man/sd_id128_get_machine.xml

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  "http://www.oasis-open.org/docbook/xml/4.2/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_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_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 irreversable (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_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 calculates HMAC-SHA256 of the application
    ID, keyed by the machine ID.</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_machine_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 reads and parses the <varname>$INVOCATION_ID</varname> environment
    variable that the service manager sets when activating a service, 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 v4 compatible IDs.  <function>sd_id128_get_machine()</function> will also return a UUID v4-compatible
    ID on new installations but might not on older. It is possible to convert the machine ID into a UUID v4-compatible
    one. For more information, see
    <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</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. In particular,
    <function>sd_id128_get_machine()</function>,
    <function>sd_id128_get_machine_app_specific()</function>, and
    <function>sd_id128_get_boot_app_specific()</function> return <constant>-ENOENT</constant> if
    <filename>/etc/machine-id</filename> is missing, and <constant>-ENOMEDIUM</constant> if is empty
    or all zeros.</para>
  </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>See Also</title>

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

</refentry>