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Kubeadm true

kubeadm Setup Tool Reference Guide

This document provides information on how to use kubeadm's advanced options.

  • TOC {:toc}

Running kubeadm init bootstraps a Kubernetes master node. This consists of the following steps:

  1. kubeadm runs a series of pre-flight checks to validate the system state before making changes. Some checks only trigger warnings, others are considered errors and will exit kubeadm until the problem is corrected or the user specifies --skip-preflight-checks.

  2. kubeadm generates a token that additional nodes can use to register themselves with the master in future. Optionally, the user can provide a token via --token, as described in the section on managing tokens below.

  3. kubeadm generates a self-signed CA to provision identities for each component (including nodes) in the cluster. It also generates client certificates to be used by various components. If the user has provided their own CA by dropping it in the cert directory configured via --cert-dir (/etc/kubernetes/pki by default) this step is skipped as described in the section on using custom certificates.

  4. kubeadm writes kubeconfig files in /etc/kubernetes/ for the kubelet, the controller-manager and the scheduler to use to connect to the API server, each one with their respective identities, as well as an additional kubeconfig file for administration.

  5. kubeadm generates static Pod manifests for the API server, controller manager and scheduler; in case an external etcd is not provided, an additional static Pod manifest will be generated for etcd.

    Static Pod manifests are written in /etc/kubernetes/manifests; the kubelet watches this directory for Pods to create on startup, as described in the section about kubelet drop-in.

    Once control plane Pods are up and running kubeadm init sequence can continue.

  6. kubeadm "labels" and "taints" the master node so that only control plane components will run there.

  7. Kubeadm makes all the necessary configurations for allowing node joining with the Bootstrap Tokens and TLS Bootstrap mechanism:

    • Write a ConfigMap for making available all the information required for joining and set up related RBAC access rules.

    • Ensure access to the CSR signing API for bootstrap tokens.

    • Configure auto approval for new CSR requests.

    See Securing your installation for hardening.

  8. kubeadm installs add-on components via the API server. Right now this is the internal DNS server and the kube-proxy DaemonSet.

  9. If kubeadm init is invoked with the alpha self-hosting feature enabled, (--feature-gates=SelfHosting=true), the static Pod based control plane will be transformed in a self-hosed control plane.

Running kubeadm join on each node in the cluster consists of the following steps:

  1. kubeadm downloads necessary cluster information from the API server. By default, it uses the bootstrap token and the CA key hash to verify the authenticity of that data. The root CA can also be discovered directly via a file or URL.

  2. Once the cluster information are known, kubelet can start the TLS bootstrapping process (in v.1.7 this step was managed by kubeadm).

    The TLS bootstrap uses the shared token to temporarily authenticate with the Kubernetes Master to submit a certificate signing request (CSR); by default the control plane will sign this CSR request automatically.

  3. Finally, kubeadm will configures the local kubelet to connect to the API server with the definitive identity assigned to the node.

Usage

Fields that support multiple values do so either with comma separation, or by specifying the flag multiple times.

The kubeadm command line interface is currently in beta. We are aiming to not break any scripted use of the main kubeadm init and kubeadm join. Exceptions to this are documented below.

kubeadm init

It is usually sufficient to run kubeadm init without any flags, but in some cases you might like to override the default behaviour. Here we specify all the flags that can be used to customise the Kubernetes installation.

Options for kubeadm init:

  • --apiserver-advertise-address

    This is the address the API Server will advertise to other members of the cluster. This is also the address used to construct the suggested kubeadm join line at the end of the init process. If not set (or set to 0.0.0.0) then IP for the default interface will be used.

    This address is also added to the certificate that the API Server uses.

  • --apiserver-bind-port

    The port that the API server will bind on. This defaults to 6443.

  • --apiserver-cert-extra-sans

    Additional hostnames or IP addresses that should be added to the Subject Alternate Name section for the certificate that the API Server will use. If you expose the API Server through a load balancer and public DNS you could specify this with

    --apiserver-cert-extra-sans=kubernetes.example.com,kube.example.com,10.100.245.1

  • --cert-dir

    The path where to save and store the certificates. The default is "/etc/kubernetes/pki".

  • --config

    A kubeadm specific config file. This can be used to specify an extended set of options including passing arbitrary command line flags to the control plane components.

    Note: Since 1.8, other flags are not allowed alongside --config except for flags used to define kubeadm behaviour (not configuration) such as --skip-preflight-checks.

