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+ -->
6 <refentry id=
"sd_id128_get_machine" xmlns:
xi=
"http://www.w3.org/2001/XInclude">
9 <title>sd_id128_get_machine
</title>
10 <productname>systemd
</productname>
14 <refentrytitle>sd_id128_get_machine
</refentrytitle>
15 <manvolnum>3</manvolnum>
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>
29 <funcsynopsisinfo>#include
<systemd/sd-id128.h
></funcsynopsisinfo>
32 <funcdef>int
<function>sd_id128_get_machine
</function></funcdef>
33 <paramdef>sd_id128_t *
<parameter>ret
</parameter></paramdef>
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>
43 <funcdef>int
<function>sd_id128_get_boot
</function></funcdef>
44 <paramdef>sd_id128_t *
<parameter>ret
</parameter></paramdef>
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>
54 <funcdef>int
<function>sd_id128_get_invocation
</function></funcdef>
55 <paramdef>sd_id128_t *
<parameter>ret
</parameter></paramdef>
62 <title>Description
</title>
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>
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>
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>
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>
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>
102 <para>Note that
<function>sd_id128_get_machine_app_specific()
</function>,
<function>sd_id128_get_boot()
</function>,
103 <function>sd_id128_get_boot_app_specific()
</function>, and
<function>sd_id128_get_invocation()
</function> always
104 return UUID v4 compatible IDs.
<function>sd_id128_get_machine()
</function> will also return a UUID v4-compatible
105 ID on new installations but might not on older. It is possible to convert the machine ID into a UUID v4-compatible
106 one. For more information, see
107 <citerefentry><refentrytitle>machine-id
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para>
109 <para>For more information about the
<literal>sd_id128_t
</literal>
111 <citerefentry><refentrytitle>sd-id128
</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
</para>
115 <title>Return Value
</title>
117 <para>Those calls return
0 on success (in which case
<parameter>ret
</parameter> is filled in),
118 or a negative errno-style error code.
</para>
121 <title>Errors
</title>
122 <para>Returned errors may indicate the following problems:
</para>
126 <term><constant>-ENOENT
</constant></term>
128 <listitem><para>Returned by
<function>sd_id128_get_machine()
</function>,
129 <function>sd_id128_get_machine_app_specific()
</function>, and
130 <function>sd_id128_get_boot_app_specific()
</function> when
<filename>/etc/machine-id
</filename> is
131 missing.
</para></listitem>
135 <term><constant>-ENOMEDIUM
</constant></term>
137 <listitem><para>Returned by
<function>sd_id128_get_machine()
</function>,
138 <function>sd_id128_get_machine_app_specific()
</function>, and
139 <function>sd_id128_get_boot_app_specific()
</function> when
<filename>/etc/machine-id
</filename> is
140 empty or all zeros.
</para></listitem>
144 <term><constant>-ENXIO
</constant></term>
146 <listitem><para>Returned by
<function>sd_id128_get_invocation()
</function> if no invocation ID is
147 set.
</para></listitem>
151 <term><constant>-EIO
</constant></term>
153 <listitem><para>Returned by any of the functions described here when the configured value has
154 invalid format.
</para></listitem>
158 <term><constant>-EPERM
</constant></term>
160 <listitem><para>Requested information could not be retrieved because of insufficient permissions.
167 <xi:include href=
"libsystemd-pkgconfig.xml" />
170 <title>Examples
</title>
173 <title>Application-specific machine ID
</title>
175 <para>First, generate the application ID:
</para>
176 <programlisting>$ systemd-id128 -p new
178 c273277323db454ea63bb96e79b53e97
181 c2732773-
23db-
454e-a63b-b96e79b53e97
183 As man:sd-id128(
3) macro:
184 #define MESSAGE_XYZ SD_ID128_MAKE(c2,
73,
27,
73,
23,db,
45,
4e,a6,
3b,b9,
6e,
79,b5,
3e,
97)
188 <para>Then use the new identifier in an example application:
</para>
190 <programlisting><xi:include href=
"id128-app-specific.c" parse=
"text" /></programlisting>
195 <title>See Also
</title>
198 <citerefentry><refentrytitle>systemd
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
199 <citerefentry><refentrytitle>systemd-id128
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
200 <citerefentry><refentrytitle>sd-id128
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
201 <citerefentry><refentrytitle>machine-id
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
202 <citerefentry><refentrytitle>systemd.exec
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
203 <citerefentry><refentrytitle>sd_id128_randomize
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
204 <citerefentry project='man-pages'
><refentrytitle>random
</refentrytitle><manvolnum>4</manvolnum></citerefentry>