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Useful Information GRUB

This page was created after digging for information in order to boot multiple linux distros from downloaded isos on a local machine and USBs. The information provided should demystify the process of modifying GRUB.



Basics

The boot process

https://opensource.com/article/17/2/linux-boot-and-startup

Booting the Linux kernel:

Basic information on how the linux kernel is booted:

Load the linux kernel:

  1. Find the "vmlinuz" file (most likely in: )

    • /live/vmlinuz (Tails, Linux Mint, Kali)
    • /casper/vmlinuz (Ubuntu, Peppermint)
    • /isolinux/vmlinuz (REHL)
    • /boot/vmlinuz-x86_64 (Manjaro)

  2. Load kernel:

    linux [vmlinuz file location] [kernel boot options]

    Example: (Ubuntu)

    linux (loop)/casper/vmlinuz boot=casper iso-scan/filename=${isofile} quiet splash

  3. Find "initrd" file (most likely in: )

  • /casper/initrd* (Ubuntu)
  • /live/initrd.img-*-amd64 (Kali, Manjaro)
  • /isolinux/vmlinuz (Fedora)

  1. Execute initrd command:

    initrd +[vmlinuz file location]

  2. Execute boot command

Check file locations for mainstream distros in: https://github.com/thias/glim/tree/master/grub2


Source:

https://www.gnu.org/software/grub/manual/grub/grub.html#GNU_002fLinux


GRUB

What is grub.cfg file? Contains "menuentrys" which act as mini scripts similar to .sh files but for grub.

List of valid commands in GRUB: https://www.gnu.org/software/grub/manual/grub/grub.html#Command_002dline-and-menu-entry-commands

Example of Ubuntu's autogenerated menuentry after flashing drive:

menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-9a6801b1-a23c-43d9-8162-563760407db3' {
        recordfail
        savedefault
        load_video
        gfxmode $linux_gfx_mode
        insmod gzio
        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
        insmod part_gpt
        insmod ext2
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root  9a6801b1-a23c-43d9-8162-563760407db3
        else
          search --no-floppy --fs-uuid --set=root 9a6801b1-a23c-43d9-8162-563760407db3
        fi
        linux   /boot/vmlinuz-5.10.0-051000-generic root=UUID=9a6801b1-a23c-43d9-8162-563760407db3 ro  quiet splash $vt_handoff
        initrd  /boot/initrd.img-5.10.0-051000-generic
}

Find current linux boot params when booted:

cat /proc/cmdline

https://www.cyberciti.biz/faq/display-view-linux-kernel-parameters-for-booted/


How to modify GRUB menu/boot options:

  1. modify:

    • /boot/grub/grub.cfg (main)
      or
    • /etc/grub.d/40_custom (designed for user-added custom boot entries. Combined into grub.cfg after update-grub )

  2. sudo update-grub

What boot options/parameters should I use?

You can view the defualt options used after iso burn/flash by viewing either grub.cfg or syslinux.cfg files depending on your config. Use 7zip to find file without extracting iso as I have found this may hide/delete file. Some distros seem to use boot options that are not included in linux master boot options list and no documentation to be found which is why looking at these files may be helpfull.

Example:
(Ubuntu 20.04)
Location: (ubuntu-20.04.1-desktop-amd648-21-2020.iso\boot\grub\grub.cfg)
Boot Options: (located after linux vmlinuz ): 

file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---

Example:
(Tails)
Location: (tails-amd64-4.3.iso\EFI\BOOT\syslinux.cfg --> live64.cfg)
Boot Options: (located after linux vmlinuz ): 

initrd=/live/initrd.img boot=live config live-media=removable nopersistence noprompt timezone=Etc/UTC block.events_dfl_poll_msecs=1000 splash noautologin module=Tails slab_nomerge slub_debug=FZP mce=0 vsyscall=none page_poison=1 init_on_alloc=1 init_on_free=1 mds=full,nosmt union=aufs  quiet

Example:
(Debian)
Location: (debian-10.5.0-amd64-xfce-8-20-2020.iso\boot\grub\grub.cfg)
Boot Options: (located after linux vmlinuz ):
/install.amd/vmlinuz desktop=xfce vga=788 --- quiet (Graphical Install)



Summary:

To create your grub file copy and modify information in grub.cfg/syslinux.cfg and GLIM github repository: https://github.com/thias/glim.


Extensive list of linux kernel boot params:

https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html

More sources for kernel boot params:

Ubuntu wiki:

https://manpages.ubuntu.com/manpages/groovy/en/man7/bootparam.7.html

https://wiki.ubuntu.com/Kernel/KernelBootParameters

Archwiki:

https://wiki.archlinux.org/index.php/Kernel_module#Using_files_in_/etc/modprobe.d/



Basic Information on Boot options/parameters:

My findings with Tails:
Default boot options: syslinux.cfg
if 64 bit system:
Options listed in live64.cfg

if 32 bit system:
Display: 

We are sorry! 
Tails can not work on this computer:
Tails now requires a 64-bit computer.