  • --dry-run

    This flag tells kubeadm to don't apply any changes; just output what would be done.

  • --feature-gates

    A set of key=value pairs that describe feature gates for alpha/experimental features. Options are:

    • SelfHosting=true|false (ALPHA - default=false)

    • StoreCertsInSecrets=true|false (ALPHA - default=false)

    See self-hosted control plane for more detail.

  • --kubernetes-version (default 'latest') the kubernetes version to initialise

    The v1.6 version of kubeadm only supports building clusters that are at least v1.6.0. There are many reasons for this including kubeadm's use of RBAC, the Bootstrap Token system, and enhancements to the Certificates API. With this flag you can try any future version of Kubernetes. Check releases page for a full list of available versions.

  • --node-name

    Allow to specify the node name, if something different than O.S. hostname should be used e.g. in case of Amazon EC2 instances.

    The node-name is also added to the certificate that the API Server uses.

  • --pod-network-cidr

    For certain networking solutions the Kubernetes master can also play a role in allocating network ranges (CIDRs) to each node. This includes many cloud providers and flannel. You can specify a subnet range that will be broken down and handed out to each node with the --pod-network-cidr flag. This should be a minimum of a /16 so controller-manager is able to assign /24 subnets to each node in the cluster. If you are using flannel with this manifest you should use --pod-network-cidr=10.244.0.0/16. Most CNI based networking solutions do not require this flag.

  • --service-cidr (default '10.96.0.0/12')

    You can use the --service-cidr flag to override the subnet Kubernetes uses to assign pods IP addresses. If you do, you will also need to update the /etc/systemd/system/kubelet.service.d/10-kubeadm.conf file to reflect this change else DNS will not function correctly.

  • --service-dns-domain (default 'cluster.local')

    By default, kubeadm init deploys a cluster that assigns services with DNS names <service_name>.<namespace>.svc.cluster.local. You can use the --service-dns-domain to change the DNS name suffix. Again, you will need to update the /etc/systemd/system/kubelet.service.d/10-kubeadm.conf file accordingly else DNS will not function correctly.

    Note: This flag has an effect (it's needed for the kube-dns Deployment manifest and the API Server's serving certificate) but not as you might expect, since you will have to modify the arguments to the kubelets in the cluster for it to work fully. Specifying DNS parameters using this flag only is not enough. Rewriting the kubelet's CLI arguments is out of scope for kubeadm as it should be agnostic to how you run the kubelet. However, making all kubelets in the cluster pick up information dynamically via the API is in scope and is a planned feature for upcoming releases.

  • --skip-preflight-checks

    By default, kubeadm runs a series of preflight checks to validate the system before making any changes. Advanced users can use this flag to bypass these if necessary.

  • --skip-token-print

    By default, kubeadm prints the token at the end of the init procedure. Advanced users can use this flag to bypass these if necessary.

  • --token

    By default, kubeadm init automatically generates the token used to initialise each new node. If you would like to manually specify this token, you can use the --token flag. The token must be of the format [a-z0-9]{6}\.[a-z0-9]{16}. A compatible random token can be generated kubeadm token generate. Tokens can be managed through the API after the cluster is created. See the section on managing tokens below.

  • --token-ttl

    This sets an expiration time for the token. This is specified as a duration from the current time. After this time the token will no longer be valid and will be removed. A value of 0 specifies that the token never expires. The default is 24 hours. See the section on managing tokens below.

kubeadm join

When joining a kubeadm initialized cluster, we need to establish bidirectional trust. This is split into discovery (having the Node trust the Kubernetes master) and TLS bootstrap (having the Kubernetes master trust the Node).

There are two main schemes for discovery:

  • Using a shared token along with the IP address of the API server and a hash of the root CA key:

    kubeadm join --discovery-token abcdef.1234567890abcdef --discovery-token-ca-cert-hash sha256:1234..cdef 1.2.3.4:6443

  • Using a file (a subset of the standard kubeconfig file). This file can be a local file or downloaded via an HTTPS URL:

    kubeadm join --discovery-file path/to/file.conf

    kubeadm join --discovery-file https://url/file.conf

Only one form can be used. If the discovery information is loaded from a URL, HTTPS must be used and the host installed CA bundle is used to verify the connection. For details on the security tradeoffs of these mechanisms, see the security model section below.

The TLS bootstrap mechanism is also driven via a shared token. This is used to temporarily authenticate with the Kubernetes master to submit a certificate signing request (CSR) for a locally created key pair. By default kubeadm will set up the Kubernetes master to automatically approve these signing requests. This token is passed in with the --tls-bootstrap-token abcdef.1234567890abcdef flag.

