machine-id — Local machine ID configuration 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
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 boot if it is found to be empty.
The machine ID does not change based on user configuration or when hardware is replaced.
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.
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
/* 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
drivers/char/random.c from the
Linux kernel sources.)
The simple configuration file format of
/etc/machine-id originates in the
introduced by D-Bus. In fact, this latter file might be a