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Combustion - Configure MicroOS on the first boot

Combustion is a minimal module for dracut, which runs a user provided script on the first boot of a system.

You can use this to create additional files, install packages, set up devices or even re-partition the hard disk. The configuration can be provided as a shell script, loaded from an external storage media and is run during boot in a new system snapshot. On success, the system will directly boot into that new snapshot, so that no reboot is needed.

How to use it

The configuration files are copied from a filesystem with the LABEL "combustion", but to be compatible and co-installable with Ignition, the LABEL "ignition" is used as fallback. It expects a directory "combustion" at the root level of the filesystem and a file "script" inside, which is executed inside a transactional-update shell.

<root directory>
├── combustion
│   ├── script
│   └── ... other files
└── ignition (optional)
    └── config.ign

If a QEMU fw_cfg blob with the name "opt/org.opensuse.combustion/script" is found, it is preferred and the content of that is used as script. Example parameter for QEMU:

-fw_cfg name=opt/org.opensuse.combustion/script,file=/var/combustion-script

You can do everything necessary for initial system configuration from this script, including addition of ssh keys, installing packages, managing users or even doing partitioning changes.

Simple example

Example for formatting a USB drive and adding a config, which installs the "vim-small" package and enables sshd:

mkfs.ext4 /dev/sdX
e2label /dev/sdX ignition
mount /dev/sdX /mnt
mkdir -p /mnt/combustion/
cat >/mnt/combustion/script <<EOF
# combustion: network
systemctl enable sshd.service
zypper --non-interactive install vim-small
echo "Hello User!" >/mnt/combustion/welcome
umount /mnt

The "# combustion: network" comment triggers networking initialization before running the script. This is equivalent to passing "rd.neednet=1" on the kernel cmdline and so the network configuration parameters (man dracut.cmdline) apply here as well. If those aren't specified, it defaults to "ip=dhcp" for each available interface.

More complex configuration example

This script additionally provides visible feedback during boot, sets a password and copies a public ssh key (which has to be in the "combustion" folder).

# combustion: network
# Redirect output to the console
exec > >(exec tee -a /dev/tty0) 2>&1
# Set a password for root, generate the hash with "openssl passwd -6"
echo 'root:$5$.wn2BZHlEJ5R3B1C$TAHEchlU.h2tvfOpOki54NaHpGYKwdNhjaBuSpDotD7' | chpasswd -e
# Add a public ssh key and enable sshd
mkdir -pm700 /root/.ssh/
cat id_rsa_new.pub >> /root/.ssh/authorized_keys
systemctl enable sshd.service
# Install vim-small
zypper --non-interactive install vim-small
# Leave a marker
echo "Configured with combustion" > /etc/issue.d/combustion

How it works

The ignition-dracut-grub2 package created a flag file which tells GRUB 2 that it's the first boot of the system and so GRUB adds "ignition.firstboot" to the kernel commandline.

If this option is found on the kernel cmdline, combustion.service's ConditionKernelCommandLine is fulfilled and it'll be required by initrd.target. This pulls in combustion-prepare.service, which runs after the config drive or QEMU fw_cfg blob appears (see combustion.rules for details). It is read and if the "network" flag comment is present, enables networking for later. After /sysroot is mounted and network is up (if enabled), combustion.service runs, which tries to activate all mountpoints in the system's /etc/fstab and then calls transactional-update in a chroot.

In this transactional-update session the script is started and the exit code recorded. If the script failed, transactional-update rollback is called and combustion.service marked as failed, which causes booting to fail. Note that a missing config drive or script is not considered a fatal error and only results in a warning.

/sysroot is unmounted and mounted again, so that the default subvolume gets reevaluated and directly booted into.

The ignition-firstboot-complete service in the final system runs, which deletes the flag file. This means that combustion is not run on subsequent boots.

Known issues

[# combustion: network] does not work (Bug in Dracut, worked around in images)