NAME

       sd_id128_get_machine, sd_id128_get_machine_app_specific, sd_id128_get_boot, 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_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 irreversable
       (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. The
       application-specific ID should be generated via a tool like journalctl --new-id128, 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.

       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() and sd_id128_get_invocation() always
       return UUID v4 compatible IDs.  sd_id128_get_machine() will also return a UUID v4-compatible ID on new
       installations but might not on older. It is possible to convert the machine ID into a UUID v4-compatible
       one. For more information, see machine-id(5).

       For more information about the "sd_id128_t" type see sd-id128(3).

RETURN VALUE

       The two calls return 0 on success (in which case ret is filled in), or a negative errno-style error code.

NOTES

       The sd_id128_get_machine(), sd_id128_get_machine_app_specific() sd_id128_get_boot() and
       sd_id128_get_invocation() interfaces are available 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

       Here's a simple example for an application specific machine ID:

           #include <systemd/sd-id128.h>
           #include <stdio.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;
           }

SEE ALSO

       systemd(1), sd-id128(3), machine-id(5), systemd.exec(5), sd_id128_randomize(3), random(4)