<?xml version="1.0"?>
<!--*-nxml-*-->
-<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
-<!--
- SPDX-License-Identifier: LGPL-2.1+
-
- This file is part of systemd.
-
- Copyright 2012 Lennart Poettering
--->
-<refentry id="systemd-random-seed.service" conditional='ENABLE_RANDOMSEED'>
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+ "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+<refentry id="systemd-random-seed.service" conditional='ENABLE_RANDOMSEED'
+ xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>systemd-random-seed.service</title>
<productname>systemd</productname>
-
- <authorgroup>
- <author>
- <contrib>Developer</contrib>
- <firstname>Lennart</firstname>
- <surname>Poettering</surname>
- <email>lennart@poettering.net</email>
- </author>
- </authorgroup>
</refentryinfo>
<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>
+ <refpurpose>Load and save the OS 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>
+ <para><filename>/usr/lib/systemd/systemd-random-seed</filename></para>
</refsynopsisdiv>
<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
+ initrd phase has finished and the <filename>/var/</filename> file system has been mounted. 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>
+
+ <xi:include href="version-info.xml" xpointer="v243"/></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>
- </para>
+ <para><simplelist type="inline">
+ <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
+ <member><citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry></member>
+ <member><citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
+ <member><citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
+ <member><citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>4</manvolnum></citerefentry></member>
+ <member><citerefentry><refentrytitle>systemd-boot-random-seed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
+ </simplelist></para>
</refsect1>
</refentry>
projects / thirdparty / systemd.git / blobdiff