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