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NAME | SYNOPSIS | DESCRIPTION | INITIALIZATION | FIRST BOOT SEMANTICS | RELATION TO OSF UUIDS | HISTORY | SEE ALSO | NOTES | COLOPHON |
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MACHINE-ID(5) machine-id MACHINE-ID(5)
machine-id - Local machine ID configuration file
/etc/machine-id
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.
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. 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.
/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.
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.)
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), systemd-firstboot(1)
1. Safely Building Images
https://systemd.io/BUILDING_IMAGES
2. RFC 4122
https://tools.ietf.org/html/rfc4122
This page is part of the systemd (systemd system and service
manager) 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
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-02-02. (At that
time, the date of the most recent commit that was found in the
repository was 2025-02-02.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
systemd 258~devel MACHINE-ID(5)
Pages that refer to this page: bootctl(1), systemd(1), systemd-creds(1), systemd-cryptenroll(1), systemd-firstboot(1), systemd-id128(1), systemd-machine-id-setup(1), sd-id128(3), sd_id128_get_machine(3), sd_id128_randomize(3), dnf4.conf(5), hostname(5), labels.conf(5), machine-info(5), networkd.conf(5), org.freedesktop.hostname1(5), os-release(5), repart.d(5), systemd.dnssd(5), systemd.link(5), systemd.netdev(5), systemd.network(5), systemd.preset(5), systemd-system.conf(5), systemd.unit(5), sysupdate.d(5), sysupdate.features(5), sysusers.d(5), tmpfiles.d(5), yum.conf(5), lvmsystemid(7), systemd.directives(7), systemd.index(7), systemd.journal-fields(7), systemd.special(7), kernel-install(8), systemd-gpt-auto-generator(8), systemd-machine-id-commit.service(8), systemd-pcrphase.service(8), systemd-repart(8), systemd-storagetm.service(8)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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