The Boot Loader Specification #
This document defines a set of file formats and naming conventions that allow the boot loader menu entries to be shared between multiple operating systems and boot loaders installed on one device.
Operating systems cooperatively manage boot loader menu entry directories that contain drop-in files, making multi-boot scenarios easy to support. Boot menu entries are defined via two simple formats that can be understood by different boot loader implementations, operating systems, and userspace programs. The same scheme can be used to prepare OS media for cases where the firmware includes a boot loader.
Target Audience #
The target audience for this specification is:
- Boot loader developers, to write a boot loader that directly reads its menu entries from these files
- Firmware developers, to add generic boot loading support directly to the firmware itself
- OS installer developers, to create appropriate partitions and set up the initial boot loader menu entries
- Distribution developers, to create appropriate menu entry snippets when installing or updating kernel packages
- UI developers, to implement user interfaces that list and select among the available boot options
The Partitions #
Everything described below is located on one or two partitions. The boot loader or user-space programs reading the boot loader menu entries should locate them in the following manner:
On disks with an MBR partition table:
- The boot partition — a partition with the type ID of
0xEA— shall be used as the single location for boot loader menu entries.
- The boot partition — a partition with the type ID of
On disks with GPT (GUID Partition Table)
The EFI System Partition (ESP for short) — a partition with a GPT type GUID of
c12a7328-f81f-11d2-ba4b-00a0c93ec93b— may be used as one of two locations for boot loader menu entries.
Optionally, an Extended Boot Loader Partition (XBOOTLDR partition for short) — a partition with GPT type GUID of
bc13c2ff-59e6-4262-a352-b275fd6f7172— may be used as the second of two locations for boot loader menu entries. This partition must be located on the same disk as the ESP.
Each partition type mentioned above can be present only once on the same disk.
Note: These partitions are shared among all OS installations on the
same disk. Instead of maintaining one boot partition per installed OS (as
/boot/ was traditionally handled), all installed OSes use the same place for
boot loader menu entries.
For systems where the firmware is able to read file systems directly, the ESP must — and the MBR boot and GPT XBOOTLDR partition should — be a file system readable by the firmware. For most systems this means VFAT (16 or 32 bit). Applications accessing both partitions should hence not assume that fancier file system features such as symlinks, hardlinks, access control or case sensitivity are supported.
$BOOT Partition Placeholder
In the text below, the placeholder
$BOOT will be used to refer to the
partition determined as follows:
On disks with an MBR partition table: → the boot partition, as described above
On disks with a GPT partition table: → the XBOOTLDR partition if it exists
Otherwise, on disks with a GPT partition table: → the ESP
$BOOT is the primary place to put boot menu entry resources into, but
typically not the only one. Most importantly, boot loaders should also pick up
menu entries from the ESP, even if XBOOTLDR exists (for details see below).
Creating These Partitions #
An installer for an operating system should use this logic when selecting or creating partitions:
If a boot partition (in case of MBR) or an XBOOTLDR partition (in case of GPT) already exists it should be used as
$BOOTand used as primary location to place boot loader menu resources in.
Otherwise, if on GPT and an ESP is found and it is large enough (let’s say at least 1G) it should be used as
$BOOTand used as primary location to place boot loader menu resources in.
Otherwise, if on GPT and neither XBOOTLDR nor ESP exist, an ESP should be created of the appropriate size and be used as
$BOOT, and used as primary location to place boot loader menu resources in.
Otherwise, a boot partition (in case of MBR) or XBOOTLDR partition (in case of GPT) should be created of an appropriate size, and be used as
$BOOT, and used as primary location to place boot loader menu resources in.
These partitions shall be determined during installation time, and
/etc/fstab entries may be created.
Mount Points #
It is recommended to mount
$BOOT (either XBOOTLDR or the ESP) to
If both XBOOTLDR and the ESP are present, the ESP should be mounted to
Effectively, this means that
/boot/ is the location where new boot entries shall be written to.
For the boot loader itself, the situation is more complicated,
because its files may be located under
depending on whether XBOOTLDR exists.
(Mounting the ESP to
/boot/efi/, as was traditionally done, is not
recommended. Such a nested setup complicates an implementation via direct
autofs mounts — as implemented by
systemd for example —, as establishing
autofs will trigger the outer one. Mounting the two partitions via
autofs is recommended because the simple VFAT file system has weak data
integrity properties and should remain unmounted whenever possible.)
