MACHINE-ID(5)                                       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.

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     MACHINE-ID(5)                                       MACHINE-ID(5)

          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:

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     MACHINE-ID(5)                                       MACHINE-ID(5)

           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 m[blue]RFC 4122m[][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),

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     MACHINE-ID(5)                                       MACHINE-ID(5)

          sd_id128_get_machine(3), systemd-firstboot(1)

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

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