machine-id

       /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 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 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.

       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[1], 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

systemd 229                                                      MACHINE-ID(5)
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