Boot Loader Entries #
This specification defines two types of boot loader entries. The first type is text based, very simple, and suitable for a variety of firmware, architecture and image types (“Type #1”). The second type is specific to EFI, but allows single-file images that embed all metadata in the kernel binary itself, which is useful to cryptographically sign them as one file for the purpose of SecureBoot (“Type #2”).
Not all boot loader entries will apply to all systems. For example, Type #1
entries that use the
efi key and all Type #2 entries only apply to EFI
systems. Entries using the
architecture key might specify an architecture that
doesn’t match the local one. Boot loaders should ignore all entries that don’t
match the local platform and what the boot loader can support, and hide them
from the user. Only entries matching the feature set of boot loader and system
shall be considered and displayed. This allows image builders to put together
images that transparently support multiple different architectures.
Note that the three partitions described above are not supposed to be the
exclusive territory of this specification. This specification only defines
semantics of the
/loader/entries/ directory (along with the companion file
/loader/entries.srel) and the
/EFI/Linux/ directory inside the file system,
but it doesn’t intend to define contents of the rest of the file system. Boot
loaders, firmware, and other software implementing this specification may
choose to place other files and directories in the same file system. For
example, boot loaders that implement this specification might install their own
boot code on the same partition; this is particularly common in the case of the
ESP. Implementations of this specification must be able to operate correctly if
files or directories other than
/EFI/Linux/ are found
in the top level directory. Implementations that add their own files or
directories to the file systems should use well-named directories, to make name
collisions between multiple users of the file system unlikely.
Type #1 Boot Loader Specification Entries #
$BOOT is the primary directory containing Type #1
drop-in snippets defining boot entries, one
.conf file for each boot menu
item. Each OS may provide one or more such entries.
If the ESP is separate from
$BOOT it may also contain a
directory, where the boot loader should look for boot entry snippets, as an
additional source. The boot loader should enumerate both directories and
present a merged list to the user. Note that this is done for compatibility
only: while boot loaders should look in both places, OSes should only add their
Note: In all cases the
/loader/entries/ directory should be located
directly in the root of the ESP or XBOOTLDR filesystem. Specifically, the
/loader/entries/ directory should not be located under the
subdirectory on the ESP.
The file name of the boot entry snippets 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
or machine ID (see
and the kernel version (as returned by
uname -r, including the OS
identifier), so that the whole filename is
In order to maximize compatibility with file system implementations and restricted boot loader environments, and to minimize conflicting character use with other programs, file names shall be chosen from a restricted character set: ASCII upper and lower case characters, digits, “+”, “-”, “_” and “.”. Also, the file names should have a length of at least one and at most 255 characters (including the file name suffix).
These boot loader menu snippets shall be UNIX-style text files (i.e. lines separated by a single newline character), in the UTF-8 encoding. The boot loader menu snippets are loosely inspired by Grub1’s configuration syntax. Lines beginning with “#” are used for comments and shall be ignored. The first word of a line is used as key and is separated by one or more spaces from the value.
Type #1 Boot Loader Entry Keys #
The following keys are recognized:
titleis a human-readable title for this menu item to be displayed in the boot menu. It is a good idea to initialize this from the
PRETTY_NAME=of 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 should show more than just the raw title in the UI, for example by appending the
versionfield. This field is optional.
title Fedora 18 (Spherical Cow)
versionis a human-readable version for this menu item. This is usually the kernel version and is intended for use by OSes to install multiple kernel versions with the same
titlefield. This field is used for sorting entries, so that the boot loader can order entries by age or select the newest one automatically. This field is optional.
See Sorting below.
machine-idis the machine ID of the OS. This can be used by boot loaders and applications to filter out boot entries, for example to show only a single newest kernel per OS, to group items by OS, or to filter out the currently booted OS when showing 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.
sort-keyis a short string used for sorting entries on display. This should typically be initialized from the
ID=fields of os-release, possibly with an additional suffix. This field is optional.
linuxis the Linux kernel image to execute and takes a path relative to the root of the file system containing the boot entry snippet itself. It is recommended that every distribution creates an entry-token/machine-id and version specific subdirectory and places its kernels and initrd images there (see below).
initrdis the initrd
cpioimage to use when executing the kernel. This key may appear more than once in which case all specified images are used, in the order they are listed.
efirefers to an arbitrary EFI program. If this key is set, and the system is not an EFI system, this entry should be hidden.
optionsshall contain kernel parameters to pass to the Linux kernel to spawn. This key is optional and may appear more than once in which case all specified parameters are combined in the order they are listed.
options root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2 quiet
devicetreerefers to the binary device tree to use when executing the kernel. This key is optional.
devicetree-overlayrefers to a list of device tree overlays that should be applied by the boot loader. Multiple overlays are separated by spaces and applied in the same order as they are listed. This key is optional but depends on the
devicetree-overlay /6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_A.dtbo /6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_B.dtbo
architecturerefers to the architecture this entry is for. The argument should be an architecture identifier, using the architecture vocabulary defined by the EFI specification (i.e.
