This version of the Boot Loader Specification is significantly altered from the upstream version. The following changes have been made:

  • Partition types. For both MS-DOS and GPT disks, the partition type is that of the EFI system partition. This provides greater consistency between BIOS and UEFI systems.
  • The "loader" directory name has been renamed to "/org/freedesktop/bls" in order to reduce the risk of namespace collisions. The UEFI specification only defines directory names under the EFI directory of the EFI system partition, and implementations are responsible for avoiding conflicts otherwise. Using a DNS-based approach minimises the risk of any future conflict.
  • No requirement that the partition be VFAT. Different firmware implementations may support different filesystems. There's no reason to enforce VFAT as long as the firmware is able to read it.
  • Support for kernels and ramdisks on other partitions. The firmware may only support legacy filesystems, but it's desirable to be able to store the kernel and ramdisk on fully featured filesystems with support for things like extended attributes in order to permit security labelling.
  • Remove the requirement that kernels be valid UEFI executables on UEFI systems. Desirable usecases (such as supporting 64-bit kernels on systems with 32-bit UEFI, or Secure Boot support without relying on being able to hook low-level firmware functions) mean that bootloaders cannot simply rely on LoadImage() and StartImage() working, and so bootloaders must be expected to be able to deal with executing kernels themselves.
  • Provide a chainload keyword for booting other operating systems on BIOS systems. BIOS systems are still widely deployed and there's a direct benefit in being able to provide configuration fragments for directly booting them.
  • Various clarifications - bootloaders must not assume that $BOOT is writable, /boot may or may not be shared between different operating systems, kernel installers may install files outside $BOOT, $BOOT is given a defined location on the filesystem.
  • multiboot and module keywords added to handle Xen and tboot use cases

The Boot Loader Specification

TL;DR: Currently there's little cooperation between multiple distributions in dual-boot (or triple, ... multi-boot) setups, and we'd like to improve this situation by getting everybody to commit to a single boot configuration format that is based on drop-in files, and thus is robust, simple, works without rewriting configuration files and is free of namespace clashes.

The Boot Loader Specification defines a scheme how different operating systems can cooperatively manage a boot loader configuration directory, that accepts drop-in files for boot menu items that are defined in a format that is shared between various boot loader implementations, operating systems, and userspace programs. The target audience for this specification is:

  • Boot loader developers, to write a boot loader that directly reads its configuration at runtime from these drop-in snippets
  • Distribution and Core OS developers, in order to create these snippets at OS/kernel package installation time
  • UI developers, for implementing a user interface that discovers the available boot options
  • OS Installer developers, for setting up the initial drop-in directory

Why is there a need for this specification?

Of course, without this specification things already work mostly fine. But here's why we think this specification is needed:

  • To make the boot more robust, as no explicit rewriting of configuration files is required any more
  • To improve dual-boot scenarios. Currently, multiple Linux installations tend to fight over which boot loader becomes the primary one in possession of the MBR, and only that one installation can then update the boot loader configuration of it freely. Other Linux installs have to be manually configured to never touch the MBR and instead install a chain-loaded boot loader in their own partition headers. In this new scheme as all installations share a loader directory no manual configuration has to take place, and all participants implicitly cooperate due to removal of name collisions and can install/remove their own boot menu entries at free will, without interfering with the entries of other installed operating systems.
  • Drop-in directories are otherwise now pretty ubiquitous on Linux as an easy way to extend configuration without having to edit, regenerate or manipulate configuration files. For the sake of uniformity we should do the same for extending the boot menu.
  • Userspace code can sanely parse boot loader configuration which is essential with modern BIOSes which do not necessarily initialize USB keyboards anymore during boot, which makes boot menus hard to reach for the user. If userspace code can parse the boot loader configuration, too, this allows for UIs that can select a boot menu item to boot into, before rebooting the machine, thus not requiring interactivity during early boot.
  • To unify and thus simplify configuration of the various boot loaders around, which makes configuration of the boot loading process easier for users, administrators and developers alike
  • For boot loaders with configuration scripts such as grub2, adopting this spec allows for mostly static scripts that are generated only once at first installation, but then do not need to be updated anymore as that is done via drop-in files exclusively.

