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diff --git a/man/systemd-random-seed.service.xml b/man/systemd-random-seed.service.xml new file mode 100644 index 0000000..3137ed0 --- /dev/null +++ b/man/systemd-random-seed.service.xml @@ -0,0 +1,92 @@ +<?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.</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> |