systemd/man/systemd-boot-random-seed.service.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<refentry id="systemd-boot-random-seed.service" conditional='ENABLE_BOOTLOADER'
          xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>systemd-boot-random-seed.service</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd-boot-random-seed.service</refentrytitle>
    <manvolnum>8</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd-boot-random-seed.service</refname>
    <refpurpose>Refresh boot loader random seed at boot</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename>systemd-boot-random-seed.service</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><filename>systemd-boot-random-seed.service</filename> is a system service that automatically
    refreshes the boot loader random seed stored in the EFI System Partition (ESP), from the Linux kernel
    entropy pool. The boot loader random seed is primarily consumed and updated by
    <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> from the
    UEFI environment (or
    <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry> if the
    former is not used, but the latter is), and passed as initial RNG seed to the OS. It is an effective way
    to ensure the OS comes up with a random pool that is fully initialized.</para>

    <para>The service also automatically generates a 'system token' to store in an EFI variable in the
    system's NVRAM. The boot loader may then combine the on-disk random seed and the system token by
    cryptographic hashing, and pass it to the OS it boots as initialization seed for its entropy pool. Note:
    the random seed stored in the ESP is refreshed on <emphasis>every</emphasis> reboot ensuring that
    multiple subsequent boots will boot with different seeds. On the other hand, the system token is
    generated randomly <emphasis>once</emphasis>, and then persistently stored in the system's EFI variable
    storage, ensuring the same disk image won't result in the same series of boot loader seed values if used
    on multiple systems in parallel.</para>

    <para>The <filename>systemd-boot-random-seed.service</filename> unit invokes the <command>bootctl
    random-seed</command> command, which updates the random seed in the ESP, and initializes the system
    token if it's not initialized yet. The service is conditionalized so that it is run only when a boot
    loader is used that implements the <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader
    Interface</ulink>.</para> <para>For further details see
    <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, regarding
    the command this service invokes.</para>

    <para>Note the relationship between <filename>systemd-boot-random-seed.service</filename> and
    <citerefentry><refentrytitle>systemd-random-seed</refentrytitle><manvolnum>8</manvolnum></citerefentry>. The
    former maintains the random seed consumed and updated by the boot environment (i.e. by
    <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> or
    <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry>), the
    latter maintains a random seed consumed and updated by the OS itself. The former ensures that the OS has
    a filled entropy pool already during earliest boot when regular disk access is not available yet
    (i.e. when the OS random seed cannot be loaded yet). The latter is processed much later, once writable
    disk access is available. Thus it cannot be used to seed the initial boot phase, but typically has much
    higher quality of entropy. Both files are consumed and updated at boot, but at different
    times. Specifically:</para>

    <orderedlist>
      <listitem><para>In UEFI mode, the <filename>systemd-boot</filename> or
      <filename>systemd-stub</filename> components load the boot loader random seed off the ESP, hash it with
      available entropy and the system token, and then update it on disk. A derived seed is passed to the
      kernel which writes it to its entropy pool.</para></listitem>

      <listitem><para>In userspace the <filename>systemd-random-seed.service</filename> service loads the OS
      random seed, writes it to the kernel entropy pool, and then updates it on disk with a new value derived
      from the kernel entropy pool.</para></listitem>

      <listitem><para>In userspace the <filename>systemd-boot-random-seed.service</filename> service updates
      the boot loader random seed with a new value derived from the kernel entropy pool.</para></listitem>
    </orderedlist>

    <para>This logic should ensure that the kernel's entropy pool is seeded during earliest bool already, if
    possible, but the highest quality entropy is propagated back to both on-disk seeds.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-random-seed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>