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-or-later -->
   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 irreversible (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>,
 103     <function>sd_id128_get_boot()</function>, <function>sd_id128_get_boot_app_specific()</function>, and
 104     <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
 106     but might not on older. It is possible to convert the machine ID into a UUID v4-compatible one. For more
 107     information, see
 108     <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>. It is
 109     hence guaranteed that thes functions will never return the ID consisting of all zero or all one bits
 110     (<constant>SD_ID128_NULL</constant>, <constant>SD_ID128_ALLF</constant>) — with the possible exception of
 111     <function>sd_id128_get_machine()</function>, as mentioned.</para>
 112
 113     <para>For more information about the <literal>sd_id128_t</literal>
 114     type see
 115     <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 116   </refsect1>
 117
 118   <refsect1>
 119     <title>Return Value</title>
 120
 121     <para>Those calls return 0 on success (in which case <parameter>ret</parameter> is filled in),
 122     or a negative errno-style error code.</para>
 123
 124     <refsect2>
 125       <title>Errors</title>
 126       <para>Returned errors may indicate the following problems:</para>
 127
 128       <variablelist>
 129         <varlistentry>
 130           <term><constant>-ENOENT</constant></term>
 131
 132           <listitem><para>Returned by <function>sd_id128_get_machine()</function>,
 133           <function>sd_id128_get_machine_app_specific()</function>, and
 134           <function>sd_id128_get_boot_app_specific()</function> when <filename>/etc/machine-id</filename> is
 135           missing.</para></listitem>
 136         </varlistentry>
 137
 138         <varlistentry>
 139           <term><constant>-ENOMEDIUM</constant></term>
 140
 141           <listitem><para>Returned by <function>sd_id128_get_machine()</function>,
 142           <function>sd_id128_get_machine_app_specific()</function>, and
 143           <function>sd_id128_get_boot_app_specific()</function> when <filename>/etc/machine-id</filename> is
 144           empty or all zeros.</para></listitem>
 145         </varlistentry>
 146
 147         <varlistentry>
 148           <term><constant>-ENXIO</constant></term>
 149
 150           <listitem><para>Returned by <function>sd_id128_get_invocation()</function> if no invocation ID is
 151           set.</para></listitem>
 152         </varlistentry>
 153
 154         <varlistentry>
 155           <term><constant>-EIO</constant></term>
 156
 157           <listitem><para>Returned by any of the functions described here when the configured value has
 158           invalid format.</para></listitem>
 159         </varlistentry>
 160
 161         <varlistentry>
 162           <term><constant>-EPERM</constant></term>
 163
 164           <listitem><para>Requested information could not be retrieved because of insufficient permissions.
 165           </para></listitem>
 166         </varlistentry>
 167       </variablelist>
 168     </refsect2>
 169   </refsect1>
 170
 171   <xi:include href="libsystemd-pkgconfig.xml" />
 172
 173   <refsect1>
 174     <title>Examples</title>
 175
 176     <example>
 177       <title>Application-specific machine ID</title>
 178
 179       <para>First, generate the application ID:</para>
 180       <programlisting>$ systemd-id128 -p new
 181 As string:
 182 c273277323db454ea63bb96e79b53e97
 183
 184 As UUID:
 185 c2732773-23db-454e-a63b-b96e79b53e97
 186
 187 As man:sd-id128(3) macro:
 188 #define MESSAGE_XYZ SD_ID128_MAKE(c2,73,27,73,23,db,45,4e,a6,3b,b9,6e,79,b5,3e,97)
 189 ...
 190 </programlisting>
 191
 192       <para>Then use the new identifier in an example application:</para>
 193
 194       <programlisting><xi:include href="id128-app-specific.c" parse="text" /></programlisting>
 195     </example>
 196   </refsect1>
 197
 198   <refsect1>
 199     <title>See Also</title>
 200
 201     <para>
 202       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 203       <citerefentry><refentrytitle>systemd-id128</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 204       <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 205       <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 206       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 207       <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 208       <citerefentry project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>
 209     </para>
 210   </refsect1>
 211
 212 </refentry>