systemd/man/sd_id128_get_machine.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
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<refentry id="sd_id128_get_machine" xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>sd_id128_get_machine</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_id128_get_machine</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_id128_get_machine</refname>
    <refname>sd_id128_get_app_specific</refname>
    <refname>sd_id128_get_machine_app_specific</refname>
    <refname>sd_id128_get_boot</refname>
    <refname>sd_id128_get_boot_app_specific</refname>
    <refname>sd_id128_get_invocation</refname>
    <refpurpose>Retrieve 128-bit IDs</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>

      <funcprototype>
        <funcdef>int <function>sd_id128_get_machine</function></funcdef>
        <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_id128_get_app_specific</function></funcdef>
        <paramdef>sd_id128_t <parameter>base</parameter></paramdef>
        <paramdef>sd_id128_t <parameter>app_id</parameter></paramdef>
        <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_id128_get_machine_app_specific</function></funcdef>
        <paramdef>sd_id128_t <parameter>app_id</parameter></paramdef>
        <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_id128_get_boot</function></funcdef>
        <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_id128_get_boot_app_specific</function></funcdef>
        <paramdef>sd_id128_t <parameter>app_id</parameter></paramdef>
        <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_id128_get_invocation</function></funcdef>
        <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
      </funcprototype>

    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><function>sd_id128_get_machine()</function> returns the machine ID of the executing host. This reads and
    parses the <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    file. This function caches the machine ID internally to make retrieving the machine ID a cheap operation. This ID
    may be used wherever a unique identifier for the local system is needed. However, it is recommended to use this ID
    as-is only in trusted environments. In untrusted environments it is recommended to derive an application specific
    ID from this machine ID, in an irreversible (cryptographically secure) way. To make this easy
    <function>sd_id128_get_machine_app_specific()</function> is provided, see below.</para>

    <para><function>sd_id128_get_app_specific()</function> returns a machine ID that is a combination of the
    <parameter>base</parameter> and <parameter>app_id</parameter> parameters. Internally, this function
    calculates HMAC-SHA256 of the <parameter>app_id</parameter> parameter keyed by the
    <parameter>base</parameter> parameter, and truncates this result to fit in
    <structname>sd_id128_t</structname> and turns it into a valid Variant 1 Version 4 UUID, in accordance
    with <ulink url="https://tools.ietf.org/html/rfc4122">RFC 4122</ulink>. Neither of the two input
    parameters can be calculated from the output parameter <parameter>ret</parameter>.</para>

    <para><function>sd_id128_get_machine_app_specific()</function> is similar to
    <function>sd_id128_get_machine()</function>, but retrieves a machine ID that is specific to the
    application that is identified by the indicated application ID. It is recommended to use this function
    instead of <function>sd_id128_get_machine()</function> when passing an ID to untrusted environments, in
    order to make sure that the original machine ID may not be determined externally. This way, the ID used
    by the application remains stable on a given machine, but cannot be easily correlated with IDs used in
    other applications on the same machine. The application-specific ID should be generated via a tool like
    <command>systemd-id128 new</command>, and may be compiled into the application. This function will return
    the same application-specific ID for each combination of machine ID and application ID. Internally, this
    function calls <function>sd_id128_get_app_specific()</function> with the result from
    <function>sd_id128_get_machine()</function> and the <parameter>app_id</parameter> parameter.</para>

    <para><function>sd_id128_get_boot()</function> returns the boot ID of the executing kernel. This reads and parses
    the <filename>/proc/sys/kernel/random/boot_id</filename> file exposed by the kernel. It is randomly generated early
    at boot and is unique for every running kernel instance. See <citerefentry
    project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry> for more
    information. This function also internally caches the returned ID to make this call a cheap operation. It is
    recommended to use this ID as-is only in trusted environments. In untrusted environments it is recommended to
    derive an application specific ID using <function>sd_id128_get_boot_app_specific()</function>, see below.</para>

    <para><function>sd_id128_get_boot_app_specific()</function> is analogous to
    <function>sd_id128_get_machine_app_specific()</function>, but returns an ID that changes between
    boots. Some machines may be used for a long time without rebooting, hence the boot ID may remain constant
    for a long time, and has properties similar to the machine ID during that time.</para>

    <para><function>sd_id128_get_invocation()</function> returns the invocation ID of the currently executed
    service. In its current implementation, this tries to read and parse the following:
    <itemizedlist>
      <listitem>
        <para>The <varname>$INVOCATION_ID</varname> environment variable that the service manager sets when
        activating a service.</para>
      </listitem>
      <listitem>
        <para>An entry in the kernel keyring that the system service manager sets when activating a service.
        </para>
      </listitem>
    </itemizedlist>
    See <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for details. The ID is cached internally. In future a different mechanism to determine the invocation ID
    may be added.</para>

