A Simple Host-Specific Container Build

zbm-builder.sh mounts a build directory (by default, the current working directory) into the container to provide a path to inject custom configuration into the container. If the system will manage ZFSBootMenu images exclusively via a build container, an obvious location for the build directory is /etc/zfsbootmenu. Start by creating this directory and populating a simple config.yaml for container builds:

mkdir -p /etc/zfsbootmenu

cat > /etc/zfsbootmenu/config.yaml <<EOF
  InitCPIO: true
  Enabled: false
  Enabled: true
  Versions: false
  Prefix: zfsbootmenu
  CommandLine: zfsbootmenu ro quiet loglevel=4 nomodeset

curl -L -O /etc/zfsbootmenu/zbm-builder.sh https://raw.githubusercontent.com/zbm-dev/zfsbootmenu/master/zbm-builder.sh
chmod 755 /etc/zfsbootmenu/zbm-builder.sh

In this configuration, mkinitcpio will be used instead of dracut. Component generation is disabled, so generate-zbm will produce only a UEFI bundle. That bundle has numeric versioning disabled, so generate-zbm will produce an unversioned zfsbootmenu.EFI; if the generator detects an existing zfsbootmenu.EFI in the output directory, it will make a single backup of that file as zfsbootmenu-backup.EFI before overwriting it. A simple kernel command-line is specified and may be overridden as necessary.

For some systems, it is necessary to tear down USB devices before ZFSBootMenu launches a boot environment. Even when this is not needed, it is generally harmless. The ZFSBootMenu repository offers a teardown hook for this purpose, and it is possible to instruct mkinitcpio to include this teardown hook straight from the version of ZFSBootMenu inside the container:

mkdir -p /etc/zfsbootmenu/mkinitcpio.conf.d
echo "zfsbootmenu_teardown=( /zbm/contrib/xhci-teardown.sh )" \
    > /etc/zfsbootmenu/mkinitcpio.conf.d/teardown.conf

The default mkinitcpio.conf in the container, which should generally not be overridden, will source all files in /etc/zfsbootmenu/mkinitcpio.conf.d.

Custom Font

On high-resolution screens, the Linux kernel does not always do a good job choosing a console font. A nice font can be explicitly specified in the ZFSBootMenu configuration for mkinitcpio. The container entrypoint must be told to install the desired font and the mkinitcpio configuration should include the necessary module and executable to set the font:

echo "BUILD_ARGS+=( -p terminus-font )" >> /etc/zfsbootmenu/zbm-builder.conf

cat > /etc/zfsbootmenu/mkinitcpio.conf.d/consolefont.conf <<EOF

This approach uses the configuration file capability of zbm-builder.sh to specify build options without requiring that they be included on the command line.

As configured, mkinitcpio will not see a configured console font and will omit the font from generated images. To make mkinitcpio aware of the desired font, it must be specified in /etc/rc.conf within the container. The "terraform" capabilities of the container entrypoint can be used to accomplish this:

mkdir -p /etc/zfsbootmenu/rc.d

cat > /etc/zfsbootmenu/rc.d/consolefont <<EOF
sed -e '/FONT=/a FONT="ter-132n"' -i /etc/rc.conf

chmod 755 /etc/zfsbootmenu/rc.d/consolefont

When the container entrypoint finds an rc.d subdirectory in the build root, it will run each executable file therein before generating a ZFSBootMenu image. If any of these executable should fail, image generation is aborted.

Host-Specific Files

By default, zbm-builder.sh will copy the file /etc/hostid from the host to the build directory so that the hostid of the generated ZFSBootMenu image will match that of your host. This is often desirable for customized builds, but it would be undesirable for copies of these files in /etc/zfsbootmenu to fall out of synchronization with the host versions. To avoid this issue, tell zbm-builder.sh to remove any copies in /etc/zfsbootmenu before determining whether the host versions should be copied in for image creation:

echo "REMOVE_HOST_FILES=yes" >> /etc/zfsbootmenu/zbm-builder.conf

If you would rather not see those files at all, it is possible to instruct generate-zbm to remove them after they are used. Edit the configuration at /etc/zfsbootmenu/config.yaml and add the following key:

  PostHooksDir: /build/cleanup.d

Alternatively, tell the build container to add this option dynamically:

echo "BUILD_ARGS+=( -e '.Global.PostHooksDir=\"/build/cleanup.d\"' )" \
    >> /etc/zfsbootmenu/zbm-builder.conf

Next, create a post-generation hook to remove the files:

mkdir -p /etc/zfsbootmenu/cleanup.d

cat > /etc/zfsbootmenu/cleanup.d/hostfiles <<EOF
rm -f /build/zpool.cache /build/hostid

chmod 755 /etc/zfsbootmenu/cleanup.d/hostfiles

The Output Directory

At this point, it should be possible to generate images by running

cd /etc/zfsbootmenu && ./zbm-builder.sh

However, these images will reside in /etc/zfsbootmenu/build and will require manual management. A better alternative is to let generate-zbm manage the ZFSBootMenu output directory directly. Assuming that ZFSBootMenu images should be installed in /boot/efi/EFI/zfsbootmenu, tell zbm-builder.sh to mount the directory inside the container, and tell the container that it should write its images to the mounted directory:

cat >> /etc/zfsbootmenu/zbm-builder.conf <<EOF
RUNTIME_ARGS+=( -v /boot/efi/EFI/zfsbootmenu:/output )
BUILD_ARGS+=( -o /output )

Now, running

cd /etc/zfsbootmenu && ./zbm-builder.sh

should create images directly in /boot/efi/EFI/zfsbootmenu and create a backup of any existing zfsbootmenu.EFI.

