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Diffstat (limited to 'upstream/fedora-40/man5/machine-id.5')
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diff --git a/upstream/fedora-40/man5/machine-id.5 b/upstream/fedora-40/man5/machine-id.5 new file mode 100644 index 00000000..a85e524b --- /dev/null +++ b/upstream/fedora-40/man5/machine-id.5 @@ -0,0 +1,253 @@ +'\" t +.TH "MACHINE\-ID" "5" "" "systemd 255" "machine-id" +.\" ----------------------------------------------------------------- +.\" * Define some portability stuff +.\" ----------------------------------------------------------------- +.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +.\" http://bugs.debian.org/507673 +.\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html +.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +.ie \n(.g .ds Aq \(aq +.el .ds Aq ' +.\" ----------------------------------------------------------------- +.\" * set default formatting +.\" ----------------------------------------------------------------- +.\" disable hyphenation +.nh +.\" disable justification (adjust text to left margin only) +.ad l +.\" ----------------------------------------------------------------- +.\" * MAIN CONTENT STARTS HERE * +.\" ----------------------------------------------------------------- +.SH "NAME" +machine-id \- Local machine ID configuration file +.SH "SYNOPSIS" +.PP +/etc/machine\-id +.SH "DESCRIPTION" +.PP +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\&. +.PP +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\&. +.PP +The machine ID may be set, for example when network booting, with the +\fIsystemd\&.machine_id=\fR +kernel command line parameter or by passing the option +\fB\-\-machine\-id=\fR +to systemd\&. An ID specified in this manner has higher priority and will be used instead of the ID stored in +/etc/machine\-id\&. +.PP +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 +\fBgethostid\fR(3) +call that POSIX specifies\&. +.PP +This machine ID adheres to the same format and logic as the D\-Bus machine ID\&. +.PP +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 +\fBsd_id128_get_machine_app_specific\fR(3) +API provides an implementation of such an algorithm\&. +.SH "INITIALIZATION" +.PP +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\&. +.PP +For normal operating system installations, where a custom image is created for a specific machine, +/etc/machine\-id +should be populated during installation\&. +.PP +\fBsystemd-machine-id-setup\fR(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\&. +.PP +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 +\m[blue]\fBSafely Building Images\fR\m[]\&\s-2\u[1]\d\s+2\&. +.PP +\fBsystemd-firstboot\fR(1) +may be used to initialize +/etc/machine\-id +on mounted (but not booted) system images\&. +.PP +When a machine is booted with +\fBsystemd\fR(1) +the ID of the machine will be established\&. If +\fIsystemd\&.machine_id=\fR +or +\fB\-\-machine\-id=\fR +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 +\fIcontainer_uuid\fR, the KVM DMI +product_uuid +or the devicetree +vm,uuid +(on KVM systems), and finally a randomly generated UUID\&. +.PP +After the machine ID is established, +\fBsystemd\fR(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\&. +.PP +\fBsystemd-machine-id-commit.service\fR(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\&. +.SH "FIRST BOOT SEMANTICS" +.PP +/etc/machine\-id +is used to decide whether a boot is the first one\&. The rules are as follows: +.sp +.RS 4 +.ie n \{\ +\h'-04' 1.\h'+01'\c +.\} +.el \{\ +.sp -1 +.IP " 1." 4.2 +.\} +The kernel command argument +\fIsystemd\&.condition\-first\-boot=\fR +may be used to override the autodetection logic, see +\fBkernel-command-line\fR(7)\&. +.RE +.sp +.RS 4 +.ie n \{\ +\h'-04' 2.\h'+01'\c +.\} +.el \{\ +.sp -1 +.IP " 2." 4.2 +.\} +Otherwise, if +/etc/machine\-id +does not exist, this is a first boot\&. During early boot, +\fBsystemd\fR +will write +"uninitialized\en" +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\&. +.RE +.sp +.RS 4 +.ie n \{\ +\h'-04' 3.\h'+01'\c +.\} +.el \{\ +.sp -1 +.IP " 3." 4.2 +.\} +If +/etc/machine\-id +contains the string +"uninitialized", a boot is also considered the first boot\&. The same mechanism as above applies\&. +.RE +.sp +.RS 4 +.ie n \{\ +\h'-04' 4.\h'+01'\c +.\} +.el \{\ +.sp -1 +.IP " 4." 4.2 +.\} +If +/etc/machine\-id +exists and is empty, a boot is +\fInot\fR +considered the first boot\&. +\fBsystemd\fR +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)\&. +.RE +.sp +.RS 4 +.ie n \{\ +\h'-04' 5.\h'+01'\c +.\} +.el \{\ +.sp -1 +.IP " 5." 4.2 +.\} +If +/etc/machine\-id +already contains a valid machine\-id, this is not a first boot\&. +.RE +.PP +If according to the above rules a first boot is detected, units with +\fIConditionFirstBoot=yes\fR +will be run and +\fBsystemd\fR +will perform additional initialization steps, in particular presetting units\&. +.SH "RELATION TO OSF UUIDS" +.PP +Note that the machine ID historically is not an OSF UUID as defined by +\m[blue]\fBRFC 4122\fR\m[]\&\s-2\u[2]\d\s+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\&. +.PP +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: +.sp +.if n \{\ +.RS 4 +.\} +.nf +/* 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; +.fi +.if n \{\ +.RE +.\} +.PP +(This code is inspired by +"generate_random_uuid()" +of +drivers/char/random\&.c +from the Linux kernel sources\&.) +.SH "HISTORY" +.PP +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\&. +.SH "SEE ALSO" +.PP +\fBsystemd\fR(1), +\fBsystemd-machine-id-setup\fR(1), +\fBgethostid\fR(3), +\fBhostname\fR(5), +\fBmachine-info\fR(5), +\fBos-release\fR(5), +\fBsd-id128\fR(3), +\fBsd_id128_get_machine\fR(3), +\fBsystemd-firstboot\fR(1) +.SH "NOTES" +.IP " 1." 4 +Safely Building Images +.RS 4 +\%https://systemd.io/BUILDING_IMAGES +.RE +.IP " 2." 4 +RFC 4122 +.RS 4 +\%https://tools.ietf.org/html/rfc4122 +.RE |