mirror of
https://github.com/systemd/systemd.git
synced 2024-12-29 15:22:07 +03:00
94 lines
5.0 KiB
XML
94 lines
5.0 KiB
XML
<?xml version="1.0"?>
|
|
<!--*-nxml-*-->
|
|
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
|
|
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
|
|
<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>
|