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mkosi - Create legacy-free OS images

A fancy wrapper around dnf --installroot, debootstrap, pacstrap and zypper that may generate disk images with a number of bells and whistles.

Supported output formats

The following output formats are supported:

  • Raw GPT disk image, with ext4 as root (raw_ext4)

  • Raw GPT disk image, with btrfs as root (raw_btrfs)

  • Raw GPT disk image, with xfs as root (raw_xfs)

  • Raw GPT disk image, with squashfs as read-only root (raw_squashfs)

  • Plain directory, containing the OS tree (directory)

  • btrfs subvolume, with separate subvolumes for /var, /home, /srv, /var/tmp (subvolume)

  • Tarball (tar)

When a GPT disk image is created, the following additional options are available:

  • A swap partition may be added in

  • The image may be made bootable on EFI systems

  • Separate partitions for /srv and /home may be added in

  • The root, /srv and /home partitions may optionally be encrypted with LUKS.

  • A dm-verity partition may be added in that adds runtime integrity data for the root partition

Compatibility

Generated images are legacy-free. This means only GPT disk labels (and no MBR disk labels) are supported, and only systemd based images may be generated. Moreover, for bootable images only EFI systems are supported (not plain MBR/BIOS).

All generated GPT disk images may be booted in a local container directly with:

systemd-nspawn -bi image.raw

Additionally, bootable GPT disk images (as created with the --bootable flag) work when booted directly by EFI systems, for example in KVM via:

qemu-kvm -m 512 -smp 2 -bios /usr/share/edk2/ovmf/OVMF_CODE.fd -drive format=raw,file=image.raw

EFI bootable GPT images are larger than plain GPT images, as they additionally carry an EFI system partition containing a boot loader, as well as a kernel, kernel modules, udev and more.

All directory or btrfs subvolume images may be booted directly with:

systemd-nspawn -bD image

Other features

  • Optionally, create an SHA256SUMS checksum file for the result, possibly even signed via gpg.

  • Optionally, place a specific .nspawn settings file along with the result.

  • Optionally, build a local project's source tree in the image and add the result to the generated image (see below).

  • Optionally, share RPM/DEB package cache between multiple runs, in order to optimize build speeds.

  • Optionally, the resulting image may be compressed with XZ.

  • Optionally, btrfs' read-only flag for the root subvolume may be set.

  • Optionally, btrfs' compression may be enabled for all created subvolumes.

  • By default images are created without all files marked as documentation in the packages, on distributions where the package manager supports this. Use the --with-docs flag to build an image with docs added.

Supported distributions

Images may be created containing installations of the following OSes.

  • Fedora

  • Debian

  • Ubuntu

  • Arch Linux

  • openSUSE

  • Mageia

  • CentOS

  • Clear Linux

In theory, any distribution may be used on the host for building images containing any other distribution, as long as the necessary tools are available. Specifically, any distro that packages debootstrap may be used to build Debian or Ubuntu images. Any distro that packages dnf may be used to build Fedora or Mageia images. Any distro that packages pacstrap may be used to build Arch Linux images. Any distro that packages zypper may be used to build openSUSE images. Any distro that packages yum (or the newer replacement dnf) may be used to build CentOS images.

Currently, Fedora packages all relevant tools as of Fedora 26.

Files

To make it easy to build images for development versions of your projects, mkosi can read configuration data from the local directory, under the assumption that it is invoked from a source tree. Specifically, the following files are used if they exist in the local directory:

  • mkosi.default may be used to configure mkosi's image building process. For example, you may configure the distribution to use (fedora, ubuntu, debian, archlinux, opensuse, mageia) for the image, or additional distribution packages to install. Note that all options encoded in this configuration file may also be set on the command line, and this file is hence little more than a way to make sure simply typing mkosi without further parameters in your source tree is enough to get the right image of your choice set up. Additionally if a mkosi.default.d directory exists, each file in it is loaded in the same manner adding/overriding the values specified in mkosi.default. Command-line arguments, as shown in the help description, have to be included in a configuration block (e.g. "[Packages]") corresponding to the argument group (e.g. "Packages"), and the argument gets converted as follows: "--with-network" becomes "WithNetwork=yes".

