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2<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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7<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
8
9<refentry id="hostname">
10  <refentryinfo>
11    <title>hostname</title>
12    <productname>systemd</productname>
13  </refentryinfo>
14
15  <refmeta>
16    <refentrytitle>hostname</refentrytitle>
17    <manvolnum>5</manvolnum>
18  </refmeta>
19
20  <refnamediv>
21    <refname>hostname</refname>
22    <refpurpose>Local hostname configuration file</refpurpose>
23  </refnamediv>
24
25  <refsynopsisdiv>
26    <para><filename>/etc/hostname</filename></para>
27  </refsynopsisdiv>
28
29  <refsect1>
30    <title>Description</title>
31
32    <para>The <filename>/etc/hostname</filename> file configures the name of the local system. Unless
33    overridden as described in the next section,
34    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will set this
35    hostname during boot using the
36    <citerefentry><refentrytitle>sethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry> system
37    call.</para>
38
39    <para>The file should contain a single newline-terminated hostname string. Comments (lines starting with
40    a <literal>#</literal>) are ignored. The hostname should be composed of up to 64 7-bit ASCII lower-case
41    alphanumeric characters or hyphens forming a valid DNS domain name. It is recommended that this name
42    contains only a single label, i.e. without any dots. Invalid characters will be filtered out in an
43    attempt to make the name valid, but obviously it is recommended to use a valid name and not rely on this
44    filtering.</para>
45
46    <para>You may use
47    <citerefentry><refentrytitle>hostnamectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to change
48    the value of this file during runtime from the command line. Use
49    <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry> to
50    initialize it on mounted (but not booted) system images.</para>
51  </refsect1>
52
53  <refsect1>
54    <title>Hostname semantics</title>
55
56    <para><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> and the
57    associated tools will obtain the hostname in the following ways:</para>
58    <itemizedlist>
59      <listitem><para>If the kernel commandline parameter <varname>systemd.hostname=</varname> specifies a
60      valid hostname,
61      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will use it
62      to set the hostname during early boot, see
63      <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
64      </para></listitem>
65
66      <listitem><para>Otherwise, the "static" hostname specified by <filename>/etc/hostname</filename> as
67      described above will be used.</para></listitem>
68
69      <listitem><para>Otherwise, a transient hostname may be set during runtime, for example based on
70      information in a DHCP lease, see
71      <citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
72      Both <ulink url="https://developer.gnome.org/NetworkManager/stable/">NetworkManager</ulink> and
73      <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
74      allow this. Note that
75      <citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
76      gives higher priority to the static hostname, so the transient hostname will only be used if the static
77      hostname is not configured.</para></listitem>
78
79      <listitem><para>Otherwise, a fallback hostname configured at compilation time will be used
80      (<literal>&FALLBACK_HOSTNAME;</literal>).</para></listitem>
81
82      <!-- what about the "linux" fallback fallback? -->
83    </itemizedlist>
84
85    <para>Effectively, the static hostname has higher priority than a transient hostname, which has higher
86    priority than the fallback hostname. Transient hostnames are equivalent, so setting a new transient
87    hostname causes the previous transient hostname to be forgotten. The hostname specified on the kernel
88    command line is like a transient hostname, with the exception that it has higher priority when the
89    machine boots. Also note that those are the semantics implemented by systemd tools, but other programs
90    may also set the hostname.</para>
91  </refsect1>
92
93  <refsect1>
94    <title>History</title>
95
96    <para>The simple configuration file format of
97    <filename>/etc/hostname</filename> originates from Debian
98    GNU/Linux.</para>
99  </refsect1>
100
101  <refsect1>
102      <title>See Also</title>
103      <para>
104        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
105        <citerefentry><refentrytitle>sethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
106        <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
107        <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
108        <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
109        <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
110        <citerefentry><refentrytitle>hostnamectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
111        <citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
112        <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
113      </para>
114  </refsect1>
115
116</refentry>
117