systemd/man/org.freedesktop.hostname1.xml

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<!-- SPDX-License-Identifier: LGPL-2.1+ -->

<refentry id="org.freedesktop.hostname1" conditional='ENABLE_HOSTNAMED'
    xmlns:xi="http://www.w3.org/2001/XInclude">
  <refentryinfo>
    <title>org.freedesktop.hostname1</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>org.freedesktop.hostname1</refentrytitle>
    <manvolnum>5</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>org.freedesktop.hostname1</refname>
    <refpurpose>The D-Bus interface of systemd-hostnamed</refpurpose>
  </refnamediv>

  <refsect1>
    <title>Introduction</title>

    <para>
    <citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    is a system service that can be used to control the hostname and related machine metadata from user
    programs. This page describes the hostname semantics and the D-Bus interface.</para>
  </refsect1>

  <refsect1>
    <title>The D-Bus API</title>

    <para>The service exposes the following interfaces on the bus:</para>

    <programlisting>
$ gdbus introspect --system \
        --dest org.freedesktop.hostname1 \
        --object-path /org/freedesktop/hostname1

node /org/freedesktop/hostname1 {
  interface org.freedesktop.hostname1 {
    methods:
      SetHostname(in  s hostname,
                  in  b interactive);
      SetStaticHostname(in  s hostname,
                        in  b interactive);
      SetPrettyHostname(in  s hostname,
                        in  b interactive);
      SetIconName(in  s icon,
                  in  b interactive);
      SetChassis(in  s chassis,
                 in  b interactive);
      SetDeployment(in  s deployment,
                    in  b interactive);
      SetLocation(in  s location,
                  in  b interactive);
      GetProductUUID(in  b interactive,
                     out ay uuid);
    properties:
      readonly s Hostname = '...';
      readonly s StaticHostname = '...';
      readonly s PrettyHostname = '...';
      readonly s IconName = '...';
      readonly s Chassis = '...';
      readonly s Deployment = '...';
      readonly s Location = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
      readonly s KernelName = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
      readonly s KernelRelease = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
      readonly s KernelVersion = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
      readonly s OperatingSystemPrettyName = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
      readonly s OperatingSystemCPEName = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
      readonly s HomeURL = '...';
  };
  interface org.freedesktop.DBus.Peer { ... };
  interface org.freedesktop.DBus.Introspectable { ... };
  interface org.freedesktop.DBus.Properties { ... };
};
    </programlisting>

    <!--method SetDeployment is not documented!-->

    <!--method SetLocation is not documented!-->

    <!--method GetProductUUID is not documented!-->

    <!--property Hostname is not documented!-->

    <!--property StaticHostname is not documented!-->

    <!--property PrettyHostname is not documented!-->

    <!--property IconName is not documented!-->

    <!--property Chassis is not documented!-->

    <!--property Deployment is not documented!-->

    <!--property Location is not documented!-->

    <!--property KernelName is not documented!-->

    <!--property KernelRelease is not documented!-->

    <!--property KernelVersion is not documented!-->

    <!--property OperatingSystemPrettyName is not documented!-->

    <!--property OperatingSystemCPEName is not documented!-->

    <!--property HomeURL is not documented!-->

    <para>Whenever the hostname or other metadata is changed via the daemon,
    <function>PropertyChanged</function> signals are sent out to subscribed clients. Changing a hostname
    using this interface is authenticated via PolicyKit.</para>
  </refsect1>

  <refsect1>
    <title>Semantics</title>

    <para>The <emphasis>static (configured) hostname</emphasis> is the one configured in
    <filename>/etc/hostname</filename>. It is chosen by the local user. It is not always in sync with the
    current hostname as returned by the
    <citerefentry project="man-pages"><refentrytitle>gethostname</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    system call. If no hostname is configured this property will be the empty string. Setting this property
    to the empty string will remove <filename>/etc/hostname</filename>. This property should be an
    internet-style hostname, 7-bit lowercase ASCII, no special chars/spaces.</para>

    <para>The <emphasis>transient (dynamic) hostname</emphasis> is the one configured via the kernel's
    <citerefentry project="man-pages"><refentrytitle>sethostname</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
    It can be different from the static hostname if DHCP or mDNS have been configured to change the name
    based on network information. <!-- FIXME: it's not DHCP that configures this... -->
    This property is never empty. If no hostname is set this will default to
    <literal>&FALLBACK_HOSTNAME;</literal> (configurable at compilation time). Setting this property to the
    empty string will reset the dynamic hostname to the static hostname. If no static hostname is
    configured the dynamic hostname will be reset to <literal>&FALLBACK_HOSTNAME;</literal>. This property
    should be an internet-style hostname, 7-bit lowercase ASCII, no special chars/spaces.</para>

    <para>The <emphasis>pretty hostname</emphasis> is a free-form UTF-8 hostname for presentation to the
    user. User interfaces should ensure that the pretty hostname and the static hostname stay in sync.
    I.e. when the former is <literal>Lennart’s Computer</literal> the latter should be
    <literal>lennarts-computer</literal>. If no pretty hostname is set this setting will be the empty
    string. Applications should then find a suitable fallback, such as the dynamic hostname.</para>

