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</style><a href="index.html">Index </a>·
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<a href="systemd.directives.html">Directives </a>·
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<a href="../python-systemd/index.html">Python </a>·
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<a href="../libudev/index.html">libudev </a>·
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<a href="../libudev/index.html">gudev </a><span style="float:right">systemd 219</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="idm140598384460224"></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
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unique machine ID of the local system that is set during
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installation. The machine ID is a single newline-terminated,
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hexadecimal, 32-character, lowercase machine ID string. When
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decoded from hexadecimal, this corresponds with a 16-byte/128-bit
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string.</p><p>The machine ID is usually generated from a random source
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during system installation and stays constant for all subsequent
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boots. Optionally, for stateless systems, it is generated during
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runtime at boot if it is found to be empty.</p><p>The machine ID does not change based on user configuration
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or when hardware is replaced.</p><p>This machine ID adheres to the same format and logic as the
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D-Bus machine ID.</p><p>Programs may use this ID to identify the host with a
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globally unique ID in the network, which does not change even if
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the local network configuration changes. Due to this and its
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greater length, it is a more useful replacement for the
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<a href="gethostid.html"><span class="citerefentry"><span class="refentrytitle">gethostid</span>(3)</span></a>
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call that POSIX specifies.</p><p>The
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<a href="systemd-machine-id-setup.html"><span class="citerefentry"><span class="refentrytitle">systemd-machine-id-setup</span>(1)</span></a>
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tool may be used by installer tools to initialize the machine ID
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at install time. Use
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<a href="systemd-firstboot.html"><span class="citerefentry"><span class="refentrytitle">systemd-firstboot</span>(1)</span></a>
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to initialize it on mounted (but not booted) system images.</p></div><div class="refsect1"><a name="idm140598382817232"></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
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defined by <a class="ulink" href="https://tools.ietf.org/html/rfc4122" target="_top">RFC
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4122</a>, nor a Microsoft GUID; however, starting with systemd
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v30, newly generated machine IDs do qualify as v4 UUIDs.</p><p>In order to maintain compatibility with existing
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installations, an application requiring a UUID should decode the
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machine ID, and then apply the following operations to turn it
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into a valid OSF v4 UUID. With "<code class="literal">id</code>" being an
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unsigned character array:</p><pre class="programlisting">/* Set UUID version to 4 --- truly random generation */
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id[6] = (id[6] & 0x0F) | 0x40;
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/* Set the UUID variant to DCE */
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id[8] = (id[8] & 0x3F) | 0x80;</pre><p>(This code is inspired by
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"<code class="literal">generate_random_uuid()</code>" of
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<code class="filename">drivers/char/random.c</code> from the Linux kernel
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sources.)</p></div><div class="refsect1"><a name="idm140598382811008"></a><h2 id="History">History<a class="headerlink" title="Permalink to this headline" href="#History">¶</a></h2><p>The simple configuration file format of
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<code class="filename">/etc/machine-id</code> originates in the
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<code class="filename">/var/lib/dbus/machine-id</code> file introduced by
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D-Bus. In fact, this latter file might be a symlink to
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<code class="varname">/etc/machine-id</code>.</p></div><div class="refsect1"><a name="idm140598382807984"></a><h2 id="See Also">See Also<a class="headerlink" title="Permalink to this headline" href="#See%20Also">¶</a></h2><p>
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<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>,
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<a href="systemd-machine-id-setup.html"><span class="citerefentry"><span class="refentrytitle">systemd-machine-id-setup</span>(1)</span></a>,
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<a href="gethostid.html"><span class="citerefentry"><span class="refentrytitle">gethostid</span>(3)</span></a>,
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<a href="hostname.html"><span class="citerefentry"><span class="refentrytitle">hostname</span>(5)</span></a>,
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<a href="machine-info.html"><span class="citerefentry"><span class="refentrytitle">machine-info</span>(5)</span></a>,
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<a href="os-release.html"><span class="citerefentry"><span class="refentrytitle">os-release</span>(5)</span></a>,
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<a href="sd-id128.html"><span class="citerefentry"><span class="refentrytitle">sd-id128</span>(3)</span></a>,
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<a href="sd_id128_get_machine.html"><span class="citerefentry"><span class="refentrytitle">sd_id128_get_machine</span>(3)</span></a>,
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<a href="systemd-firstboot.html"><span class="citerefentry"><span class="refentrytitle">systemd-firstboot</span>(1)</span></a>
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</p></div></div></body></html>
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