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: 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;
           }

SEE ALSO

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