vmdb2 builds disk images with Debian installed. The images can be used for virtual machines, or can be written to USB flash memory devices, and hardware computers can be booted off them.
This manual is published as HTML and as PDF.
You can get vmdb2 by getting the source code from git, either author’s server or gitlab.com.
You can then run it from the source tree:
$ sudo /path/to/vmdb2/vmdb2 ...In Debian 10 (“buster”) and its derivatives, you can also install the vmdb2 package:
$ apt install vmdb2For any other systems, we have no instructions. If you figure it out, please tell us how.
vmdb2 is a successor of the vmdebootstrap program, written by the same author, to fix a number of architectural problems and limitations with the old program. The new program is not compatible with the old one; that would’ve required keeping the problems, as well.
vmdebootstrap was the first attempt by it author to write a tool to build system images. It turned out to not be well designed. Specifically, it was not easily extensible to be as flexible as a tool of this sort should be.
The author likes to write tools for himself and had some free time. He sometimes prefers to write his own tools rather than spend time and energy evaluating and improving existing tools. He admits this is a character flaw.
Also, he felt ashamed of how messy vmdebootstrap turned out to be.
If nobody else likes vmdb2, that just means the author had some fun on his own.
You need to make a specification file (in YAML) that tells vmdb2 what kind of image to build, and how. An example:
steps:
- mkimg: "{{ output }}"
size: 4G
- mklabel: msdos
device: "{{ output }}"
- mkpart: primary
device: "{{ output }}"
start: 0%
end: 100%
tag: /
- kpartx: "{{ output }}"
- mkfs: ext4
partition: /
- mount: /
- debootstrap: buster
mirror: http://deb.debian.org/debian
target: /
- apt: install
packages:
- linux-image-amd64
tag: /
- fstab: /
- grub: bios
tag: /The source repository of vmdb2 has more examples, which are also automatically tested, unlike the above one.
The list of steps builds the kind of image that the user wants. The specification file can easily be shared, and put under version control.
Every action in a step is provided by a plugin to vmdb2. Each action is a well-defined task, which may be parameterised by some of the key/value pairs in the step. For example, mkimg would create a raw disk image file. The image is 4 gigabytes in size. mkpart creates a partition, and mkfs an ext4 filesystem in the partition. And so on.
Steps may need to clean up after themselves. For example, a step that mounts a filesystem will need to unmount it at the end of the image creation. Also, if a later step fails, then the unmount needs to happen as well. This is called a “teardown”.
By providing well-defined steps that the user may combine as they wish, vmdb2 gives great flexibility without much complexity, but at the cost of forcing the user to write a longer specification file than a simple command line invocation.
To use this, save the specification into test.vmdb, and run the following command:
$ sudo vmdb2 test.vmdb --output test.img --verboseAlternatively the specification can be passed in via stdin by setting the file name to -, like so:
$ cat test.vmdb | sudo vmdb2 - --output test.img --verboseThis will take a long time, mostly at the debootstrap step. See below for speeding that up by caching the result.
Due to the kinds of things vmdb2 does (such as mounting, creating device nodes, etc), it needs to be run using root privileges. For the same reason, it probably can’t be run in an unprivileged container.
vmdb2 uses debootstrap, which copies the host’s /etc/hostname file into the image. You probably want to set the hostname for the image you’re creating. You can do this by overwriting the /etc/hostname file in the image, for example with the following step:
- chroot: rootfs
shell: |
echo myhostname > /etc/hostnameAt this time, vmdb2 does not support building partitioned images without partition, or images without a partition table. Such support may be added later. If this would be useful, do tell the authors.
Instead of device filenames, which vary from run to run, vmdb2 steps refer to block devices inside the image, and their mount points, by symbolic names called tags. Tags are any names that the user likes, and vmdb2 does not assign meaning to them. They’re just strings.
To refer to the filename specified with the --output or --image command line options, you can use Jinja2 templating. The variables output and image can be used.
- mkimg: "{{ output }}"
- mklabel: "{{ image }}"The difference is that --output creates a new file, or truncates an existing file, whereas --images requires the file to already exist. The former is better for image file, the latter for real block devices.
Building an image can take several minutes, and that’s with fast access to a Debian mirror and an SSD. The slowest part is typically running debootstrap, and that always results in the same output, for a given Debian release. This means its easy to cache.
vmdb2 has the two actions cache-roots and unpack-rootfs and the command line option --rootfs-tarball to allow user to cache. The user uses the option to name a file. cache-rootfs takes the root filesystem and stores it into the file as a compress tar archive (“tarball”). unpack-rootfs unpacks the tarball. This allows vmdb2 to skip running debootstrap needlessly.
The specify which steps should be skipped, the unless field can be used: unpack-rootfs sets the rootfs_unpacked flag if it actually unpacks a tarball, and unless allows checking for that flag. If the tarball doesn’t exist, the flag is not set.
