123 lines
6.7 KiB
Diff
123 lines
6.7 KiB
Diff
From 07ba9da0bc0d4e3309e766d3734ac56683d3298d Mon Sep 17 00:00:00 2001
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From: =?UTF-8?q?Zbigniew=20J=C4=99drzejewski-Szmek?= <zbyszek@in.waw.pl>
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Date: Wed, 18 Oct 2017 18:47:34 +0200
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Subject: [PATCH] man: describe how machine-id should be initialized (#7051)
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(cherry picked from commit 74a79c657e17fc43172a59f1f158d4a26aec2be1)
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---
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man/machine-id.xml | 90 +++++++++++++++++++++++++++++++++++++++++-------------
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1 file changed, 69 insertions(+), 21 deletions(-)
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diff --git a/man/machine-id.xml b/man/machine-id.xml
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index 3c261bffcc..e434c89de9 100644
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--- a/man/machine-id.xml
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+++ b/man/machine-id.xml
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@@ -53,18 +53,26 @@
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<refsect1>
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<title>Description</title>
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- <para>The <filename>/etc/machine-id</filename> file contains the unique machine ID of the local
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- system that is set during installation. The machine ID is a single newline-terminated,
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- hexadecimal, 32-character, lowercase ID. When decoded from hexadecimal, this corresponds to a
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- 16-byte/128-bit value.</para>
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+ <para>The <filename>/etc/machine-id</filename> file contains the unique machine ID of
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+ the local system that is set during installation or boot. The machine ID is a single
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+ newline-terminated, hexadecimal, 32-character, lowercase ID. When decoded from
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+ hexadecimal, this corresponds to a 16-byte/128-bit value. This ID may not be all
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+ zeros.</para>
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- <para>The machine ID is usually generated from a random source
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- during system installation and stays constant for all subsequent
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- boots. Optionally, for stateless systems, it is generated during
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- runtime at early boot if it is found to be empty.</para>
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+ <para>The machine ID is usually generated from a random source during system
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+ installation or first boot and stays constant for all subsequent boots. Optionally,
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+ for stateless systems, it is generated during runtime during early boot if necessary.
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+ </para>
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- <para>The machine ID does not change based on local or network configuration or when hardware is
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- replaced. Due to this and its greater length, it is a more useful replacement for the
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+ <para>The machine ID may be set, for example when network booting, with the
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+ <varname>systemd.machine_id=</varname> kernel command line parameter or by passing the
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+ option <option>--machine-id=</option> to systemd. An ID is specified in this manner
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+ has higher priority and will be used instead of the ID stored in
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+ <filename>/etc/machine-id</filename>.</para>
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+
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+ <para>The machine ID does not change based on local or network configuration or when
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+ hardware is replaced. Due to this and its greater length, it is a more useful
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+ replacement for the
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<citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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call that POSIX specifies.</para>
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@@ -79,19 +87,59 @@
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the original machine ID from the application-specific one. The
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<citerefentry><refentrytitle>sd_id128_get_machine_app_specific</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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API provides an implementation of such an algorithm.</para>
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+ </refsect1>
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- <para>The
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+ <refsect1>
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+ <title>Initialization</title>
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+
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+ <para>Each machine should have a non-empty ID in normal operation. The ID of each
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+ machine should be unique. To achive those objectives,
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+ <filename>/etc/machine-id</filename> can be initialized in a few different ways.
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+ </para>
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+
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+ <para>For normal operating system installations, where a custom image is created for a
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+ specific machine, <filename>/etc/machine-id</filename> should be populated during
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+ installation.</para>
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+
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+ <para>
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<citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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- tool may be used by installer tools to initialize the machine ID
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- at install time. Use
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- <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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- to initialize it on mounted (but not booted) system images.</para>
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-
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- <para>The machine-id may also be set, for example when network
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- booting, by setting the <varname>systemd.machine_id=</varname>
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- kernel command line parameter or passing the option
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- <option>--machine-id=</option> to systemd. A machine-id may not
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- be set to all zeros.</para>
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+ may be used by installer tools to initialize the machine ID at install time, but
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+ <filename>/etc/machine-id</filename> may also be written using any other means.
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+ </para>
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+
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+ <para>For operating system images which are created once and used on multiple
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+ machines, for example for containers or in the cloud,
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+ <filename>/etc/machine-id</filename> should be an empty file in the generic file
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+ system image. An ID will be generated during boot and saved to this file if
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+ possible. Having an empty file in place is useful because it allows a temporary file
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+ to be bind-mounted over the real file, in case the image is used read-only.</para>
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+
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+ <para><citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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+ may be used to to initialize <filename>/etc/machine-id</filename> on mounted (but not
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+ booted) system images.</para>
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+
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+ <para>When a machine is booted with
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+ <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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+ the ID of the machine will be established. If <varname>systemd.machine_id=</varname>
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+ or <option>--machine-id=</option> options (see first section) are specified, this
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+ value will be used. Otherwise, the value in <filename>/etc/machine-id</filename> will
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+ be used. If this file is empty or missing, <filename>systemd</filename> will attempt
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+ to use the D-Bus machine ID from <filename>/var/lib/dbus/machine-id</filename>, the
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+ value of the kernel command line option <varname>container_uuid</varname>, the KVM DMI
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+ <filename>product_uuid</filename> (on KVM systems), and finally a randomly generated
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+ UUID.</para>
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+
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+ <para>After the machine ID is established,
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+ <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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+ will attempt to save it to <filename>/etc/machine-id</filename>. If this fails, it
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+ will attempt to bind-mount a temporary file over <filename>/etc/machine-id</filename>.
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+ It is an error if the file system is read-only and does not contain a (possibly empty)
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+ <filename>/etc/machine-id</filename> file.</para>
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+
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+ <para><citerefentry><refentrytitle>systemd-machine-id-commit.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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+ will attempt to write the machine ID to the file system if
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+ <filename>/etc/machine-id</filename> or <filename>/etc</filename> are read-only during
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+ early boot but become writable later on.</para>
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</refsect1>
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<refsect1>
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