In EC2 and OpenStack cloud environments user data can be passed to the cloud instance to customize the cloud instance on the first boot. But what if your virtual machine doesn’t run in the cloud environment? In this article we’re going to configure our virtual machines with user data regardless if they’re running in the cloud or not.

Introducing Cloud-Init

Cloud-Init is a tool that handles early initialization of a cloud instance. The cloud-init RPM package should be installed on the disk image which the cloud instance is going to boot up from. The package installs init scripts into /etc/rc.d/init.d that makes Cloud-Init run early during the system initialization. Cloud-Init obtains user data passed to it by the cloud software and executes them. User data contains a set of configuration tasks for the cloud instance. For example, Cloud-Init can update machine’s hostname, configure /etc/hosts, create users, configure SSH authorized keys, resize filesystems, manage disk mounts, run user-defined scripts and much more.

Even if you’re not running your virtual machines in the cloud environment it’s worth it to deploy Cloud-Init.

Every cloud software comes with its own mechanism of how to pass the user data to the cloud instance. For example, EC2 provides a magic IP from which the instance can download its user data. OpenStack cloud attaches a special config drive to the cloud instance containing the user data to be consumed by Clout-Init. In order to pass the user data to our virtual machine let’s go the OpenStack way and assemble a minimum config drive.

Config drive assembly

First, we’re going to prepare the following file structure for our config drive:

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config_drive
config_drive/openstack
config_drive/openstack/latest
config_drive/openstack/2012-08-10
config_drive/openstack/2012-08-10/meta_data.json
config_drive/openstack/2012-08-10/user_data

Start by creating directories and the latest symbolic link like this:

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$ mkdir config_drive
$ mkdir -p config_drive/openstack/2012-08-10
$ ln -s 2012-08-10 config_drive/openstack/latest

Next create a minimum metadata file required by Cloud-Init. I’m using a fully qualified domain name of the virtual machine as its UUID:

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$ cat > config_drive/openstack/latest/meta_data.json << EOF
{
    "uuid": "myinstance.mydomain.com"
}
EOF

Cloud-Init supports many formats for scripts within user data. One of the most popular formats is the cloud-config file format. Let’s create a cloud-config script that adds our SSH public key to the authorized keys for the user root on the virtual machine. We can then login into the virtual machine as user root without using a password. If you don’t have a public-private SSH key pair you can quickly generate it using ssh-keygen command:

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$ ssh-keygen -f mykey

Now create a user_data file with the configuration instructions for Cloud-Init. In the following code block replace the value of the ssh-authorized-keys field with the content of your generated mykey.pub file:

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$ cat > config_drive/openstack/latest/user_data << EOF
#cloud-config
fqdn: myinstance.mydomain.com
users:
  - name: root
    ssh-authorized-keys:
      - ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDNH8Qwn4raGR1f9fvjbZe/GXM2N9Mh+eWlsFoYpcU4H5qf5YxT5CUo7BaTOgeE5geHyzxJQmCQlvoxcW3qkcjBJvVgEsTrrnX7KYS8BszvT4AMIuG2Za8f7myubXd6zYfj74XYhutUsPz7x2TEp9ZqbVkWcaElrQFxF2AzF7dV1RGntpPKyISqem70En8RYpGY514OLZ9TQDBYjbw8tfPuDd9mznXnWOZ34fPtP7+QDvOMFuA4tXsBpHj99/cbC0ViwzZtvb1QtY7dv9OFDgCRadw81+SKtzXctQ2rCYkb0huc0BCE7kLzinzlO62Znd+N1d+tpLAwP6i8Z5ZMXIJj user@machine
EOF

The file structure for our config drive is ready. Let’s generate an ext2 filesystem and copy the files to it. The virt-make-fs utility from the libguestfs-tools package can help us with that:

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$ virt-make-fs config_drive disk.config

In order for Cloud-Init to detect the attached drive as config drive the filesystem on the config drive needs to be labeled config-2. You can use e2label command from the e2fsprogs package to label your config drive:

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$ e2label disk.config config-2

Cloud-Init in action

On my Linux host I’m running libvirt to ease the management of virtual machines. You can install it by running sudo yum install libvirt. There is a handy command-line utility virsh which comes with libvirt in the extra package libvirt-client.

Let’s create a virtual machine with the config drive attached. As a virtual machine boot image I’m using a CentOS-6 image from cloud.centos.org which comes with Cloud-Init built in. Make sure that your virtual machine boot image has Cloud-Init installed. Following is a virtual machine definition file for the CentoOS-6 virtual machine. You might need to change the location of the disk image files and save it as CentOS-6.xml:

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<domain type='kvm'>
  <name>CentOS-6</name>
  <memory unit='KiB'>2097152</memory>
  <os>
    <type>hvm</type>
  </os>
  <devices>
    <disk type='file' device='disk'>
      <driver name="qemu" type="qcow2"/>
      <source file='/tmp/CentOS-6-x86_64-GenericCloud.qcow2'/>
      <target bus="virtio" dev="vda"/>
    </disk>
    <disk type='file' device='disk'>
      <driver name="qemu" type="raw"/>
      <source file='/tmp/disk.config'/>
      <target bus="virtio" dev="vdb"/>
    </disk>
    <interface type='network'>
      <source network='default'/>
    </interface>
    <serial type="file">
      <source path="/tmp/CentOS-6.log"/>
    </serial>
  </devices>
</domain>

Okay, everything is ready, let’s launch our Cloud-Init enabled CentOS-6 virtual machine:

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$ sudo virsh define CentOS-6.xml
$ sudo virsh start CentOS-6

If everything went fine you can watch the console output of the booting virtual machine at /tmp/CentOS-6.log. Cloud-Init will print out the IP address obtained by the virtual machine (192.168.122.165 in my case) where we can login as root using the generated private key:

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$ ssh -i testkey root@192.168.122.165

Congratulations, your virtual machine has just been configured by Cloud-Init the same way as any other virtual machine running in the cloud environment.