man/sd_id128_get_machine.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4
   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7
   8   This file is part of systemd.
   9
  10   Copyright 2012 Lennart Poettering
  11 -->
  12
  13 <refentry id="sd_id128_get_machine" xmlns:xi="http://www.w3.org/2001/XInclude">
  14
  15   <refentryinfo>
  16     <title>sd_id128_get_machine</title>
  17     <productname>systemd</productname>
  18
  19     <authorgroup>
  20       <author>
  21         <contrib>Developer</contrib>
  22         <firstname>Lennart</firstname>
  23         <surname>Poettering</surname>
  24         <email>lennart@poettering.net</email>
  25       </author>
  26     </authorgroup>
  27   </refentryinfo>
  28
  29   <refmeta>
  30     <refentrytitle>sd_id128_get_machine</refentrytitle>
  31     <manvolnum>3</manvolnum>
  32   </refmeta>
  33
  34   <refnamediv>
  35     <refname>sd_id128_get_machine</refname>
  36     <refname>sd_id128_get_machine_app_specific</refname>
  37     <refname>sd_id128_get_boot</refname>
  38     <refname>sd_id128_get_invocation</refname>
  39     <refpurpose>Retrieve 128-bit IDs</refpurpose>
  40   </refnamediv>
  41
  42   <refsynopsisdiv>
  43     <funcsynopsis>
  44       <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
  45
  46       <funcprototype>
  47         <funcdef>int <function>sd_id128_get_machine</function></funcdef>
  48         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  49       </funcprototype>
  50
  51       <funcprototype>
  52         <funcdef>int <function>sd_id128_get_machine_app_specific</function></funcdef>
  53         <paramdef>sd_id128_t <parameter>app_id</parameter></paramdef>
  54         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  55       </funcprototype>
  56
  57       <funcprototype>
  58         <funcdef>int <function>sd_id128_get_boot</function></funcdef>
  59         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  60       </funcprototype>
  61
  62       <funcprototype>
  63         <funcdef>int <function>sd_id128_get_invocation</function></funcdef>
  64         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  65       </funcprototype>
  66
  67     </funcsynopsis>
  68   </refsynopsisdiv>
  69
  70   <refsect1>
  71     <title>Description</title>
  72
  73     <para><function>sd_id128_get_machine()</function> returns the machine ID of the executing host. This reads and
  74     parses the <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  75     file. This function caches the machine ID internally to make retrieving the machine ID a cheap operation. This ID
  76     may be used wherever a unique identifier for the local system is needed. However, it is recommended to use this ID
  77     as-is only in trusted environments. In untrusted environments it is recommended to derive an application specific
  78     ID from this machine ID, in an irreversable (cryptographically secure) way. To make this easy
  79     <function>sd_id128_get_machine_app_specific()</function> is provided, see below.</para>
  80
  81     <para><function>sd_id128_get_machine_app_specific()</function> is similar to
  82     <function>sd_id128_get_machine()</function>, but retrieves a machine ID that is specific to the application that is
  83     identified by the indicated application ID. It is recommended to use this function instead of
  84     <function>sd_id128_get_machine()</function> when passing an ID to untrusted environments, in order to make sure
  85     that the original machine ID may not be determined externally. The application-specific ID should be generated via
  86     a tool like <command>journalctl --new-id128</command>, and may be compiled into the application. This function will
  87     return the same application-specific ID for each combination of machine ID and application ID. Internally, this
  88     function calculates HMAC-SHA256 of the application ID, keyed by the machine ID.</para>
  89
  90     <para><function>sd_id128_get_boot()</function> returns the boot ID
  91     of the executing kernel. This reads and parses the
  92     <filename>/proc/sys/kernel/random/boot_id</filename> file exposed
  93     by the kernel. It is randomly generated early at boot and is
  94     unique for every running kernel instance. See
  95     <citerefentry project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>
  96     for more information. This function also internally caches the
  97     returned ID to make this call a cheap operation.</para>
  98
  99     <para><function>sd_id128_get_invocation()</function> returns the invocation ID of the currently executed
 100     service. In its current implementation, this reads and parses the <varname>$INVOCATION_ID</varname> environment
 101     variable that the service manager sets when activating a service, see
 102     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
 103     ID is cached internally. In future a different mechanism to determine the invocation ID may be added.</para>
 104
 105     <para>Note that <function>sd_id128_get_machine_app_specific()</function>, <function>sd_id128_get_boot()</function>
 106     and <function>sd_id128_get_invocation()</function> always return UUID v4 compatible IDs.
 107     <function>sd_id128_get_machine()</function> will also return a UUID v4-compatible ID on new installations but might
 108     not on older.  It is possible to convert the machine ID into a UUID v4-compatible one. For more information, see
 109     <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
 110
 111     <para>For more information about the <literal>sd_id128_t</literal>
 112     type see
 113     <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 114   </refsect1>
 115
 116   <refsect1>
 117     <title>Return Value</title>
 118
 119     <para>Those calls return 0 on success (in which case <parameter>ret</parameter> is filled in),
 120     or a negative errno-style error code. In particular, <function>sd_id128_get_machine()</function>
 121     and <function>sd_id128_get_machine_app_specific()</function> return <constant>-ENOENT</constant>
 122     if <filename>/etc/machine-id</filename> is missing, and <constant>-ENOMEDIUM</constant> if is
 123     empty or all zeros.</para>
 124   </refsect1>
 125
 126   <xi:include href="libsystemd-pkgconfig.xml" />
 127
 128   <refsect1>
 129     <title>Examples</title>
 130
 131     <example>
 132       <title>Application-specific machine ID</title>
 133
 134       <para>Here's a simple example for an application specific machine ID:</para>
 135
 136       <programlisting>#include &lt;systemd/sd-id128.h&gt;
 137 #include &lt;stdio.h&gt;
 138
 139 #define OUR_APPLICATION_ID SD_ID128_MAKE(c2,73,27,73,23,db,45,4e,a6,3b,b9,6e,79,b5,3e,97)
 140
 141 int main(int argc, char *argv[]) {
 142         sd_id128_t id;
 143         sd_id128_get_machine_app_specific(OUR_APPLICATION_ID, &amp;id);
 144         printf("Our application ID: " SD_ID128_FORMAT_STR "\n", SD_ID128_FORMAT_VAL(id));
 145         return 0;
 146 }</programlisting>
 147     </example>
 148   </refsect1>
 149
 150   <refsect1>
 151     <title>See Also</title>
 152
 153     <para>
 154       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 155       <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 156       <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 157       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 158       <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 159       <citerefentry project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>
 160     </para>
 161   </refsect1>
 162
 163 </refentry>