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NAME | SYNOPSIS | DESCRIPTION | INITIALIZATION | RELATION TO OSF UUIDS | HISTORY | SEE ALSO | NOTES | COLOPHON |
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 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 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.
Note that the machine ID historically is not an OSF UUID as defined
by RFC 4122[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.)
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. 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 2020-08-13. (At that
time, the date of the most recent commit that was found in the repos‐
itory was 2020-08-11.) 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 man-pages@man7.org
systemd 246 MACHINE-ID(5)
Pages that refer to this page: systemd-firstboot(1) , systemd-firstboot.service(1) , systemd-id128(1) , systemd-machine-id-setup(1) , sd-id128(3) , sd_id128_const_str(3) , SD_ID128_CONST_STR(3) , sd_id128_equal(3) , sd_id128_format_str(3) , SD_ID128_FORMAT_STR(3) , sd_id128_format_val(3) , SD_ID128_FORMAT_VAL(3) , sd_id128_get_boot(3) , sd_id128_get_boot_app_specific(3) , sd_id128_get_invocation(3) , sd_id128_get_machine(3) , sd_id128_get_machine_app_specific(3) , sd_id128_is_null(3) , sd_id128_make(3) , SD_ID128_MAKE(3) , sd_id128_make_str(3) , SD_ID128_MAKE_STR(3) , sd_id128_null(3) , SD_ID128_NULL(3) , sd_id128_randomize(3) , sd_id128_t(3) , sd_id128_uuid_format_str(3) , SD_ID128_UUID_FORMAT_STR(3) , hostname(5) , machine-info(5) , networkd.conf(5) , networkd.conf.d(5) , os-release(5) , repart.d(5) , systemd.dnssd(5) , systemd.netdev(5) , systemd.network(5) , systemd.unit(5) , sysusers.d(5) , tmpfiles.d(5) , 30-systemd-environment-d-generator(7) , lvmsystemid(7) , systemd.directives(7) , systemd.index(7) , systemd.journal-fields(7) , kernel-install(8) , systemd-gpt-auto-generator(8) , systemd-machine-id-commit.service(8) , systemd-repart(8) , systemd-repart.service(8)