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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>machine-id</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><style>
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</style><a href="index.html">Index </a>·
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<a href="../libudev/index.html">gudev </a><span style="float:right">systemd 215</span><hr><div class="refentry"><a name="machine-id"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>machine-id — Local machine ID configuration file</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><p><code class="filename">/etc/machine-id</code></p></div><div class="refsect1"><a name="idm214174731056"></a><h2 id="Description">Description<a class="headerlink" title="Permalink to this headline" href="#Description">¶</a></h2><p>The <code class="filename">/etc/machine-id</code> file
contains the unique machine ID of the local system
that is set during installation. The machine ID is a
single newline-terminated, hexadecimal, 32-character,
lowercase machine ID string. When decoded from
hexadecimal, this corresponds with a 16-byte/128-bit
string.</p><p>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.</p><p>The machine ID does not change based on user
configuration or when hardware is replaced.</p><p>This machine ID adheres to the same format and
logic as the D-Bus machine ID.</p><p>Programs may use this ID to identify the host
with a globally unique ID in the network, which 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
<a href="gethostid.html"><span class="citerefentry"><span class="refentrytitle">gethostid</span>(3)</span></a>
call that POSIX specifies.</p><p>The
<a href="systemd-machine-id-setup.html"><span class="citerefentry"><span class="refentrytitle">systemd-machine-id-setup</span>(1)</span></a>
tool may be used by installer tools to initialize the
machine ID at install time.</p></div><div class="refsect1"><a name="idm214175404608"></a><h2 id="Relation to OSF UUIDs">Relation to OSF UUIDs<a class="headerlink" title="Permalink to this headline" href="#Relation%20to%20OSF%20UUIDs">¶</a></h2><p>Note that the machine ID historically is not an
OSF UUID as defined by <a class="ulink" href="https://tools.ietf.org/html/rfc4122" target="_top">RFC
4122</a>, nor a Microsoft GUID; however, starting with
systemd v30, newly generated machine IDs do
qualify as v4 UUIDs.</p><p>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
"<code class="literal">id</code>" being an unsigned character
array:</p><pre class="programlisting">/* 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;</pre><p>(This code is inspired by
"<code class="literal">generate_random_uuid()</code>" of
<code class="filename">drivers/char/random.c</code> from the
Linux kernel sources.)</p></div><div class="refsect1"><a name="idm214178157136"></a><h2 id="History">History<a class="headerlink" title="Permalink to this headline" href="#History">¶</a></h2><p>The simple configuration file format of
<code class="filename">/etc/machine-id</code> originates in the
<code class="filename">/var/lib/dbus/machine-id</code> file
introduced by D-Bus. In fact, this latter file might be a
symlink to
<code class="varname">/etc/machine-id</code>.</p></div><div class="refsect1"><a name="idm214177106384"></a><h2 id="See Also">See Also<a class="headerlink" title="Permalink to this headline" href="#See%20Also">¶</a></h2><p>
<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>,
<a href="systemd-machine-id-setup.html"><span class="citerefentry"><span class="refentrytitle">systemd-machine-id-setup</span>(1)</span></a>,
<a href="gethostid.html"><span class="citerefentry"><span class="refentrytitle">gethostid</span>(3)</span></a>,
<a href="hostname.html"><span class="citerefentry"><span class="refentrytitle">hostname</span>(5)</span></a>,
<a href="machine-info.html"><span class="citerefentry"><span class="refentrytitle">machine-info</span>(5)</span></a>,
<a href="os-release.html"><span class="citerefentry"><span class="refentrytitle">os-release</span>(5)</span></a>,
<a href="sd-id128.html"><span class="citerefentry"><span class="refentrytitle">sd-id128</span>(3)</span></a>,
<a href="sd_id128_get_machine.html"><span class="citerefentry"><span class="refentrytitle">sd_id128_get_machine</span>(3)</span></a>
</p></div></div></body></html>
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