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k3OS is a Linux distribution designed to remove as much as possible OS maintenance in a Kubernetes cluster. It is specifically designed to only have what is need to run k3s. Additionally the OS is designed to be managed by kubectl once a cluster is bootstrapped. Nodes only need to join a cluster and then all aspects of the OS can be managed from Kubernetes. Both k3OS and k3s upgrades are handled by the k3OS operator.

  1. Quick Start
  2. Design
  3. Installation
  4. Configuration
  5. Upgrade/Maintenance
  6. Building
  7. Configuration Reference

Quick Start

Download the ISO from the latest release and run in VMware, VirtualBox, or KVM. The server will automatically start a single node Kubernetes cluster. Log in with the user rancher and run kubectl. This is a "live install" running from the ISO media and changes will not persist after reboot.

To copy k3OS to local disk, after logging in as rancher run sudo os-config. Then remove the ISO from the virtual machine and reboot.

Live install (boot from ISO) requires at least 1GB of RAM. Local install requires 512MB RAM.


Core design goals of k3OS are

  1. Minimal OS for running Kubernetes by way of k3s
  2. Ability to upgrade and configure using kubectl
  3. Versatile installation to allow easy creation of OS images.

File System Structure

Critical to the design of k3OS is how that file system is structured. A booted system will look as follows

/etc - ephemeral
/usr - read-only (except /usr/local is writable and persistent)
/k3os - system files
/home - persistent
/var - persistent
/opt - persistent
/usr/local - persistent


All configuration in the system is intended to be ephemeral. If you change anything in /etc it will revert on next reboot. If you wish to persist changes to the configuration they must be done in the k3OS config.yaml which will be applied on each boot.


The entire user space is stored in /usr and as read-only. The only way to change /usr is to change versions of k3OS. The directory /usr/local is a symlink to /var/local and therefore writable.


The k3OS directory contains the core operating system files references on boot to construct the file system. It contains a squashfs images and binaries for k3OS, k3s, and the Linux kernel. On boot the appropriate version for all three will be chosen and configured.

/var, /usr/local, /home, /opt

Persistent changes should be kept in /var, '/usr/local', '/home', or '/opt'.

Upstream Distros

Most of the user-space binaries comes from Alpine and are repackaged for k3OS. Currently the kernel source is coming from Ubuntu 18.04 LTS. Some code and a lot of inspiration came from LinuxKit


Interactive Installation

Interactive installation is done from booting from the ISO. The installation is done by running os-config. The os-config script is only available systems booted live. An installation to disk will not have os-config. Follow the prompts to install k3OS to disk.

The installation will format an entire disk. If you have a single hard disk attached to the system it will not ask which disk but just pick the first and only one.

Automated Installation

Installation can be automated by using kernel cmdline parameters. There are a lot of creative solutions to booting a machine with cmdline args. You can remaster the k3OS ISO, PXE boot, use qemu/kvm, or automate input with packer. The kernel and initrd are available in the k3OS release artifacts, along with the ISO.

The cmdline value k3os.mode=install or k3os.fallback_mode=install is required to enable automated installations. Below is a reference of all cmdline args used to automate installation

cmdline Default Example Description
k3os.mode install Boot k3OS to the installer, not an interactive session
k3os.fallback_mode install If a valid K3OS_STATE partition is not found to boot from, run the installation
k3os.install.silent false true Ensure no questions will be asked
k3os.install.force_efi false true Force EFI installation even when EFI is not detected
k3os.install.device /dev/vda Device to partition and format (/dev/sda, /dev/vda)
k3os.install.config_url The URL of the config to be installed at /k3os/system/config.yaml
k3os.install.iso_url ISO to download and install from if booting from kernel/vmlinuz and not ISO.
k3os.install.no_format true Do not partition and format, assume layout exists already
k3os.install.tty auto ttyS0 The tty device used for console
k3os.install.debug false true Run installation with more logging and configure debug for installed system
k3os.install.power_off false true Shutdown the machine after install instead of rebooting

Custom partition layout

By default k3OS expects one partitions to exist labeled K3OS_STATE. K3OS_STATE is expected to be an ext4 formatted filesystem with at least 2GB of disk space. The installer will create this partitions and file system automatically, or you can create them manually if you have a need for an advanced file system layout.

