man/sd_id128_get_machine.xml

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  10   Copyright 2012 Lennart Poettering
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  25
  26 <refentry id="sd_id128_get_machine">
  27
  28   <refentryinfo>
  29     <title>sd_id128_get_machine</title>
  30     <productname>systemd</productname>
  31
  32     <authorgroup>
  33       <author>
  34         <contrib>Developer</contrib>
  35         <firstname>Lennart</firstname>
  36         <surname>Poettering</surname>
  37         <email>lennart@poettering.net</email>
  38       </author>
  39     </authorgroup>
  40   </refentryinfo>
  41
  42   <refmeta>
  43     <refentrytitle>sd_id128_get_machine</refentrytitle>
  44     <manvolnum>3</manvolnum>
  45   </refmeta>
  46
  47   <refnamediv>
  48     <refname>sd_id128_get_machine</refname>
  49     <refname>sd_id128_get_machine_app_specific</refname>
  50     <refname>sd_id128_get_boot</refname>
  51     <refname>sd_id128_get_invocation</refname>
  52     <refpurpose>Retrieve 128-bit IDs</refpurpose>
  53   </refnamediv>
  54
  55   <refsynopsisdiv>
  56     <funcsynopsis>
  57       <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
  58
  59       <funcprototype>
  60         <funcdef>int <function>sd_id128_get_machine</function></funcdef>
  61         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  62       </funcprototype>
  63
  64       <funcprototype>
  65         <funcdef>int <function>sd_id128_get_machine_app_specific</function></funcdef>
  66         <paramdef>sd_id128_t <parameter>app_id</parameter></paramdef>
  67         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  68       </funcprototype>
  69
  70       <funcprototype>
  71         <funcdef>int <function>sd_id128_get_boot</function></funcdef>
  72         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  73       </funcprototype>
  74
  75       <funcprototype>
  76         <funcdef>int <function>sd_id128_get_invocation</function></funcdef>
  77         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
  78       </funcprototype>
  79
  80     </funcsynopsis>
  81   </refsynopsisdiv>
  82
  83   <refsect1>
  84     <title>Description</title>
  85
  86     <para><function>sd_id128_get_machine()</function> returns the machine ID of the executing host. This reads and
  87     parses the <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  88     file. This function caches the machine ID internally to make retrieving the machine ID a cheap operation. This ID
  89     may be used wherever a unique identifier for the local system is needed. However, it is recommended to use this ID
  90     as-is only in trusted environments. In untrusted environments it is recommended to derive an application specific
  91     ID from this machine ID, in an irreversable (cryptographically secure) way. To make this easy
  92     <function>sd_id128_get_machine_app_specific()</function> is provided, see below.</para>
  93
  94     <para><function>sd_id128_get_machine_app_specific()</function> is similar to
  95     <function>sd_id128_get_machine()</function>, but retrieves a machine ID that is specific to the application that is
  96     identified by the indicated application ID. It is recommended to use this function instead of
  97     <function>sd_id128_get_machine()</function> when passing an ID to untrusted environments, in order to make sure
  98     that the original machine ID may not be determined externally. The application-specific ID should be generated via
  99     a tool like <command>journalctl --new-id128</command>, and may be compiled into the application. This function will
 100     return the same application-specific ID for each combination of machine ID and application ID. Internally, this
 101     function calculates HMAC-SHA256 of the application ID, keyed by the machine ID.</para>
 102
 103     <para><function>sd_id128_get_boot()</function> returns the boot ID
 104     of the executing kernel. This reads and parses the
 105     <filename>/proc/sys/kernel/random/boot_id</filename> file exposed
 106     by the kernel. It is randomly generated early at boot and is
 107     unique for every running kernel instance. See
 108     <citerefentry project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>
 109     for more information. This function also internally caches the
 110     returned ID to make this call a cheap operation.</para>
 111
 112     <para><function>sd_id128_get_invocation()</function> returns the invocation ID of the currently executed
 113     service. In its current implementation, this reads and parses the <varname>$INVOCATION_ID</varname> environment
 114     variable that the service manager sets when activating a service, see
 115     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
 116     ID is cached internally. In future a different mechanism to determine the invocation ID may be added.</para>
 117
 118     <para>Note that <function>sd_id128_get_machine_app_specific()</function>, <function>sd_id128_get_boot()</function>
 119     and <function>sd_id128_get_invocation()</function> always return UUID v4 compatible IDs.
 120     <function>sd_id128_get_machine()</function> will also return a UUID v4-compatible ID on new installations but might
 121     not on older.  It is possible to convert the machine ID into a UUID v4-compatible one. For more information, see
 122     <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
 123
 124     <para>For more information about the <literal>sd_id128_t</literal>
 125     type see
 126     <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 127   </refsect1>
 128
 129   <refsect1>
 130     <title>Return Value</title>
 131
 132     <para>The two calls return 0 on success (in which case
 133     <parameter>ret</parameter> is filled in), or a negative
 134     errno-style error code.</para>
 135   </refsect1>
 136
 137   <refsect1>
 138     <title>Notes</title>
 139
 140     <para>The <function>sd_id128_get_machine()</function>, <function>sd_id128_get_machine_app_specific()</function>
 141     <function>sd_id128_get_boot()</function> and <function>sd_id128_get_invocation()</function> interfaces are
 142     available as a shared library, which can be compiled and linked to with the
 143     <literal>libsystemd</literal> <citerefentry
 144     project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry> file.</para>
 145   </refsect1>
 146
 147   <refsect1>
 148     <title>Examples</title>
 149
 150     <example>
 151       <title>Application-specific machine ID</title>
 152
 153       <para>Here's a simple example for an application specific machine ID:</para>
 154
 155       <programlisting>#include &lt;systemd/sd-id128.h&gt;
 156 #include &lt;stdio.h&gt;
 157
 158 #define OUR_APPLICATION_ID SD_ID128_MAKE(c2,73,27,73,23,db,45,4e,a6,3b,b9,6e,79,b5,3e,97)
 159
 160 int main(int argc, char *argv[]) {
 161         sd_id128_t id;
 162         sd_id128_get_machine_app_specific(OUR_APPLICATION_ID, &amp;id);
 163         printf("Our application ID: " SD_ID128_FORMAT_STR "\n", SD_ID128_FORMAT_VAL(id));
 164         return 0;
 165 }</programlisting>
 166     </example>
 167   </refsect1>
 168
 169   <refsect1>
 170     <title>See Also</title>
 171
 172     <para>
 173       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 174       <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 175       <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 176       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 177       <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 178       <citerefentry project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>
 179     </para>
 180   </refsect1>
 181
 182 </refentry>