Boot options from syslinux.cfg:

initrd=/live/initrd.img boot=live config live-media=removable nopersistence noprompt timezone=Etc/UTC block.events_dfl_poll_msecs=1000 splash noautologin module=Tails slab_nomerge slub_debug=FZP mce=0 vsyscall=none page_poison=1 init_on_alloc=1 init_on_free=1 mds=full,nosmt union=aufs noapic noapm nodma nomce nolapic nomodeset nosmp vga=normal

For booting iso from local machine (not flashed USB), use the following boot options: (taken from combination of GLIM and syslinux.cfg)

root=(loop) boot=live findiso=${isofile} config apparmor=1 nopersistence noprompt timezone=Etc/UTC block.events_dfl_poll_msecs=1000 splash noautologin module=Tails slab_nomerge slub_debug=FZP mce=0 vsyscall=none page_poison=1 init_on_alloc=1 init_on_free=1 mds=full,nosmt quiet

Whole menuentry:

  menuentry "Tails ISO" --class tails {
    set isofile="/images/tails-amd64-4.14.iso"
    loopback loop $isofile
    linux (loop)/live/vmlinuz root=(loop) boot=live findiso=${isofile} config apparmor=1 nopersistence noprompt timezone=Etc/UTC block.events_dfl_poll_msecs=1000 splash noautologin module=Tails slab_nomerge slub_debug=FZP mce=0 vsyscall=none page_poison=1 init_on_alloc=1 init_on_free=1 mds=full,nosmt quiet
    initrd (loop)/live/initrd.img
  }

Source: https://www.howtogeek.com/196933/how-to-boot-linux-iso-images-directly-from-your-hard-drive/

Boot iso from current install:

If you want to boot an iso without flashing iage to usb/dvd, use following instructions:

  1. Download isos (move them to easy-to-find location such as root)
  2. GRUB:
    • Create/Modify "40_custom" file located in: /etc/grub.d/
    • Add desired menuentries using above instructions for your distros
    • Execute update-grub
      Examples:
menuentry "Ubuntu ISO" --class ubuntu {
  set isofile="/images/ubuntu-20.04.2.0-desktop-amd64.iso"
  loopback loop $isofile
  linux (loop)/casper/vmlinuz boot=casper iso-scan/filename=${isofile} quiet splash
  initrd (loop)/casper/initrd*
}

menuentry "Tails ISO" --class tails {
  set isofile="/images/tails-amd64-4.15.1.iso"
  loopback loop $isofile
  linux (loop)/live/vmlinuz root=(loop) boot=live findiso=${isofile} config apparmor=1 nopersistence noprompt timezone=Etc/UTC block.events_dfl_poll_msecs=1000 splash noautologin module=Tails slab_nomerge slub_debug=FZP mce=0 vsyscall=none page_poison=1 init_on_alloc=1 init_on_free=1 mds=full,nosmt quiet
  initrd (loop)/live/initrd.img
}

menuentry "Kali Live" {
  set isofile="/images/kali-linux-2020.4-live-amd64.iso"
  loopback loop $isofile
  linux (loop)/live/vmlinuz-5.9.0-kali1-amd64 boot=live components quiet splash noeject findiso=${isofile}
  initrd (loop)/live/initrd.img-5.9.0-kali1-amd64
}
  1. Test your configuration

Boot isos from USB:

1. Create partitions (use fdisk or GTK's "Disks")

  • Ensure you use GPT partition table
  • Create ~500MB+ partition formatted in FAT32 for EFI parition (will contain grub's EFI and /boot)
  • Create EXT4 partition to store all isos

2. Install GRUB

  • mount EFI partition
  • execute: grub-install --efi-directory=(directory of mounted drive) --boot-directory=(directory of mounted drive)/boot --removable
    eg. grub-install --efi-directory=/mnt/usb --boot-directory=/mnt/usb/boot --removable

3. Add GRUB config files + isos

  • Open (directory of mounted drive)/EFI/BOOT/grub.cfg
  • Add menuentries using instructions above
    Ensure "loopback loop" line references your iso using GRUB's drive numbering scheme
    eg. loopback loop (hd0,gpt2)/ubuntu.iso

4. Test

  • You may need multiple tries to get everything set up properly and boot iso. If you get to initramfs and error: "unable to find medium containing live filesystem", check your loopback loop line. This had me stuck for days due to lack of documentation.