Bootstrapped Installation

You can install k3OS to a block device from any modern Linux distribution. Just download and run This script will run the same installation as the ISO but it a bit more raw and will not prompt for configuration.

Usage: ./ [--force-efi] [--debug] [--tty TTY] [--poweroff] [--takeover] [--no-format] [--config https://.../config.yaml] DEVICE ISO_URL

Example: ./ /dev/vda

DEVICE must be the disk that will be partitioned (/dev/vda). If you are using --no-format it should be the device of the K3OS_STATE partition (/dev/vda2)

The parameters names refer to the same names used in the cmdline, refer to for
more info.

Remastering ISO

To remaster the ISO all you need to do is copy /k3os and /boot from the ISO to a new folder. Then modify /boot/grub/grub.cfg to add whatever kernel cmdline args for auto-installation. To build a new ISO just use the utility grub-mkrescue as follows:

mount -o loop k3os.iso /mnt
mkdir -p iso/boot/grub
cp -rf /mnt/k3os iso/
cp /mnt/k3os/boot/grub/grub.cfg iso/boot/grub/

# Edit iso/boot/grub/grub.cfg

grub-mkrescue -o k3os-new.iso iso/ -V K3OS

Takeover Installation

A special mode of installation is designed to install to a current running Linux system. This only works on ARM64 and x86_64. Download and run with the --takeover flag. This will install k3OS to the current root and override the grub.cfg. After you reboot the system k3OS will then delete all files on the root partition that are not k3OS and then shutdown. This mode is particularly handy when creating cloud images. This way you can use an existing base image like Ubuntu and install k3OS over the top, snapshot, and create a new image.

In order for this to work a couple of assumptions are made. First the root (/) is assumed to be a ext4 partition. Also it is assumed that grub2 is installed and looking for the configuration at /boot/grub/grub.cfg. When running --takeover ensure that you also set --no-format and DEVICE must be set to the partition of /. Refer to the AWS packer template to see this mode in action, below is any example of how to run a takeover installation.

./ --takeover --debug --tty ttyS0 --config /tmp/config.yaml --no-format /dev/vda1

ARM Overlay Installation

If you have a custom ARMv7 or ARM64 device you can easily use an existing bootable ARM image to create an k3OS setup. All you must do is boot the ARM system and then extract k3os-rootfs-arm.tar.gz to the root (stripping one path, look at the example below) and then place your cloud-config at /k3os/system/config.yaml. For example:

curl -sfL | tar xvf - --strip-components=1 -C /
cp myconfig.yaml /k3os/system/config.yaml
reboot -f

This method places k3OS on disk and also overwrites /sbin/init. On next reboot your ARM bootloader and kernel should be loaded, but then when user space is to be initialized k3OS should take over. One important consideration at the moment is that k3OS assume the root device is not read only. This typically means you need to remove ro from the kernel cmdline. This should be fixed in a future release.


All configuration is done through a single cloud-init style config file that is either packaged in the image, downloaded though cloud-init or managed by Kubernetes. The configuration file is found at


The /k3os/system/config.yaml file is reserved for the system installation and should not be modified on a running system. This file is usually populated by during the image build or installation process and contains important bootstrap information (such as networking or cloud-init data sources).

The /var/lib/rancher/k3os/config.yaml or config.d/* files are intended to be used at runtime. These files can be manipulated manually, through scripting, or managed with the Kubernetes operator.

Sample config.yaml

A full example of the k3OS configuration file is as below.

