machine-id(5) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | INITIALIZATION | 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 or boot. The machine ID is a
       single newline-terminated, hexadecimal, 32-character, lowercase ID.
       When decoded from hexadecimal, this corresponds to a 16-byte/128-bit
       value. This ID may not be all zeros.

       The machine ID is usually generated from a random source during
       system installation or first boot and stays constant for all
       subsequent boots. Optionally, for stateless systems, it is generated
       during runtime during early boot if necessary.

       The machine ID may be set, for example when network booting, with the
       systemd.machine_id= kernel command line parameter or by passing the
       option --machine-id= to systemd. An ID specified in this manner has
       higher priority and will be used instead of the ID stored in
       /etc/machine-id.

       The machine ID does not change based on local or network
       configuration or when hardware is replaced. Due to this and its
       greater length, it is a more useful replacement for the gethostid(3)
       call that POSIX specifies.

       This machine ID adheres to the same format and logic as the D-Bus
       machine ID.

       This ID uniquely identifies the host. It should be considered
       "confidential", and must not be exposed in untrusted environments, in
       particular on the network. If a stable unique identifier that is tied
       to the machine is needed for some application, the machine ID or any
       part of it must not be used directly. Instead the machine ID should
       be hashed with a cryptographic, keyed hash function, using a fixed,
       application-specific key. That way the ID will be properly unique,
       and derived in a constant way from the machine ID but there will be
       no way to retrieve the original machine ID from the
       application-specific one. The sd_id128_get_machine_app_specific(3)
       API provides an implementation of such an algorithm.

INITIALIZATION         top

       Each machine should have a non-empty ID in normal operation. The ID
       of each machine should be unique. To achieve those objectives,
       /etc/machine-id can be initialized in a few different ways.

       For normal operating system installations, where a custom image is
       created for a specific machine, /etc/machine-id should be populated
       during installation.

       systemd-machine-id-setup(1) may be used by installer tools to
       initialize the machine ID at install time, but /etc/machine-id may
       also be written using any other means.

       For operating system images which are created once and used on
       multiple machines, for example for containers or in the cloud,
       /etc/machine-id should be an empty file in the generic file system
       image. An ID will be generated during boot and saved to this file if
       possible. Having an empty file in place is useful because it allows a
       temporary file to be bind-mounted over the real file, in case the
       image is used read-only.

       systemd-firstboot(1) may be used to initialize /etc/machine-id on
       mounted (but not booted) system images.

       When a machine is booted with systemd(1) the ID of the machine will
       be established. If systemd.machine_id= or --machine-id= options (see
       first section) are specified, this value will be used. Otherwise, the
       value in /etc/machine-id will be used. If this file is empty or
       missing, systemd will attempt to use the D-Bus machine ID from
       /var/lib/dbus/machine-id, the value of the kernel command line option
       container_uuid, the KVM DMI product_uuid or the devicetree vm,uuid
       (on KVM systems), and finally a randomly generated UUID.

       After the machine ID is established, systemd(1) will attempt to save
       it to /etc/machine-id. If this fails, it will attempt to bind-mount a
       temporary file over /etc/machine-id. It is an error if the file
       system is read-only and does not contain a (possibly empty)
       /etc/machine-id file.

       systemd-machine-id-commit.service(8) will attempt to write the
       machine ID to the file system if /etc/machine-id or /etc are
       read-only during early boot but become writable later on.

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)
       project.  Information about the project can be found at 
       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a bug
       report for this manual page, see
       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨https://github.com/systemd/systemd.git⟩ on 2020-07-14.  (At that
       time, the date of the most recent commit that was found in the repos‐
       itory was 2020-07-14.)  If you discover any rendering problems in
       this HTML version of the page, or you believe there is a better or
       more up-to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not part
       of the original manual page), send a mail to man-pages@man7.org

systemd 246                                                    MACHINE-ID(5)

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