    <para>Note that <function>sd_id128_get_machine_app_specific()</function>,
    <function>sd_id128_get_boot()</function>, <function>sd_id128_get_boot_app_specific()</function>, and
    <function>sd_id128_get_invocation()</function> always return UUID Variant 1 Version 4 compatible IDs.
    <function>sd_id128_get_machine()</function> will also return a UUID Variant 1 Version 4 compatible ID on
    new installations but might not on older. It is possible to convert the machine ID non-reversibly into a
    UUID Variant 1 Version 4 compatible one. For more information, see
    <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>. It is
    hence guaranteed that these functions will never return the ID consisting of all zero or all one bits
    (<constant>SD_ID128_NULL</constant>, <constant>SD_ID128_ALLF</constant>) — with the possible exception of
    <function>sd_id128_get_machine()</function>, as mentioned.</para>

    <para>For more information about the <literal>sd_id128_t</literal>
    type see
    <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
  </refsect1>

  <refsect1>
    <title>Return Value</title>

    <para>Those calls return 0 on success (in which case <parameter>ret</parameter> is filled in),
    or a negative errno-style error code.</para>

    <refsect2>
      <title>Errors</title>
      <para>Returned errors may indicate the following problems:</para>

      <variablelist>
        <varlistentry>
          <term><constant>-ENOENT</constant></term>

          <listitem><para>Returned by <function>sd_id128_get_machine()</function> and
          <function>sd_id128_get_machine_app_specific()</function> when <filename>/etc/machine-id</filename>
          is missing.</para>

          <xi:include href="version-info.xml" xpointer="v242"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><constant>-ENOMEDIUM</constant></term>

          <listitem><para>Returned by <function>sd_id128_get_machine()</function> and
          <function>sd_id128_get_machine_app_specific()</function> when <filename>/etc/machine-id</filename>
          is empty or all zeros. Also returned by <function>sd_id128_get_invocation()</function> when the
          invocation ID is all zeros.</para>

          <xi:include href="version-info.xml" xpointer="v242"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><constant>-ENOPKG</constant></term>

          <listitem><para>Returned by <function>sd_id128_get_machine()</function> and
          <function>sd_id128_get_machine_app_specific()</function> when the content of
          <filename>/etc/machine-id</filename> is <literal>uninitialized</literal>.</para>

          <xi:include href="version-info.xml" xpointer="v253"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><constant>-ENOSYS</constant></term>

          <listitem><para>Returned by <function>sd_id128_get_boot()</function> and
          <function>sd_id128_get_boot_app_specific()</function> when <filename>/proc/</filename> is not
          mounted.</para>

          <xi:include href="version-info.xml" xpointer="v253"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><constant>-ENXIO</constant></term>

          <listitem><para>Returned by <function>sd_id128_get_invocation()</function> if no invocation ID is
          set. Also returned by <function>sd_id128_get_app_specific()</function>,
          <function>sd_id128_get_machine_app_specific()</function>, and
          <function>sd_id128_get_boot_app_specific()</function> when the <parameter>app_id</parameter>
          parameter is all zeros.</para>

          <xi:include href="version-info.xml" xpointer="v242"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><constant>-EUCLEAN</constant></term>

          <listitem><para>Returned by any of the functions described here when the configured value has
          invalid format.</para>

          <xi:include href="version-info.xml" xpointer="v253"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><constant>-EPERM</constant></term>

          <listitem><para>Requested information could not be retrieved because of insufficient permissions.
          </para>

          <xi:include href="version-info.xml" xpointer="v242"/></listitem>
        </varlistentry>
      </variablelist>
    </refsect2>
  </refsect1>

  <xi:include href="libsystemd-pkgconfig.xml" />

  <refsect1>
    <title>Examples</title>

    <example>
      <title>Application-specific machine ID</title>

      <para>First, generate the application ID:</para>
      <programlisting>$ systemd-id128 -p new
As string:
c273277323db454ea63bb96e79b53e97

As UUID:
c2732773-23db-454e-a63b-b96e79b53e97

As man:sd-id128(3) macro:
#define MESSAGE_XYZ SD_ID128_MAKE(c2,73,27,73,23,db,45,4e,a6,3b,b9,6e,79,b5,3e,97)
...
</programlisting>

      <para>Then use the new identifier in an example application:</para>

      <programlisting><xi:include href="id128-app-specific.c" parse="text" /></programlisting>
    </example>
  </refsect1>

  <refsect1>
    <title>History</title>
    <para><function>sd_id128_get_machine()</function> and
    <function>sd_id128_get_boot()</function> were added in version 187.</para>
    <para><function>sd_id128_get_invocation()</function> was added in version 232.</para>
    <para><function>sd_id128_get_machine_app_specific()</function> was added in version 233.</para>
    <para><function>sd_id128_get_boot_app_specific()</function> was added in version 240.</para>
    <para><function>sd_id128_get_app_specific()</function> was added in version 255.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>

    <para><simplelist type="inline">
      <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd-id128</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry></member>
    </simplelist></para>
  </refsect1>

</refentry>