SYSTEMD-RANDOM-SEED.SERVICsystemd-random-seed.seSYSTEMD-RANDOM-SEED.SERVICE(8)
NAME
systemd-random-seed.service, systemd-random-seed - Load and save the
system random seed at boot and shutdown
SYNOPSIS
systemd-random-seed.service
/usr/lib/systemd/random-seed
DESCRIPTION
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 its 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.
ENVIRONMENT
$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.
SEE ALSO
systemd(1), random(4), systemd-boot(7), bootctl(4)
NOTES
1. Random Seeds
https://systemd.io/RANDOM_SEEDS
systemd 245 SYSTEMD-RANDOM-SEED.SERVICE(8)