Cloud-init

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Cloud-init is a package that contains utilities for early initialization of cloud instances. It is needed in arch linux images that are built with the intention of being launched in cloud like OpenStack, AWS etc.

Installation

Install cloud-init from the official repositories.

Configuration

In order to make an Arch image cloud ready, a few steps have to be followed:

  • Create a default user. You can login to your instance as this user. We will create and use a user called arch.
  • Install sudo and add the default user to sudo group. This can be used later to execute commands as privileged user.
  • Enable password less sudo for the default user.
  • Configure cloud-init to pull instance metadata to configure the instance. This includes but not limited to:
    • Setting up the instance hostname
    • Configuring instance's resolv.conf
    • Configuring ~/.ssh/authorized_keys file for the default user

The cloud-init's main configuration file is /etc/cloud/cloud.cfg. Optionally the user can place *.cfg files in /etc/cloud/cloud.cfg.d to be loaded.

Default user configuration

As of February 2016, the default /etc/cloud/cloud.cfg shipped with the package has not been adapted to Arch, and still states that the distro is Ubuntu, therefore it requires editing.

Edit /etc/cloud/cloud.cfg to have the following contents:

users:
  - default

This tells cloud-init to use the user configured under system_info > default_user as the default user:

system_info:
   distro: arch
   default_user:
     name: arch 
     lock_passwd: true
     gecos: Arch
     groups: [adm, audio, cdrom, dialout, dip, floppy, netdev, plugdev, sudo, video]
     sudo: ["ALL=(ALL) NOPASSWD:ALL"]
     shell: /bin/bash

Under system_info we specify the distro as "arch". This will instruct cloud-init to use arch.py module for configuration. Further we specify that:

  • the default user's name would be arch
  • the default user is password locked, which means you can not login to the instance without the ssh keys configured during boot
  • the default user should be added to the groups adm, audio, cdrom, dialout, dip, floppy, netdev, plugdev, sudo, video
  • the default user is allow passwordless sudo usage
  • the default user's shell is /bin/bash

Disable login as root

To do this you can specify in /etc/cloud/cloud.cfg:

disable_root: true

You may also delete the root user password:

# passwd -d root

Do not perform this step unless you are sure the configuration in previous section works. Otherwise you might be completely locked out of your instance.

Configuring data sources

Data Sources define how the instance metadata is pulled during boot. This depends on what cloud (OpenStack, AWS, OpenNebula etc.) you are running your instance on. Under the hood this translates to a corresponding module which implement a few methods defined in a common interface. Edit your /etc/cloud/cloud.cfg to have the below contents:

datasource_list: [ NoCloud, ConfigDrive, OpenNebula, Azure, AltCloud, OVF, MAAS, GCE, OpenStack, CloudSigma, Ec2, CloudStack, None ]

This instructs cloud-init what modules to load while trying to download instance metadata. Optionally further configuration parameters may be passed specific to each datasource as below:

datasource:
  OpenStack:
    metadata_urls: [ 'http://169.254.169.254:80' ]
    dsmode: net

The above configuration tells OpenStack datasource to use the url http://169.254.169.254:80 to download metadata and to run after network initialization, both of which are the default behaviour and may be omitted.

Other sections in cloud.cfg

cloud.cfg defines several other sections which includes but not limited to cloud_init_modules, cloud_config_modules and cloud_final_modules that define the modules that would be run at various stages during instance initialization. These modules are loaded dynamically from the path /usr/lib/python2.7/site-packages/cloudinit/config/ and run at boot time. The user may define their own modules and configure them to be called during every boot like say to:

  • perform disk resize
  • perform package update

etc.

The various modules declare to cloud-init which distros they have been verified for. Even if you specify that you want to run them, they will refuse to run unless the distro specified in cloud.cfg is one of the verified distros for the given module. If you want to run a module on Arch anyway that does not specify arch, add the module to the unverified_modules: section in cloud.cfg, e.g.:

unverified_modules: ['ssh-import-id']

Systemd integration

Package cloud-init provides 4 systemd services, and a systemd target, whose dependencies are constructed in a way that they are activated in the sequence listed:

  • cloud-init-local.service. Only requires the filesystems to be up. Executes cloud-init init --local
  • cloud-init.service. Requires the network to be up. Executes cloud-init init
  • cloud-config.target. Corresponds to the cloud-config upstart event "to inform third parties that cloud-config is available"
  • cloud-config.service. Executes cloud-init modules --mode=config
  • cloud-final.service. Executes cloud-init modules --mode=final

The Uplink Labs EC2 images have all of them enabled, although that appears to be overkill due to the dependencies.