---
-title: The Boot Loader Specification
+title: Boot Loader Specification
+category: Booting
+layout: default
---
# 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:
+_TL;DR: Currently there's no common boot scheme across architectures and
+platforms for open-source operating systems. There's also little cooperation
+between multiple distributions in dual-boot (or triple, … multi-boot)
+setups. 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 same scheme can be used to prepare OS media for cases where the
+firmware includes a boot loader. 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
+* Firmware developers, to add generic boot loading support directly to the firmware itself
* 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
+* OS Installer developers, to prepare their installation media and 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 allow an out of the box boot experience on any platform without the need of traditional firmware mechanisms (e.g. BIOS calls, UEFI Boot Services)
* 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.
## 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:
+EFI is not ubiquitous, especially not in embedded systems. If you have an EFI
+system, it 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.
+* Harddisk images should be movable 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.
Everything described below is located on a placeholder file system `$BOOT`. The installer program should pick `$BOOT` according to the following rules:
-* On disks with MBR disk labels
- * If the OS is installed on a disk with MBR disk label, and a partition with the MBR type id of 0xEA already exists it should be used as `$BOOT`.
- * Otherwise, if the OS is installed on a disk with MBR disk label, a new partition with MBR type id of 0xEA shall be created, of a suitable size (let's say 500MB), and it should be used as `$BOOT`.
-* On disks with GPT disk labels
- * If the OS is installed on a disk with GPT disk label, and a partition with the GPT type GUID of bc13c2ff-59e6-4262-a352-b275fd6f7172 already exists, it should be used as `$BOOT`.
- * Otherwise, if the OS is installed on a disk with GPT disk label, and an ESP partition (i.e. with the GPT type UID of c12a7328-f81f-11d2-ba4b-00a0c93ec93b) already exists and is large enough (let's say 250MB) and otherwise qualifies, it should be used as `$BOOT`.
- * Otherwise, if the OS is installed on a disk with GPT disk label, and if the ESP partition already exists but is too small, a new suitably sized (let's say 500MB) partition with GPT type GUID of bc13c2ff-59e6-4262-a352-b275fd6f7172 shall be created and 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`.
+* On disks with an MBR partition table:
+ * If the OS is installed on a disk with an MBR partition table, and a partition with the type id of 0xEA already exists it should be used as `$BOOT`.
+ * Otherwise, if the OS is installed on a disk with an MBR partition table, a new partition with type id of 0xEA shall be created, of a suitable size (let's say 500MB), and it should be used as `$BOOT`.
+* On disks with GPT (GUID Partition Table)
+ * If the OS is installed on a disk with GPT, and an Extended Boot Loader Partition or XBOOTLDR partition for short, i.e. a partition with GPT type GUID of `bc13c2ff-59e6-4262-a352-b275fd6f7172`, already exists, it should be used as `$BOOT`.
+ * Otherwise, if the OS is installed on a disk with GPT, and an EFI System Partition or ESP for short, i.e. a partition with GPT type UID of `c12a7328-f81f-11d2-ba4b-00a0c93ec93b`) already exists and is large enough (let's say 250MB) and otherwise qualifies, it should be used as `$BOOT`.
+ * Otherwise, if the OS is installed on a disk with GPT, and if the ESP partition already exists but is too small, a new suitably sized (let's say 500MB) XBOOTLDR partition shall be created and used as `$BOOT`.
+ * Otherwise, if the OS is installed on a disk with GPT, and no ESP partition exists yet, a new suitably sized (let's say 500MB) ESP should be created and used as `$BOOT`.
This placeholder file system shall be determined during _installation time_, and an fstab entry may be created. It should be mounted to either `/boot/` or `/efi/`. Additional locations like `/boot/efi/`, with `/boot/` being a separate file system, might be supported by implementations. This is not recommended because the mounting of `$BOOT` is then dependent on and requires the mounting of the intermediate file system.
**Note:** _`$BOOT` should be considered **shared** among all OS installations of a system. Instead of maintaining one `$BOOT` per installed OS (as `/boot/` was traditionally handled), all installed OS share the same place to drop in their boot-time configuration._
-For systems where the firmware is able to read file systems directly, `$BOOT` must be a file system 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.
+For systems where the firmware is able to read file systems directly, `$BOOT`
+must be a file system readable by the firmware. For other systems and generic
+installation and live media, `$BOOT` must be a VFAT (16 or 32) file
+system. Applications accessing `$BOOT` should hence not assume that fancier
+file system features such as symlinks, hardlinks, access control or case
+sensitivity are supported.
This specification defines two types of boot loader entries. The first type is
text based, very simple and suitable for a variety of firmware, architecture
shall be considered and displayed. This allows image builders to put together
images that transparently support multiple different architectures.
+Note that the `$BOOT` partition is not supposed to be exclusive territory of
+this specification. This specification only defines semantics of the `/loader/`
+directory inside the file system (see below), but it doesn't intend to define
+ownership of the whole file system exclusively. 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 into the `$BOOT`
+partition. On systems where `$BOOT` is the ESP this is a particularly common
+setup. Implementations of this specification must be able to operate correctly
+if files or directories other than `/loader/` 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
We define two directories below `$BOOT`:
Inside the `$BOOT/loader/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/loader/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 encoding. 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:
+These configuration snippets shall be Unix-style text files (i.e. line separation with a single newline character), in the UTF-8 encoding. 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 one or more spaces 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`.
* `devicetree` refers to the binary device tree to use when executing the
kernel. This also shall be a path relative to the `$BOOT` directory. This
key is optional. Example: `6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.armv7hl/tegra20-paz00.dtb`.
+* `devicetree-overlay` refers 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` key. Example:
+`/6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_A.dtbo
+/6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_B.dtbo`
* `architecture` refers to the architecture this entry is defined for. The argument should be an architecture identifier, using the architecture vocabulary defined by the EFI specification (i.e. `IA32`, `x64`, `IA64`, `ARM`, `AA64`, …). If specified and this does not match (case insensitively) the local system architecture this entry should be hidden.
Each configuration drop-in snippet must include at least a `linux` or an `efi` key and is otherwise not valid. Here's an example for a complete drop-in file:
linux /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux
initrd /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd
-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.
+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.
Note that these configuration snippets may only reference kernels (and EFI programs) that reside on the same file system as the configuration snippets, i.e. everything referenced must be contained in the same file system. This is by design, as referencing other partitions or devices would require a non-trivial language for denoting device paths. If kernels/initrds are to be read from other partitions/disks the boot loader can do this in its own native configuration, using its own specific device path language, and this is out of focus for this specification. More specifically, on non-EFI systems configuration snippets following this specification cannot be used to spawn other operating systems (such as Windows).
## Links
+[GUID Partition Table](https://en.wikipedia.org/wiki/GUID_Partition_Table)<br>
+[Boot Loader Interface](https://systemd.io/BOOT_LOADER_INTERFACE)<br>
+[Discoverable Partitions Specification](https://systemd.io/DISCOVERABLE_PARTITIONS)<br>
[systemd-boot(7)](https://www.freedesktop.org/software/systemd/man/systemd-boot.html)<br>
-[bootctl(1)](https://www.freedesktop.org/software/systemd/man/bootctl.html)
+[bootctl(1)](https://www.freedesktop.org/software/systemd/man/bootctl.html)<br>
+[systemd-gpt-auto-generator(8)](https://www.freedesktop.org/software/systemd/man/systemd-gpt-auto-generator.html)
projects / thirdparty / systemd.git / blobdiff