]>
Commit | Line | Data |
---|---|---|
edcf89be LP |
1 | <?xml version="1.0"?> |
2 | <!--*-nxml-*--> | |
3a54a157 ZJS |
3 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" |
4 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> | |
0307f791 | 5 | <!-- SPDX-License-Identifier: LGPL-2.1+ --> |
c35b956d | 6 | <refentry id="systemd-random-seed.service" conditional='ENABLE_RANDOMSEED'> |
edcf89be | 7 | |
798d3a52 ZJS |
8 | <refentryinfo> |
9 | <title>systemd-random-seed.service</title> | |
10 | <productname>systemd</productname> | |
798d3a52 | 11 | </refentryinfo> |
edcf89be | 12 | |
798d3a52 ZJS |
13 | <refmeta> |
14 | <refentrytitle>systemd-random-seed.service</refentrytitle> | |
15 | <manvolnum>8</manvolnum> | |
16 | </refmeta> | |
edcf89be | 17 | |
798d3a52 ZJS |
18 | <refnamediv> |
19 | <refname>systemd-random-seed.service</refname> | |
20 | <refname>systemd-random-seed</refname> | |
21 | <refpurpose>Load and save the system random seed at boot and shutdown</refpurpose> | |
22 | </refnamediv> | |
edcf89be | 23 | |
798d3a52 ZJS |
24 | <refsynopsisdiv> |
25 | <para><filename>systemd-random-seed.service</filename></para> | |
621a2c80 | 26 | <para><filename>/usr/lib/systemd/random-seed</filename></para> |
798d3a52 | 27 | </refsynopsisdiv> |
edcf89be | 28 | |
798d3a52 ZJS |
29 | <refsect1> |
30 | <title>Description</title> | |
edcf89be | 31 | |
39867bb9 LP |
32 | <para><filename>systemd-random-seed.service</filename> is a service that loads an on-disk random seed |
33 | into the kernel entropy pool during boot and saves it at shutdown. See | |
34 | <citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry> for | |
35 | details. By default, no entropy is credited when the random seed is written into the kernel entropy pool, | |
36 | but this may be changed with <varname>$SYSTEMD_RANDOM_SEED_CREDIT</varname>, see below. On disk the random | |
37 | seed is stored in <filename>/var/lib/systemd/random-seed</filename>.</para> | |
38 | ||
39 | <para>Note that this service runs relatively late during the early boot phase, i.e. generally after the | |
40 | initial RAM disk (initrd) completed its work, and the <filename>/var/</filename> file system has been | |
41 | mounted writable. Many system services require entropy much earlier than this — this service is hence of | |
42 | limited use for complex system. It is recommended to use a boot loader that can pass an initial random | |
43 | seed to the kernel to ensure that entropy is available from earliest boot on, for example | |
44 | <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>, with | |
45 | its <command>bootctl random-seed</command> functionality.</para> | |
46 | ||
e9dd6984 | 47 | <para>When loading the random seed from disk, the file is immediately updated with a new seed retrieved |
39867bb9 LP |
48 | from the kernel, in order to ensure no two boots operate with the same random seed. This new seed is |
49 | retrieved synchronously from the kernel, which means the service will not complete start-up until the | |
50 | random pool is fully initialized. On entropy-starved systems this may take a while. This functionality is | |
51 | intended to be used as synchronization point for ordering services that require an initialized entropy | |
52 | pool to function securely (i.e. services that access <filename>/dev/urandom</filename> without any | |
53 | further precautions).</para> | |
54 | ||
55 | <para>Care should be taken when creating OS images that are replicated to multiple systems: if the random | |
56 | seed file is included unmodified each system will initialize its entropy pool with the same data, and | |
57 | thus — if otherwise entropy-starved — generate the same or at least guessable random seed streams. As a | |
58 | safety precaution crediting entropy is thus disabled by default. It is recommended to remove the random | |
59 | seed from OS images intended for replication on multiple systems, in which case it is safe to enable | |
60 | entropy crediting, see below.</para> | |
93f59100 LP |
61 | |
62 | <para>See <ulink url="https://systemd.io/RANDOM_SEEDS">Random Seeds</ulink> for further | |
63 | information.</para> | |
39867bb9 LP |
64 | </refsect1> |
65 | ||
66 | <refsect1> | |
67 | <title>Environment</title> | |
68 | ||
69 | <variablelist class='environment-variables'> | |
70 | <varlistentry> | |
71 | <term><varname>$SYSTEMD_RANDOM_SEED_CREDIT</varname></term> | |
72 | <listitem><para>By default, <filename>systemd-random-seed.service</filename> does not credit any | |
73 | entropy when loading the random seed. With this option this behaviour may be changed: it either takes | |
74 | a boolean parameter or the special string <literal>force</literal>. Defaults to false, in which case | |
75 | no entropy is credited. If true, entropy is credited if the random seed file and system state pass | |
76 | various superficial concisistency checks. If set to <literal>force</literal> entropy is credited, | |
77 | regardless of these checks, as long as the random seed file exists.</para></listitem> | |
78 | </varlistentry> | |
79 | </variablelist> | |
798d3a52 | 80 | </refsect1> |
edcf89be | 81 | |
798d3a52 ZJS |
82 | <refsect1> |
83 | <title>See Also</title> | |
84 | <para> | |
85 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
39867bb9 LP |
86 | <citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>, |
87 | <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>, | |
88 | <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>4</manvolnum></citerefentry> | |
798d3a52 ZJS |
89 | </para> |
90 | </refsect1> | |
edcf89be LP |
91 | |
92 | </refentry> |