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

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<refentry id="systemd-random-seed.service" conditional='ENABLE_RANDOMSEED'>

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

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

  <refnamediv>
    <refname>systemd-random-seed.service</refname>
    <refname>systemd-random-seed</refname>
    <refpurpose>Load and save the system random seed at boot and shutdown</refpurpose>
  </refnamediv>

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

  <refsect1>
    <title>Description</title>

    <para><filename>systemd-random-seed.service</filename> is a service that loads an on-disk random seed
    into the kernel entropy pool during boot and saves it at shutdown. See
    <citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry> 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 <varname>$SYSTEMD_RANDOM_SEED_CREDIT</varname>, see below.  On disk the random
    seed is stored in <filename>/var/lib/systemd/random-seed</filename>.</para>

    <para>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 <filename>/var/</filename> 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
    <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>, with
    its <command>bootctl random-seed</command> functionality.</para>

    <para>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 <filename>/dev/urandom</filename> without any
    further precautions).</para>

    <para>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 <ulink url="https://systemd.io/BUILDING_IMAGES">Safely Building
    Images</ulink>.</para>

    <para>See <ulink url="https://systemd.io/RANDOM_SEEDS">Random Seeds</ulink> for further
    information.</para>
  </refsect1>

  <refsect1>
    <title>Environment</title>

    <variablelist class='environment-variables'>
      <varlistentry>
        <term><varname>$SYSTEMD_RANDOM_SEED_CREDIT</varname></term>
        <listitem><para>By default, <filename>systemd-random-seed.service</filename> 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 <literal>force</literal>. 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 <literal>force</literal> entropy is credited,
        regardless of these checks, as long as the random seed file exists.</para></listitem>
      </varlistentry>
    </variablelist>
  </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>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>4</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>