+---
+title: The Boot Loader Interface
+category: Booting
+layout: default
+---
+
# The Boot Loader Interface
systemd can interface with the boot loader to receive performance data and
ESP the boot loader was run from formatted as NUL-terminated UTF16 string, in
normal GUID syntax.
-* The EFI variable `LoaderConfigTimeout` contains the boot menu time-out
+* The EFI variable `LoaderConfigTimeout` contains the boot menu timeout
currently in use. It may be modified both by the boot loader and by the
host. The value should be formatted as numeric, NUL-terminated, decimal
string, in UTF-16. The time is specified in µs.
* Similarly, the EFI variable `LoaderConfigTimeoutOneShot` contains a boot menu
- time-out for a single following boot. It is set by the OS in order to request
+ timeout for a single following boot. It is set by the OS in order to request
display of the boot menu on the following boot. When set overrides
`LoaderConfigTimeout`. It is removed automatically after being read by the
boot loader, to ensure it only takes effect a single time. This value is
formatted the same way as `LoaderConfigTimeout`. If set to `0` the boot menu
- time-out is turned off, and the menu is shown indefinitely.
+ timeout is turned off, and the menu is shown indefinitely.
* The EFI variable `LoaderEntries` may contain a series of boot loader entry
identifiers, one after the other, each individually NUL terminated. This may
* `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