|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
|
SYSTEM...SERVICE(8) systemd-boot-random-seed.service SYSTEM...SERVICE(8)
systemd-boot-random-seed.service - Refresh boot loader random
seed at boot
systemd-boot-random-seed.service
systemd-boot-random-seed.service is a system service that
automatically refreshes the boot loader random seed stored in the
EFI System Partition (ESP), from the Linux kernel entropy pool.
The boot loader random seed is primarily consumed and updated by
systemd-boot(7) from the UEFI environment (or systemd-stub(7) if
the former is not used, but the latter is), and passed as initial
RNG seed to the OS. It is an effective way to ensure the OS comes
up with a random pool that is fully initialized.
The service also automatically generates a 'system token' to
store in an EFI variable in the system's NVRAM. The boot loader
may then combine the on-disk random seed and the system token by
cryptographic hashing, and pass it to the OS it boots as
initialization seed for its entropy pool. Note: the random seed
stored in the ESP is refreshed on every reboot ensuring that
multiple subsequent boots will boot with different seeds. On the
other hand, the system token is generated randomly once, and then
persistently stored in the system's EFI variable storage,
ensuring the same disk image won't result in the same series of
boot loader seed values if used on multiple systems in parallel.
The systemd-boot-random-seed.service unit invokes the bootctl
random-seed command, which updates the random seed in the ESP,
and initializes the system token if it's not initialized yet. The
service is conditionalized so that it is run only when a boot
loader is used that implements the Boot Loader Interface[1].
For further details see bootctl(1), regarding the command this
service invokes.
Note the relationship between systemd-boot-random-seed.service
and systemd-random-seed(8). The former maintains the random seed
consumed and updated by the boot environment (i.e. by
systemd-boot(7) or systemd-stub(7)), the latter maintains a
random seed consumed and updated by the OS itself. The former
ensures that the OS has a filled entropy pool already during
earliest boot when regular disk access is not available yet (i.e.
when the OS random seed cannot be loaded yet). The latter is
processed much later, once writable disk access is available.
Thus it cannot be used to seed the initial boot phase, but
typically has much higher quality of entropy. Both files are
consumed and updated at boot, but at different times.
Specifically:
1. In UEFI mode, the systemd-boot(7) or systemd-stub(7)
components load the boot loader random seed from the ESP,
hash it with available entropy and the system token, and then
update it on disk. A derived seed is passed to the kernel
which writes it to its entropy pool.
2. In userspace the systemd-random-seed.service service loads
the OS random seed, writes it to the kernel entropy pool, and
then updates it on disk with a new value derived from the
kernel entropy pool.
3. In userspace the systemd-boot-random-seed.service service
updates the boot loader random seed with a new value derived
from the kernel entropy pool.
This logic should ensure that the kernel's entropy pool is seeded
during earliest bool already, if possible, but the highest
quality entropy is propagated back to both on-disk seeds.
systemd(1), random(4), bootctl(1), systemd-boot(7),
systemd-stub(7), systemd-random-seed.service(8)
1. Boot Loader Interface
https://systemd.io/BOOT_LOADER_INTERFACE
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have
a bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
systemd 257~devel SYSTEM...SERVICE(8)
Pages that refer to this page: bootctl(1), systemd-boot(7), systemd.directives(7), systemd.index(7), systemd-random-seed.service(8)
|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|