machine-id - Local machine ID configuration file
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
(3) API provides an implementation of
such an algorithm.
(1) tool may be used by installer tools to
initialize the machine ID at install time. Use systemd-firstboot
initialize it on mounted (but not booted) system images.
The machine-id may also be set, for example when network booting, by setting the
kernel command line parameter or passing the option
to systemd. A machine-id may not be set to all zeros.
Note that the machine ID historically is not an OSF UUID as defined by RFC
, 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 = (id & 0x0F) | 0x40;
/* Set the UUID variant to DCE */
id = (id & 0x3F) | 0x80;
(This code is inspired by "generate_random_uuid()" of
drivers/char/random.c from the Linux kernel sources.)
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.
- RFC 4122