man/sd_id128_get_machine.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1+ -->
   5
   6 <refentry id="sd_id128_get_machine" xmlns:xi="http://www.w3.org/2001/XInclude">
   7
   8   <refentryinfo>
   9     <title>sd_id128_get_machine</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>sd_id128_get_machine</refentrytitle>
  15     <manvolnum>3</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>sd_id128_get_machine</refname>
  20     <refname>sd_id128_get_machine_app_specific</refname>
  21     <refname>sd_id128_get_boot</refname>
  22     <refname>sd_id128_get_boot_app_specific</refname>
  23     <refname>sd_id128_get_invocation</refname>
  24     <refpurpose>Retrieve 128-bit IDs</refpurpose>
  25   </refnamediv>
  26
  27   <refsynopsisdiv>
  28     <funcsynopsis>
  29       <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
  30
  31       <funcprototype>
  32         <funcdef>int <function>sd_id128_get_machine</function></funcdef>
  33         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  34       </funcprototype>
  35
  36       <funcprototype>
  37         <funcdef>int <function>sd_id128_get_machine_app_specific</function></funcdef>
  38         <paramdef>sd_id128_t <parameter>app_id</parameter></paramdef>
  39         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  40       </funcprototype>
  41
  42       <funcprototype>
  43         <funcdef>int <function>sd_id128_get_boot</function></funcdef>
  44         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  45       </funcprototype>
  46
  47       <funcprototype>
  48         <funcdef>int <function>sd_id128_get_boot_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_invocation</function></funcdef>
  55         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  56       </funcprototype>
  57
  58     </funcsynopsis>
  59   </refsynopsisdiv>
  60
  61   <refsect1>
  62     <title>Description</title>
  63
  64     <para><function>sd_id128_get_machine()</function> returns the machine ID of the executing host. This reads and
  65     parses the <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  66     file. This function caches the machine ID internally to make retrieving the machine ID a cheap operation. This ID
  67     may be used wherever a unique identifier for the local system is needed. However, it is recommended to use this ID
  68     as-is only in trusted environments. In untrusted environments it is recommended to derive an application specific
  69     ID from this machine ID, in an irreversable (cryptographically secure) way. To make this easy
  70     <function>sd_id128_get_machine_app_specific()</function> is provided, see below.</para>
  71
  72     <para><function>sd_id128_get_machine_app_specific()</function> is similar to
  73     <function>sd_id128_get_machine()</function>, but retrieves a machine ID that is specific to the application that is
  74     identified by the indicated application ID. It is recommended to use this function instead of
  75     <function>sd_id128_get_machine()</function> when passing an ID to untrusted environments, in order to make sure
  76     that the original machine ID may not be determined externally. This way, the ID used by the application remains
  77     stable on a given machine, but cannot be easily correlated with IDs used in other applications on the same
  78     machine. The application-specific ID should be generated via a tool like <command>systemd-id128 new</command>,
  79     and may be compiled into the application. This function will return the same application-specific ID for each
  80     combination of machine ID and application ID. Internally, this function calculates HMAC-SHA256 of the application
  81     ID, keyed by the machine ID.</para>
  82
  83     <para><function>sd_id128_get_boot()</function> returns the boot ID of the executing kernel. This reads and parses
  84     the <filename>/proc/sys/kernel/random/boot_id</filename> file exposed by the kernel. It is randomly generated early
  85     at boot and is unique for every running kernel instance. See <citerefentry
  86     project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry> for more
  87     information. This function also internally caches the returned ID to make this call a cheap operation. It is
  88     recommended to use this ID as-is only in trusted environments. In untrusted environments it is recommended to
  89     derive an application specific ID using <function>sd_id128_get_machine_app_specific()</function>, see below.</para>
  90
  91     <para><function>sd_id128_get_boot_app_specific()</function> is analogous to
  92     <function>sd_id128_get_machine_app_specific()</function> but returns an ID that changes between boots. Some
  93     machines may be used for a long time without rebooting, hence the boot ID may remain constant for a long time, and
  94     has properties similar to the machine ID during that time.</para>
  95
  96     <para><function>sd_id128_get_invocation()</function> returns the invocation ID of the currently executed
  97     service. In its current implementation, this reads and parses the <varname>$INVOCATION_ID</varname> environment
  98     variable that the service manager sets when activating a service, see
  99     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
 100     ID is cached internally. In future a different mechanism to determine the invocation ID may be added.</para>
 101
 102     <para>Note that <function>sd_id128_get_machine_app_specific()</function>, <function>sd_id128_get_boot()</function>,
 103     <function>sd_id128_get_boot_app_specific()</function>, and <function>sd_id128_get_invocation()</function> always
 104     return UUID v4 compatible IDs.  <function>sd_id128_get_machine()</function> will also return a UUID v4-compatible
 105     ID on new installations but might not on older. It is possible to convert the machine ID into a UUID v4-compatible
 106     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,
 119     <function>sd_id128_get_machine()</function>,
 120     <function>sd_id128_get_machine_app_specific()</function>, and
 121     <function>sd_id128_get_boot_app_specific()</function> return <constant>-ENOENT</constant> if
 122     <filename>/etc/machine-id</filename> is missing, and <constant>-ENOMEDIUM</constant> if
 123     <filename>/etc/machine-id</filename> is 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>First, generate the application ID:</para>
 135       <programlisting>$ systemd-id128 -p new
 136 As string:
 137 c273277323db454ea63bb96e79b53e97
 138
 139 As UUID:
 140 c2732773-23db-454e-a63b-b96e79b53e97
 141
 142 As man:sd-id128(3) macro:
 143 #define MESSAGE_XYZ SD_ID128_MAKE(c2,73,27,73,23,db,45,4e,a6,3b,b9,6e,79,b5,3e,97)
 144 ...
 145 </programlisting>
 146
 147       <para>Then use the new identifier in an example application:</para>
 148
 149       <programlisting><xi:include href="id128-app-specific.c" parse="text" /></programlisting>
 150     </example>
 151   </refsect1>
 152
 153   <refsect1>
 154     <title>See Also</title>
 155
 156     <para>
 157       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 158       <citerefentry><refentrytitle>systemd-id128</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 159       <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 160       <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 161       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 162       <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 163       <citerefentry project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>
 164     </para>
 165   </refsect1>
 166
 167 </refentry>