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 machine ID string. When decoded from
hexadecimal, this corresponds with a 16-byte/128-bit string.
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 user configuration or when hardware is
This machine ID adheres to the same format and logic as the D-Bus machine ID.
Programs may use this ID to identify the host with a globally unique ID in the
network, which does not change even if the local network configuration
changes. Due to this and its greater length, it is a more useful replacement
for the gethostid(3)
call that POSIX specifies.
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.
RELATION TO OSF UUIDS¶
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