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Disk_Image_Builder_techniques.md

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Some Techniques used with Disk Image Builder

Introduction

This doc lists a few techniques I used when creating my own images using disk image builder.

Copy files from your host to the image

Think of this an "injecting" files into an image -- e.g., ssh keys, and other scripts that might be useful to the user of the resultant image.

See the Phase Subdirectories doc for information on each element directory mentioned below.

  • Put the files you want to inject into some subdirectory on the local host running disk-image-create. This can be done via manual operation or via some ansible role that creates a nodepool machine.
  • write an element that uses these directories:
  • exta-data.d: In this directory, create a script that will copy the files from the local host to the $TMP_HOOKS_PATH
  • install.d: In this directory, create a script that will:
    • Create the directory on the image
    • Copy the files from $TMP_HOOKS_PATH to the directory on the image

Here's is my example of a file in /etc/mykeys/id_rsa.pub on the local host that I want copied to my resultant image using disk-image create. My element will be called 'copy_keys'. And I will make it so that the /etc/mykeys/id.pub.rsa file will be copied to /root/.ssh/authorized_keys on the image.

The element directory structure will be a subdirectory called 'copy_keys' and two subdirectories underneath it called extra-data.d and install.d. There will be one script inside each directory -- 99-copy-keys and 20-install-keys.

  • elements
    • copy_keys
      • extra-data.d
        • 99-copy-keys
      • install.d
        • 20-install-keys

The 99-copy-keys script:

$ cat 99-copy-keys
#!/bin/bash

# This command is exected outside of chroot.
#
cp /etc/mykeys/id_rsa.pub $TMP_HOOKS_PATH

Note that the extra-data.d scripts are executed outside of chroot and so it has access to the local host files. You copy files to $TMP_HOOKS_PATH which is used by disk-image-create.

The 20-install-keys script:

$ cat 20-install-keys
#!/bin/bash

# These commands are exected in chroot on the image.
# The /tmp/in_target.d is where the keys are copied to when 99-copy-keys was executed.
#
mkdir /etc/mykeys
cp /tmp/in_target.d/id_rsa.pub /etc/mykeys

Note that the install.d scripts are exected in chroot so everything you do will affect the resultant image. The /tmp/in_target.d directory is available inside the image for files you copied using the extra-data.d scripts.

The scripts must be executable -- if they are not, they are skipped.

Remeber to set your ELEMENTS_PATH environment variable to include the directory called elements above so that disk-image-create can find your element.

You can run the disk-image-create command like this example:

$ disk-image-create --image-size 4G -o ~/myimage vm ubuntu copy_keys