Networking in Rootfull Containers

Manipulating files in /etc/zfsbootmenu and /boot/efi/EFI/zfsbootmenu may require root privileges, which means that zbm-builder.sh and the build container will need to run as root. In some configurations, podman may not provide working networking for rootfull containers by default. A simple fix is to allow the containers to use the host network stack, which can be accomplished by running

echo "RUTNIME_ARGS+=( --net=host )" >> /etc/zfsbootmenu/zbm-builder.conf

Adding Remote Access Capabilities

The process for including dropbear for remote access to container-built ZFSBootMenu images is largely the same as the process for host-built images, but care must be taken to ensure that all necessary components are available within the build directory.

  • The core configuration changes should be ignored. They are unnecessary with the container configuration described above.

  • The basic network access and dropbear instructions are generally applicable, except no changes should be made to /etc/zfsbootmenu/mkinitcpio.conf and all references to paths in /etc/dropbear should be replaced with corresponding references to paths in /etc/zfsbootmenu/dropbear.

Specific alterations are noted below.

Configuring Basic Network Access

Commands to fetch and unpack the mkinitcpio-rclocal module and create an /etc/zfsbootmenu/initcpio/rc.local script still apply as described to containerized builds. Subsequent sed and echo commands that write to /etc/zfsbootmenu/mkinitcpio.conf should be ignored because this file should not exist. Instead, create a configuration snippet that will add network configuration to the ZFSBootMenu image:

cat > /etc/zfsbootmenu/mkinitcpio.conf.d/network.conf <<EOF
BINARIES+=(ip dhclient dhclient-script)


If a static IP address will be configured, it is acceptable to leave dhclient and dhclient-script out of the BINARIES array.

Next, edit /etc/zfsbootmenu/config.yaml to add a hook directory configuration telling mkinitcpio where to find custom modules:

    - /build/initcpio
    - /usr/lib/initcpio

Configuring Dropbear

The commands to fetch and unpack the mkinitcpio-dropbear module still apply to containerized builds. Instead of adding dropbear to the non-existent configuration /etc/zfsbootmenu/mkinitcpio.conf, create a snippet:

cat > /etc/zfsbootmenu/mkinitcpio.conf.d/dropbear.conf <<EOF

Rather than creating keys (and optional configuration) in /etc/dropbear, create the keys and configuration in /etc/zfsbootmenu/dropbear:

mkdir -p /etc/zfsbootmenu/dropbear

## Not strictly required; see note below
for keytype in rsa ecdsa ed25519; do
    dropbearkey -t "${keytype}" -f "/etc/dropbear/dropbear_${keytype}_host_key"

## If desired
echo 'dropbear_listen=2222' > /etc/zfsbootmenu/dropbear/dropbear.conf


Generating keys is not strictly necessary and can be skipped if dropbearkey is not available on the host. The build container will generally lack SSH host keys, so the mkinitcpio-dropbear module will default to creating new, random keys in the build directory. These keys will persist for subsequent use.

The file /etc/zfsbootmenu/dropbear/root_key is required to provide a list of authorized keys in the ZFSBootMenu image. Unlike with host builds, this may not be a symlink to a user's authorized_keys file because that path will be unavailble in the container. Instead, simply copy a desired authorized_keys file to /etc/zfsbootmenu/dropbear/root_key. Alternatively, dynamism can be preserved by relying on bind-mounting a specific authorized_keys file into the build container:

echo "RUNTIME_ARGS+=( -v /home/${dropbear_user}/.ssh/authorized_keys:/authorized_keys:ro ) >> /etc/zfsbootmenu/zbm-builder.conf
ln -s /authorized_keys /etc/zfsbootmenu/dropbear/root_key

Replace ${dropbear_user} with the desired user whose authorized_keys file should govern access to ZFSBootMenu.

Make sure that the build container installs the packages necessary to provide dropbear:

echo "BUILD_ARGS+=( -p dropbear -p psmisc )" >> /etc/zfsbootmenu/zbm-builder.conf

Finally, add a "terraform" script to link the expected /etc/dropbear directory to that in the build directory:

cat > /etc/zfsbootmenu/rc.d/dropbear <<EOF

[ -d /build/dropbear ] || exit 0

if [ -d /etc/dropbear ] && [ ! -L /etc/dropbear ]; then
    if ! rmdir /etc/dropbear; then
        echo "ERROR: failed to remove existing /etc/dropbear directory"
        exit 1

if ! ln -Tsf /build/dropbear /etc/dropbear; then
    echo "ERROR: failed to make /etc/dropbear symlink"
    exit 1

chmod 755 /etc/zfsbootmenu/rc.d/dropbear