. Write the kubelet base configuration into the kubelet-base-config-v1.9 ConfigMap in the kube-system namespace.
  2. Creates RBAC rules for granting read access to that ConfigMap to all bootstrap tokens and all kubelets (system:bootstrappers:kubeadm:default-node-token and system:nodes groups).
  3. Enable the dynamic kubelet configuration feature for the initial master node by pointing Node.spec.configSource to the newly-created configmap.

Saves kubeadm MasterConfiguration in a ConfigMap for later reference

kubeadm saves the configuration passed to kubeadm init, either via flags or the config file, in a ConfigMap named kubeadm-config under kube-system namespace.

This will ensure that kubeadm actions executed in future (e.g kubeadm upgrade) will be able to determine the actual/current cluster state and make new decisions based on that data.

Please note that

  1. Before uploading, sensitive information like e.g. the token are stripped from the configuration.
  2. Upload of master configuration can be invoked individually with the kubeadm alpha phase upload-config command.
  3. If you initialized your cluster using kubeadm v1.7.x or lower, you must create manually the master configuration ConfigMap before kubeadm upgrade to v1.8 . In order to facilitate this task, the kubeadm config upload (from-flags|from-file) was implemented.

Mark master

As soon as the control plane is available, kubeadm executes following actions:

  • Label the master with node-role.kubernetes.io/master=""
  • Taints the master with node-role.kubernetes.io/master:NoSchedule

Please note that

  1. Mark master phase can be invoked individually with the kubeadm alpha phase mark-master command.

Configure TLS-Bootstrapping for node joining

Kubeadm uses Authenticating with Bootstrap Tokens for joining new nodes to an existing cluster; for more details see also design proposal.

kubeadm init ensures that everything is properly configured for this process, and this includes following steps as well as setting API server and controller flags as already described in previous paragraphs.

Please note that

  1. TLS bootstrapping for nodes can be configured with the kubeadm alpha phase bootstrap-token all command, executing configuration steps described in following paragraphs; alternatively, each step can be invoked individually.

Create a bootstrap token

kubeadm init create a first bootstrap token, either generated automatically or provided by the user with the --token flag; as documented in bootstrap token specification, token should be saved as secrets with name bootstrap-token-<token-id> under kube-system namespace.

Please note that

  1. The default token created by kubeadm init will be used to validate temporary user during TLS bootstrap process; those users will be member of system:bootstrappers:kubeadm:default-node-token group.
  2. The token has a limited validity, default 24 hours (the interval may be changed with the —token-ttl flag)
  3. Additional tokens can be created with the kubeadm token command, that provide as well other useful functions for token management .

Allow joining nodes to call CSR API

kubeadm ensure that users in system:bootstrappers:kubeadm:default-node-token group are able to access the certificate signing API.

This is implemented by creating a ClusterRoleBinding named kubeadm:kubelet-bootstrap between the group above and the default RBAC role system:node-bootstrapper.

Setup auto approval for new bootstrap tokens

kubeadm ensures that the Bootstrap Token will get its CSR request automatically approved by the csrapprover controller.

This is implemented by creating ClusterRoleBinding named kubeadm:node-autoapprove-bootstrap between the system:bootstrappers:kubeadm:default-node-token group and the default role system:certificates.k8s.io:certificatesigningrequests:nodeclient.

The role system:certificates.k8s.io:certificatesigningrequests:nodeclient should be created as well, granting POST permission to /apis/certificates.k8s.io/certificatesigningrequests/nodeclient (in v1.8 role will be automatically created by default).

Setup nodes certificate rotation with auto approval

kubeadm ensures that certificate rotation is enabled for nodes, and that new certificate request for nodes will get its CSR request automatically approved by the csrapprover controller.

This is implemented by creating ClusterRoleBinding named kubeadm:node-autoapprove-certificate-rotation between the system:nodes group and the default role system:certificates.k8s.io:certificatesigningrequests:selfnodeclient.

Create the public cluster-info ConfigMap

This phase creates the cluster-info ConfigMap in the kube-public namespace.

