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Quick Reference

Alpine Boot Process

Alpine Boot Process

I want to create a USB stick that will boot into Alpine connect to the network using a static IP address and gateway then connect to a VPN with OpenVPN... so how does init in Alpine work?

initramfs

Alpine, like other Linux, loads a kernel and a filesystem into RAM then executes the init file on that mini RAM filesystem. For Alpine Linux 3.11.6 Standard that initial filesystem is initramfs-lts. This file is a gzip compressed cpio archive.

The init script in the initramfs shows that the boot process calls nlplug-findfs which searches for apkovls on filesystems that can be applied to the run-from-RAM tmpfs root created by the init script.

Custom Bootable Drive

To begin boot into Alpine, these steps could be performed with another Linux installation but that would just complicate the tutorial.

From here the process is simple:

  • Create a new partition table (with two partitons if you want UEFI)
  • Copy the CDROM directories to the data partition
  • Setup the boot sector (for BIOS booting) with syslinux
  • Setup UEFI for syslinux
  • Update the kernel command line arguments
  • Boot the new OS to see that it works
  • Configure system using setup-alpine
  • Commit the changes using lbu

Create new Partition Table

It should be possible to create a system that will boot using both UEFI and BIOS with a GPT partition table. Unfortunately I have not been able to do it so this first section is in two parts, one for BIOS and MBR and one for EFI and GPT.

Tools required for these steps are not all available on the Alpine Standard ISO but are available on Alpine Extended. I recommend using Alpine Extended since it has all the packages required in the ISO.

BIOS and MBR

Creating a bootable Alpine installation with BIOS and MBR requires syslinux which is unfortunately not present in Alpine Standard but easily added after setup-alpine or use Alpine Extended.

setup-alpine # if using Alpine Standard
apk add sfdisk e2fsprogs syslinux
sfdisk /dev/sda
label:dos
start=1M,size=255M,type=c,bootable
,,L
write
mkfs.vfat /dev/sda1
mkfs.ext2 /dev/sda2
modprobe vfat
modprobe ext4
mkdir /media/sda1
mkdir /media/sda2
mount /dev/sda1 /media/sda1
mount /dev/sda2 /media/sda2

ext2 partitions can be used as boot media by syslinux but vfat is fine and is similar to the GPT setup.

Install the kernel and syslinux configuration to sda1

mkdir -p /media/sda1/boot/syslinux
cp /media/cdrom/boot/syslinux/syslinux.cfg /media/sda1/boot/syslinux
cd /media/cdrom/boot
cp vmlinuz-lts initramfs-lts modloop-lts /media/sda1/boot
cd /media/sda1
syslinux --directory boot/syslinux --install /dev/sda1

Finally install the syslinux boot sector.

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

Booting this disk now should present an emergency boot prompt. To boot into Alpine the APKs must be copied to the drive and the kernel must be able to access the vfat file system.

Add the two kernel modules to the kernel arguments provided by syslinux (/media/sda1/boot/syslinux/syslinux.cfg):

TIMEOUT 20
PROMPT 1
DEFAULT lts

LABEL lts
MENU LABEL Linux lts
KERNEL /boot/vmlinuz-lts
INITRD /boot/initramfs-lts
FDTDIR /boot/dtbs-lts
APPEND modules=loop,squashfs,sd-mod,usb-storage,vfat,ext4 quiet nomodeset

And copy the apks to the media from the CDROM:

cp -r /media/cdrom/apks /media/sda1/

EFI and GPT

To support UEFI boot process the partition table should be GPT, some hardware does not support UEFI booting from a DOS/MBR partition. Unfortunately fdisk included with Alpine does not support GPT and so the sfdisk utility or parted must be installed. To install sfdisk run setup-alpine, choose none when asked about disks and leave the apk cache as /var/cache/apk.

setup-alpine
apk add sfdisk e2fsprogs syslinux
sfdisk /dev/sda
label:gpt
1M,255M,U,*
,,L
write
mkfs.vfat /dev/sda1
mkfs.ext2 /dev/sda2
modprobe vfat
modprobe ext4
mkdir /media/sda1
mkdir /media/sda2
mount /dev/sda1 /media/sda1
mount /dev/sda2 /media/sda2

To allow UEFI to boot create the special UEFI boot directories and files:

mkdir -p /media/sda1/EFI/BOOT
cp /usr/share/syslinux/efi64/syslinux.efi /media/sda1/EFI/BOOT/bootx64.efi
cp /usr/share/syslinux/efi64/ldlinux.e64 /media/sda1/EFI/BOOT/ldlinux.e64

Just these files will allow UEFI to boot into syslinux. Of course there is no kernel or syslinux configuration so those files will need adding also.

mkdir -p /media/sda1/boot/syslinux
cd /media/cdrom/boot
cp vmlinuz-lts initramfs-lts modloop-lts /media/sda1/boot
cp syslinux/syslinux.cfg /media/sda1/boot/syslinux

By default the kernel included does not include support for a VFAT file system or an EXT2 file system, to allow the boot process to load the APKs from the VFAT partition alter the kernel arguments in syslinux.cfg. (Note: the ext4 kernel module provides support for reading and writing EXT2 file systems).

vi /media/sda1/boot/syslinux/syslinux.cfg
TIMEOUT 20
PROMPT 1
DEFAULT lts

LABEL lts
MENU LABEL Linux lts
KERNEL /boot/vmlinuz-lts
INITRD /boot/initramfs-lts
FDTDIR /boot/dtbs-lts
APPEND modules=loop,squashfs,sd-mod,usb-storage,vfat,ext4 quiet nomodeset

If you boot this now the system will boot to emergency rescue prompt. The last step is to copy the apk files from the CDROM.

cp -r /media/cdrom/apks /media/sda1

The system is now ready to boot from sda. Unmount the drives and poweroff the system then remove the CDROM the system should now boot using UEFI to the Alpine login prompt.

umount /media/sda1
umount /media/sda2
poweroff

References

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