157 lines
7.9 KiB
XML
157 lines
7.9 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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SPDX-License-Identifier: LGPL-2.1+
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-->
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<refentry id="sd_id128_get_machine" xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_id128_get_machine</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_id128_get_machine</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_id128_get_machine</refname>
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<refname>sd_id128_get_machine_app_specific</refname>
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<refname>sd_id128_get_boot</refname>
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<refname>sd_id128_get_boot_app_specific</refname>
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<refname>sd_id128_get_invocation</refname>
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<refpurpose>Retrieve 128-bit IDs</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-id128.h></funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>sd_id128_get_machine</function></funcdef>
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<paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_id128_get_machine_app_specific</function></funcdef>
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<paramdef>sd_id128_t <parameter>app_id</parameter></paramdef>
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<paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_id128_get_boot</function></funcdef>
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<paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_id128_get_boot_app_specific</function></funcdef>
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<paramdef>sd_id128_t <parameter>app_id</parameter></paramdef>
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<paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_id128_get_invocation</function></funcdef>
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<paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><function>sd_id128_get_machine()</function> returns the machine ID of the executing host. This reads and
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parses the <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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file. This function caches the machine ID internally to make retrieving the machine ID a cheap operation. This ID
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may be used wherever a unique identifier for the local system is needed. However, it is recommended to use this ID
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as-is only in trusted environments. In untrusted environments it is recommended to derive an application specific
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ID from this machine ID, in an irreversable (cryptographically secure) way. To make this easy
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<function>sd_id128_get_machine_app_specific()</function> is provided, see below.</para>
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<para><function>sd_id128_get_machine_app_specific()</function> is similar to
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<function>sd_id128_get_machine()</function>, but retrieves a machine ID that is specific to the application that is
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identified by the indicated application ID. It is recommended to use this function instead of
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<function>sd_id128_get_machine()</function> when passing an ID to untrusted environments, in order to make sure
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that the original machine ID may not be determined externally. This way, the ID used by the application remains
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stable on a given machine, but cannot be easily correlated with IDs used in other applications on the same
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machine. The application-specific ID should be generated via a tool like <command>journalctl --new-id128</command>,
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and may be compiled into the application. This function will return the same application-specific ID for each
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combination of machine ID and application ID. Internally, this function calculates HMAC-SHA256 of the application
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ID, keyed by the machine ID.</para>
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<para><function>sd_id128_get_boot()</function> returns the boot ID of the executing kernel. This reads and parses
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the <filename>/proc/sys/kernel/random/boot_id</filename> file exposed by the kernel. It is randomly generated early
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at boot and is unique for every running kernel instance. See <citerefentry
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project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry> for more
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information. This function also internally caches the returned ID to make this call a cheap operation. It is
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recommended to use this ID as-is only in trusted environments. In untrusted environments it is recommended to
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derive an application specific ID using <function>sd_id128_get_machine_app_specific()</function>, see below.</para>
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<para><function>sd_id128_get_boot_app_specific()</function> is analogous to
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<function>sd_id128_get_machine_app_specific()</function> but returns an ID that changes between boots. Some
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machines may be used for a long time without rebooting, hence the boot ID may remain constant for a long time, and
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has properties similar to the machine ID during that time.</para>
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<para><function>sd_id128_get_invocation()</function> returns the invocation ID of the currently executed
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service. In its current implementation, this reads and parses the <varname>$INVOCATION_ID</varname> environment
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variable that the service manager sets when activating a service, see
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
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ID is cached internally. In future a different mechanism to determine the invocation ID may be added.</para>
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<para>Note that <function>sd_id128_get_machine_app_specific()</function>, <function>sd_id128_get_boot()</function>,
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<function>sd_id128_get_boot_app_specific()</function>, and <function>sd_id128_get_invocation()</function> always
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return UUID v4 compatible IDs. <function>sd_id128_get_machine()</function> will also return a UUID v4-compatible
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ID on new installations but might not on older. It is possible to convert the machine ID into a UUID v4-compatible
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one. For more information, see
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<citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
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<para>For more information about the <literal>sd_id128_t</literal>
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type see
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<citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>Those calls return 0 on success (in which case <parameter>ret</parameter> is filled in),
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or a negative errno-style error code. In particular,
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<function>sd_id128_get_machine()</function>,
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<function>sd_id128_get_machine_app_specific()</function>, and
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<function>sd_id128_get_boot_app_specific()</function> return <constant>-ENOENT</constant> if
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<filename>/etc/machine-id</filename> is missing, and <constant>-ENOMEDIUM</constant> if is empty
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or all zeros.</para>
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</refsect1>
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<xi:include href="libsystemd-pkgconfig.xml" />
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<refsect1>
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<title>Examples</title>
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<example>
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<title>Application-specific machine ID</title>
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<para>Here's a simple example for an application specific machine ID:</para>
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<programlisting><xi:include href="id128-app-specific.c" parse="text" /></programlisting>
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</example>
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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>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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