systemd-random-seed.service - Man Page

systemd-random-seed​.service

/usr/lib/systemd/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 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​.

$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 consistency checks​. If set to "force" entropy is credited, regardless of these checks, as long as the random seed file exists​.

Added in version 243​.

systemd(1), random(4), systemd-boot(7), systemd-stub(7), bootctl(1), systemd-boot-random-seed.service(8)

1. Safely Building Images
   https://systemd.io/BUILDING_IMAGES
2. Random Seeds
   https://systemd.io/RANDOM_SEEDS