  • mkosi.extra/ or mkosi.extra.tar may be respectively a directory or archive. If any exist all files contained in it are copied over the directory tree of the image after the OS was installed. This may be used to add in additional files to an image, on top of what the distribution includes in its packages. When using a directory file ownership is not preserved: all files copied will be owned by root. To preserve ownership use a tar archive.

  • mkosi.skeleton/ or mkosi.skeleton.tar may be respectively a directory or archive, and they work in the same way as mkosi.extra/mkosi.skeleton.tar. However the files are copied before anything else so to have a skeleton tree for the OS. This allows to change the package manager and create files that need to be there before anything is installed. When using a directory file ownership is not preserved: all files copied will be owned by root. To preserve ownership use a tar archive.

  • mkosi.build may be an executable script. If it exists the image will be built twice: the first iteration will be the development image, the second iteration will be the final image. The development image is used to build the project in the current working directory (the source tree). For that the whole directory is copied into the image, along with the mkosi.build build script. The script is then invoked inside the image (via systemd-nspawn), with $SRCDIR pointing to the source tree. $DESTDIR points to a directory where the script should place any files generated it would like to end up in the final image. Note that make/automake based build systems generally honour $DESTDIR, thus making it very natural to build source trees from the build script. After the development image was built and the build script ran inside of it, it is removed again. After that the final image is built, without any source tree or build script copied in. However, this time the contents of $DESTDIR are added into the image.

    When the source tree is copied into the build image, all files are copied, except for mkosi.builddir/, mkosi.cache/ and mkosi.output/. That said, .gitignore is respected if the source tree is a git checkout. If multiple different images shall be built from the same source tree it's essential to exclude their output files from this copy operation, as otherwise a version of an image built earlier might be included in a later build, which is usually not intended. An alternative to excluding these built images via .gitignore entries is making use of the mkosi.output/ directory (see below), which is an easy way to exclude all build artifacts.

  • mkosi.postinst may be an executable script. If it exists it is invoked as last step of preparing an image, from within the image context. It is once called for the development image (if this is enabled, see above) with the "build" command line parameter, right before invoking the build script. It is called a second time for the final image with the "final" command line parameter, right before the image is considered complete. This script may be used to alter the images without any restrictions, after all software packages and built sources have been installed. Note that this script is executed directly in the image context with the final root directory in place, without any $SRCDIR/$DESTDIR setup.

  • mkosi.nspawn may be an nspawn settings file. If this exists it will be copied into the same place as the output image file. This is useful since nspawn looks for settings files next to image files it boots, for additional container runtime settings.

  • mkosi.cache/ may be a directory. If so, it is automatically used as package download cache, in order to speed repeated runs of the tool.

  • mkosi.builddir/ may be a directory. If so, it is automatically used as out-of-tree build directory, if the build commands in the mkosi.build script support it. Specifically, this directory will be mounted into the build countainer, and the $BUILDDIR environment variable will be set to it when the build script is invoked. The build script may then use this directory as build directory, for automake-style or ninja-style out-of-tree builds. This speeds up builds considerably, in particular when mkosi is used in incremental mode (-i): not only the disk images but also the build tree is reused between subsequent invocations. Note that if this directory does not exist the $BUILDDIR environment variable is not set, and it is up to build script to decide whether to do in in-tree or an out-of-tree build, and which build directory to use.

  • mkosi.rootpw may be a file containing the password for the root user of the image to set. The password may optionally be followed by a newline character which is implicitly removed. The file must have an access mode of 0600 or less. If this file does not exist the distribution's default root password is set (which usually means access to the root user is blocked).

