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NAME | SYNOPSIS | DESCRIPTION | ENVIRONMENT | SEE ALSO | NOTES | COLOPHON |
SYSTEMD-RANDOM-SEED.SERVICE(8)md-random-seed.serviceD-RANDOM-SEED.SERVICE(8)
systemd-random-seed.service, systemd-random-seed - Load and save the
system random seed at boot and shutdown
systemd-random-seed.service
/usr/lib/systemd/random-seed
systemd-random-seed.service is a service that loads an on-disk random
seed into the kernel entropy pool during boot and saves it at
shutdown. See random(4) for details. By default, no entropy is
credited when the random seed is written into the kernel entropy
pool, but this may be changed with $SYSTEMD_RANDOM_SEED_CREDIT, see
below. On disk the random seed is stored in
/var/lib/systemd/random-seed.
Note that this service runs relatively late during the early boot
phase, i.e. generally after the initial RAM disk (initrd) completed
its work, and the /var/ file system has been mounted writable. Many
system services require entropy much earlier than this — this service
is hence of limited use for complex system. It is recommended to use
a boot loader that can pass an initial random seed to the kernel to
ensure that entropy is available from earliest boot on, for example
systemd-boot(7), with its bootctl random-seed functionality.
When loading the random seed from disk, the file is immediately
updated with a new seed retrieved from the kernel, in order to ensure
no two boots operate with the same random seed. This new seed is
retrieved synchronously from the kernel, which means the service will
not complete start-up until the random pool is fully initialized. On
entropy-starved systems this may take a while. This functionality is
intended to be used as synchronization point for ordering services
that require an initialized entropy pool to function securely (i.e.
services that access /dev/urandom without any further precautions).
Care should be taken when creating OS images that are replicated to
multiple systems: if the random seed file is included unmodified each
system will initialize its entropy pool with the same data, and thus
— if otherwise entropy-starved — generate the same or at least
guessable random seed streams. As a safety precaution crediting
entropy is thus disabled by default. It is recommended to remove the
random seed from OS images intended for replication on multiple
systems, in which case it is safe to enable entropy crediting, see
below.
See Random Seeds[1] for further information.
$SYSTEMD_RANDOM_SEED_CREDIT
By default, systemd-random-seed.service does not credit any
entropy when loading the random seed. With this option this
behaviour may be changed: it either takes a boolean parameter or
the special string "force". Defaults to false, in which case no
entropy is credited. If true, entropy is credited if the random
seed file and system state pass various superficial concisistency
checks. If set to "force" entropy is credited, regardless of
these checks, as long as the random seed file exists.
systemd(1), random(4), systemd-boot(7), bootctl(4)
1. Random Seeds
https://systemd.io/RANDOM_SEEDS
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 2020-08-13. (At that
time, the date of the most recent commit that was found in the repos‐
itory was 2020-08-11.) 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 man-pages@man7.org
systemd 246 SYSTEMD-RANDOM-SEED.SERVICE(8)
Pages that refer to this page: 30-systemd-environment-d-generator(7) , systemd.index(7)