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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
<!ENTITY % entities SYSTEM "custom-entities.ent" >
%entities;
]>
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<refentry id="hostname">
<refentryinfo>
<title>hostname</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>hostname</refentrytitle>
<manvolnum>5</manvolnum>
</refmeta>
<refnamediv>
<refname>hostname</refname>
<refpurpose>Local hostname configuration file</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>/etc/hostname</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>The <filename>/etc/hostname</filename> file configures the name of the local system. Unless
overridden as described in the next section,
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will set this
hostname during boot using the
<citerefentry><refentrytitle>sethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry> system
call.</para>
<para>The file should contain a single newline-terminated hostname string. Comments (lines starting with
a <literal>#</literal>) are ignored. The hostname should be composed of up to 64 7-bit ASCII lower-case
alphanumeric characters or hyphens forming a valid DNS domain name. It is strongly recommended that this
name contains only a single DNS label, i.e does not contain any dots. This recommendation reflects both
usual expectations of applications, and the fact that the Linux kernel limits the length of the system
hostname to 64 (i.e. close to the maximum DNS label length of 63) rather than 255 (the maximum DNS domain
name length). When applied, invalid characters will be filtered out in an attempt to make the name valid,
but obviously it is recommended to use a valid name and not rely on this filtering.</para>
<para id="question-mark-hostname-pattern">If the question mark character <literal>?</literal> appears in
the hostname, it is automatically substituted by a hexadecimal character derived from the
<citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry> when
applied, securely and deterministically by cryptographic hashing. Example:
<literal>foobar-????-????</literal> will automatically expand to <literal>foobar-92a9-061c</literal> or
similar, depending on the local machine ID.</para>
<para>You may use
<citerefentry><refentrytitle>hostnamectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to change
the value of this file during runtime from the command line. Use
<citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry> to
initialize it on mounted (but not booted) system images.</para>
</refsect1>
<refsect1>
<title>Hostname semantics</title>
<para><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> and the
associated tools will obtain the hostname in the following ways:</para>
<itemizedlist>
<listitem><para>If the kernel command line parameter <varname>systemd.hostname=</varname> specifies a
valid hostname,
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will use it
to set the hostname during early boot, see
<citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
</para></listitem>
<listitem><para>Otherwise, the "static" hostname specified by <filename>/etc/hostname</filename> as
described above will be used.</para></listitem>
<listitem><para>Otherwise, a transient hostname may be set during runtime, for example based on
information in a DHCP lease, see
<citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
Both <ulink url="https://developer.gnome.org/NetworkManager/stable/">NetworkManager</ulink> and
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
allow this. Note that
<citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
gives higher priority to the static hostname, so the transient hostname will only be used if the static
hostname is not configured.</para></listitem>
<listitem><para>Otherwise, a fallback hostname configured at compilation time will be used
(<literal>&FALLBACK_HOSTNAME;</literal>).</para></listitem>
<!-- what about the "linux" fallback fallback? -->
</itemizedlist>
<para>Effectively, the static hostname has higher priority than a transient hostname, which has higher
priority than the fallback hostname. Transient hostnames are equivalent, so setting a new transient
hostname causes the previous transient hostname to be forgotten. The hostname specified on the kernel
command line is like a transient hostname, with the exception that it has higher priority when the
machine boots. Also note that those are the semantics implemented by systemd tools, but other programs
may also set the hostname.</para>
</refsect1>
<refsect1>
<title>History</title>
<para>The simple configuration file format of
<filename>/etc/hostname</filename> originates from Debian
GNU/Linux.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para><simplelist type="inline">
<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>hostnamectl</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
</simplelist></para>
</refsect1>
</refentry>
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