Provided by: libsystemd-dev_255.4-1ubuntu8.4_amd64 
NAME
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
SYNOPSIS
#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);
DESCRIPTION
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.
Note that sd_id128_get_machine_app_specific(), sd_id128_get_boot(),
sd_id128_get_boot_app_specific(), and sd_id128_get_invocation() 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).
RETURN VALUE
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.
NOTES
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.
EXAMPLES
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;
}
HISTORY
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.
SEE ALSO
systemd(1), systemd-id128(1), sd-id128(3), machine-id(5), systemd.exec(5),
sd_id128_randomize(3), random(4)
NOTES
1. RFC 4122
https://tools.ietf.org/html/rfc4122