Often times the same token is used for both parts. In this case, the --token flag can be used instead of specifying the each token individually.

Here's an example on how to use it:

kubeadm join --token=abcdef.1234567890abcdef --discovery-token-ca-cert-hash sha256:1234..cdef 192.168.1.1:6443

Options for kubeadm join:

  • --config

    Extended options are specified in the kubeadm specific config file.

  • --discovery-file

    A local file path or HTTPS URL. The file specified must be a kubeconfig file with nothing but an unnamed cluster entry. This is used to find both the location of the API server to join along with a root CA bundle to use when talking to that server.

    This might look something like this:

    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: []

  • --discovery-token

    The discovery token is used along with the address of the API server (as an unnamed argument) to download and verify information about the cluster. The most critical part of the cluster information is the root CA bundle used to verify the identity of the server during subsequent TLS connections.

  • --discovery-token-ca-cert-hash

    The CA key hash is used to verify the full root CA certificate discovered during token-based bootstrapping. It has the format sha256:<hex_encoded_hash>. By default, the hash value is returned in the kubeadm join command printed at the end of kubeadm init. It is in a standard format (see RFC7469) and can also be calculated by 3rd party tools or provisioning systems. For example, using the OpenSSL CLI: openssl x509 -pubkey -in /etc/kubernetes/pki/ca.crt | openssl rsa -pubin -outform der 2>&/dev/null | openssl dgst -sha256 -hex

    Skipping this flag is allowed in Kubernetes 1.8, but makes certain spoofing attacks possible. See the security model for details. Pass --discovery-token-unsafe-skip-ca-verification to silence warnings (which will become errors in Kubernetes 1.9).

  • --discovery-token-unsafe-skip-ca-verification

    Disable the warning/error that occurs when --discovery-token-ca-cert-hash is not provided. Passing this flag is an acknowledgement of the security tradeoffs involved in skipping this verification (which may or may not be appropriate in your environment).

  • --node-name

    Specify the Node name. The default is to use the OS hostname. This is useful on some cloud providers such as AWS. This name is also added to the node's TLS certificate.

  • --tls-bootstrap-token

    The token used to authenticate to the API server for the purposes of TLS bootstrapping.

  • --token=<token>

    Often times the same token is used for both --discovery-token and --tls-bootstrap-token. This option specifies the same token for both. Other flags override this flag if present.

  • --skip-preflight-checks

    By default, kubeadm runs a series of preflight checks to validate the system before making any changes. Advanced users can use this flag to bypass these if necessary.

kubeadm completion

Output shell completion code for the specified shell (bash or zsh).

kubeadm config

Kubeadm v1.8.0+ automatically creates a ConfigMap with all the parameters used during kubeadm init.

If you initialized your cluster using kubeadm v1.7.x or lower, you must use the kubeadm config upload command to create this ConfigMap in order for kubeadm upgrade to be able to configure your upgraded cluster correctly.

kubeadm reset

Reverts any changes made to this host by kubeadm init or kubeadm join.

kubeadm token

Manage tokens on a running cluster. See managing tokens below for further details.

kubeadm alpha phases

WARNING: While kubeadm command line interface is in beta, commands under this entry is still considered alpha and may change in future versions.

kubeadm phase introduces a set of kubeadm CLI commands allowing to invoke individually each phase of the kubeadm init sequence; phases provide a reusable and composable API/toolbox for building your own automated cluster installer.

Options for kubeadm phases:

Each kubeadm phase exposes a subset of relevant options from kubeadm init.

Using kubeadm with a configuration file

WARNING: While kubeadm command line interface is in beta, the config file is still considered alpha and may change in future versions.

It's possible to configure kubeadm with a configuration file instead of command line flags, and some more advanced features may only be available as configuration file options. This file is passed in to the --config option on both kubeadm init and kubeadm join.