AA64, …). If specified and it does not match the local system architecture this entry should be hidden. The comparison should be done case-insensitively.
Each boot loader menu entry drop-in snippet must include at least a
linux or an
key. Here is an example for a complete drop-in file:
title Fedora 19 (Rawhide)
options root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2 quiet
On EFI systems all Linux kernel images should be EFI images. In order to increase compatibility with EFI systems it is highly recommended only to install EFI kernel images, even on non-EFI systems, if that’s applicable and supported on the specific architecture.
Conversely, in order to increase compatibility it is recommended to install generic kernel images that make few assumptions about the firmware they run on, i.e. it is a good idea that both images shipped as UEFI PE images and those which are not don’t make unnecessary assumption on the underlying firmware, i.e. don’t hard depend on legacy BIOS calls or UEFI boot services.
When Type #1 boot loader menu entry snippets refer to other files (for
devicetree-overlay), those files must be
located on the same partition, and the paths must be absolute paths relative to
the root of that file system. The naming of those files can be chosen by the
installer. A recommended scheme is described in the next section. Paths should
be normalized, i.e. not include
. or a sequence of more than one
/. Paths may be prefixed with a
/, but this is optional and has the same
effect as paths without it: all paths are always relative to the root directory
of the partition they are referenced from.
Even though the backing file system is typically case-insensitive (i.e. VFAT) it is strongly recommended to reference files in the casing actually used for the directories/files, so that placing these files on other file systems is still safe and robust.
Recommended Directory Layout for Additional Files #
It is recommended to place the kernel and other other files comprising a single
boot loader entry in a separate directory:
/<entry-token-or-machine-id>/<version>/. This naming scheme uses the same
elements as the boot loader menu entry snippet, providing the same level of
Other naming schemes are possible. In particular, traditionally a flat naming scheme with files in the root directory was used. This is not recommended because it is hard to avoid conflicts in a multi-boot installation.
Standard-conformance Marker File #
Unfortunately, there are implementations of boot loading infrastructure that
are also using the
/loader/entries/ directory, but install files that do not
follow this specification. In order to minimize confusion, a boot loader
implementation may place the file
/loader/entries.srel next to the
/loader/entries/ directory containing the ASCII string
type1 (followed by a
UNIX newline). Tools that need to determine whether an existing directory
implements the semantics described here may check for this file and contents:
if it exists and contains the mentioned string, it shall assume a
standards-compliant implementation is in place. If it exists but contains a
different string it shall assume other semantics are implemented. If the file
does not exist, no assumptions should be made.
Type #2 EFI Unified Kernel Images #
A unified kernel image is a single EFI PE executable combining an EFI stub
loader, a kernel image, an initrd image, and the kernel command line. See
for details. The primary place for such unified images is the
$BOOT. Operating systems should place unified EFI kernels only
$BOOT partition. Boot loaders should also look in the
the ESP — if it is different from
$BOOT — and present a merged list of menu
entries from both partitions. Regardless if placed in the primary or secondary
location: the files must have the extension
.efi. Support for images of this
type is of course specific to systems with EFI firmware. Ignore this section if
you work on systems not supporting EFI.
Type #2 file names should be chosen from the same restricted character set as
Type #1 described above (but with the file name suffix of
.efi instead of
Images of this type have the advantage that all metadata and payload that makes up the boot entry is contained in a single PE file that can be signed cryptographically as one for the purpose of EFI SecureBoot.
A valid unified kernel image in the
/EFI/Linux/ directory must contain two PE sections:
.cmdlinesection with the kernel command line,
.osrelsection with an embedded copy of the os-release file describing the image.
VERSION_ID= fields in the embedded
are used the same as
version in the Type #1 entries. The
.cmdline section is used instead of the
fields are not necessary, and there is no counterpart for the
On EFI, any such images shall be added to the list of valid boot entries.
Additional Notes #
Note that these boot entry snippets and unified kernels do not need to be the only menu entry sources 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.