Why not simply rely on the EFI boot menu logic?

The EFI specification provides a boot options logic that can offer similar functionality. Here's why we think that it is not enough for our uses:

  • The various EFI implementations implement the boot order/boot item logic to different levels. Some firmware implementations do not offer a boot menu at all and instead unconditionally follow the EFI boot order, booting the first item that is working.
  • If the firmware setup is used to reset all data usually all EFI boot entries are lost, making the system entirely unbootable, as the firmware setups generally do not offer a UI to define additional boot items. By placing the menu item information on disk it is always available, regardless if the BIOS setup data is lost.
  • Harddisk images should be moveable between machines and be bootable without requiring explicit EFI variables to be set. This also requires that the list of boot options is defined on disk, and not in EFI variables alone.
  • EFI is not universal yet (especially on non-x86 platforms), this specification is useful both for EFI and non-EFI boot loaders.
  • Many EFI systems disable USB support during early boot to optimize boot times, thus making keyboard input unavailable in the EFI menu. It is thus useful if the OS UI has a standardized way to discover available boot options which can be booted to.

Technical Details

We define two directories:

  • $BOOT/org/freedesktop/bls is the directory containing all files defined by this specification
  • $BOOT/org/freedesktop/bls/entries/ is the directory containing the drop-in snippets. This directory contains one .conf file for each boot menu item.

These directories are defined below the placeholder file system $BOOT. This placeholder file system shall be determined during installation time, and an fstab entry for it shall be created mounting it to /boot. The installer program should pick $BOOT according to the following rules:

  • If the OS is installed on a disk with MBR disk label, and a partition with the MBR type id of 0xEF already exists it should be used as $BOOT.
  • Otherwise, if the the OS is installed on a disk with MBR disk label, a new partition with MBR type id of 0xEF shall be created, of a suitable size (let's say 500MB), and it should be used as $BOOT.
  • If the OS is installed on a disk with GPT disk label, and an ESP partition (i.e. with the GPT type UUID of c12a7328-f81f-11d2-ba4b-00a0c93ec93b) already exists it should be used as $BOOT.
  • Otherwise, if the OS is installed on a disk with GPT disk label, and no ESP partition exists yet, a new suitably sized (let's say 500MB) ESP should be created and should be used as $BOOT.

Note: In all cases the /org/freedesktop/bls directory should be located directly in the root of the file system. Specifically, if $BOOT is the ESP, then /org/freedesktop/bls directory should be located directly in the root directory of the ESP, and not in the EFI/ subdirectory.

Note: $BOOT should be considered shared among all OS installations of a system. Instead of maintaining one $BOOT per installed OS (as /boot was traditionally handled), all installed OS share the same place to drop in their boot-time configuration.

For systems where the firmware is able to read filesystems directly, $BOOT must be a filesystem readable by the firmware. For other systems, $BOOT must be a VFAT (16 or 32) file system. Applications accessing $BOOT should hence not assume that fancier file system features such as symlinks, hardlinks, access control or case sensitivity are supported.

Inside the $BOOT/org/freedesktop/bls/entries/ directory each OS vendor may drop one or more configuration snippets with the suffix ".conf", one for each boot menu item. The file name of the file is used for identification of the boot item, but shall never be presented to the user in the UI. The file name may be chosen freely but should be unique enough to avoid clashes between OS installations. More specifically it is suggested to include the machine ID (/etc/machine-id or the D-Bus machine ID for OSes that lack /etc/machine-id), the kernel version (as returned by uname -r) and an OS identifier (The ID field of /etc/os-release). Example: $BOOT/org/freedesktop/bls/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf`.

