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.

Installation

Run make install to install it manually. Usually combustion is packaged up and installed during image builds already.

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 (https://github.com/coreos/ignition), the LABEL "ignition" is used as fallback. All-uppercase labels are accepted as well. Alternatively, if a KIWI selfinstall .iso is used for deployment (LABEL "INSTALL"), this is used as a 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

If the VMware guestinfo parameter "guestinfo.combustion.script" is set and nonempty, it is treated as a base64 encoded gzipped script.

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

Simple example

Example for formatting a USB drive and adding a config, which installs the "vim-small" package and creates a /root/welcome file:

mkfs.ext4 /dev/sdX
e2label /dev/sdX ignition
mount /dev/sdX /mnt
mkdir -p /mnt/combustion/
cat >/mnt/combustion/script <<EOF
#!/bin/sh
# combustion: network
systemctl enable sshd.service
zypper --non-interactive install vim-small
cp welcome /root/welcome
EOF
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).

#!/bin/bash
# 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

Perform modifications in the initrd environment

Using the # combustion: prepare marker, the initrd environment can be modified for instance to perform tasks before /sysroot is mounted or to write NetworkManager connection configuration into /etc/NetworkManager/system-connections/. If the marker is present, the script is invoked with --prepare as parameter from combustion-prepare.service, in addition to the main invocation inside the transaction later. Example:

#!/bin/bash
# combustion: network prepare
set -euxo pipefail

nm_config() {
    umask 077 # Required for NM config
    mkdir -p /etc/NetworkManager/system-connections/
    cat >/etc/NetworkManager/system-connections/static.nmconnection <<-EOF
    [connection]
    id=static
    type=ethernet
    autoconnect=true

    [ipv4]
    method=manual
    dns=192.168.100.1
    address1=192.168.100.42/24,192.168.100.1
EOF
}

if [ "${1-}" = "--prepare" ]; then
    nm_config # Configure NM in the initrd
    exit 0
fi

# Redirect output to the console
exec > >(exec tee -a /dev/tty0) 2>&1

nm_config # Configure NM in the system
curl example.com
# Leave a marker
echo "Configured with combustion" > /etc/issue.d/combustion

How it works

Firstboot detection

Combustion ships a firstboot dracut module which introduces firstboot.target as well as a firstboot-detect.service. This service mounts /sysroot/etc early in the initrd (in a private namespace, to not trigger .mount units and dependencies out of order). It then checks for the following conditions:

• combustion.firstboot or ignition.firstboot are present on the kernel cmdline
• /etc/machine-id does not exist in /sysroot. Note: Unlike systemd's ConditionFirstBoot, this is not triggered by "uninitialized" in the machine-id file or influcenced by systemd.firstboot= on the kernel cmdline.

If one of them applies, it enables and starts firstboot.target. It's important that all units started by firstboot.target are effectively ordered after firstboot-detect.service to prevent loops. Add some Before= to firstboot-detect.service if necessary.

If any of the firstboot configuration mechanisms (combustion, ignition) find a user specified config, they delete /var/lib/YaST2/reconfig_system. The result is that if (and only if) no configuration for those was provided, the file still exists in the real system and triggers interactive setup (jeos-firstboot, YaST Firstboot) if present.

The final system eventually reaches first-boot-complete.target and after that systemd-machine-id-commit.service, which commits /etc/machine-id to disk and thus firstboot-detect.service no longer triggers firstboot.target on subsequent boots.

Combustion

The combustion dracut module is included by default, but omitted if dracut is run on an already configured system in hostonly mode.

firstboot.target pulls in combustion.service and combustion-prepare.service. The latter runs after the config drive or QEMU fw_cfg blob appears (see combustion.rules for details). The combustion configuration is copied from the config source into /dev/shm/combustion/config (this is accessible in transactional-update shell later). If the script contains the prepare flag, it's executed now with the --prepare option. If the network flag is present, networking is enabled in the initrd. 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.

Now, initrd-parse-etc.service can evaluate the final /sysroot/etc/fstab and create the matching sysroot-FOO.mount units which are started before switching into the root filesystem.