man/sd_id128_get_machine.xml

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