machine-id

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



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



systemd 247                                                        MACHINE-ID(5)