machine-id

Index · Directives systemd 234

## Name

machine-id — Local machine ID configuration file

## Synopsis

`/etc/machine-id`

## Description

The `/etc/machine-id` file contains the unique machine ID of the local system that is set during installation. The machine ID is a single newline-terminated, hexadecimal, 32-character, lowercase ID. When decoded from hexadecimal, this corresponds to a 16-byte/128-bit value.

The machine ID is usually generated from a random source during system installation and stays constant for all subsequent boots. Optionally, for stateless systems, it is generated during runtime at early boot if it is found to be empty.

The machine ID does not change based on local or network configuration or when hardware is replaced. Due to this and its greater length, it is a more useful replacement for the gethostid(3) call that POSIX specifies.

This machine ID adheres to the same format and logic as the D-Bus machine ID.

This ID uniquely identifies the host. It should be considered "confidential", and must not be exposed in untrusted environments, in particular on the network. If a stable unique identifier that is tied to the machine is needed for some application, the machine ID or any part of it must not be used directly. Instead the machine ID should be hashed with a cryptographic, keyed hash function, using a fixed, application-specific key. That way the ID will be properly unique, and derived in a constant way from the machine ID but there will be no way to retrieve the original machine ID from the application-specific one. The sd_id128_get_machine_app_specific(3) API provides an implementation of such an algorithm.

The systemd-machine-id-setup(1) tool may be used by installer tools to initialize the machine ID at install time. Use systemd-firstboot(1) to initialize it on mounted (but not booted) system images.

The machine-id may also be set, for example when network booting, by setting the `systemd.machine_id=` kernel command line parameter or passing the option `--machine-id=` to systemd. A machine-id may not be set to all zeros.

## Relation to OSF UUIDs

Note that the machine ID historically is not an OSF UUID as defined by RFC 4122, nor a Microsoft GUID; however, starting with systemd v30, newly generated machine IDs do qualify as v4 UUIDs.

In order to maintain compatibility with existing installations, an application requiring a UUID should decode the machine ID, and then apply the following operations to turn it into a valid OSF v4 UUID. With "`id`" being an unsigned character array:

    /* Set UUID version to 4 --- truly random generation */
    id[6] = (id[6] & 0x0F) | 0x40;
    /* Set the UUID variant to DCE */
    id[8] = (id[8] & 0x3F) | 0x80;

(This code is inspired by "`generate_random_uuid()`" of `drivers/char/random.c` from the Linux kernel sources.)

## History

The simple configuration file format of `/etc/machine-id` originates in the `/var/lib/dbus/machine-id` file introduced by D-Bus. In fact, this latter file might be a symlink to `/etc/machine-id`.

## See Also

systemd(1), systemd-machine-id-setup(1), gethostid(3), hostname(5), machine-info(5), os-release(5), sd-id128(3), sd_id128_get_machine(3), systemd-firstboot(1)