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70fc4f5790
This adds an API for retrieving an app-specific machine ID to sd-id128. Internally it calculates HMAC-SHA256 with an 128bit app-specific ID as payload and the machine ID as key. (An alternative would have been to use siphash for this, which is also cryptographically strong. However, as it only generates 64bit hashes it's not an obvious choice for generating 128bit IDs.) Fixes: #4667
149 lines
6.5 KiB
XML
149 lines
6.5 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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This file is part of systemd.
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Copyright 2010 Lennart Poettering
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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-->
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<refentry id="machine-id">
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<refentryinfo>
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<title>machine-id</title>
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<productname>systemd</productname>
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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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</refentryinfo>
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<refmeta>
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<refentrytitle>machine-id</refentrytitle>
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<manvolnum>5</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>machine-id</refname>
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<refpurpose>Local machine ID configuration file</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>/etc/machine-id</filename></para>
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</refsynopsisdiv>
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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 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 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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<citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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call that POSIX specifies.</para>
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<para>This machine ID adheres to the same format and logic as the
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D-Bus machine ID.</para>
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<para>This ID uniquely identifies the host. It should be considered "confidential", and must not be exposed in
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untrusted environments, in particular on the network. If a stable unique identifier that is tied to the machine is
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needed for some application, the machine ID or any part of it must not be used directly. Instead the machine ID
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should be hashed with a cryptographic, keyed hash function, using a fixed, application-specific key. That way the
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ID will be properly unique, and derived in a constant way from the machine ID but there will be no way to retrieve
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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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<para>The
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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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<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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</refsect1>
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<refsect1>
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<title>Relation to OSF UUIDs</title>
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<para>Note that the machine ID historically is not an OSF UUID as
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defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
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4122</ulink>, nor a Microsoft GUID; however, starting with systemd
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v30, newly generated machine IDs do qualify as v4 UUIDs.</para>
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<para>In order to maintain compatibility with existing
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installations, an application requiring a UUID should decode the
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machine ID, and then apply the following operations to turn it
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into a valid OSF v4 UUID. With <literal>id</literal> being an
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unsigned character array:</para>
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<programlisting>/* Set UUID version to 4 --- truly random generation */
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id[6] = (id[6] & 0x0F) | 0x40;
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/* Set the UUID variant to DCE */
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id[8] = (id[8] & 0x3F) | 0x80;</programlisting>
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<para>(This code is inspired by
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<literal>generate_random_uuid()</literal> of
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<filename>drivers/char/random.c</filename> from the Linux kernel
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sources.)</para>
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</refsect1>
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<refsect1>
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<title>History</title>
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<para>The simple configuration file format of
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<filename>/etc/machine-id</filename> originates in the
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<filename>/var/lib/dbus/machine-id</filename> file introduced by
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D-Bus. In fact, this latter file might be a symlink to
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<filename>/etc/machine-id</filename>.</para>
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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>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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