'\" t
.\"     Title: machine-id
.\"    Author: Lennart Poettering <lennart@poettering.net>
.\" Generator: DocBook XSL Stylesheets v1.76.1 <http://docbook.sf.net/>
.\"      Date: 10/07/2013
.\"    Manual: /etc/machine-id
.\"    Source: systemd
.\"  Language: English
.\"
.TH "MACHINE\-ID" "5" "10/07/2013" "systemd" "/etc/machine-id"
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.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\&. The machine ID is a single newline\-terminated, hexadecimal, lowercase 32 character machine ID string\&. (When decoded from hexadecimal this corresponds with a 16 byte/128 bit string\&.)
.PP
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 boot if it is found to be empty\&.
.PP
The machine ID does not change based on user configuration, or when hardware is replaced\&.
.PP
This machine ID adheres to the same format and logic as the D\-Bus machine ID\&.
.PP
Programs may use this ID to identify the host with a globally unique ID in the network, that does not change even if the local network configuration changes\&. Due to this and its greater length it is a more useful replacement for the
\fBgethostid\fR(3)
call POSIX specifies\&.
.PP
The
\fBsystemd-machine-id-setup\fR(1)
tool may be used by installer tools to initialize the machine ID at install time\&.
.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[1]\d\s+2, nor a Microsoft GUID\&. Starting with systemd v30 newly generated machine IDs however do qualify as v4 UUIDs\&.
.PP
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:
.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 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
\fI/etc/machine\-id\fR\&.
.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)
.SH "AUTHOR"
.PP
\fBLennart Poettering\fR <\&lennart@poettering\&.net\&>
.RS 4
Developer
.RE
.SH "NOTES"
.IP " 1." 4
RFC 4122
.RS 4
\%http://tools.ietf.org/html/rfc4122
.RE