Implementation details

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 provides additional details on what happen under the hood, with the aim of sharing knowledge on Kubernetes cluster best practices.

Core design principles

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
  • lock-down the kubelet API
  • locking down access to the API for system components like the kube-proxy and CoreDNS
  • locking down what a Bootstrap Token can access
• User-friendly: The user should not have to run anything more than a couple of commands:
  • kubeadm init
  • export KUBECONFIG=/etc/kubernetes/admin.conf
  • kubectl apply -f <network-of-choice.yaml>
  • kubeadm join --token <token> <endpoint>:<port>
• Extendable:
  • It should not favor any particular network provider. Configuring the cluster network is out-of-scope
  • It should provide the possibility to use a config file for customizing various parameters

Constants and well-known values and paths

In order to reduce complexity and to simplify development of higher level tools that build on top of kubeadm, it uses a limited set of constant values for well-known paths and file names.

The Kubernetes directory /etc/kubernetes is a constant in the application, since it is clearly the given path in a majority of cases, and the most intuitive location; other constants paths and file names are:

• /etc/kubernetes/manifests as the path where kubelet should look for static Pod manifests. Names of static Pod manifests are:
  • etcd.yaml
  • kube-apiserver.yaml
  • kube-controller-manager.yaml
  • kube-scheduler.yaml
• /etc/kubernetes/ as the path where kubeconfig files with identities for control plane components are stored. Names of kubeconfig files are:
  • kubelet.conf (bootstrap-kubelet.conf during TLS bootstrap)
  • controller-manager.conf
  • scheduler.conf
  • admin.conf for the cluster admin and kubeadm itself
• Names of certificates and key files :
  • ca.crt, ca.key for the Kubernetes certificate authority
  • apiserver.crt, apiserver.key for the API server certificate
  • apiserver-kubelet-client.crt, apiserver-kubelet-client.key for the client certificate used by the API server to connect to the kubelets securely
  • sa.pub, sa.key for the key used by the controller manager when signing ServiceAccount
  • front-proxy-ca.crt, front-proxy-ca.key for the front proxy certificate authority
  • front-proxy-client.crt, front-proxy-client.key for the front proxy client

kubeadm init workflow internal design

The kubeadm init internal workflow consists of a sequence of atomic work tasks to perform, as described in kubeadm init.

The kubeadm init phase command allows users to invoke each task individually, and ultimately offers a reusable and composable API/toolbox that can be used by other Kubernetes bootstrap tools, by any IT automation tool or by an 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. The user can skip specific preflight checks or all of them with the --ignore-preflight-errors option.

• [warning] If the Kubernetes version to use (specified 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 Kernel is older than the minimum required version
    • [error] if required cgroups subsystem aren't in set up
  • if using docker:
    • [warning/error] if Docker service does not exist, if it is disabled, if it is not active.
    • [error] if Docker endpoint does not exist or does not work
    • [warning] if docker version is not in the list of validated docker versions
  • If using other cri engine:
    • [error] if crictl socket does not answer
• [error] if user is not root
• [error] if the machine 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 or if it is disabled
• [warning] if firewalld is active
• [error] if API server 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 advertise address is ipv6 and /proc/sys/net/bridge/bridge-nf-call-ip6tables does not exist/does not contain 1.
• [Error] if swap is on
• [Error] if conntrack, 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 through proxy
• [warning] if connection to services subnet goes through proxy (only first address checked)
• [warning] if connection to Pods subnet goes through proxy (only first address checked)
• If external etcd is provided:
  • [Error] if etcd version is older than the minimum required version
  • [Error] if etcd certificates or keys are specified, but not provided
• If external etcd is NOT provided (and thus local etcd will be installed):
  • [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

Please note that:

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

Generate the necessary certificates

Kubeadm generates certificate and private key pairs for different purposes:

• A self signed certificate authority for the Kubernetes cluster saved into ca.crt file and ca.key private key file
• A serving certificate for the API server, generated using ca.crt as the CA, and saved into apiserver.crt file with its private key apiserver.key. This certificate should contain following alternative names:
  • 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 flag value is cluster.local, plus default DNS names kubernetes.default.svc, kubernetes.default, kubernetes
  • The node-name
  • The --apiserver-advertise-address
  • Additional alternative names specified by the user
• A client certificate for the API server to connect to the kubelets securely, generated using ca.crt as the CA and saved into apiserver-kubelet-client.crt file with its private key apiserver-kubelet-client.key. This certificate should be in the system:masters organization
• A private key for signing ServiceAccount Tokens saved into sa.key file along with its public key sa.pub
• A certificate authority for the front proxy saved into front-proxy-ca.crt file with its key front-proxy-ca.key
• A client cert for the front proxy client, generate using front-proxy-ca.crt as the CA and saved into front-proxy-client.crt file with its private keyfront-proxy-client.key

Certificates are stored by default in /etc/kubernetes/pki, but this directory is configurable using the --cert-dir flag.

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 kubeadm will use those files for signing the rest of the certs. See also using custom certificates
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 external CA mode; all the certificates must be provided by the user, because kubeadm cannot generate them by itself
4. In case of kubeadm is executed in the --dry-run mode, certificates files are written in a temporary folder
5. Certificate generation can be invoked individually with the kubeadm init phase certs all command

Generate kubeconfig files for control plane components

Kubeadm generates kubeconfig files with identities for control plane components:

• A kubeconfig file for the kubelet to use during TLS bootstrap - /etc/kubernetes/bootstrap-kubelet.conf. Inside this file there is a bootstrap-token or embedded client certificates for authenticating this node with the cluster. This client cert should:
  • Be in the system:nodes organization, as required by the Node Authorization module
  • Have the Common Name (CN) system:node:<hostname-lowercased>
• A kubeconfig file for controller-manager, /etc/kubernetes/controller-manager.conf; inside this file is embedded a client certificate with controller-manager identity. This client cert should have the CN system:kube-controller-manager, as defined by default RBAC core components roles
• A kubeconfig file for scheduler, /etc/kubernetes/scheduler.conf; inside this file is embedded a client certificate with scheduler identity. This client cert should have the CN system:kube-scheduler, as defined by default RBAC core components roles

Additionally, a kubeconfig file for kubeadm itself and the admin is generated and saved into the /etc/kubernetes/admin.conf file. The "admin" here is defined as the actual person(s) that is administering the cluster and wants to have full control (root) over the cluster. The embedded client certificate for admin should be in the system:masters organization, as defined by default RBAC user facing role bindings. It should also include a CN. Kubeadm uses the kubernetes-admin CN.

Please note that:

1. ca.crt certificate 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 is 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 init phase kubeconfig all command

Generate static Pod manifests for control plane components

Kubeadm writes static Pod manifest files for control plane components to /etc/kubernetes/manifests. The kubelet watches this directory for Pods to create on startup.

Static Pod manifest share a set of common properties:

• All static Pods are deployed on kube-system namespace
• All static Pods get tier:control-plane and component:{component-name} labels
• All static Pods use the system-node-critical priority class
• hostNetwork: true is set on all static Pods to allow control plane startup before a network is configured; as a consequence:
  • 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 as described in passing custom arguments to control plane components
• All static Pods get any extra Volumes specified by the user (Host path)

Please note that:

1. All images will be pulled from k8s.gcr.io by default. See using custom images for customizing the image repository
2. In case of kubeadm is executed in the --dry-run mode, static Pods files are written in a temporary folder
3. Static Pod manifest generation for control plane components can be invoked individually with the kubeadm init phase control-plane all command

API server

The static Pod manifest for the API server is affected by following parameters provided by the users:

• The apiserver-advertise-address and apiserver-bind-port to bind to; if not provided, those value defaults to the IP address of the default network interface on the machine and port 6443
• The service-cluster-ip-range to use for services
• If an external etcd server is specified, the etcd-servers address and related TLS settings (etcd-cafile, etcd-certfile, etcd-keyfile); if an external etcd server is not 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 (this is experimental, alpha and will be removed in a future version)

Other API server flags that are set unconditionally are:

• --insecure-port=0 to avoid insecure connections to the api server
• --enable-bootstrap-token-auth=true to enable the BootstrapTokenAuthenticator authentication module. See TLS Bootstrapping for more details
• --allow-privileged to true (required e.g. by kube proxy)
• --requestheader-client-ca-file to front-proxy-ca.crt
• --enable-admission-plugins to:
  • NamespaceLifecycle e.g. to avoid deletion of system reserved namespaces
  • LimitRanger and ResourceQuota to enforce limits on namespaces
  • ServiceAccount to enforce service account automation
  • PersistentVolumeLabel attaches region or zone labels to PersistentVolumes as defined by the cloud provider (This admission controller is deprecated and will be removed in a future version. It is not deployed by kubeadm by default with v1.9 onwards when not explicitly opting into using gce or aws as cloud providers)
  • DefaultStorageClass to enforce default storage class on PersistentVolumeClaim objects
  • DefaultTolerationSeconds
  • NodeRestriction to limit what a kubelet can modify (e.g. only pods on this node)
• --kubelet-preferred-address-types to InternalIP,ExternalIP,Hostname; this makes kubectl logs and other API server-kubelet communication work in environments where the hostnames of the nodes aren't resolvable
• Flags for using certificates generated in previous steps:
  • --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 tofront-proxy-ca.crt
  • --proxy-client-cert-file to front-proxy-client.crt
  • --proxy-client-key-file to front-proxy-client.key
• Other flags for securing the front proxy (API Aggregation) communications:
  • --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

Controller manager

The static Pod manifest for the controller manager is affected by following parameters provided by the users:

• 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 according to the given CIDR
• If a cloud provider is specified, the corresponding --cloud-provider is specified, together with the --cloud-config path if such configuration file exists (this is experimental, alpha and will be removed in a future version)

Other flags that are set unconditionally are:

• --controllers enabling all the default controllers plus BootstrapSigner and TokenCleaner controllers for TLS bootstrap. See TLS Bootstrapping for more details
• --use-service-account-credentials to true
• Flags for using certificates generated in previous steps:
  • --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

Scheduler

The static Pod manifest for the scheduler is not affected by parameters provided by the users.

Generate static Pod manifest for local etcd

If the user specified an external etcd this step will be skipped, otherwise kubeadm generates a static Pod manifest file 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 k8s.gcr.io by default. See using custom images for customizing the image repository
2. in case of kubeadm is 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 init phase etcd local command

Wait for the control plane to come up

kubeadm waits (upto 4m0s) until localhost:6443/healthz (kube-apiserver liveness) 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 within 40s and 60s respectively.

kubeadm relies on the kubelet to pull the control plane images and run them properly as static Pods. After the control plane is up, kubeadm completes the tasks described in following paragraphs.

Save the kubeadm ClusterConfiguration in a ConfigMap for later reference

kubeadm saves the configuration passed to kubeadm init 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 saving the ClusterConfiguration, sensitive information like the token is stripped from the configuration
2. Upload of control plane node configuration can be invoked individually with the kubeadm init phase upload-config command

Mark the node as control-plane

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

• Labels the node as control-plane with node-role.kubernetes.io/master=""
• Taints the node with node-role.kubernetes.io/master:NoSchedule

Please note that:

1. Mark control-plane phase phase can be invoked individually with the kubeadm init phase mark-control-plane 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 init phase bootstrap-token command, executing all the 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 ensures 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.

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 creates a Role and a RoleBinding granting access 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 cluster's API server 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 addons

Kubeadm installs the internal DNS server and the kube-proxy addon components via the API server. Please note that:

1. This phase can be invoked individually with the kubeadm init phase addon all command.

proxy

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 control plane come from the ServiceAccount
• The location (URL) of the API server comes from a ConfigMap
• The kube-proxy ServiceAccount is bound to the privileges in the system:node-proxier ClusterRole

DNS

• The CoreDNS service is named kube-dns. This is done to prevent any interruption in service when the user is switching the cluster DNS from kube-dns to CoreDNS the --config method described here.
• A ServiceAccount for CoreDNS is created in the kube-system namespace.
• The coredns ServiceAccount is bound to the privileges in the system:coredns ClusterRole

In Kubernetes version 1.21, support for using kube-dns with kubeadm was removed. You can use CoreDNS with kubeadm even when the related Service is named kube-dns.

kubeadm join phases internal design

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 or the corresponding 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 (that is 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 (this is possible because kubeadm init granted access to cluster-info users for system:unauthenticated )
• Then the CA certificate goes trough 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, as described in kubeadm join reference doc; 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, thus allowing kubelet to do TLS Bootstrapping.

The TLS bootstrap mechanism uses the shared token to temporarily authenticate with the Kubernetes API server 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