Additionally it is created a role and a RoleBinding granting access for to the ConfigMap for unauthenticated users (i.e. users in RBAC group system:unauthenticated)

Please note that

  1. The access to the cluster-info ConfigMap is not rate-limited. This may or may not be a problem if you expose your master to the internet; worst-case scenario here is a DoS attack where an attacker uses all the in-flight requests the kube-apiserver can handle to serving the cluster-info ConfigMap.

Install kube-proxy addon

A ServiceAccount for kube-proxy is created in the kube-system namespace; then kube-proxy is deployed as a DaemonSet:

  • the credentials (ca.crt and token) to the master come from the ServiceAccount
  • the location of the master comes from a ConfigMap
  • the kube-proxy ServiceAccount is bound to the privileges in the system:node-proxier ClusterRole

Please note that

  1. This phase can be invoked individually with the kubeadm alpha phase addon kube-proxy command.

Install dns addon

A ServiceAccount for kube-dns is created in the kube-system namespace.

Deploy the kube-dns Deployment and Service:

  • it's the upstream kube-dns deployment relatively unmodified
  • the kube-dns ServiceAccount is bound to the privileges in the system:kube-dns ClusterRole

Please note that

  1. If kubeadm is invoked with --feature-gates=CoreDNS, CoreDNS is installed instead of kube-dns.
  2. This phase can be invoked individually with the kubeadm alpha phase addon kube-dns command.

(Optional and alpha in v1.9) self-hosting

This phase is performed only if kubeadm init is invoked with —features-gates=self-hosting

The self hosting phase basically replaces static Pods for control plane components with DaemonSets; this is achieved by executing following procedure for API server, scheduler and controller manager static Pods:

  • Load the static Pod specification from disk
  • Extract the PodSpec from that static Pod specification
  • Mutate the PodSpec to be compatible with self-hosting, and more in detail:
    • add node selector attribute targeting nodes withnode-role.kubernetes.io/master="" label,
    • add a toleration for node-role.kubernetes.io/master:NoSchedule taint,
    • set spec.DNSPolicy to ClusterFirstWithHostNet
  • Build a new DaemonSet object for the self-hosted component in question. Use the above mentioned PodSpec
  • Create the DaemonSet resource in kube-system namespace. Wait until the Pods are running.
  • Remove the static Pod manifest file. The kubelet will stop the original static Pod-hosted component that was running

Please note that:

  1. Self hosting is not yet resilient to node restarts; this can be fixed with external checkpointing or with kubelet checkpointing for the control plane Pods.

  2. If invoked with —features-gates=StoreCertsInSecrets following additional steps will be executed

    • creation of ca, apiserver, apiserver-kubelet-client, sa, front-proxy-ca, front-proxy-client TLS secrets in kube-system namespace with respective certificates and keys

      Note: Please note that storing the CA key in a Secret might have security implications.

    • creation of scheduler.conf and controller-manager.conf secrets inkube-system namespace with respective kubeconfig files

    • mutation of all the Pod specs by replacing host path volumes with projected volumes from the secrets above

  3. This phase can be invoked individually with the kubeadm alpha phase selfhosting convert-from-staticpods command.

kubeadm join phases

Similarly to kubeadm init, also kubeadm join internal workflow consists of a sequence of atomic work tasks to perform.

This is split into discovery (having the Node trust the Kubernetes Master) and TLS bootstrap (having the Kubernetes Master trust the Node).

see Authenticating with Bootstrap Tokens , design proposal.

Preflight checks

kubeadm executes a set of preflight checks before starting the join, with the aim to verify preconditions and avoid common cluster startup problems.

Please note that:

  1. kubeadm join preflight checks are basically a subset kubeadm init preflight checks
  2. Starting from 1.9, kubeadm provides better support for CRI-generic functionality; in that case, docker specific controls are skipped or replaced by similar controls for crictl.
  3. Starting from 1.9, kubeadm provides support for joining nodes running on Windows; in that case, linux specific controls are skipped.
  4. In any case the user can skip specific preflight checks (or eventually all preflight checks) with the --ignore-preflight-errors option.

