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 Variant 1 Version 4 compatible IDs.
 105     <function>sd_id128_get_machine()</function> will also return a UUID Variant 1 Version 4 compatible ID on
 106     new installations but might not on older. It is possible to convert the machine ID non-reversibly into a
 107     UUID Variant 1 Version 4 compatible one. For more information, see
 108     <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>. It is
 109     hence guaranteed that these 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> and
 133           <function>sd_id128_get_machine_app_specific()</function> when <filename>/etc/machine-id</filename>
 134           is missing.</para></listitem>
 135         </varlistentry>
 136
 137         <varlistentry>
 138           <term><constant>-ENOMEDIUM</constant></term>
 139
 140           <listitem><para>Returned by <function>sd_id128_get_machine()</function> and
 141           <function>sd_id128_get_machine_app_specific()</function> when <filename>/etc/machine-id</filename>
 142           is empty or all zeros. Also returned by <function>sd_id128_get_invocation()</function> when the
 143           invocation ID is all zeros.</para></listitem>
 144         </varlistentry>
 145
 146         <varlistentry>
 147           <term><constant>-ENOPKG</constant></term>
 148
 149           <listitem><para>Returned by <function>sd_id128_get_machine()</function> and
 150           <function>sd_id128_get_machine_app_specific()</function> when the content of
 151           <filename>/etc/machine-id</filename> is <literal>uninitialized</literal>.</para></listitem>
 152         </varlistentry>
 153
 154         <varlistentry>
 155           <term><constant>-ENOSYS</constant></term>
 156
 157           <listitem><para>Returned by <function>sd_id128_get_boot()</function> and
 158           <function>sd_id128_get_boot_app_specific()</function> when <filename>/proc/</filename> is not
 159           mounted.</para></listitem>
 160         </varlistentry>
 161
 162         <varlistentry>
 163           <term><constant>-ENXIO</constant></term>
 164
 165           <listitem><para>Returned by <function>sd_id128_get_invocation()</function> if no invocation ID is
 166           set.</para></listitem>
 167         </varlistentry>
 168
 169         <varlistentry>
 170           <term><constant>-EUCLEAN</constant></term>
 171
 172           <listitem><para>Returned by any of the functions described here when the configured value has
 173           invalid format.</para></listitem>
 174         </varlistentry>
 175
 176         <varlistentry>
 177           <term><constant>-EPERM</constant></term>
 178
 179           <listitem><para>Requested information could not be retrieved because of insufficient permissions.
 180           </para></listitem>
 181         </varlistentry>
 182       </variablelist>
 183     </refsect2>
 184   </refsect1>
 185
 186   <xi:include href="libsystemd-pkgconfig.xml" />
 187
 188   <refsect1>
 189     <title>Examples</title>
 190
 191     <example>
 192       <title>Application-specific machine ID</title>
 193
 194       <para>First, generate the application ID:</para>
 195       <programlisting>$ systemd-id128 -p new
 196 As string:
 197 c273277323db454ea63bb96e79b53e97
 198
 199 As UUID:
 200 c2732773-23db-454e-a63b-b96e79b53e97
 201
 202 As man:sd-id128(3) macro:
 203 #define MESSAGE_XYZ SD_ID128_MAKE(c2,73,27,73,23,db,45,4e,a6,3b,b9,6e,79,b5,3e,97)
 204 ...
 205 </programlisting>
 206
 207       <para>Then use the new identifier in an example application:</para>
 208
 209       <programlisting><xi:include href="id128-app-specific.c" parse="text" /></programlisting>
 210     </example>
 211   </refsect1>
 212
 213   <refsect1>
 214     <title>See Also</title>
 215
 216     <para>
 217       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 218       <citerefentry><refentrytitle>systemd-id128</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 219       <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 220       <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 221       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 222       <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 223       <citerefentry project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>
 224     </para>
 225   </refsect1>
 226
 227 </refentry>