1SYSTEMD-BOOT-RANDOM-SEEDs.ySsEtReVmIdC-Eb(o8o)t-random-sSeYeSdT.EsMeDr-vBiOcOeT-RANDOM-SEED.SERVICE(8)
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6 systemd-boot-random-seed.service - Refresh boot loader random seed at
7 boot
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10 systemd-boot-random-seed.service
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13 systemd-boot-random-seed.service is a system service that automatically
14 refreshes the boot loader random seed stored in the EFI System
15 Partition (ESP), from the Linux kernel entropy pool. The boot loader
16 random seed is primarily consumed and updated by systemd-boot(7) from
17 the UEFI environment (or systemd-stub(7) if the former is not used, but
18 the latter is), and passed as initial RNG seed to the OS. It is an
19 effective way to ensure the OS comes up with a random pool that is
20 fully initialized.
21 The service also automatically generates a 'system token' to store in
23 an EFI variable in the system's NVRAM. The boot loader may then combine
24 the on-disk random seed and the system token by cryptographic hashing,
25 and pass it to the OS it boots as initialization seed for its entropy
26 pool. Note: the random seed stored in the ESP is refreshed on every
27 reboot ensuring that multiple subsequent boots will boot with different
28 seeds. On the other hand, the system token is generated randomly once,
29 and then persistently stored in the system's EFI variable storage,
30 ensuring the same disk image won't result in the same series of boot
31 loader seed values if used on multiple systems in parallel.
32 The systemd-boot-random-seed.service unit invokes the bootctl
34 random-seed command, which updates the random seed in the ESP, and
35 initializes the system token if it's not initialized yet. The service
36 is conditionalized so that it is run only when a boot loader is used
37 that implements the Boot Loader Interface[1].
38 For further details see bootctl(1), regarding the command this service
40 invokes.
41 Note the relationship between systemd-boot-random-seed.service and
43 systemd-random-seed(8). The former maintains the random seed consumed
44 and updated by the boot environment (i.e. by systemd-boot(7) or
45 systemd-stub(7)), the latter maintains a random seed consumed and
46 updated by the OS itself. The former ensures that the OS has a filled
47 entropy pool already during earliest boot when regular disk access is
48 not available yet (i.e. when the OS random seed cannot be loaded yet).
49 The latter is processed much later, once writable disk access is
50 available. Thus it cannot be used to seed the initial boot phase, but
51 typically has much higher quality of entropy. Both files are consumed
52 and updated at boot, but at different times. Specifically:
53 1. In UEFI mode, the systemd-boot(7) or systemd-stub(7) components
55 load the boot loader random seed from the ESP, hash it with
56 available entropy and the system token, and then update it on disk.
57 A derived seed is passed to the kernel which writes it to its
58 entropy pool.
59 2. In userspace the systemd-random-seed.service service loads the OS
61 random seed, writes it to the kernel entropy pool, and then updates
62 it on disk with a new value derived from the kernel entropy pool.
63 3. In userspace the systemd-boot-random-seed.service service updates
65 the boot loader random seed with a new value derived from the
66 kernel entropy pool.
67 This logic should ensure that the kernel's entropy pool is seeded
69 during earliest bool already, if possible, but the highest quality
70 entropy is propagated back to both on-disk seeds.
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73 systemd(1), random(4), bootctl(1), systemd-boot(7), systemd-stub(7),
74 systemd-random-seed.service(8)
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77 1. Boot Loader Interface
78 https://systemd.io/BOOT_LOADER_INTERFACE
79systemd 254 SYSTEMD-BOOT-RANDOM-SEED.SERVICE(8)