Bootloaders

A bootloader is a computer program that is responsible for booting a computer. In the case where it also provides an interactive menu with multiple boot choices, then it's often called a boot manager. This page shows the basic steps that you need to perform if, for any reason, you want to switch bootloaders or to apply some manual configuration.

The following bootloaders are available in Alpine Linux:-

Syslinux is the default lightweight bootloader used in Alpine Linux.
rEFInd is an easy-to-use EFI boot menu that allows booting different operating systems.
GRUB is a standard Linux bootloader.
EFI Boot Stub allows for booting Linux directly from a motherboard supporting UEFI or from another bootloader.
UEFI Secure Boot uses systemd-efistub or stubbyboot-efistub.

A Unified Kernel Image (UKI) is additionally supported, available for UEFI only. It is a UEFI executable that can be useful in certain use cases, including secure boot, clouds and containers.

Syslinux

If you want to switch from another bootloader back to Syslinux, or if for some reason you want to install Syslinux manually, then the following steps are required.

Install the syslinux package:

# apk add syslinux

If you're using GPT partitions, then install the GPT MBR onto the drive that you want to install the bootloader on (in this case, /dev/sda):

# dd bs=440 count=1 conv=notrunc if=/usr/share/syslinux/gptmbr.bin of=/dev/sda

Or, if you're using DOS partitions, then install the DOS MBR instead:

# dd bs=440 count=1 conv=notrunc if=/usr/share/syslinux/mbr.bin of=/dev/sda

Next, install the required Syslinux binaries. Despite being called extlinux, Syslinux supports booting from FAT12/16/32, NTFS, ext2/3/4, btrfs, XFS and UFS/FFS filesystems.

# extlinux --install /boot

The configuration file is located in /boot/extlinux.conf.

Alpine Linux ships with a script, update-extlinux, that automatically (re)generates this file, for example, on updates to Syslinux. The settings for this script can be found in /etc/update-extlinux.conf, including the option to disable automatic overwriting of /boot/extlinux.conf.

You can also place additional menu entries in the /etc/update-extlinux.d/ directory e.g. for dual booting.

Using EFI with syslinux

The Alpine Linux installer automatically uses GRUB if EFI mode is detected. The section below is specifically about using EFI with Syslinux:

Assuming that /mnt is a FAT32 partition of type EF00 and that /boot belongs to the rootfs created after running setup-disk:

# mkdir -p /mnt/EFI/syslinux # cp /usr/share/syslinux/efi64/* /mnt/EFI/syslinux/ # cp /boot/extlinux.conf /mnt/EFI/syslinux/syslinux.cfg # cp /boot/vmlinuz* /mnt/EFI/syslinux/ # cp /boot/initramfs* /mnt/EFI/syslinux/

You may need to modify /mnt/EFI/syslinux/syslinux.cfg to change the paths to absolute paths (just add a / in front of the vmlinuz/initramfs entries), or copy the files to /mnt/EFI/syslinux instead (XXX: untested).

In the end, the /mnt/EFI/syslinux/syslinux.cfg file should look like this:

Contents of /mnt/EFI/syslinux/syslinux.cfg

...
DEFAULT menu.c32
PROMPT 0
MENU TITLE Alpine/Linux Boot Menu
MENU HIDDEN
MENU AUTOBOOT Alpine will be booted automatically in # seconds
TIMEOUT 10
LABEL lts
  MENU DEFAULT
  MENU LABEL Linux lts
  LINUX /vmlinuz-lts
  INITRD /initrd-lts
  APPEND root=/dev/sda3 modules=sd-load,usb-storage,ext4 quiet

Finally, add syslinux to the EFI boot menu. Assuming that /dev/sda is your hard drive —be careful to check first what is its path on your device and to adjust accordingly:

# apk add efibootmgr # efibootmgr -c -d /dev/sda -p 1 -l \\EFI\\syslinux\\syslinux.efi -L "ALPINE-SYSLINUX"

You can now verify that the boot entry has been added:

efibootmgr

BootCurrent: 0001
Timeout: 0 seconds
BootOrder: 0001,0000,0002,...
Boot001* ALPINE-SYSLINUX HD(1,GPT,xxxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)/FILE(\EFI\syslinux\syslinux.efi)

rEFInd

rEFInd provides a graphical boot menu for UEFI systems.

