MACHINE-ID(5) /etc/machine-id MACHINE-ID(5)NAMEmachine-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, lowercase 32 character 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 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, that 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 POSIX specifies.
The systemd-machine-id-setup(1) tool may be used by installer tools to
initialize the machine ID at install time.
RELATION TO OSF UUIDS
Note that the machine ID historically is not an OSF UUID as defined by
RFC 4122[1], nor a Microsoft GUID. Starting with systemd v30 newly
generated machine IDs however 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 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 ALSOsystemd(1), systemd-machine-id-setup(1), gethostid(3), hostname(5),
machine-info(5), os-release(5)AUTHOR
Lennart Poettering <lennart@poettering.net>
Developer
NOTES
1. RFC 4122
http://tools.ietf.org/html/rfc4122
systemd 03/16/2012 MACHINE-ID(5)