  • mkosi.passphrase may be a passphrase file to use when LUKS encryption is selected. It should contain the passphrase literally, and not end in a newline character (i.e. in the same format as cryptsetup and /etc/crypttab expect the passphrase files). The file must have an access mode of 0600 or less. If this file does not exist and encryption is requested the user is queried instead.

  • mkosi.secure-boot.crt and mkosi.secure-boot.key may contain an X509 certificate and PEM private key to use when UEFI SecureBoot support is enabled. All EFI binaries included in the image's ESP are signed with this key, as a late step in the build process.

  • mkosi.output/ may be a directory. If it exists, and the image output path is not configured (i.e. no --output= setting specified), or configured to a filename (i.e. a path containing no / character) all build artifacts (that is: the image itself, the root hash file in case Verity is used, the checksum and its signature if that's enabled, and the nspawn settings file if there is any) are placed in this directory. Note that this directory is not used if the image output path contains at least one slash, and has no effect in that case. This setting is particularly useful if multiple different images shall be built from the same working directory, as otherwise the build result of a preceeding run might be copied into a build image as part of the source tree (see above).

All these files are optional.

Note that the location of all these files may also be configured during invocation via command line switches, and as settings in mkosi.default, in case the default settings are not acceptable for a project.

Examples

Create and run a raw GPT image with ext4, as image.raw:

# mkosi
# systemd-nspawn -b -i image.raw

Create and run a bootable btrfs GPT image, as foobar.raw:

# mkosi -t raw_btrfs --bootable -o foobar.raw
# systemd-nspawn -b -i foobar.raw
# qemu-kvm -m 512 -smp 2 -bios /usr/share/edk2/ovmf/OVMF_CODE.fd -drive format=raw,file=foobar.raw

Create and run a Fedora image into a plain directory:

# mkosi -d fedora -t directory -o quux
# systemd-nspawn -b -D quux

Create a compressed image image.raw.xz and add a checksum file, and install SSH into it:

# mkosi -d fedora -t raw_squashfs --checksum --xz --package=openssh-clients

Inside the source directory of an automake-based project, configure mkosi so that simply invoking mkosi without any parameters builds an OS image containing a built version of the project in its current state:

# cat > mkosi.default <<EOF
[Distribution]
Distribution=fedora
Release=24

[Output]
Format=raw_btrfs
Bootable=yes

[Packages]
Packages=openssh-clients httpd
BuildPackages=make gcc libcurl-devel
EOF
# cat > mkosi.build <<EOF
#!/bin/sh
cd $SRCDIR
./autogen.sh
./configure --prefix=/usr
make -j `nproc`
make install
EOF
# chmod +x mkosi.build
# mkosi
# systemd-nspawn -bi image.raw

To create a Fedora image with hostname:

# mkosi -d fedora --hostname image

Also you could set hostname in configuration file:

# cat mkosi.default
...
[Output]
Hostname=image
...

Requirements

mkosi is packaged for various distributions: Debian, Ubuntu, Arch (in AUR), Fedora. It is usually easiest to use the distribution package.

The current version requires systemd 233 (or actually, systemd-nspawn of it).

When not using distribution packages make sure to install the necessary dependencies. For example, on Fedora you need:

dnf install arch-install-scripts btrfs-progs debootstrap dosfstools edk2-ovmf e2fsprogs squashfs-tools gnupg python3 tar veritysetup xfsprogs xz zypper

On Debian/Ubuntu it might be necessary to install the ubuntu-keyring, ubuntu-archive-keyring and/or debian-archive-keyring packages explicitly, in addition to debootstrap, depending on what kind of distribution images you want to build. debootstrap on Debian only pulls in the Debian keyring on its own, and the version on Ubuntu only the one from Ubuntu.

Note that the minimum required Python version is 3.5.

If SecureBoot signing is to be used, then the "sbsign" tool needs to be installed as well, which is currently not available on Fedora, but in a COPR repository:

dnf copr enable msekleta/sbsigntool
dnf install sbsigntool

References