GRUB

To install GRUB in BIOS mode, (optionally) remove the Syslinux package and install the required GRUB packages:

# apk del syslinux # apk add grub grub-bios

For EFI, install GRUB's EFI package instead. Note that /boot has to be an EFI-compatible filesystem, such as FAT32.

# apk add grub-efi efibootmgr

Next, install the MBR and GRUB binaries to disk for BIOS mode:

# grub-install /dev/vda

For EFI mode:

# grub-install --target=x86_64-efi --efi-directory=/boot

Then, add this line to /etc/default/grub:

Contents of /etc/default/grub

# GRUB_CMDLINE_LINUX_DEFAULT="quiet rootfstype=ext4 modules=sd-mod,usb-storage,ext4"

GRUB ships with an automatic configuration generator, including some automatic detection of other operating systems installed on the device:

# grub-mkconfig -o /boot/grub/grub.cfg

This script can be configured via the /etc/default/grub file.

If the font in the GRUB boot screen appears too small, then change the GRUB font.

Consult gnu.org's online manual for a list of available grub-mkconfig configuration options.

EFI Boot Stub

To boot directly from your motherboard's UEFI boot menu, a boot entry needs to be created either with a UEFI shell or with efibootmgr.

efibootmgr

Install efibootmgr:

# apk add efibootmgr

Create a boot entry. It is recommended to do this in a script, as efibootmgr does not allow for editing entries.

Contents of add-bootentry

#!/bin/sh

params="root=/dev/sdXZ rootfstype=ext4 rw \
  initrd=\intel-ucode.img \
  initrd=\initramfs-lts"

efibootmgr --create --label "Alpine Linux" \
  --disk /dev/sdX --part Y \
  --loader /vmlinuz-lts \
  --unicode "${params}" \
  --verbose

Where /dev/sdXY contains the EFI partition and /dev/sdXZ contains the root partition. If you are using linux-edge, then replace lts with edge in the script.

Note: The kernel contains the exhaustive list of ways to specify the block device. For a more robust boot entry, it is recommended to use a persistent name such as the PARTUUID.

Optionally, set the newly-created entry as the default:

# efibootmgr -n XXXX

Where XXXX is the boot number of the new entry.

Note: The loader and initrd file arguments are relative to the EFI partition. In a default installation, Alpine Linux places these files in /boot/, while EFI is mounted to /boot/efi/. You can either update fstab to mount EFI at /boot/, or manually copy them to /boot/efi/.

Using a UKI

Unified Kernel Image (UKI) is supported in UEFI only. It is possible to boot directly into a UKI. A UKI is a single file that contains the initfs, kernel and cmdline.

The UEFI Secure Boot page contains the instructions for setting up an a UKI. While this is typically done in order to SecureBoot, it is perfectly feasible to skip enrolling the custom keys and to leave SecureBoot off.

Additionally, it is possible to install the UKI in the default fallback path used by most UEFI implementations. By installing the UKI into this path, the system will automatically boot into it if no other entries are defined. This can be automated as part of the kernel hook by adding the following to /etc/kernel-hooks.d/secureboot.conf :

Contents of /etc/kernel-hooks.d/secureboot.conf

# For the edge kernel, install the UKI into the default UEFI path.
if [ "$1" == "edge" ]; then
  output_dir="/efi/EFI/Boot/"
  output_name="bootx64.efi"
fi

Note: bootx64.efi is only correct for x86_64 systems. For other architectures, the exact name will vary.