auto chain grub

A scripted GRUB for easy and automatic chainloading of disks (not partitions!). EFI- and secure-boot supported.

why?

I needed an easy way to multiboot server operating systems and distros on different disks or RAID-volumes. Most server OSes or distros do not allow for automatic multibooting (configuring grub/sylinux right); and if they do, if volumes are removed or erased, bootloader configurations do not update automatically. This GRUB "variant" is intended to be a system wide first bootloader, residing on an SD-card or USB-drive.

Requirements:

• chainload volumes with their own bootloader
• detect bootable volumes and OSes
• automatic update of boot options
• store default boot option
• show defined behavior when removing or erasing volumes

using

Write the .img file produced by the build to a drive - using dd or your favorite tool - and set the drive as first boot device. You may as well try to use the .iso image and write it using e.g. rufus. For testing, a .vhd and .vhdx image are provided.

OS booting

select boot drive

OSes or bootable disks will be detected

Select a disk (OS) to boot from and optionally set as default boot option

booting default selection

A disk/OS selected as default boot option will boot automatically after a (configurable) timeout. If the disk has been removed, further booting is stopped and the main menu is shown.

configuration

A set of default modules can be de-/activated in the configure options menu. Custom modules and custom commands to execute before loading the main menu can be configured as well. Options are saved and persisted in grubenv.

OS detection

Following OSes should be detected

• esxi
• Debian generic
• Ubuntu
• Arch
• xcp-ng
• CentOS
• XenServer
• Windows Server
• Windows 10
• Windows 8
• Windows NT + (likely 7)

(for heuristics, see ./notes/heuristics.md)

Bootloader & EFI detection

Detected bootloaders are, in different installation locations.

• syslinux
• GRUB

(for heuristics, see ./notes/heuristics.md)

If GRUB is run in EFI mode, it searches for a partition including bootable EFI-images

building

Build and run the docker container. Container has to be run using --privileged as mounting of loopback devices must be possible to create disk images. Mount the repositories root to the container (example mounts at /grub).

docker build --tag grub-builder .
docker create -v $(pwd):/grub/ -w /grub/ --name grub-builder --privileged -it grub-builder bash

Run and attach to the container, change terminal to repository root in container and:

./make.sh

Building will produce following artifacts:

• .iso-file: Bootable iso created by grub-mkrescue
• .img-file: Raw disk image with partitions for MBR- and EFI-booting
• .vhd-file: virtual disk from the raw image
• .vhdx-file: virtual disk from the raw image

customization

You may customize the build by adding/enabling default options or adding keymaps in make.sh. You may as well configure the timeout for the default boot option while building. Default boot option timeout can be modified from a live installation, using set timeout=<seconds>; save_env timeout. Loading custom modules, or adding custom commands to execute before loading, can be done in the configure options menu.

GRUB scripts

For anybody who wants to script their own flavour of GRUB, the functions.cfg in disk/boot/grub includes some neat functions, including:

• file-path manipulation
• string manipulation
• list operations
• code execution based on variable state
• "safe" testing for bool values of variables
• random string/char generator

info

booting ISO images is prepared, but not useable per now

Licensed unter MPL v2.0. Copyright (C) 2020 Arne Wendt