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94 lines
5.0 KiB
XML
94 lines
5.0 KiB
XML
<?xml version="1.0"?>
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<!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="systemd-random-seed.service" conditional='ENABLE_RANDOMSEED'>
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<refentryinfo>
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<title>systemd-random-seed.service</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd-random-seed.service</refentrytitle>
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<manvolnum>8</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd-random-seed.service</refname>
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<refname>systemd-random-seed</refname>
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<refpurpose>Load and save the system random seed at boot and shutdown</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>systemd-random-seed.service</filename></para>
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<para><filename>/usr/lib/systemd/random-seed</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><filename>systemd-random-seed.service</filename> is a service that loads an on-disk random seed
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into the kernel entropy pool during boot and saves it at shutdown. See
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<citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry> for
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details. By default, no entropy is credited when the random seed is written into the kernel entropy pool,
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but this may be changed with <varname>$SYSTEMD_RANDOM_SEED_CREDIT</varname>, see below. On disk the random
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seed is stored in <filename>/var/lib/systemd/random-seed</filename>.</para>
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<para>Note that this service runs relatively late during the early boot phase, i.e. generally after the
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initial RAM disk (initrd) completed its work, and the <filename>/var/</filename> file system has been
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mounted writable. Many system services require entropy much earlier than this — this service is hence of
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limited use for complex system. It is recommended to use a boot loader that can pass an initial random
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seed to the kernel to ensure that entropy is available from earliest boot on, for example
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<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>, with
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its <command>bootctl random-seed</command> functionality.</para>
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<para>When loading the random seed from disk, the file is immediately updated with a new seed retrieved
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from the kernel, in order to ensure no two boots operate with the same random seed. This new seed is
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retrieved synchronously from the kernel, which means the service will not complete start-up until the
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random pool is fully initialized. On entropy-starved systems this may take a while. This functionality is
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intended to be used as synchronization point for ordering services that require an initialized entropy
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pool to function securely (i.e. services that access <filename>/dev/urandom</filename> without any
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further precautions).</para>
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<para>Care should be taken when creating OS images that are replicated to multiple systems: if the random
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seed file is included unmodified each system will initialize its entropy pool with the same data, and
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thus — if otherwise entropy-starved — generate the same or at least guessable random seed streams. As a
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safety precaution crediting entropy is thus disabled by default. It is recommended to remove the random
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seed from OS images intended for replication on multiple systems, in which case it is safe to enable
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entropy crediting, see below. Also see <ulink url="https://systemd.io/BUILDING_IMAGES">Safely Building
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Images</ulink>.</para>
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<para>See <ulink url="https://systemd.io/RANDOM_SEEDS">Random Seeds</ulink> for further
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information.</para>
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</refsect1>
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<refsect1>
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<title>Environment</title>
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<variablelist class='environment-variables'>
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<varlistentry>
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<term><varname>$SYSTEMD_RANDOM_SEED_CREDIT</varname></term>
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<listitem><para>By default, <filename>systemd-random-seed.service</filename> does not credit any
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entropy when loading the random seed. With this option this behaviour may be changed: it either takes
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a boolean parameter or the special string <literal>force</literal>. Defaults to false, in which case
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no entropy is credited. If true, entropy is credited if the random seed file and system state pass
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various superficial concisistency checks. If set to <literal>force</literal> entropy is credited,
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regardless of these checks, as long as the random seed file exists.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>4</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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