preppi

PrepPi

PrepPi is a simple binary designed to run early in the boot process of a Raspberry Pi (and specifically, the Raspbian OS) which copies files from one location to another.

The intended use is to copy files from a directory under /boot - which in Raspbian is the mountpoint of the FAT-formatted boot partition used by the Rapsberry Pi bootloader - to arbitrary locations inside of the OS partition. The rationale is that almost any OS which can write an SD card (and certainly all of the most prevalent desktop OSes) can write to a FAT volume, while relatively few can write to EXT4 volumes, so providing a simple mechanism to configure the OS via writes to the FAT volume is highly desirable.

The secondary motivation for this approach is the idea that editing config files is easy, and since updating a binary in an OS image is relatively difficult, we shouldn't encode the logic for configuration in a binary. For this reason, the preppi binary itself is fairly simple, and should remain that way. Any complicated logic for configuration (ie, template expansion, macros, etc) should exist elsewhere, and preppi should only concern itself with the output of such a tool.

The .deb package produced by the build scripts includes a systemd service which executes preppi after local filesystems are mounted but before the system enters multi-user mode to minimize the chances of the configs having already been read. This means that very-early boot configuration is still impossible with PrepPi.

Using PrepPi

1. Flash an OS image with PrepPi installed to the SD card
2. Mount the boot volume locally
3. Create a directory preppi/ and file preppi/preppi.conf, and populate preppi.conf
4. Boot the OS - PrepPi will place the files where you specify

Example preppi.conf

To set the hostname of the system, the PrepPi configuration might be:

{
  "map": [
    {
      "source": "/boot/preppi/etc-hostname",
      "destination": "/etc/hostname",
      "mode": 420,
      "dirmode": 493,
      "uid": 0,
      "gid": 0,
      "clobber": true
    },
    {
      "source": "/boot/preppi/etc-hosts",
      "destination": "/etc/hosts",
      "mode": 420,
      "dirmode": 493,
      "uid": 0,
      "gid": 0,
      "clobber": true
    }
  ]
}

Note that the mode and dirmode are standard unix file modes, expressed as decimal values. This is because JSON parsers - and specifically, the Go JSON implementation - don't handle octal very well. In this case, 420 = 0644 and 493 = 0755.

Versions

The versions and notable changes are listed below.

v0.1.1 - 2017-09-25

• Refactored for subcommands
• Now includes a bake mode, which generates configs from templates

v0.1 - 2017-08-28

• Basic functionality
  • Applies files to FS, and then reboots the system to force changes to take effect
  • UID/GID enforcement works, but is not unit tested or checked due to FS abstraction shortcomings

Planned Future Features

• Early boot configuration (ie, filesystem mounting)
• Package installation / Script execution

Contributors

• Christian Funkhouser (cfunkhouser)

Alternatives

• device-init - The device-init package is well established and aims to accomplish a similar goal, but with configuration expansion in the binary.