- unpack-rootfs: root
- debootstrap: buster
target: root
unless: rootfs_unpacked
- cache-rootfs: root
unless: rootfs_unpackedIf the tarball exists, it is unpacked, and the debootstrap and cache-rootfs steps are skipped. If the tarball doesn’t exist, the unpack step is silently skipped, and the debootstrap and caching steps are performed instead.
It’s possible to have any number of steps between the unpack and the cache steps. However, note that if you change anything within those steps, or time passes and you want to include the new packages that have made it into Debian, you need to delete the tarball so it is run again.
This chapter documents the user-level acceptance criteria for vmdb2, and how they are to be verified. It’s meant to be processed with the Subplot tool, but understood by all users of and contributors to the vmdb2 software. The criteria and their verification are expressed as scenarios.
For reasons of speed, security, and reliability, these scenarios test only the core functionality of vmdb2. All the useful steps for actually building images are left out.. Those are tested by actually building images. However, those useful steps are not useful, if the core that invokes them is rotten.
The first case we look at is one for the happy path: a specification with two echo steps, and nothing else. It’s very simple, and nothing goes wrong when executing it. In addition to the actual thing to do, each step also defines a “teardown” thing to do. We check that all the steps and teardown steps are performed, in the right order.
Note that the “echo” step is provided by vmdb2 explicitly for this kind of testing, and that the teardown field in the step is implemented by the echo step. It’s not a generic feature.
given a specification file called happy.vmdb
when user runs vmdb2 -v happy.vmdb --output=happy.img
then exit code is 0
and stdout contains "foo" followed by "bar"
and stdout contains "bar" followed by "bar_teardown"
and stdout contains "bar_teardown" followed by "foo_teardown"vmdb2 allows values in specification files to be processed by the Jinja2 templating engine. This allows users to do thing such as write specifications that use configuration values to determine what happens. For our simple echo/error steps, we will write a rule that outputs the image file name given by the user. A more realistic specification file would instead do thing like create the file.
given a specification file called j2.vmdb
when user runs vmdb2 -v j2.vmdb --output=foo.img
then exit code is 0
and stdout contains "image is foo.img" followed by "bar"Sometimes things do not quite go as they should. Does vmdb2 do things in the right order then? This scenario uses the “error” step provided for testing this kind of thing.
given a specification file called unhappy.vmdb
when user runs vmdb2 -v unhappy.vmdb --output=unhappy.img
then exit code is 1
and stdout contains "foo" followed by "yikes"
and stdout contains "yikes" followed by "WAT?!"
and stdout contains "WAT?!" followed by "foo_teardown"
and stdout does NOT contain "bar_step"
and stdout does NOT contain "bar_teardown"steps:
- echo: foo
teardown: foo_teardown
- error: yikes
teardown: "WAT?!"
- echo: bar
teardown: bar_teardownRun Ansible using a provided playbook, to configure the image. vmdb2 sets up Ansible so that it treats the image as the host being configured (via the chroot connecion). The image MUST have Python installed (version 2 or 3 depending on Ansible version).
Step keys:
ansible — REQUIRED; value is the tag of the root filesystem.
playbook — REQUIRED; value is the filename of the Ansible playbook, relative to the .vmdb file.
Example (in the .vmdb file):
- apt: install
tag: root
packages: [python]
- ansible: root
playbook: foo.ymlExample (foo.yml):
- hosts: image
tasks:
- name: "set /etc/hostname"
shell: |
echo "{{ hostname }}" > /etc/hostname
- name: "unset root password"
shell: |
sed -i '/^root:[^:]*:/s//root::/' /etc/passwd
- name: "configure networking"
copy:
content: |
auto eth0
iface eth0 inet dhcp
iface eth0 inet6 auto
dest: /etc/network/interfaces.d/wired
vars:
hostname: discworldInstall packages using apt, which needs to already have been installed.
Step keys:
apt — REQUIRED; value MUST be install.
tag — REQUIRED; value is the tag for the root filesystem.
packages — REQUIRED; value is a list of packages to install.
Example (in the .vmdb file):
- apt: install
tag: root
packages:
- python
- linux-image-amd64Create a tarball of the root filesystem in the image.
Step keys:
cache-rootfs — REQUIRED; tag of root filesystem on image.Example (in the .vmdb file):
- cache-rootfs: root
unless: rootfs_unpackedRun a shell snippet in a chroot inside the image.
Step keys:
chroot — REQUIRED; value is the tag for the root filesystem.
shell — REQUIRED; the shell snippet to run
Example (in the .vmdb file):
- chroot: root
shell: |
echo I am in chrootCopy a file from outside into the target filesystem.
