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_boot_app_specific</refname>
  26     <refname>sd_id128_get_invocation</refname>
  27     <refpurpose>Retrieve 128-bit IDs</refpurpose>
  28   </refnamediv>
  29
  30   <refsynopsisdiv>
  31     <funcsynopsis>
  32       <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
  33
  34       <funcprototype>
  35         <funcdef>int <function>sd_id128_get_machine</function></funcdef>
  36         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  37       </funcprototype>
  38
  39       <funcprototype>
  40         <funcdef>int <function>sd_id128_get_machine_app_specific</function></funcdef>
  41         <paramdef>sd_id128_t <parameter>app_id</parameter></paramdef>
  42         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  43       </funcprototype>
  44
  45       <funcprototype>
  46         <funcdef>int <function>sd_id128_get_boot</function></funcdef>
  47         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  48       </funcprototype>
  49
  50       <funcprototype>
  51         <funcdef>int <function>sd_id128_get_boot_app_specific</function></funcdef>
  52         <paramdef>sd_id128_t <parameter>app_id</parameter></paramdef>
  53         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  54       </funcprototype>
  55
  56       <funcprototype>
  57         <funcdef>int <function>sd_id128_get_invocation</function></funcdef>
  58         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  59       </funcprototype>
  60
  61     </funcsynopsis>
  62   </refsynopsisdiv>
  63
  64   <refsect1>
  65     <title>Description</title>
  66
  67     <para><function>sd_id128_get_machine()</function> returns the machine ID of the executing host. This reads and
  68     parses the <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  69     file. This function caches the machine ID internally to make retrieving the machine ID a cheap operation. This ID
  70     may be used wherever a unique identifier for the local system is needed. However, it is recommended to use this ID
  71     as-is only in trusted environments. In untrusted environments it is recommended to derive an application specific
  72     ID from this machine ID, in an irreversable (cryptographically secure) way. To make this easy
  73     <function>sd_id128_get_machine_app_specific()</function> is provided, see below.</para>
  74
  75     <para><function>sd_id128_get_machine_app_specific()</function> is similar to
  76     <function>sd_id128_get_machine()</function>, but retrieves a machine ID that is specific to the application that is
  77     identified by the indicated application ID. It is recommended to use this function instead of
  78     <function>sd_id128_get_machine()</function> when passing an ID to untrusted environments, in order to make sure
  79     that the original machine ID may not be determined externally. This way, the ID used by the application remains
  80     stable on a given machine, but cannot be easily correlated with IDs used in other applications on the same
  81     machine. The application-specific ID should be generated via a tool like <command>systemd-id128 new</command>,
  82     and may be compiled into the application. This function will return the same application-specific ID for each
  83     combination of machine ID and application ID. Internally, this function calculates HMAC-SHA256 of the application
  84     ID, keyed by the machine ID.</para>
  85
  86     <para><function>sd_id128_get_boot()</function> returns the boot ID of the executing kernel. This reads and parses
  87     the <filename>/proc/sys/kernel/random/boot_id</filename> file exposed by the kernel. It is randomly generated early
  88     at boot and is unique for every running kernel instance. See <citerefentry
  89     project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry> for more
  90     information. This function also internally caches the returned ID to make this call a cheap operation. It is
  91     recommended to use this ID as-is only in trusted environments. In untrusted environments it is recommended to
  92     derive an application specific ID using <function>sd_id128_get_machine_app_specific()</function>, see below.</para>
  93
  94     <para><function>sd_id128_get_boot_app_specific()</function> is analogous to
  95     <function>sd_id128_get_machine_app_specific()</function> but returns an ID that changes between boots. Some
  96     machines may be used for a long time without rebooting, hence the boot ID may remain constant for a long time, and
  97     has properties similar to the machine ID during that time.</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     <function>sd_id128_get_boot_app_specific()</function>, and <function>sd_id128_get_invocation()</function> always
 107     return UUID v4 compatible IDs.  <function>sd_id128_get_machine()</function> will also return a UUID v4-compatible
 108     ID on new installations but might not on older. It is possible to convert the machine ID into a UUID v4-compatible
 109     one. For more information, see
 110     <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
 111
 112     <para>For more information about the <literal>sd_id128_t</literal>
 113     type see
 114     <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 115   </refsect1>
 116
 117   <refsect1>
 118     <title>Return Value</title>
 119
 120     <para>Those calls return 0 on success (in which case <parameter>ret</parameter> is filled in),
 121     or a negative errno-style error code. In particular,
 122     <function>sd_id128_get_machine()</function>,
 123     <function>sd_id128_get_machine_app_specific()</function>, and
 124     <function>sd_id128_get_boot_app_specific()</function> return <constant>-ENOENT</constant> if
 125     <filename>/etc/machine-id</filename> is missing, and <constant>-ENOMEDIUM</constant> if
 126     <filename>/etc/machine-id</filename> is empty or all zeros.</para>
 127   </refsect1>
 128
 129   <xi:include href="libsystemd-pkgconfig.xml" />
 130
 131   <refsect1>
 132     <title>Examples</title>
 133
 134     <example>
 135       <title>Application-specific machine ID</title>
 136
 137       <para>First, generate the application ID:</para>
 138       <programlisting>$ systemd-id128 -p new
 139 As string:
 140 c273277323db454ea63bb96e79b53e97
 141
 142 As UUID:
 143 c2732773-23db-454e-a63b-b96e79b53e97
 144
 145 As man:sd-id128(3) macro:
 146 #define MESSAGE_XYZ SD_ID128_MAKE(c2,73,27,73,23,db,45,4e,a6,3b,b9,6e,79,b5,3e,97)
 147 ...
 148 </programlisting>
 149
 150       <para>Then use the new identifier in an example application:</para>
 151
 152       <programlisting><xi:include href="id128-app-specific.c" parse="text" /></programlisting>
 153     </example>
 154   </refsect1>
 155
 156   <refsect1>
 157     <title>See Also</title>
 158
 159     <para>
 160       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 161       <citerefentry><refentrytitle>systemd-id128</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 162       <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 163       <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 164       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 165       <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 166       <citerefentry project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>
 167     </para>
 168   </refsect1>
 169
 170 </refentry>