<refsect1>
<title>Description</title>
- <para><filename>systemd-random-seed.service</filename> is a
- service that restores the random seed of the system at early boot
- and saves it at shutdown. See
- <citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>
- for details. Saving/restoring the random seed across boots
- increases the amount of available entropy early at boot. On disk
- the random seed is stored in
- <filename>/var/lib/systemd/random-seed</filename>.</para>
+ <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 its 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.</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>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>