These configuration snippets shall be Unix-style text files (i.e. line separation with a single newline character), in the UTF-8 character sets. The configuration snippets are loosely inspired on Grub1's configuration syntax. Lines beginning with '#' shall be ignored and used for commenting. The first word of a line is used as key, and shall be separated by a space from its value. The following keys are known:

  • title shall contain a human readable title string for this menu item. This will be displayed in the boot menu for the item. It is a good idea to initialize this from the PRETTY_NAME of /etc/os-release. This name should be descriptive and does not have to be unique. If a boot loader discovers two entries with the same title it is a good idea to show more than just the raw title in the UI, for example by appending the version field. This field is optional. Example: "Fedora 18 (Spherical Cow)".
  • version shall contain a human readable version string for this menu item. This is usually the kernel version and is intended for use by OSes to install multiple kernel versions at the same time with the same title field. This field shall be in a syntax that is useful for Debian-style version sorts, so that the boot loader UI can determine the newest version easily and show it first or preselect it automatically. This field is optional. Example: 3.7.2-201.fc18.x86_64
  • machine-id shall contain the machine ID of the OS /etc/machine-id. This is useful for boot loaders and applications to filter out boot entries, for example to show only a single newest kernel per OS, or to group items by OS, or to maybe filter out the currently booted OS in UIs that want to show only other installed operating systems. This ID shall be formatted as 32 lower case hexadecimal characters (i.e. without any UUID formatting). This key is optional. Example: 4098b3f648d74c13b1f04ccfba7798e8
  • filesystem defines the disk and partition forming the base of the path for the chainload, linux, initrd, devicetree and efi parameters. This may be a filesystem UUID (eg, 7fd988cf-57aa-4684-9a29-324b37d668d3 for an ext4 filesystem or B720-C7C4 for a FAT filesystem). In addition, for BIOS systems this may be a hex value containing the drive number (eg, 0x80 for the first hard drive), optionally followed by a comma and a 1-indexed partition number (eg, 0x80,1 for the first partition on the first hard drive). If no comma is present, the filesystem shall be considered to be the entire drive.
  • chainload indicates that the bootloader should execute code from the boot sector of the drive or partition defined by filesystem. Providing chainload without providing filesystem shall be an error. The definition of "boot sector" may be firmware dependent.
  • linux refers to the kernel to spawn. If filesystem is defined the path shall be relative to this - otherwise it shall be a path relative to the $BOOT directory. See the pathnames section for discussion of paths. Example: /boot/vmlinuz-3.8.0-2.fc19.x86_64
  • initrd refers to the initrd to use when executing the kernel. If filesystem is defined the path shall be relative to this - otherwise it shall be a path relative to the $BOOT directory. See the pathnames section for discussion of paths. This key is optional. Example: initrd-3.8.0-2.fc19.x86_64
  • efi to spawn arbitrary EFI programs. If filesystem is defined the path shall be relative to this - otherwise it shall be a path relative to the $BOOT directory. This key is only available on EFI systems.
  • devicetree refers to the binary device tree to use when executing the kernel. If filesystem is defined the path shall be relative to this - otherwise it shall be a path relative to the $BOOT directory. This key is optional. Example: /boot/dtb-3.8.0-2.fc19.armv7hl/tegra20-paz00.dtb
  • multiboot refers to a kernel compatible with the multiboot specification. If filesystem is defined the path shall be relative to this - otherwise it shall be a path relative to the $BOOT directory. See the pathnames section for discussion of paths. If the linux, initrd or efi keywords are provided after a multiboot keyword, they shall be interpreted as synonyms for module. Example: /boot/tboot.gz.
  • module refers to an additional multoboot module. If filesystem is defined the path shall be relative to this - otherwise it shall be a path relative to the $BOOT directory. See the pathnames section for discussion of paths. Multiple module keywords may be provided. Example: /boot/GM45_GS45_PM45_SINIT_51.BIN
  • options shall contain parameters to pass to the target of the immediately preceeding linux, multiboot, module or efi keyword.

Each configuration drop-in snippet must include at least a linux, multiboot, chainload or efi key, and is otherwise not valid. Here's an example for a complete drop-in file:

title      Fedora 19 (Rawhide)
version    3.8.0-2.fc19.x86_64
machine-id 6a9857a393724b7a981ebb5b8495b9ea
filesystem 6d3376e4-fc93-4509-95ec-a21d68011da2
linux      /boot/vmlinux-3.8.0-2.fc19.x86_64
options    root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2
initrd     /boot/initrd-3.8.0-2.fc19.x86_64

And here's an example of one using multiboot:

title      Fedora 19 (tboot) (Rawhide)
version    3.8.0-2.fc19.x86_64
machine-id 6a9857a393724b7a981ebb5b8495b9ea
filesystem 6d3376e4-fc93-4509-95ec-a21d68011da2
multiboot  /boot/tboot.gz
options    logging=serial,vga,memory
linux      /boot/vmlinux-3.8.0-2.fc19.x86_64
options    root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2
initrd     /boot/initrd-3.8.0-2.fc19.x86_64

Note that these configurations snippets do not need to be the only configuration source for a boot loader. It may extend this list of entries with additional items from other configuration files (for example its own native configuration files) or automatically detected other entries without explicit configuration.

