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b66a6e1a58
In many places we spelled out the phrase behind "initrd" in full, but this isn't terribly useful. In fact, no "RAM disk" is used, so emphasizing this is just confusing to the reader. Let's just say "initrd" everywhere, people understand what this refers to, and that it's in fact an initramfs image. Also, s/i.e./e.g./ where appropriate. Also, don't say "in RAM", when in fact it's virtual memory, whose pages may or may not be loaded in page frames in RAM, and we have no control over this. Also, add <filename></filename> and other minor cleanups.
94 lines
4.9 KiB
XML
94 lines
4.9 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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initrd phase has finished and the <filename>/var/</filename> file system has been mounted. Many system
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services require entropy much earlier than this — this service is hence of limited use for complex
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system. It is recommended to use a boot loader that can pass an initial random seed to the kernel to
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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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