sd_id128_get_machine

elogind
sd_id128_get_machine 3
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

    #include <elogind/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. Neither of the two input parameters can be calculated from the output parameter ret.

`sd_id128_get_machine_app_specific()` is similar to machine. This function will return the same application-specific ID for each `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 `elogind-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 variable that the service manager has to set when activating a service. If `$INVOCATION_ID` was not set by the service manager, the function returns -ENXIO. The 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 elogind.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.

`-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.

`-ENOPKG`

  Returned by `sd_id128_get_machine()` and `sd_id128_get_machine_app_specific()` when the content of `/etc/machine-id` is `uninitialized`.

`-ENOSYS`

  Returned by `sd_id128_get_boot()` and `sd_id128_get_boot_app_specific()` when `/proc/` is not mounted.

`-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.

`-EUCLEAN`

  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.

## 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

elogind 8, sd-id128 3, machine-id 5, sd_id128_randomize 3, random 4