NAME | SYNOPSIS | DESCRIPTION | RELATION TO OSF UUIDS | HISTORY | SEE ALSO | NOTES | COLOPHON

MACHINE-ID(5)                    machine-id                    MACHINE-ID(5)

NAME         top

       machine-id - Local machine ID configuration file

SYNOPSIS         top

       /etc/machine-id

DESCRIPTION         top

       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         top

       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         top

       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 ALSO         top

       systemd(1), systemd-machine-id-setup(1), gethostid(3), hostname(5),
       machine-info(5), os-release(5), sd-id128(3), sd_id128_get_machine(3),
       systemd-firstboot(1)

NOTES         top

        1. RFC 4122
           https://tools.ietf.org/html/rfc4122

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
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systemd 231                                                    MACHINE-ID(5)