Discovery cluster-info

There are 2 main schemes for discovery. The first is to use a shared token along with the IP address of the API server. The second is to provide a file (a subset of the standard kubeconfig file).

Shared token discovery

If kubeadm join is invoked with --discovery-token, token discovery is used; in this case the node basically retrieves the cluster CA certificates from the cluster-info ConfigMap in the kube-public namespace.

In order to prevent "man in the middle" attacks, several steps are taken:

  • First, the CA certificate is retrieved via insecure connection (note: this is possible because kubeadm init granted access to cluster-info users for system:unauthenticated )
  • Then the CA certificate goes through following validation steps:
    • "Basic validation", using the token ID against a JWT signature
    • "Pub key validation", using provided --discovery-token-ca-cert-hash. This value is available in the output of "kubeadm init" or can be calculated using standard tools (the hash is calculated over the bytes of the Subject Public Key Info (SPKI) object as in RFC7469). The --discovery-token-ca-cert-hash flag may be repeated multiple times to allow more than one public key.
    • as a additional validation, the CA certificate is retrieved via secure connection and then compared with the CA retrieved initially

Please note that:

  1. "Pub key validation" can be skipped passing --discovery-token-unsafe-skip-ca-verification flag; This weakens the kubeadm security model since others can potentially impersonate the Kubernetes Master.

File/https discovery

If kubeadm join is invoked with --discovery-file, file discovery is used; this file can be a local file or downloaded via an HTTPS URL; in case of HTTPS, the host installed CA bundle is used to verify the connection.

With file discovery, the cluster CA certificates is provided into the file itself; in fact, the discovery file is a kubeconfig file with only server and certificate-authority-data attributes set, e.g.:

apiVersion: v1
clusters:
- cluster:
    certificate-authority-data: <really long certificate data>
    server: https://10.138.0.2:6443
  name: ""
contexts: []
current-context: ""
kind: Config
preferences: {}
users: []

Finally, when the connection with the cluster is established, kubeadm try to access the cluster-info ConfigMap, and if available, uses it.

TLS Bootstrap

Once the cluster info are known, the file bootstrap-kubelet.conf is written, allowing kubelet to do TLS Bootstrapping (conversely in v.1.7 TLS were managed by kubeadm).

The TLS bootstrap mechanism uses the shared token to temporarily authenticate with the Kubernetes Master to submit a certificate signing request (CSR) for a locally created key pair.

The request is then automatically approved and the operation completes saving ca.crt file and kubelet.conf file to be used by kubelet for joining the cluster, whilebootstrap-kubelet.conf is deleted.

Please note that:

  • The temporary authentication is validated against the token saved during the kubeadm init process (or with additional tokens created with kubeadm token)
  • The temporary authentication resolve to a user member of system:bootstrappers:kubeadm:default-node-token group which was granted access to CSR api during the kubeadm init process
  • The automatic CSR approval is managed by the csrapprover controller, according with configuration done the kubeadm init process

(optional) Write init kubelet configuration

If kubeadm is invoked with --feature-gates=DynamicKubeletConfig:

  1. Read the kubelet base configuration from the kubelet-base-config-v1.9 ConfigMap in the kube-system namespace using the Bootstrap Token credentials, and write it to disk as kubelet init configuration file /var/lib/kubelet/config/init/kubelet.
  2. As soon as kubelet starts with the Node's own credential (/etc/kubernetes/kubelet.conf), update current node configuration specifying that the source for the node/kubelet configuration is the above ConfigMap.

Please note that:

  1. to make dynamic kubelet configuration work, flag --dynamic-config-dir=/var/lib/kubelet/config/dynamic should be specified in /etc/systemd/system/kubelet.service.d/10-kubeadm.conf.

Extending kubeadm

There are two primary ways to extend kubeadm:

  • By setting CLI arguments or editing the lightweight kubeadm init API.
  • By running the phases you need separately and giving every phase the arguments it needs

The kubeadm init and kubeadm join APIs respectively are very limited in scope by design; That is where kubeadm alpha phase comes in, which gives you full power of the cluster creation.