- ssh-rsa AAAAB3NzaC1yc2EAAAADAQAB
- github:ibuildthecloud
- encoding: ""
  content: |-
    echo hi
  owner: root
  path: /etc/rc.local
  permissions: '0755'
hostname: myhost
- "echo hi && echo bye"
- "echo hi && echo bye"
- "echo hi && echo bye"

  - aws
  - cdrom
  - kvm
  - nvme
    kernel.printk: 4 4 1 7
    kernel.kptr_restrict: 1
  - ssid: home
    passphrase: mypassword
  - ssid: nothome
    passphrase: somethingelse
  password: rancher
  server_url: https://someserver:6443
  token: TOKEN_VALUE
    region: us-west-1
    somekey: somevalue
  - server
  - "--disable-agent"
    http_proxy: http://myserver
    http_proxys: http://myserver
  - key1=value1:NoSchedule
  - key1=value1:NoExecute

Refer to the configuration reference for full details of each configuration key.


Since k3OS is built on k3s all Kubernetes configuration is done by configuring k3s. This is primarily done through environment and k3s_args keys in config.yaml. The write_files key can be used to populate the /var/lib/rancher/k3s/server/manifests folder with apps you'd like to deploy on boot.

Refer to k3s docs for more information as to how to configure Kubernetes.

Kernel cmdline

All configuration can be passed as kernel cmdline parameters too. The keys are dot separated. For example k3os.token=TOKEN. If the key is a slice multiple values are set by repeating the key, for example k3os.dns_nameserver= k3os.dns_nameserver= You can use the plural or singular form of the name, just ensure you consistently use the same form. For map values the form key[key]=value form is used, for example k3os.sysctl[kernel.printk]="4 4 1 7". If the value has spaces in it ensure that the value is quoted. Boolean keys expect a value of true or false or no value at all means true. For example k3os.install.efi is the same as k3os.install.efi=true.


Configuration is applied in three distinct phases: initrd, boot, runtime. initrd is ran during the initrd phase before the root disk has been mounted. boot is ran after the root disk is mounted an the file system is setup, but before any services have started. There is no networking available yet at this point. The final stage runtime is executed after networking has come online. If you are using a configuration from a cloud provider (like AWS userdata) it will only be ran in the runtime phase. Below is a table of which config keys are supported in each phase.

Key initrd boot runtime
ssh_authorized_keys x x
write_files x x x
hostname x x x
runcmd x
bootcmd x
initcmd x
k3os.data_sources x
k3os.modules x x x
k3os.sysctls x x x
k3os.ntp_services x x
k3os.dns_nameservers x x
k3os.wifi x x
k3os.password x x x
k3os.server_url x x
k3os.token x x
k3os.labels x x
k3os.k3s_args x x
k3os.environment x x x
k3os.taints x x
k3os.token x x


Networking is powered by connman. To configure networking a couple helper keys are available: k3os.dns_nameserver, k3os.ntp_servers, k3os.wifi. Refer to the reference for a full explanation of those keys. If you wish to configure a HTTP proxy set the http_proxy, and https_proxy fields in k3os.environment. All other networking configuration should be done by configuring connman directly by using the write_files key to create connman service files.

Upgrade and Maintenance

Upgrading and reconfiguring k3OS is all handled through the Kubernetes operator. The operator is still in development. More details to follow. The basic design is that one can set the desired k3s and k3OS versions, plus there configuration and the operator will roll that out to the cluster.


To build k3OS you just need Docker and then run make. All artifacts will be put in ./dist/artifacts. If you are running on Linux you can run ./scripts/run to run a VM of k3OS in the terminal. To exit the instance type CTRL+a c to get the qemu console and then q for quit.

The source for the kernel is in and similarly you just need to have Docker and run make to compile the kernel.

Configuration Reference

Below is a reference of all keys available in the config.yaml


A list of SSH authorized keys that should be added to the rancher user. k3OS primarily has one user, rancher. root account is always disabled, has no password, and is never assigned a ssh key. SSH keys can be obtained from GitHub user accounts by using the format github:${USERNAME}. This is done by downloading the keys from${USERNAME}.keys.