Note that all paths used in the configuration snippets use a Unix-style "/" as path separator. This needs to be converted to an EFI-style "\" separator in EFI boot loaders.


It is valid for operating systems to install kernels, initrds, devicetree files or EFI executables under $BOOT/org/freedesktop/bls, although care must be taken to avoid collisions. It is recommended that operating systems use their machine id as a unique parent directory. Example:

title      Fedora 19 (Rawhide)
version    3.8.0-2.fc19.x86_64
machine-id 6a9857a393724b7a981ebb5b8495b9ea
options    root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2
linux      /org/freedesktop/bls/6a9857a393724b7a981ebb5b8495b9ea/vmlinux-3.8.0-2.fc19.x86_64
initrd     /org/freedesktop/bls/6a9857a393724b7a981ebb5b8495b9ea/initrd-3.8.0-2.fc19.x86_64
module     /boot/GM45_GS45_PM45_SINIT_51.BIN

If an operating system installs its boot files in an operating system-specific partition, there is no need to embed the machine id in any paths.


A boot loader needs a file system driver to discover and read $BOOT, then simply reads all files $BOOT/org/freedesktop/bls/entries/*.conf, and populates its boot menu with this. Optionally it might sort the menu based on the machine-id and version fields, and possibly others. It uses the file name to identify specific items, for example in case it supports storing away default entry information somewhere. A boot loader should generally not modify these files and must not assume that the filesystem is writable.

A kernel installer package installs the kernel and initrd images to either $BOOT (it is recommended to place these files in a vendor and OS and installation specific directory) or an operating system-specific partition and then generates a configuration snippet for it, placing this in $BOOT/org/freedesktop/bls/entries/xyz.conf, with xyz as concatenation of machine id and version information (see above). The files created by a kernel package are private property of the package, and should be deleted along with it.

A UI application intended to show available boot options shall operate similar to a boot loader, but might apply additional filters, for example by filtering out the booted OS via the machine ID, or by suppressing all but the newest kernel versions.

An OS installer picks the right place for $BOOT as defined above (possibly creating a partition and file system for it), and pre-creates the /org/freedesktop/bls/entries/ directory in it. It then installs an appropriate boot loader that can read these snippets. Finally it installs one or more kernel packages. On POSIX-style operating systems, $BOOT must be accessible via /boot/bls - this may be either a mountpoint or a symlink (eg, /boot/bls may be a symlink to /boot/efi). It is acceptable to create boot fragments to chainload operating systems that are unaware of this specification. Such entries should be created using a consistent naming scheme that does not embed the machine-id - for example, use the OS name followed by the chained filesystem ID (eg, macosx-9004e695-2e0f-4230-9a9e-e90bb1e88e17.conf) or "ESP" in the case of $BOOT (eg, ubuntu-ESP.conf).

Out of Focus

There are a couple of items that are out of focus for this specifications:

  • If userspace can figure out the available boot options, then this is only useful so much: we'd still need to come up with a way how userspace could communicate to the boot loader the default boot loader entry temporarily or persistently. Defining a common scheme for this is certainly a good idea, but out of focus for this specifications.
  • This specification does not define whether /boot on POSIX-style operating systems should be shared between multiple operating systems. Nothing in the specification prevents this, but operating systems that wish to support this case must ensure that they write files in unique locations (eg, /boot/machine-id/vmlinux-3.8.0-2.fc19.x86_64)
  • This specification leaves undefined what to do about systems which are upgraded from an OS that does not implement this specification. As the previous boot loader logic was largely handled by in distribution-specific ways we probably should leave the upgrade path (and whether there actually is one) to the distributions. The simplest solution might be to simply continue with the old scheme for old installations and use this new scheme only for new installations.