Sample Master Configuration

apiVersion: kubeadm.k8s.io/v1alpha1
kind: MasterConfiguration
api:
  advertiseAddress: <address|string>
  bindPort: <int>
etcd:
  endpoints:
  - <endpoint1|string>
  - <endpoint2|string>
  caFile: <path|string>
  certFile: <path|string>
  keyFile: <path|string>
  dataDir: <path|string>
  extraArgs:
    <argument>: <value|string>
    <argument>: <value|string>
  image: <string>
networking:
  dnsDomain: <string>
  serviceSubnet: <cidr>
  podSubnet: <cidr>
kubernetesVersion: <string>
cloudProvider: <string>
nodeName: <string>
authorizationModes:
- <authorizationMode1|string>
- <authorizationMode2|string>
token: <string>
tokenTTL: <time duration>
selfHosted: <bool>
apiServerExtraArgs:
  <argument>: <value|string>
  <argument>: <value|string>
controllerManagerExtraArgs:
  <argument>: <value|string>
  <argument>: <value|string>
schedulerExtraArgs:
  <argument>: <value|string>
  <argument>: <value|string>
apiServerCertSANs:
- <name1|string>
- <name2|string>
certificatesDir: <string>
imageRepository: <string>
unifiedControlPlaneImage: <string>
featureGates:
  <feature>: <bool>
  <feature>: <bool>

In addition, if authorizationMode is set to ABAC, you should write the config to /etc/kubernetes/abac_policy.json. However, if authorizationMode is set to Webhook, you should write the config to /etc/kubernetes/webhook_authz.conf.

Sample Node Configuration

apiVersion: kubeadm.k8s.io/v1alpha1
kind: NodeConfiguration
caCertPath: <path|string>
discoveryFile: <path|string>
discoveryToken: <string>
discoveryTokenAPIServers:
- <address|string>
- <address|string>
nodeName: <string>
tlsBootstrapToken: <string>
token: <string>
discoveryTokenCACertHashes:
- <SHA-256 hash|string>
- <SHA-256 hash|string>
discoveryTokenUnsafeSkipCAVerification: <bool>

Securing your installation even more

The defaults for kubeadm may not work for everyone. This section documents how to tighten up a kubeadm install at the cost of some usability.

Turning off auto-approval of Node Client Certificates

By default, there is an CSR auto-approver enabled that basically approves any client certificate request for a kubelet when a Bootstrap Token was used when authenticating. If you don't want the cluster to automatically approve kubelet client certs, you can turn it off by executing this command:

$ kubectl delete clusterrole kubeadm:node-autoapprove-bootstrap

After that, kubeadm join will block until the admin has manually approved the CSR in flight:

$ kubectl get csr
NAME                                                   AGE       REQUESTOR                 CONDITION
node-csr-c69HXe7aYcqkS1bKmH4faEnHAWxn6i2bHZ2mD04jZyQ   18s       system:bootstrap:878f07   Pending

$ kubectl certificate approve node-csr-c69HXe7aYcqkS1bKmH4faEnHAWxn6i2bHZ2mD04jZyQ
certificatesigningrequest "node-csr-c69HXe7aYcqkS1bKmH4faEnHAWxn6i2bHZ2mD04jZyQ" approved

$ kubectl get csr
NAME                                                   AGE       REQUESTOR                 CONDITION
node-csr-c69HXe7aYcqkS1bKmH4faEnHAWxn6i2bHZ2mD04jZyQ   1m        system:bootstrap:878f07   Approved,Issued

Only after kubectl certificate approve has been run, kubeadm join can proceed.

Turning off public access to the cluster-info ConfigMap

In order to achieve the joining flow using the token as the only piece of validation information, a public ConfigMap with some data needed for validation of the master's identity is exposed publicly by default. While there is no private data in this ConfigMap, some users are sensitive and wish to turn it off regardless. Doing so will disable the ability to use the --discovery-token flag of the kubeadm join flow. Here are the steps to do so:

Fetch the cluster-info file from the API Server:

$ kubectl -n kube-public get cm cluster-info -oyaml | grep "kubeconfig:" -A11 | grep "apiVersion" -A10 | sed "s/    //" | tee cluster-info.yaml
apiVersion: v1
clusters:
- cluster:
    certificate-authority-data: <ca-cert>
    server: https://<ip>:<port>
  name: ""
contexts: []
current-context: ""
kind: Config
preferences: {}
users: []

You can then use the cluster-info.yaml file as an argument to kubeadm join --discovery-file.

Turning of public access to the cluster-info ConfigMap:

$ kubectl -n kube-public delete rolebinding kubeadm:bootstrap-signer-clusterinfo

These command should be run after kubeadm init but before kubeadm join.

Managing Tokens

You can use the kubeadm tool to manage tokens on a running cluster. It will automatically grab the default admin credentials on a master from a kubeadm created cluster (/etc/kubernetes/admin.conf). You can specify an alternate kubeconfig file for credentials with the --kubeconfig to the following commands.

