This document provides information on how to use kubeadm’s advanced options.
Running kubeadm init
bootstraps a Kubernetes cluster. This consists of the
following steps:
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
.
kubeadm generates a token that additional nodes can use to register themselves with the master in future. Optionally, the user can provide a token.
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
, by default
/etc/kubernetes/pki
), this step is skipped.
Outputting a kubeconfig file for the kubelet to use to connect to the API server, as well as an additional kubeconfig file for administration.
kubeadm generates Kubernetes Static Pod manifests for the API server,
controller manager and scheduler. It places them in
/etc/kubernetes/manifests
. The kubelet watches this directory for Pods to
create on startup. These are the core components of Kubernetes. Once they are
up and running kubeadm can set up and manage any additional components.
kubeadm “taints” the master node so that only control plane components will run there. It also sets up the RBAC authorization system and writes a special ConfigMap that is used to bootstrap trust with the kubelets.
kubeadm installs add-on components via the API server. Right now this is the internal DNS server and the kube-proxy DaemonSet.
Running kubeadm join
on each node in the cluster consists of the following
steps:
kubeadm downloads root CA information from the API server. It uses the token to verify the authenticity of that data.
kubeadm creates a local key pair. It prepares a certificate signing request (CSR) and sends that off to the API server for signing. The bootstrap token is used to authenticate. The API server is configured to sign this automatically.
kubeadm configures the local kubelet to connect to the API server
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
. The
single exception here is the format of the kubeadm config file as detailed
below. That format is still considered alpha and may change.
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.
--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 certifcate 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: When providing configuration values using both a configuration file and flags, the file will take precedence. For example, if a file exists with:
apiVersion: kubeadm.k8s.io/v1alpha1
kind: MasterConfiguration
token: 1234
and the user ran kubeadm init --config file.yaml --token 5678
,
the chosen token value will be 1234
.
--kubernetes-version
(default ‘latest’) the kubernetes version to initialiseThe 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.
--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.
--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. 0 is the default. 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 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). This file can be a local file or downloaded
via an HTTPS URL. The forms are kubeadm join --discovery-token
abcdef.1234567890abcdef 1.2.3.4:6443
, kubeadm join --discovery-file
path/to/file.conf
or 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.
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 use 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 192.168.1.1:6443
Specific options:
--config
Extended options a specified in the kubeadm specific config file.
--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.
--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.
--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.
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
.
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>
networking:
dnsDomain: <string>
serviceSubnet: <cidr>
podSubnet: <cidr>
kubernetesVersion: <string>
cloudProvider: <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>
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
.
apiVersion: kubeadm.k8s.io/v1alpha1
kind: NodeConfiguration
caCertPath: <path|string>
discoveryFile: <path|string>
discoveryToken: <string>
# Currently only the first server is used as a target for the cluster
# bootstrap flow.
discoveryTokenAPIServers:
- <address|string>
- <address|string>
tlsBootstrapToken: <string>
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
Lists the tokens along with when they expire and what the
approved usages are.kubeadm token create
Creates a new token.
--description
Set the description on the new token.--ttl duration
Set expiration time of the token as a delta from “now”.
Default is 0 for no expiration. The unit of the duration is seconds.--usages
Set the ways that the token can be used. The default is
signing,authentication
. These are the usages as described above.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.In addition, you can use the kubeadm token generate
command to locally creates
a new token. 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.
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.
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
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.
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
.
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 build-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:
Install runtime shim on every node. You will need to follow the installation document in the runtime shim project listing above.
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.
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.
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.
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