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)