  • kubeadm token list - List tokens (along with their expirations, usages, and groups).

  • kubeadm token create - Create a new token.

    • --description <description>

      Set the human-readable description for the new token.

    • --ttl <duration>

      Set the expiration time-to-live of the token relative to the current time. Default is 24 hours. A value of 0 means "never expire". The default unit of the duration is seconds but other units can be specified (e.g., 15m, 1h).

    • --usages <usage>[,<usage>...]

      Set the ways that the token can be used. The default is signing,authentication. These are the usages as described above.

    • --groups <group>[,<group>...]

      Add extra bootstrap groups that the new token will authenticate as. Can be specified multiple times. Each extra group must start with system:bootstrappers:. This is an advanced feature meant for custom bootstrap scenarios where you want to change how CSR approval works for different groups of nodes.

  • kubeadm token delete <token id>|<token id>.<token secret> - Delete a token.

    The token can either be identified with just an ID or with the entire token value. Only the ID is used; the token is still deleted if the secret does not match.

  • kubeadm token generate - Generate a token locally.

    Locally generate a token but do not create it on the server. This token is of the correct form for specifying with the --token argument to kubeadm init.

For the gory details on how the tokens are implemented (including managing them outside of kubeadm) see the Bootstrap Token docs.

Automating kubeadm

Rather than copying the token you obtained from kubeadm init to each node, as in the basic kubeadm tutorial, you can parallelize the token distribution for easier automation. To implement this automation, you must know the IP address that the master will have after it is started.

  1. Generate a token. This token must have the form <6 character string>.<16 character string>. More formally, it must match the regex: [a-z0-9]{6}\.[a-z0-9]{16}.

    kubeadm can generate a token for you:

    kubeadm token generate

  2. Start both the master node and the worker nodes concurrently with this token. As they come up they should find each other and form the cluster. The same --token argument can be used on both kubeadm init and kubeadm join.

Once the cluster is up, you can grab the admin credentials from the master node at /etc/kubernetes/admin.conf and use that to talk to the cluster.

Note that this style of bootstrap has some relaxed security guarantees because it does not allow the root CA hash to be validated with --discovery-token-ca-cert-hash (since it's not generated when the nodes are provisioned). For details, see the security model.

Security model

The kubeadm discovery system has several options, each with security tradeoffs. The right method for your environment depends on how you provision nodes and the security expectations you have about your network and node lifecycles.

Token-based discovery with CA pinning

This is the default mode in Kubernetes 1.8. In this mode, kubeadm downloads the cluster configuration (including root CA) and validates it using the token as well as validating that the root CA public key matches the provided hash and that the API server certificate is valid under the root CA.

Example kubeadm join command:

  • kubeadm join --discovery-token abcdef.1234567890abcdef --discovery-token-ca-cert-hash sha256:1234..cdef 1.2.3.4:6443

Advantages:

  • Allows bootstrapping nodes to securely discover a root of trust for the master even if other worker nodes or the network are compromised.

  • Convenient to execute manually since all of the information required fits into a single kubeadm join command that is easy to copy and paste.

Disadvantages:

  • The CA hash is not normally known until the master has been provisioned, which can make it more difficult to build automated provisioning tools that use kubeadm.

Token-based discovery without CA pinning

This was the default in Kubernetes 1.7 and earlier, but comes with some important caveats. This mode relies only on the symmetric token to sign (HMAC-SHA256) the discovery information that establishes the root of trust for the master. It's still possible in Kubernetes 1.8 and above using the --discovery-token-unsafe-skip-ca-verification flag, but you should consider using one of the other modes if possible.

Example kubeadm join command:

  • kubeadm join --discovery-token abcdef.1234567890abcdef --discovery-token-unsafe-skip-ca-verification 1.2.3.4:6443

Advantages:

  • Still protects against many network-level attacks.

  • The token can be generated ahead of time and shared with the master and worker nodes, which can then bootstrap in parallel without coordination. This allows it to be used in many provisioning scenarios.

Disadvantages:

  • If an attacker is able to steal a bootstrap token via some vulnerability, they can use that token (along with network-level access) to impersonate the master to other bootstrapping nodes. This may or may not be an appropriate tradeoff in your environment.

File or HTTPS-based discovery

This provides an out-of-band way to establish a root of trust between the master and bootstrapping nodes. Consider using this mode if you are building automated provisioning using kubeadm.