To make this explicitly clear: this specification is designed with “free” operating systems in mind, starting Windows or MacOS is out of focus with these boot loader menu entry snippets, use boot-loader specific solutions for that. In the text above, if we say “OS” we hence imply “free”, i.e. primarily Linux (though this could be easily be extended to the BSDs and whatnot).
Note that all paths used in the boot loader menu entry snippets use a Unix-style “/” as path separator. This needs to be converted to an EFI-style “\” separator in EFI boot loaders.
Locating Boot Entries #
A boot loader locates the XBOOTLDR partition and the ESP, then simply reads
all the files
/loader/entries/*.conf in them, and populates its boot menu
(and handle gracefully if one of the two partitions is missing). On EFI, it
then extends this with any unified kernel images found in
the two partitions. It may also add additional entries, for example a “Reboot
into firmware” option. Optionally it may sort the menu based on the
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.
For “Boot Loader Specification Entries” (Type #1), the kernel package
installer installs the kernel and initrd images to
$BOOT. It is recommended
to place these files in a vendor and OS and installation specific directory. It
then generates a boot loader menu entry snippet, placing it in
$BOOT/loader/entries/xyz.conf, with “xyz” as concatenation of
entry-token/machine-id and version information (see above). The files created
by a kernel package are tied to the kernel package and should be removed along
For “EFI Unified Kernel Images” (Type #2), the vendor or kernel package
installer should create the combined image and drop it into
$BOOT/EFI/Linux/. This file is also tied to the kernel package and should be
removed along with it.
A UI application intended to show available boot options shall operate
similarly to a boot loader (and thus search both
$BOOT and the ESP if
distinct), but might apply additional filters, for example by filtering the
booted OS via the machine ID, or by suppressing all but the newest kernel
An OS installer picks the right place for
$BOOT as defined above (possibly
creating a partition and file system for it) and creates the
directory and the
/loader/entries.srel file in it (the latter only if the
directory didn’t exist yet). It then installs an appropriate boot loader that
can read these snippets. Finally, it installs one or more kernel packages.
Boot counting #
The main idea is that when boot entries are initially installed, they are marked as “indeterminate” and assigned a number of boot attempts. Each time the boot loader tries to boot an entry, it decreases this count by one. If the operating system considers the boot as successful, it removes the counter altogether and the entry becomes “good”. Otherwise, once the assigned number of boots is exhausted, the entry is marked as “bad”.
Which boots are “successful” is determined by the operating system. systemd provides a generic mechanism that can be extended with arbitrary checks and actions, see Automatic Boot Assessment, but the boot counting mechanism described in this specification can also be used with other implementations.
The boot counting data is stored in the name of the boot loader entry. A boot
loader entry file name may contain a plus (
+) followed by a number. This may
optionally be followed by a minus (
-) followed by a second number. The dot
.) and file name suffix (
efi) must immediately follow. Boot
counting is enabled for entries which match this pattern.
The first number is the “tries left” counter signifying how many attempts to boot this entry shall still be made. The second number is the “tries done” counter, showing how many failed attempts to boot it have already been made. Each time a boot loader entry marked this way is booted, the first counter is decremented, and the second one incremented. (If the second counter is missing, then it is assumed to be equivalent to zero.) If the “tries left” counter is above zero the entry is still considered “indeterminate”. A boot entry with the “tries left” counter at zero is considered “bad”.
If the boot attempt completed successfully the entry’s counters are removed from the name (entry state becomes “good”), thus turning off boot counting for this entry.
The boot loader menu should generally show entries in some order meaningful to
the user. The
title key is free-form and not suitable to be used as the
primary sorting key. Instead, the boot loader should use the following rules:
Entries which are subject to boot counting and are marked as “bad”, should be sorted later than all other entries. Entries which are marked as “indeterminate” or “good” (or were not subject to boot counting at all), are thus sorted earlier.
sort-keyis set on both entries, use in order of priority, the
sort-key(A-Z, increasing alphanumerical order),
machine-id(A-Z, increasing alphanumerical order), and
versionkeys (decreasing version order).
sort-keyis set on one entry, it sorts earlier.
At the end, if necessary, when
sort-keyis not set or those fields are not set or are all equal, the boot loader should sort using the file name of the entry (decreasing version sort), with the suffix removed.
Note: This description assumes that the boot loader shows entries in a traditional menu, with newest and “best” entries at the top, thus entries with a higher version number are sorter earlier. The boot loader is free to use a different direction (or none at all) during display.
Note: The boot loader should allow booting “bad” entries, e.g. in case no other entries are left or they are unusable for other reasons. It may deemphasize or hide such entries by default.
