Customize with Cloud-Config

CoreOS allows you to configure networking, create users, launch systemd units on startup and more. We've designed our implementation to allow the same cloud-config file to work across all of our supported platforms.

Only a subset of cloud-config functionality is implemented. A set of custom parameters were added to the cloud-config format that are specific to CoreOS. An example file containing all available options can be found at the bottom of this page.

CoreOS Parameters

coreos.etcd

The coreos.etcd.* options are translated to a partial systemd unit acting as an etcd configuration file. coreos-cloudinit will also replace the strings $private_ipv4 and $public_ipv4 with the values generated by CoreOS based on a given provider.

For example, the following cloud-config document...

#cloud-config

coreos:
    etcd:
        name: node001
        discovery: https://discovery.etcd.io/3445fa65423d8b04df07f59fb40218f8
        addr: $public_ipv4:4001
        peer-addr: $private_ipv4:7001

...will generate a systemd unit drop-in like this:

[Service]
Environment="ETCD_NAME=node001""
Environment="ETCD_DISCOVERY=https://discovery.etcd.io/3445fa65423d8b04df07f59fb40218f8"
Environment="ETCD_ADDR=203.0.113.29:4001"
Environment="ETCD_PEER_ADDR=192.0.2.13:7001"

For more information about the available configuration options, see the etcd documentation. Note that hyphens in the coreos.etcd.* keys are mapped to underscores.

coreos.units

Arbitrary systemd units may be provided in the coreos.units attribute. coreos.units is a list of objects with the following fields:

name: string representing unit's name
runtime: boolean indicating whether or not to persist the unit across reboots. This is analagous to the --runtime flag to systemd enable.
content: plaintext string representing entire unit file

#cloud-config

coreos:
    units:
      - name: docker-redis.service
        content: |
          [Unit]
          Description=Redis container
          Author=Me
          After=docker.service

          [Service]
          Restart=always
          ExecStart=/usr/bin/docker start -a redis_server
          ExecStop=/usr/bin/docker stop -t 2 redis_server
          
          [Install]
          WantedBy=local.target

Cloud-Config Parameters

ssh_authorized_keys

Provided public SSH keys will be authorized for the core user.

The keys will be named "coreos-cloudinit" by default. Override this with the --ssh-key-name flag when calling coreos-cloudinit.

#cloud-config

ssh_authorized_keys:
  - ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC0g+ZTxC7weoIJLUafOgrm+h...

hostname

The provided value will be used to set the system's hostname. This is the local part of a fully-qualified domain name (i.e. foo in foo.example.com).

#cloud-config

hostname: coreos1

users

Add or modify users with the users directive by providing a list of user objects, each consisting of the following fields. Each field is optional and of type string unless otherwise noted. All but the passwd and ssh-authorized-keys fields will be ignored if the user already exists.

name: Required. Login name of user
gecos: GECOS comment of user
passwd: Hash of the password to use for this user
homedir: User's home directory. Defaults to /home/
no-create-home: Boolean. Skip home directory creation.
primary-group: Default group for the user. Defaults to a new group created named after the user.
groups: Add user to these additional groups
no-user-group: Boolean. Skip default group creation.
ssh-authorized-keys: List of public SSH keys to authorize for this user
coreos-ssh-import-github: Authorize SSH keys from Github user
system: Create the user as a system user. No home directory will be created.
no-log-init: Boolean. Skip initialization of lastlog and faillog databases.

The following fields are not yet implemented:

inactive: Deactivate the user upon creation
lock-passwd: Boolean. Disable password login for user
sudo: Entry to add to /etc/sudoers for user. By default, no sudo access is authorized.
selinux-user: Corresponding SELinux user
ssh-import-id: Import SSH keys by ID from Launchpad.

#cloud-config

users:
  - name: elroy
  passwd: $6$5s2u6/jR$un0AvWnqilcgaNB3Mkxd5yYv6mTlWfOoCYHZmfi3LDKVltj.E8XNKEcwWm...
  groups:
    - staff
    - docker
  ssh-authorized-keys:
    - ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC0g+ZTxC7weoIJLUafOgrm+h...

Generating a password hash

If you choose to use a password instead of an SSH key, generating a safe hash is extremely important to the security of your system. Simplified hashes like md5crypt are trivial to crack on modern GPU hardware. Here are a few ways to generate secure hashes:

# On Debian/Ubuntu (via the package "whois")
mkpasswd --method=SHA-512 --rounds=4096

# OpenSSL (note: this will only make md5crypt.  While better than plantext it should not be considered fully secure)
openssl passwd -1

# Python (change password and salt values)
python -c "import crypt, getpass, pwd; print crypt.crypt('password', '\$6\$SALT\$')"

# Perl (change password and salt values)
perl -e 'print crypt("password","\$6\$SALT\$") . "\n"'

Using a higher number of rounds will help create more secure passwords, but given enough time, password hashes can be reversed. On most RPM based distributions there is a tool called mkpasswd available in the expect package, but this does not handle "rounds" nor advanced hashing algorithms.

write_files

Inject an arbitrary set of files to the local filesystem. Provide a list of objects with the following attributes:

path: Absolute location on disk where contents should be written
content: Data to write at the provided path
permissions: String representing file permissions in octal notation (i.e. '0644')
owner: User and group that should own the file written to disk. This is equivalent to the <user>:<group> argument to chown <user>:<group> <path>.

user-data Script

Simply set your user-data to a script where the first line is a shebang:

#!/bin/bash

echo 'Hello, world!'

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