Example kubeadm join commands:

  • kubeadm join --discovery-file path/to/file.conf (local file)

  • kubeadm join --discovery-file https://url/file.conf (remote HTTPS URL)

Advantages:

  • Allows bootstrapping nodes to securely discover a root of trust for the master even if other worker nodes or the network are compromised.

Disadvantages:

  • Requires that you have some way to carry the discovery information from the master to the bootstrapping nodes. This might be possible, for example, via your cloud provider or provisioning tool. The information in this file is not secret, but HTTPS or equivalent is required to ensure its integrity.

  • Less convenient to use manually since the file is difficult to copy and paste between nodes.

Use kubeadm with other CRI runtimes

Since Kubernetes 1.6 release, Kubernetes container runtimes have been transferred to using CRI by default. Currently, the build-in container runtime is Docker which is enabled by built-in dockershim in kubelet.

Using other CRI based runtimes with kubeadm is very simple, and currently supported runtimes are:

After you have successfully installed kubeadm and kubelet, please follow these two steps:

  1. Install runtime shim on every node. You will need to follow the installation document in the runtime shim project listing above.

  2. Configure kubelet to use remote CRI runtime. Please remember to change RUNTIME_ENDPOINT to your own value like /var/run/{your_runtime}.sock:

  $ cat > /etc/systemd/system/kubelet.service.d/20-cri.conf <<EOF
Environment="KUBELET_EXTRA_ARGS=--container-runtime=remote --container-runtime-endpoint=$RUNTIME_ENDPOINT --feature-gates=AllAlpha=true"
EOF
  $ systemctl daemon-reload

Now kubelet is ready to use the specified CRI runtime, and you can continue with kubeadm init and kubeadm join workflow to deploy Kubernetes cluster.

Using custom images

By default, kubeadm will pull images from gcr.io/google_containers, unless requested kubernetes version is a ci version; in this case gcr.io/kubernetes-ci-image will be used.

This behaviour can be overridden by using kubeadm with a configuration file. Allowed customization are:

  • provide an alternative imageRepository to be used instead of gcr.io/google_containers (NB. does not works for ci version)
  • provide an unifiedControlPlaneImage to be used instead of single images for control plane components
  • provide an etcd.image name to be used

Running kubeadm without an internet connection

All of the control plane components run in Pods started by the kubelet and the following images are required for the cluster works will be automatically pulled by the kubelet if they don't exist locally while kubeadm init is initializing your master:

Image Name v1.7 release branch version v1.8 release branch version
gcr.io/google_containers/kube-apiserver-${ARCH} v1.7.x v1.8.x
gcr.io/google_containers/kube-controller-manager-${ARCH} v1.7.x v1.8.x
gcr.io/google_containers/kube-scheduler-${ARCH} v1.7.x v1.8.x
gcr.io/google_containers/kube-proxy-${ARCH} v1.7.x v1.8.x
gcr.io/google_containers/etcd-${ARCH} 3.0.17 3.0.17
gcr.io/google_containers/pause-${ARCH} 3.0 3.0
gcr.io/google_containers/k8s-dns-sidecar-${ARCH} 1.14.4 1.14.4
gcr.io/google_containers/k8s-dns-kube-dns-${ARCH} 1.14.4 1.14.4
gcr.io/google_containers/k8s-dns-dnsmasq-nanny-${ARCH} 1.14.4 1.14.4

Here v1.7.x means the "latest patch release of the v1.7 branch".

${ARCH} can be one of: amd64, arm, arm64, ppc64le or s390x.

Managing the kubeadm drop-in file for the kubelet

The kubeadm deb package ships with configuration for how the kubelet should be run. Note that the kubeadm CLI command will never touch this drop-in file. This drop-in file belongs to the kubeadm deb/rpm package.

This is what it looks like in v1.7:

[Service]
Environment="KUBELET_KUBECONFIG_ARGS=--kubeconfig=/etc/kubernetes/kubelet.conf --require-kubeconfig=true"
Environment="KUBELET_SYSTEM_PODS_ARGS=--pod-manifest-path=/etc/kubernetes/manifests --allow-privileged=true"
Environment="KUBELET_NETWORK_ARGS=--network-plugin=cni --cni-conf-dir=/etc/cni/net.d --cni-bin-dir=/opt/cni/bin"
Environment="KUBELET_DNS_ARGS=--cluster-dns=10.96.0.10 --cluster-domain=cluster.local"
Environment="KUBELET_AUTHZ_ARGS=--authorization-mode=Webhook --client-ca-file=/etc/kubernetes/pki/ca.crt"
Environment="KUBELET_CADVISOR_ARGS=--cadvisor-port=0"
ExecStart=
ExecStart=/usr/bin/kubelet $KUBELET_KUBECONFIG_ARGS $KUBELET_SYSTEM_PODS_ARGS $KUBELET_NETWORK_ARGS $KUBELET_DNS_ARGS $KUBELET_AUTHZ_ARGS $KUBELET_CADVISOR_ARGS $KUBELET_EXTRA_ARGS