Step keys:
copy-file — REQUIRED; the full (starting from the new filesystem root) path name of the file to create. Any missing directories will be created (owner root, group root, mode 0511).src — REQUIRED; filename on the host filesystem, outside the chroot, relative to the current working directory of the vmdb2 process.perm — OPTIONAL; the numeric (octal) representation of the file’s permissions. Defaults to 0644.uid — OPTIONAL; the numeric user ID of the file’s owner. Defaults to 0 (root).gid — OPTIONAL; the numeric user ID of the file’s group. Defaults to 0 (root).Create a directory in the target filesystem
Step keys:
create-dir — REQUIRED; the full (starting from the new filesystem root) path name of the directory to create. It will work as a mkdir -p — Any intermediate directories that do not yet exist will be created.perm — OPTIONAL; the numeric (octal) representation of the directory’s permissions. Defaults to 0755.uid — OPTIONAL; the numeric user ID of the directory’s owner. Defaults to 0 (root).gid — OPTIONAL; the numeric user ID of the directory’s group. Defaults to 0 (root).Create an empty file in the target filesystem.
Step keys:
create-file — REQUIRED; the full (starting from the new filesystem root) path name of the file to create. It will not create any directories; if they need to be created, please use create-dir first.contents — REQUIRED; the contents to be written to the generated file.perm — OPTIONAL; the numeric (octal) representation of the file’s permissions. Defaults to 0644.uid — OPTIONAL; the numeric user ID of the file’s owner. Defaults to 0 (root).gid — OPTIONAL; the numeric user ID of the file’s group. Defaults to 0 (root).Create a directory tree with a basic Debian installation. This does not include a boot loader.
A side effect of this step is that apt-get update is always run, even if the step is otherwise skipped using unless.
Step keys:
debootstrap — REQUIRED; value is the codename of the Debian release to install: stretch, buster, etc.
target — REQUIRED; value is the tag for the root filesystem.
mirror — REQUIRED; which Debian mirror to use
keyring — OPTIONAL; which gpg keyring to use to verify the packages. This is useful when using a non-official Debian repository (e.g. Raspbian) as by default debootstrap will use the keys provided by the “debian-archive-keyring” package.
Example (in the .vmdb file):
- debootstrap: buster
target: root
mirror: http://mirror.example.com/debian
keyring: /etc/apt/trusted.gpgCreate /etc/fstab inside the the image.
Step keys:
fstab — REQUIRED; value is the tag for the root filesystem.Example (in the .vmdb file):
- fstab: rootInstall the GRUB bootloader to the image. Works on a PC for traditional BIOS booting, PC and ARM machines for modern UEFI booting, and PowerPC machines for IEEE1275 booting. Supports Secure Boot for amd64 UEFI.
Warning: This is the least robust part of vmdb2.
Step keys:
grub — REQUIRED; value MUST be one of uefi and bios, for a UEFI or a BIOS boot, respectively. Only PC systems support the bios option.
tag — REQUIRED; value is the tag for the root filesystem.
efi — REQUIRED for UEFI; value is the tag for the EFI partition.
prep — REQUIRED for IEEE1275; value is the tag for the PReP partition.
console — OPTIONAL; set to serial to configure the image to use a serial console.
image-dev — OPTIONAL; which device to install GRUB onto; this is needed when installing to a real hard drive, instead of an image.
quiet — OPTIONAL; should the kernel be configured to boot quietly? Default is no.
timeout — OPTIONAL; set the grub menu timeout, in seconds. Defaults to 0 seconds.
Example (in the .vmdb file):
- grub: bios
tag: rootSame, but for UEFI, assuming that a FAT32 filesystem exists on the partition with tag efi:
- grub: uefi
tag: root
efi: efi
console: serialOr for IEEE1275, assuming that a partition with tag prep exists:
- grub: ieee1275
tag: root
prep: prep
console: serialInstall to a real hard disk (named with the --image option):
- grub: uefi
tag: root
efi: efi
image-dev: "{{ image }}"Create loop devices for partitions in an image file. Not needed when installing to a real block device, instead of an image file.
Step keys:
kpartx — REQUIRED; filename of block device with partitions.Example (in the .vmdb file):
- kpartx: "{{ output }}"Set up disk encryption using LUKS with the cryptsetup utility. The encryption passphrase is read from a file or from the output of a command. The encrypted disk gets opened and can be mounted using a separate tag for the cleartext view.
Step keys:
cryptsetup — REQUIRED; value is the tag for the encrypted block device. This is not directly useable by users, or mountable.
tag — REQUIRED; the tag for the de-crypted block device. This is what gets mounted and visible to users.
key-file — OPTIONAL; file from where passphrase is read.
key-cmd — OPTIONAL; command to run, passphrase is the first line of its standard output.
