machine-id(5) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | INITIALIZATION | FIRST BOOT SEMANTICS | 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 either missing or an empty file in the
       generic file system image (the difference between the two options
       is described under "First Boot Semantics" below). 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.

FIRST BOOT SEMANTICS         top

       /etc/machine-id is used to decide whether a boot is the first
       one. The rules are as follows:

        1. If /etc/machine-id does not exist, this is a first boot.
           During early boot, systemd will write "uninitialized\n" to
           this file and overmount a temporary file which contains the
           actual machine ID. Later (after first-boot-complete.target
           has been reached), the real machine ID will be written to
           disk.

        2. If /etc/machine-id contains the string "uninitialized", a
           boot is also considered the first boot. The same mechanism as
           above applies.

        3. If /etc/machine-id exists and is empty, a boot is not
           considered the first boot.  systemd will still bind-mount a
           file containing the actual machine-id over it and later try
           to commit it to disk (if /etc/ is writable).

        4. If /etc/machine-id already contains a valid machine-id, this
           is not a first boot.

       If by any of the above rules, a first boot is detected, units
       with ConditionFirstBoot=yes will be run.

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
       Variant 1 Version 4 UUIDs, as per RFC 4122.

       In order to maintain compatibility with existing installations,
       an application requiring a strictly RFC 4122 compliant UUID
       should decode the machine ID, and then (non-reversibly) apply the
       following operations to turn it into a valid RFC 4122 Variant 1
       Version 4 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 2021-06-20.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2021-06-19.)  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 249                                                MACHINE-ID(5)

Pages that refer to this page: bootctl(1)systemd-firstboot(1)systemd-id128(1)systemd-machine-id-setup(1)sd-id128(3)sd_id128_get_machine(3)sd_id128_randomize(3)hostname(5)labels.conf(5)machine-info(5)networkd.conf(5)os-release(5)repart.d(5)systemd.dnssd(5)systemd.netdev(5)systemd.network(5)systemd-system.conf(5)systemd.unit(5)sysusers.d(5)tmpfiles.d(5)lvmsystemid(7)systemd.directives(7)systemd.index(7)systemd.journal-fields(7)systemd.special(7)kernel-install(8)systemd-gpt-auto-generator(8)systemd-machine-id-commit.service(8)systemd-repart(8)