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de31bbc6b1
The semantics were significantly changed inc779a44222("hostnamed: Fix the way that static and transient host names interact", Feb. 2014), but when the dbus api documentation was imported much later, it wasn't properly adjusted to describe those new semantics.34293dfafdwhich added systemd.hostname= also added new behaviour. Let's ove various bits and pieces around so that they are in more appropriate places. Drop recommendations to set the hostname for DHCP or mDNS purposes. Nowadays we expect tools that want to expose some different hostname to the outside to manage that internally without affecting visible state. Also drop mentions of DHCP or mDNS directly setting the hostname, since nowadays network management software is expected to (and does) go through hostnamed. Also, add a high-level description of semantics. It glosses over the details of handling of localhost-style names. Later commits will remove this special handling anyway.
117 lines
5.7 KiB
XML
117 lines
5.7 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
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<!ENTITY % entities SYSTEM "custom-entities.ent" >
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%entities;
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]>
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="hostname">
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<refentryinfo>
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<title>hostname</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>hostname</refentrytitle>
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<manvolnum>5</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>hostname</refname>
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<refpurpose>Local hostname configuration file</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>/etc/hostname</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>The <filename>/etc/hostname</filename> file configures the name of the local system. Unless
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overridden as described in the next section,
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will set this
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hostname during boot using the
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<citerefentry><refentrytitle>sethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry> system
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call.</para>
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<para>The file should contain a single newline-terminated hostname string. Comments (lines starting with
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a <literal>#</literal>) are ignored. The hostname should be composed of up to 64 7-bit ASCII lower-case
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alphanumeric characters or hyphens forming a valid DNS domain name. It is recommended that this name
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contains only a single label, i.e. without any dots. Invalid characters will be filtered out in an
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attempt to make the name valid, but obviously it is recommended to use a valid name and not rely on this
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filtering.</para>
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<para>You may use
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<citerefentry><refentrytitle>hostnamectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to change
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the value of this file during runtime from the command line. Use
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<citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry> to
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initialize it on mounted (but not booted) system images.</para>
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</refsect1>
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<refsect1>
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<title>Hostname semantics</title>
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<para><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> and the
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associated tools will obtain the hostname in the following ways:</para>
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<itemizedlist>
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<listitem><para>If the kernel commandline parameter <varname>systemd.hostname=</varname> specifies a
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valid hostname,
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will use it
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to set the hostname during early boot, see
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<citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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</para></listitem>
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<listitem><para>Otherwise, the "static" hostname specified by <filename>/etc/hostname</filename> as
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described above will be used.</para></listitem>
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<listitem><para>Otherwise, a transient hostname may be set during runtime, for example based on
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information in a DHCP lease, see
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<citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
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Both <ulink url="https://developer.gnome.org/NetworkManager/stable/">NetworkManager</ulink> and
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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allow this. Note that
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<citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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gives higher priority to the static hostname, so the transient hostname will only be used if the static
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hostname is not configured.</para></listitem>
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<listitem><para>Otherwise, a fallback hostname configured at compilation time will be used
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(<literal>&FALLBACK_HOSTNAME;</literal>).</para></listitem>
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<!-- what about the "linux" fallback fallback? -->
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</itemizedlist>
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<para>Effectively, the static hostname has higher priority than a transient hostname, which has higher
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priority than the fallback hostname. Transient hostnames are equivalent, so setting a new transient
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hostname causes the previous transient hostname to be forgotten. The hostname specified on the kernel
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command line is like a transient hostname, with the exception that it has higher priority when the
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machine boots. Also note that those are the semantics implemented by systemd tools, but other programs
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may also set the hostname.</para>
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</refsect1>
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<refsect1>
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<title>History</title>
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<para>The simple configuration file format of
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<filename>/etc/hostname</filename> originates from Debian
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GNU/Linux.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>hostnamectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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