файл конфигурации идентификатора локального компьютера (Local machine ID configuration file)
Имя (Name)
machine-id - Local machine ID configuration file
Синопсис (Synopsis)
/etc/machine-id
Описание (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 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.
FIRST BOOT SEMANTICS
/etc/machine-id is used to decide whether a boot is the first
one. The rules are as follows:
1. 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.
2. If /etc/machine-id contains the string "uninitialized", a
boot is also considered the first boot. The same mechanism as
above applies.
3. 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).
4. If /etc/machine-id already contains a valid machine-id, this
is not a first boot.
If by any of the above rules, a first boot is detected, units
with ConditionFirstBoot=yes will be run.
RELATION TO OSF UUIDS
Note that the machine ID historically is not an OSF UUID as
defined by 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.)
История (History)
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.
Смотри также (See also)
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)
Примечание (Note)
1. RFC 4122
https://tools.ietf.org/html/rfc4122
