+---
+title: Boot Loader Interface
+category: Booting
+layout: default
+---
+
# The Boot Loader Interface
systemd can interface with the boot loader to receive performance data and
* `1 << 2` → The boot loader honours `LoaderEntryDefault` when set.
* `1 << 3` → The boot loader honours `LoaderEntryOneShot` when set.
* `1 << 4` → The boot loader supports boot counting as described in [Automatic Boot Assessment](https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT).
+ * `1 << 5` → The boot loader supports looking for boot menu entries in the Extended Boot Loader Partition.
+ * `1 << 6` → The boot loader supports passing a random seed to the OS.
+
+* The EFI variable `LoaderRandomSeed` contains a binary random seed if set. It
+ is set by the boot loader to pass an entropy seed read from the ESP partition
+ to the OS. The system manager then credits this seed to the kernel's entropy
+ pool. It is the responsibility of the boot loader to ensure the quality and
+ integrity of the random seed.
+
+* The EFI variable `LoaderSystemToken` contains binary random data,
+ persistently set by the OS installer. Boot loaders that support passing
+ random seeds to the OS should use this data and combine it with the random
+ seed file read from the ESP. By combining this random data with the random
+ seed read off the disk before generating a seed to pass to the OS and a new
+ seed to store in the ESP the boot loader can protect itself from situations
+ where "golden" OS images that include a random seed are replicated and used
+ on multiple systems. Since the EFI variable storage is usually independent
+ (i.e. in physical NVRAM) of the ESP file system storage, and only the latter
+ is part of "golden" OS images, this ensures that different systems still come
+ up with different random seeds. Note that the `LoaderSystemToken` is
+ generally only written once, by the OS installer, and is usually not touched
+ after that.
If `LoaderTimeInitUSec` and `LoaderTimeExecUSec` are set, `systemd-analyze`
will include them in its boot-time analysis. If `LoaderDevicePartUUID` is set,
that directory is empty, and only if no other file systems are mounted
there. The `systemctl reboot --boot-loader-entry=…` and `systemctl reboot
--boot-loader-menu=…` commands rely on the `LoaderFeatures` ,
-`LoaderConfigTimeoutOneShot`, `LoaderEntries`, `LoaderEntryOneShot` variables.
+`LoaderConfigTimeoutOneShot`, `LoaderEntries`, `LoaderEntryOneShot`
+variables. `LoaderRandomSeed` is read by PID during early boot and credited to
+the kernel's random pool.
## Boot Loader Entry Identifiers
6. If a boot menu entry encapsulates a reboot into EFI firmware setup feature,
it should use the identifier `reboot-to-firmware-setup` (or
`auto-reboot-to-firmware-setup` in case it is automatically discovered).
+
+## Links
+
+[Boot Loader Specification](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)<br>
+[systemd-gpt-auto-generator(8)](https://www.freedesktop.org/software/systemd/man/systemd-gpt-auto-generator.html)