A breakdown of what/why:

  • --kubeconfig=/etc/kubernetes/kubelet.conf points to the kubeconfig file that tells the kubelet where the API server is. This file also has the kubelet's credentials.

  • --require-kubeconfig=true the kubelet should fail fast if the kubeconfig file is not present. This makes the kubelet crash loop during the time the service is started until kubeadm init is run.

  • --pod-manifest-path=/etc/kubernetes/manifests specifies from where to read Static Pod manifests used for spinning up the control plane

  • --allow-privileged=true allows this kubelet to run privileged Pods

  • --network-plugin=cni uses CNI networking

  • --cni-conf-dir=/etc/cni/net.d specifies where to look for the CNI spec file(s)

  • --cni-bin-dir=/opt/cni/bin specifies where to look for the actual CNI binaries

  • --cluster-dns=10.96.0.10 use this cluster-internal DNS server for nameserver entries in Pods' /etc/resolv.conf

  • --cluster-domain=cluster.local uses this cluster-internal DNS domain for search entries in Pods' /etc/resolv.conf

  • --client-ca-file=/etc/kubernetes/pki/ca.crt authenticates requests to the Kubelet API using this CA certificate.

  • --authorization-mode=Webhook authorizes requests to the Kubelet API by POST-ing a SubjectAccessReview to the API Server

  • --cadvisor-port=0 disables cAdvisor from listening to 0.0.0.0:4194 by default. cAdvisor will still be run inside of the kubelet and its API can be accessed at https://{node-ip}:10250/stats/. If you want to enable cAdvisor to listen on a wide-open port, run:

    sed -e "/cadvisor-port=0/d" -i /etc/systemd/system/kubelet.service.d/10-kubeadm.conf
systemctl daemon-reload
systemctl restart kubelet

Cloud provider integrations (experimental)

Enabling specific cloud providers is a common request. This currently requires manual configuration and is therefore not yet fully supported. If you wish to do so, edit the kubeadm drop-in for the kubelet service (/etc/systemd/system/kubelet.service.d/10-kubeadm.conf) on all nodes, including the master. If your cloud provider requires any extra packages installed on the host, for example for volume mounting/unmounting, install those packages.

Specify the --cloud-provider flag for the kubelet and set it to the cloud of your choice. If your cloud provider requires a configuration file, create the file /etc/kubernetes/cloud-config on every node. The exact format and content of that file depends on the requirements imposed by your cloud provider. If you use the /etc/kubernetes/cloud-config file, you must append it to the kubelet arguments as follows: --cloud-config=/etc/kubernetes/cloud-config

Note that there is most likely other per-provider configuration that may be needed (IAM roles for AWS) that is currently underdocumented.

Next, specify the cloud provider in the kubeadm config file. Create a file called kubeadm.conf with the following contents:

kind: MasterConfiguration
apiVersion: kubeadm.k8s.io/v1alpha1
cloudProvider: <cloud provider>

Lastly, run kubeadm init --config=kubeadm.conf to bootstrap your cluster with the cloud provider.

This workflow is not yet fully supported, however we hope to make it extremely easy to spin up clusters with cloud providers in the future. (See this proposal for more information) The Kubelet Dynamic Settings feature may also help to fully automate this process in the future.

Environment variables

Note: These environment variables are deprecated and will stop functioning in v1.8!

There are some environment variables that modify the way that kubeadm works. Most users will have no need to set these. These environment variables are a short-term solution, eventually they will be integrated in the kubeadm configuration file.

Variable Default Description
KUBE_KUBERNETES_DIR /etc/kubernetes Where most configuration files are written to and read from
KUBE_HYPERKUBE_IMAGE If set, use a single hyperkube image with this name. If not set, individual images per server component will be used.
KUBE_ETCD_IMAGE gcr.io/google_containers/etcd-<arch>:3.0.17 The etcd container image to use.
KUBE_REPO_PREFIX gcr.io/google_containers The image prefix for all images that are used.