Example (in the .vmdb file):
- cryptsetup: root
tag: root_crypt
key-file: disk.passSame, except run a command to get passphrase (in this case pass):
- cryptsetup: root
tag: root_crypt
key-cmd: pass show disk-encryptionCreate an LVM2 logical volume (LV) in an existing volume group.
Step keys:
lvcreate — REQUIRED; value is the tag for the volume group.
name — REQUIRED; tag for the new LV block device.
size — REQUIRED; size of the new LV.
Example (in the .vmdb file):
- lvcreate: rootvg
name: rootfs
size: 1GCreate a filesystem.
Step keys:
mkfs — REQUIRED; filesystem type, such as ext4 or vfat.
partition — REQUIRED; tag for the block device to use.
Example (in the .vmdb file):
- mkfs: ext4
partition: rootCreate a new image file of a desired size.
Step keys:
mkimage — REQUIRED; name of file to create.
size — REQUIRED; size of the image.
Example (in the .vmdb file):
- mkimg: "{{ output }}"
size: 4GCreate a partition table on a block device.
Step keys:
mklabel — REQUIRED; type of partition table, MUST be one of msdos and gpt.
device — REQUIRED; tag for the block device.
Example (in the .vmdb file):
- mklabel: msdos
device: "{{ output }}"Create a partition.
Step keys:
mkpart — REQUIRED; type of partition to create: use primary (but any value acceped by parted is OK).
device — REQUIRED; filename of block device where to create partition.
start — REQUIRED; where does the partition start?
end — REQUIRED; where does the partition end?
tag — REQUIRED; tag for the new partition.
Example (in the .vmdb file):
- mkpart: primary
device: "{{ output }}"
start: 0%
end: 100%
tag: rootMount a filesystem.
Step keys:
mount — REQUIRED; tag of filesystem to mount.
dirname — OPTIONAL; the mount point.
mount-on — OPTIONAL; tag of already mounted filesystem in image. (FIXME: this may be wrong?)
Example (in the .vmdb file):
- mount: rootInstall packages using apt, which needs to already have been installed, for a different architecture than the host where vmdb2 is being run. For example, for building an image for a Raspberry Pi on an Intel PC.
Step keys:
qemu-debootstrap — REQUIRED; value is the codename of the Debian release to install: stretch, buster, etc.
target — REQUIRED; value is the tag for the root filesystem.
mirror — OPTIONAL; which Debian mirror to use.
keyring — OPTIONAL; which gpg keyring to use to verify the packages. This is useful when using a non-official Debian repository (e.g. Raspbian) as by default qemu-debootstrap will use the keys provided by the “debian-archive-keyring” package.
arch — REQUIRED; the foreign architecture to use.
variant — OPTIONAL; the variant for debootstrap.
Example (in the .vmdb file):
- qemu-debootstrap: buster
target: root
mirror: http://mirror.example.com/debian
keyring: /etc/apt/trusted.gpg
arch: arm64
variant: builddConfigure the system on the image so that it automatically resizes itself to fill the actual disk, upon first boot. For this to work, the root file system MUST be the last partition on the image.
Also, the image MUST have the parted package installed for the partprobe command.
Step keys:
resize-rootfs — REQUIRED; value MUST be the tag for the root filesystem.This is based on reading the changes by Peter Lawler to the image-specs repository to do the same thing.
Example:
- resize-rootfs: rootRun a shell snippet on the host. This is not run in a chroot, and can access the host system.
Step keys:
root-fs — REQUIRED; value is the tag for the root filesystem.
shell — REQUIRED; the shell snippet to run
Example (in the .vmdb file):
- root-fs: root
shell: |
echo I am in NOT in chroot.Unpack a tarball of the root filesystem to the image, and set the rootfs_unpacked condition to true. If the tarball doesn’t exist, do nothing and leave the rootfs_unpacked condition to false.
Step keys:
unpack-rootfs — REQUIRED; tag for the root filesystem.Example (in the .vmdb file):
- unpack-rootfs: rootCreate an LVM2 volume group (VG), and also initialise the physical volumes for it.
Step keys:
vgcreate — REQUIRED; value is the tag for the volume group. This gets initialised with vgcreate.
physical — REQUIRED; list of tags for block devices (partitions) to use as physical volumes. These get initialised with pvcreate.
Example (in the .vmdb file):
- vgcreate: rootvg
physical:
- my_partition
- other_partitionMount the usual Linux virtual filesystems in the chroot:
/proc/dev/dev/pts/dev/shm/run/run/lock/sysThey will be automatically unmounted at the end.
Often, the virtual filesystems are unnecessary, but some Debian packages won’t install without them. The grub boot loader needs them as well, but mounts what it needs itself, if necessary.
Step keys:
virtual-filesystems — REQUIRED; value is the tag of the root filesystem.Example (in the .vmdb file):
- virtual-filesystem: rootfs