Note: “Bad” boot entries have a suffix of “+0-
n is the
number of failed boot attempts. Removal of the suffix is not necessary for
comparisons described by the last point above. In the unlikely scenario that we
have multiple such boot entries that differ only by the boot counting data, we
would sort them by
Alphanumerical Order #
Free-form strings and machine IDs should be compared using a method equivalent to strcmp(3) on their UTF-8 representations. If just one of the strings is unspecified or empty, it compares lower. If both strings are unspecified or empty, they compare equal.
Version Order #
The version format and the comparison algorithm are defined in the Version Format Specification. A specific separator character between the version and other tokens in a string is not mandated by this specification, as it often depends on existing kernel packages which differ between distributions and packaging formats.
Additional discussion #
Why is there a need for this specification? #
This specification brings the following advantages:
Installation of new boot entries is more robust, as no explicit rewriting of configuration files is required.
It allows an out-of-the-box boot experience on any platform without the need of traditional firmware mechanisms (e.g. BIOS calls, UEFI Boot Services).
It improves dual-boot scenarios. Without cooperation, multiple Linux installations tend to fight over which boot loader becomes the primary one in possession of the MBR or the boot partition, and only that one installation can then update the boot loader configuration. 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 all installations share a loader directory and no manual configuration has to take place. All participants implicitly cooperate due to removal of name collisions and can install/remove their own boot menu entries without interfering with the entries of other installed operating systems.
Drop-in directories are now pretty ubiquitous on Linux as an easy way to extend boot loader menus without having to edit, regenerate or manipulate configuration files. For the sake of uniformity, we should do the same for the boot menu.
Userspace code can sanely parse boot loader menu entries which is essential with modern firmware which does not necessarily initialize USB keyboards during boot, which makes boot menus hard to reach for the user. If userspace code can parse the boot loader menu entries too, UI can be written that select a boot menu item to boot into before rebooting the machine, thus not requiring interactivity during early boot.
To unify and thus simplify menu entries of the various boot loaders, 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? #
EFI is not ubiquitous, especially not in embedded systems. But even on systems with EFI, which provides a boot options logic that can offer similar functionality, this specification is still needed for the following reasons:
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 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, even if the firmware configuration is lost.
Harddisk images should be movable between machines and be bootable without requiring firmware configuration. 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.
Why is the version comparison logic so complicated? #
sort-key allows us to group entries by “operating system”, e.g. all
versions of Fedora together, no matter if they identify themselves as “Fedora
Workstation” or “Fedora Rawhide (prerelease)”. The
sort-key was introduced
only recently, so we need to provide a meaningful order for entries both with
and without it. Since it is a new concept, it is assumed that entries with
sort-key are newer.
In a traditional menu with entries displayed vertically, we want names to be sorter alpabetically (CentOS, Debian, Fedora, OpenSUSE, …), it would be strange to have them in reverse order. But when multiple kernels are available for the same installation, we want to display the latest kernel with highest priority, i.e. earlier in the list.
Why do you use file renames to store the counter? Why not a regular file? #
Mainly two reasons: it’s relatively likely that renames can be implemented atomically even in simpler file systems, as renaming generally avoids allocating or releasing data blocks. Writing to file contents has a much bigger chance to be result in incomplete or corrupt data. Moreover renaming has the benefit that the boot count metadata is directly attached to the boot loader entry file, and thus the lifecycle of the metadata and the entry itself are bound together. This means no additional clean-up needs to take place to drop the boot loader counting information for an entry when it is removed.
Why not use EFI variables for storing the boot counter? #
The memory chips used to back the persistent EFI variables are generally not of the highest quality, hence shouldn’t be written to more than necessary. This means we can’t really use it for changes made regularly during boot, but should use it only for seldom-made configuration changes.
Out of Focus #
There are a couple of items that are out of focus for this specification:
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 specification.
This specification is just about “Free” Operating systems. Hooking in other operating systems (like Windows and macOS) into the boot menu is a different story and should probably happen outside of this specification. For example, boot loaders might choose to detect other available OSes dynamically at runtime without explicit configuration (like
systemd-bootdoes it), or via native configuration (for example via explicit Grub2 configuration generated once at installation).
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.
Referencing kernels or initrds on other partitions other than the partition containing the Type #1 boot loader entry. This is by design, as specifying other partitions or devices would require a non-trivial language for denoting device paths. In particular this means that on non-EFI systems boot loader menu entry snippets following this specification cannot be used to spawn other operating systems (such as Windows).