- "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC2TBZGjE+J8ag11dzkFT58J3XPONrDVmalCNrKxsfADfyy0eqdZrG8hcAxAR/5zuj90Gin2uBR4Sw6Cn4VHsPZcFpXyQCjK1QDADj+WcuhpXOIOY3AB0LZBly9NI0ll+8lo3QtEaoyRLtrMBhQ6Mooy2M3MTG4JNwU9o3yInuqZWf9PvtW6KxMl+ygg1xZkljhemGZ9k0wSrjqif+8usNbzVlCOVQmZwZA+BZxbdcLNwkg7zWJSXzDIXyqM6iWPGXQDEbWLq3+HR1qKucTCSxjbqoe0FD5xcW7NHIME5XKX84yH92n6yn+rxSsyUfhJWYqJd+i0fKf5UbN6qLrtd/D"
- "github:ibuildthecloud"


A list of files to write to disk on boot. These files can be either plain text, gziped, base64 encoded, or base64+gzip encoded.


- encoding: b64
  content: CiMgVGhpcyBmaWxlIGNvbnRyb2xzIHRoZSBzdGF0ZSBvZiBTRUxpbnV4...
  owner: root:root
  path: /etc/connman/main.conf
  permissions: '0644'
- content: |
    # My new /etc/sysconfig/samba file

  path: /etc/sysconfig/samba
- content: !!binary |
  path: /bin/arch
  permissions: '0555'
- content: |
    15 * * * * root ship_logs
  path: /etc/crontab


Set the system hostname. This value will be overwritten by DHCP if DHCP supplies a hostname for the system.


hostname: myhostname

initcmd, bootcmd, runcmd

All three keys are used to run arbitrary commands on startup in the respective phases of initrd, boot and runtime. Commands are ran after write_files so it is possible to write a script to disk and run it from these commands. That often makes it easier to do longer form setup.


These are the data sources used for download config from cloud provider. The valid options are


More than one can be supported at a time, for example:

  - openstack
  - cdrom


A list of kernel modules to be loaded on start.


  - kvm
  - nvme


Kernel sysctl to setup on start. These are the same configuration you'd typically find in /etc/sysctl.conf.

    kernel.printk: 4 4 1 7
    kernel.kptr_restrict: 1


Fallback ntp servers to use if NTP is not configured elsewhere in connman.




Fallback DNS name servers to use if DNS is not configured by DHCP or in a connman service config.




Simple wifi configuration. All that is accepted is SSID and Passphrase. If you require more complex configuration then you should use write_files to write a connman service config.


  - ssid: home
    passphrase: mypassword
  - ssid: nothome
    passphrase: somethingelse


The password for the rancher user. By default there is no password for the rancher user. If you set a password at runtime it will be reset on next boot because /etc is ephemeral. The value of the password can be clear text or an encrypted form. The easiest way to get this encrypted form is to just change your password on a Linux system and copy the value of the second field from /etc/shadow. You can also encrypt a password using openssl passwd -1.


  password: "$1$tYtghCfK$QHa51MS6MVAcfUKuOzNKt0"

Or clear text

  password: supersecure


The URL of the k3s server to join as an agent.


  server_url: https://myserver:6443


The cluster secret or node token. If the value matches the format of a node token it will automatically be assume to be a node token. Otherwise it is treated as a cluster secret.


  token: myclustersecret

Or a node token

  token: "K1074ec55daebdf54ef48294b0ddf0ce1c3cb64ee7e3d0b9ec79fbc7baf1f7ddac6::node:77689533d0140c7019416603a05275d4"


Labels to be assigned to this node in Kubernetes on registration. After the node is first registered in Kubernetes the value of this setting will be ignored.


    region: us-west-1
    somekey: somevalue


Arguments to be passed to the k3s process. The arguments should start with server or agent to be valid.


  - server
  - "--disable-agent"


Environment variables to be set on k3s an other processes like the boot process. Primary use of this field is to set the http proxy.


    http_proxy: http://myserver
    http_proxys: http://myserver


Taints to set on the current node when it is first registered. After the node is first registered the value of this field is ignored.

  - "key1=value1:NoSchedule"
  - "key1=value1:NoExecute"


Copyright (c) 2014-2019 Rancher Labs, Inc.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

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