User Data Script

A Bash script is another common way to inject user data into an instance. The file must be a valid Bash script, so it must begin with a #!/bin/bash line. For example, Listing 5 shows an instance configuration input file run on boot that installs two RPM packages, enables the Apache HTTP service, and creates the index.html file in the server's root directory.

#!/bin/bash
# install additional packages
sudo yum install -y httpd vim
# enable httpd service
sudo systemctl enable httpd
sudo systemctl start httpd
# create file in http document root
sudo echo "<p>hello world</p>" > /var/www/html/index.html

The command to launch an instance with a user data Bash script would look like Listing 6.

root@controller ~(keystone_gjuszczak)]# openstack server create --flavor m2.tiny --image CentOS_7_cloud_init --nic net-id=int_net --key-name cloud --security-group default --user-data user_data.sh CentOS_7_CI_UD

Image Metadata Properties

Worth mentioning is that images also can contain metadata properties that determine the purpose and nature of a particular image. OpenStack offers a variety of image properties that can be set for the image to define its purpose (e.g., CPU pinning, compute CPU architecture, or Cinder volume type required by the image).

Figure 1 shows the image properties configuration screen with the hypervisor_type parameter set to kvm, which means that launching an instance from this particular image requires a KVM-based compute node. Otherwise, the instance won't boot. Booting the instance on compute nodes other than KVM (e.g., LXC- or XEN-based nodes) results in an error.

Figure 1: Defining the image metadata properties.

You can also set an image property from the command line with the --property <key>=<type> parameter. For example,

[root@controller ~(keystone_admin)]# openstack image set --property hypervisor_type= kvm cirros-0.4.0

sets the hypervisor_type parameter to kvm.

Passing Metadata with a Config Drive

You can force OpenStack to write instance metadata to a special drive called a configuration drive, or simply config drive, that is attached to the instance during the boot process.

Thanks to the cloud-init script, a config drive is automatically mounted on the instance's OS, and its files can be read easily, as if they were coming from the metadata service. With a config drive, you can pass different user data formats, as well as ordinary files, to the instance. The main requirement for using a config drive is passing the --config-drive true parameter in the openstack server create command. You can also configure the compute node always to use a config drive by setting the following parameter in /etc/nova/nova.conf:

force_config_drive = true

The example command in Listing 7 enables a config drive and passes a previously created user data script and two files to the instance on boot. The first --file replaces the original /etc/hosts with a file with new content, and the second --file copies content to the /tmp directory on the instance:

[root@controller ~(keystone_gjuszczak)]# openstack server create --config-drive true --flavor m2.tiny --image CentOS_7_cloud_init --nic net-id=int_net --key-name cloud --security-group default --user-data user_data.sh --file /etc/hosts=/root/hosts --file /tmp/example_file.txt=/root/example_file.txt CentOS_7_CI_CD