If KUBE_KUBERNETES_DIR is specified, you may need to rewrite the arguments of the kubelet. (e.g. --kubeconfig, --pod-manifest-path)

If KUBE_REPO_PREFIX is specified, you may need to set the kubelet flag --pod-infra-container-image which specifies which pause image to use.

Defaults to gcr.io/google_containers/pause-${ARCH}:3.0 where ${ARCH} can be one of amd64, arm, arm64, ppc64le or s390x.

cat > /etc/systemd/system/kubelet.service.d/20-pod-infra-image.conf <<EOF
[Service]
Environment="KUBELET_EXTRA_ARGS=--pod-infra-container-image=<your-image>"
EOF
systemctl daemon-reload
systemctl restart kubelet

If you want to use kubeadm with an http proxy, you may need to configure it to support http_proxy, https_proxy, or no_proxy.

For example, if your kube master node IP address is 10.18.17.16 and you have a proxy which supports both http/https on 10.18.17.16 port 8080, you can use the following command:

export PROXY_PORT=8080
export PROXY_IP=10.18.17.16
export http_proxy=http://$PROXY_IP:$PROXY_PORT
export HTTP_PROXY=$http_proxy
export https_proxy=$http_proxy
export HTTPS_PROXY=$http_proxy
export no_proxy="localhost,127.0.0.1,localaddress,.localdomain.com,example.com,10.18.17.16"

Remember to change proxy_ip and add a kube master node IP address to no_proxy.

Using custom certificates

By default kubeadm will generate all the certificates needed for a cluster to run. You can override this behaviour by providing your own certificates.

To do so, you must place them in whatever directory is specified by the --cert-dir flag or CertificatesDir configuration file key. By default this is /etc/kubernetes/pki.

If a given certificate and private key pair both exist, kubeadm will skip the generation step and those files will be validated and used for the prescribed use-case.

This means you can, for example, prepopulate /etc/kubernetes/pki/ca.crt and /etc/kubernetes/pki/ca.key with an existing CA, which then will be used for signing the rest of the certs.

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 recognise this condition and activates the so called "ExternalCA" mode, which also implies the csrsignercontroller in controller-manager won't be started.

Self-hosting the Kubernetes control plane

As of 1.8, kubeadm can experimentally create a self-hosted Kubernetes control plane. This means that key components such as the API server, controller manager, and scheduler run as DaemonSet pods configured via the Kubernetes API instead of static pods configured in the kubelet via static files.

Self-hosting is alpha in kubeadm 1.8 but is expected to become the default in a future version. To create a self-hosted cluster, pass the --feature-gates=SelfHosting=true flag to kubeadm init.

Caveats

Kubeadm self-hosting in 1.8 has some important limitations. In particular, a self-hosted cluster cannot currently recover from a reboot of the master node without manual intervention. This and other limitations are expected to be resolved before self-hosting graduates from alpha.

By default, self-hosted control plane pods rely on credentials loaded from hostPath volumes. Except for initial creation, these credentials are not managed by kubeadm. You can use --feature-gates=StoreCertsInSecrets=true to enable an experimental mode where control plane credentials are loaded from Secrets instead. This requires very careful control over the authentication and authorization configuration for your cluster, and may not be appropriate for your environment.

In 1.8, the self-hosted portion of the control plane does not include etcd, which still runs as a static pod.

Process

The self-hosting bootstrap process is documented in the kubeadm 1.8 design document. In summary, kubeadm init --feature-gates=SelfHosting=true works as follows:

  1. As usual, kubeadm creates static pod YAML files in /etc/kubernetes/manifests/.

  2. Kubelet loads these files and launches the initial static control plane. Kubeadm waits for this initial static control plane to be running and healthy. This is identical to the kubeadm init process without self-hosting.

  3. Kubeadm uses the static control plane pod manifests to construct a set of DaemonSet manifests that will run the self-hosted control plane.

  4. Kubeadm creates DaemonSets in the kube-system namespace and waits for the resulting pods to be running.

  5. Once the new control plane is running (but not yet active), kubeadm deletes the static pod YAML files. This triggers kubelet to stop those static pods.

  6. When the original static control plane stops, the new self-hosted control plane is able to bind to listening ports and become active.

This process (steps 3-6) can also be triggered with kubeadm phase selfhosting convert-from-staticpods.

Releases and release notes

If you already have kubeadm installed and want to upgrade, run apt-get update && apt-get upgrade or yum update to get the latest version of kubeadm.

Refer to the CHANGELOG.md for more information.