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, 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), bootctl(4)

NOTES

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

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