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