The /etc/machine-id file contains the unique machine ID of the local
system that is set during installation. 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
The machine ID is usually generated from a random source during
system installation and stays constant for all subsequent boots.
Optionally, for stateless systems, it is generated during runtime at
early boot if it is found to be empty.
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
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.
The systemd-machine-id-setup(1) tool may be used by installer tools
to initialize the machine ID at install time. Use
systemd-firstboot(1) to initialize it on mounted (but not booted)
The machine-id may also be set, for example when network booting, by
setting the systemd.machine_id= kernel command line parameter or
passing the option --machine-id= to systemd. A machine-id may not be
set to all zeros.
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.)
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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
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COLOPHON (which is not part of the original manual page), send a mail
systemd 233 MACHINE-ID(5)