man 8 systemd-random-seed

SYSTEMD-RANDOM-SEED.SERVICsystemd-random-seed.seSYSTEMD-RANDOM-SEED.SERVICE(8)

NAME
       systemd-random-seed.service, systemd-random-seed - Load and save the OS
       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 initrd phase has finished and the /var/
       file system has been mounted. 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. Also see Safely
       Building Images[1].

       See Random Seeds[2] 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), systemd-stub(7), bootctl(4),
       systemd-boot-random-seed.service(8)

NOTES
        1. Safely Building Images
           https://systemd.io/BUILDING_IMAGES

        2. Random Seeds
           https://systemd.io/RANDOM_SEEDS

systemd 252                                     SYSTEMD-RANDOM-SEED.SERVICE(8)