|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | EXAMPLES | HISTORY | SEE ALSO | NOTES | COLOPHON |
|
|
|
SD_ID128_GET_MACHINE(3) sd_id128_get_machine SD_ID128_GET_MACHINE(3)
sd_id128_get_machine, sd_id128_get_app_specific,
sd_id128_get_machine_app_specific, sd_id128_get_boot,
sd_id128_get_boot_app_specific, sd_id128_get_invocation - Retrieve
128-bit IDs
#include <systemd/sd-id128.h>
int sd_id128_get_machine(sd_id128_t *ret);
int sd_id128_get_app_specific(sd_id128_t base, sd_id128_t app_id,
sd_id128_t *ret);
int sd_id128_get_machine_app_specific(sd_id128_t app_id,
sd_id128_t *ret);
int sd_id128_get_boot(sd_id128_t *ret);
int sd_id128_get_boot_app_specific(sd_id128_t app_id,
sd_id128_t *ret);
int sd_id128_get_invocation(sd_id128_t *ret);
int sd_id128_get_invocation_app_specific(sd_id128_t app_id,
sd_id128_t *ret);
sd_id128_get_machine() returns the machine ID of the executing
host. This reads and parses the machine-id(5) 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
sd_id128_get_machine_app_specific() is provided, see below.
sd_id128_get_app_specific() returns a machine ID that is a
combination of the base and app_id parameters. Internally, this
function calculates HMAC-SHA256 of the app_id parameter keyed by
the base parameter, and truncates this result to fit in sd_id128_t
and turns it into a valid Variant 1 Version 4 UUID, in accordance
with RFC 4122[1]. Neither of the two input parameters can be
calculated from the output parameter ret.
sd_id128_get_machine_app_specific() is similar to
sd_id128_get_machine(), 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
sd_id128_get_machine() 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 systemd-id128 new, 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
sd_id128_get_app_specific() with the result from
sd_id128_get_machine() and the app_id parameter.
sd_id128_get_boot() returns the boot ID of the executing kernel.
This reads and parses the /proc/sys/kernel/random/boot_id file
exposed by the kernel. It is randomly generated early at boot and
is unique for every running kernel instance. See random(4) 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 sd_id128_get_boot_app_specific(), see below.
sd_id128_get_boot_app_specific() is analogous to
sd_id128_get_machine_app_specific(), 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.
sd_id128_get_invocation() returns the invocation ID of the
currently executed service. In its current implementation, this
tries to read and parse the following:
• The $INVOCATION_ID environment variable that the service
manager sets when activating a service.
• An entry in the kernel keyring that the system service manager
sets when activating a service.
See systemd.exec(5) for details. The ID is cached internally. In
future a different mechanism to determine the invocation ID may be
added.
sd_id128_get_invocation_app_specific() derives an
application-specific ID from the invocation ID.
Note that sd_id128_get_machine_app_specific(),
sd_id128_get_boot(), sd_id128_get_boot_app_specific(),
sd_id128_get_invocation() and sd_id128_get_invocation_app_specific
always return UUID Variant 1 Version 4 compatible IDs.
sd_id128_get_machine() 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
machine-id(5). It is hence guaranteed that these functions will
never return the ID consisting of all zero or all one bits
(SD_ID128_NULL, SD_ID128_ALLF) — with the possible exception of
sd_id128_get_machine(), as mentioned.
For more information about the "sd_id128_t" type see sd-id128(3).
Those calls return 0 on success (in which case ret is filled in),
or a negative errno-style error code.
Errors
Returned errors may indicate the following problems:
-ENOENT
Returned by sd_id128_get_machine() and
sd_id128_get_machine_app_specific() when /etc/machine-id is
missing.
Added in version 242.
-ENOMEDIUM
Returned by sd_id128_get_machine() and
sd_id128_get_machine_app_specific() when /etc/machine-id is
empty or all zeros. Also returned by sd_id128_get_invocation()
when the invocation ID is all zeros.
Added in version 242.
-ENOPKG
Returned by sd_id128_get_machine() and
sd_id128_get_machine_app_specific() when the content of
/etc/machine-id is "uninitialized".
Added in version 253.
-ENOSYS
Returned by sd_id128_get_boot() and
sd_id128_get_boot_app_specific() when /proc/ is not mounted.
Added in version 253.
-ENXIO
Returned by sd_id128_get_invocation() if no invocation ID is
set. Also returned by sd_id128_get_app_specific(),
sd_id128_get_machine_app_specific(), and
sd_id128_get_boot_app_specific() when the app_id parameter is
all zeros.
Added in version 242.
-EUCLEAN
Returned by any of the functions described here when the
configured value has invalid format.
Added in version 253.
-EPERM
Requested information could not be retrieved because of
insufficient permissions.
Added in version 242.
Functions described here are available as a shared library, which
can be compiled against and linked to with the
libsystemd pkg-config(1) file.
The code described here uses getenv(3), which is declared to be
not multi-thread-safe. This means that the code calling the
functions described here must not call setenv(3) from a parallel
thread. It is recommended to only do calls to setenv() from an
early phase of the program when no other threads have been
started.
Example 1. Application-specific machine ID
First, generate the application ID:
$ 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)
...
Then use the new identifier in an example application:
/* SPDX-License-Identifier: MIT-0 */
#include <stdio.h>
#include <systemd/sd-id128.h>
#define OUR_APPLICATION_ID SD_ID128_MAKE(c2,73,27,73,23,db,45,4e,a6,3b,b9,6e,79,b5,3e,97)
int main(int argc, char *argv[]) {
sd_id128_t id;
sd_id128_get_machine_app_specific(OUR_APPLICATION_ID, &id);
printf("Our application ID: " SD_ID128_FORMAT_STR "\n", SD_ID128_FORMAT_VAL(id));
return 0;
}
sd_id128_get_machine() and sd_id128_get_boot() were added in
version 187.
sd_id128_get_invocation() was added in version 232.
sd_id128_get_machine_app_specific() was added in version 233.
sd_id128_get_boot_app_specific() was added in version 240.
sd_id128_get_app_specific() was added in version 255.
sd_id128_get_invocation_app_specific() was added in version 256.
systemd(1), systemd-id128(1), sd-id128(3), machine-id(5),
systemd.exec(5), sd_id128_randomize(3), random(4)
1. RFC 4122
https://tools.ietf.org/html/rfc4122
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
systemd 258~rc2 SD_ID128_GET_MACHINE(3)
Pages that refer to this page: systemd-id128(1), sd_bus_message_get_monotonic_usec(3), sd-id128(3), sd_id128_randomize(3), sd_journal_get_cutoff_realtime_usec(3), sd_journal_get_realtime_usec(3), machine-id(5), networkd.conf(5), systemd.network(5), systemd.directives(7), systemd.index(7), pam_systemd(8)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|