SD_ID128_GET_MACHINE - Online Linux Manual Page

Section : 3
Updated :
Source : systemd 250
Note : sd_id128_get_machine

NAME

sd_id128_get_machine, 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_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_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 calculates HMAC−SHA256 of the application ID, keyed by the machine ID.

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_machine_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 reads and parses the

$INVOCATION_ID

environment variable that the 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(

)

sd_id128_get_machine_app_specific(

)

, and

sd_id128_get_boot_app_specific(

)

when /etc/machine−id is missing.

−ENOMEDIUM

Returned by sd_id128_get_machine(

)

sd_id128_get_machine_app_specific(

)

, and

sd_id128_get_boot_app_specific(

)

when /etc/machine−id is empty or all zeros.

−ENXIO

Returned by sd_id128_get_invocation(

)

if no invocation ID is set.

−EIO

Returned by any of the functions described here when the configured value has invalid format.

−EPERM

Requested information could not be retrieved because of insufficient permissions.

NOTES

These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd

pkg-config

(1) file.

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: CC0−1.0 */ #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; }