MACHINE-ID(5) machine-id MACHINE-ID(5)
machine-id - Local machine ID configuration file
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 is specified in this manner has
higher priority and will be used instead of the ID stored in
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
This machine ID adheres to the same format and logic as the D-Bus
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.
Each machine should have a non-empty ID in normal operation. The ID of
each machine should be unique. To achive 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
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 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 (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
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
Note that the machine ID historically is not an OSF UUID as defined by
RFC 4122, 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 = (id & 0x0F) | 0x40;
/* Set the UUID variant to DCE */
id = (id & 0x3F) | 0x80;
(This code is inspired by "generate_random_uuid()" of
drivers/char/random.c from the Linux kernel sources.)
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.
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),
1. RFC 4122
systemd 237 MACHINE-ID(5)
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