# machine-id

## 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 recommended because it allows a
           temporary file to be bind-mounted over the real file, in case the
           image is used read-only and when /etc is mounted read-only in the
           early boot. Also see **Safely Building Images******[1].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), the Xen
           hypervisor uuid, and finally a randomly generated UUID.
           _systemd.machine_id=firmware_ can be set to generate the machine ID
           from the firmware.
    
           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. The kernel command argument _systemd.condition_first_boot=_ may
               be used to override the autodetection logic, see
               kernel-command-line(7).
    
            2. Otherwise, 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.
    
            3. If /etc/machine-id contains the string "uninitialized", a boot
               is also considered the first boot. The same mechanism as above
               applies.
    
            4. 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).
    
            5. If /etc/machine-id already contains a valid machine-id, this
               is not a first boot.
    
           If according to the above rules a first boot is detected, units
           with _ConditionFirstBoot=yes_ will be run and **systemd** will perform
           additional initialization steps, in particular presetting units.
    


## RELATION TO OSF UUIDS



           Note that the machine ID historically is not an OSF UUID as
           defined by **RFC 4122******[2], 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.)
    


## NOTES



            1. Safely Building Images
               https://systemd.io/BUILDING_IMAGES
    
            2. RFC 4122
               https://tools.ietf.org/html/rfc4122
    


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