    <para>The <emphasis>icon name</emphasis> is a name following the XDG icon naming spec. If not set,
    information such as the chassis type (see below) is used to find a suitable fallback icon name
    (i.e. <literal>computer-laptop</literal> vs. <literal>computer-desktop</literal> is picked based on the
    chassis information). If no such data is available, the empty string is returned. In that case an application
    should fall back to a replacement icon, for example <literal>computer</literal>. If this property is set
    to the empty string, the automatic fallback name selection is enabled again.</para>

    <para>The <emphasis>chassis type</emphasis> should be one of the currently defined chassis types:
    <literal>desktop</literal>, <literal>laptop</literal>, <literal>server</literal>,
    <literal>tablet</literal>, <literal>handset</literal>, as well as the special chassis types
    <literal>vm</literal> and <literal>container</literal> for virtualized systems. Note that in most cases
    the chassis type will be determined automatically from DMI/SMBIOS/ACPI firmware information. Writing to
    this setting is hence useful only to override misdetected chassis types, or to configure the chassis type if
    it could not be auto-detected. Set this property to the empty string to reenable the automatic detection of
    the chassis type from firmware information.</para>

    <para>A client that wants to change the local hostname for DHCP/mDNS should invoke
    <code>SetHostname("newname", false)</code> as soon as the name is available and afterwards reset it via
    <code>SetHostname("")</code>.</para>

    <para>Note that <filename>systemd-hostnamed</filename> starts only on request and terminates after a
    short idle period. This effectively means that <function>PropertyChanged</function> messages are not sent
    out for changes made directly on the files (as in: administrator edits the files with vi). This is
    the intended behavior: manual configuration changes should require manual reloading.</para>

    <para>The transient (dynamic) hostname maps directly to the kernel hostname. This hostname should be
    assumed to be highly dynamic, and hence should be watched directly, without depending on
    <function>PropertyChanged</function> messages from <filename>systemd-hostnamed</filename>. To accomplish
    this, open <filename>/proc/sys/kernel/hostname</filename> and
    <citerefentry project="man-pages"><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    for <constant>SIGHUP</constant> which is triggered by the kernel every time the hostname changes. Again:
    this is special for the transient (dynamic) hostname, and does not apply to the configured (fixed)
    hostname.</para>

    <para>Applications may read the hostname data directly if hostname change notifications
    are not necessary. Use
    <citerefentry project="man-pages"><refentrytitle>gethostname</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    <filename>/etc/hostname</filename> (possibly with per-distribution fallbacks), and
    <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    for that. For more information on these files and syscalls see the respective man pages.</para>

    <para>The <varname>user_interaction</varname> boolean parameters can be used to control whether PolicyKit
    should interactively ask the user for authentication credentials if required.</para>

    <para>The PolicyKit action for <function>SetHostname()</function> is
    <interfacename>org.freedesktop.hostname1.set-hostname</interfacename>. For
    <function>SetStaticHostname()</function> and <function>SetPrettyHostname()</function> it is
    <interfacename>org.freedesktop.hostname1.set-static-hostname</interfacename>. For
    <function>SetIconName()</function> and <function>SetChassis()</function> it is
    <interfacename>org.freedesktop.hostname1.set-machine-info</interfacename>.</para>

    <para>Here are three examples show how the pretty hostname and the icon name should be used:
    <itemizedlist>
      <listitem><para>When registering DNS-SD services: use the pretty hostname in the service name, and
      pass the icon name in the TXT data, if there is an icon name. Browsing clients can then show the server
      icon on each service. This is especially useful for WebDAV applications or UPnP media sharing.
      </para></listitem>

      <listitem><para>Set the bluetooth name to the pretty hostname.</para></listitem>

      <listitem><para>When your file browser has a "Computer" icon, replace the name with the pretty hostname if set, and the icon with the icon name, if it is set.</para></listitem>
    </itemizedlist></para>

    <para>To properly handle name lookups with changing local hostnames without having to edit
    <filename>/etc/hosts</filename>, we recommend using <filename>systemd-hostnamed</filename> in
    combination with <citerefentry><refentrytitle>nss-myhostname</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
    </para>

    <para>Here are some recommendations to follow when generating a static (internet) hostname from a pretty
    name:
    <itemizedlist>
      <listitem><para>Generate a single DNS label only, not an FQDN. That means no dots allowed. Strip them,
      or replace them with <literal>-</literal>.</para></listitem>

      <listitem><para>It's probably safer to not use any non-ASCII chars, even if DNS allows this in some way
      these days. In fact, restrict your charset to <literal>a-zA-Z0-9</literal> and <literal>-</literal>.
      Strip other chars, or try to replace them in some smart way with chars from this set, for example
      <literal>ä</literal> → <literal>ae</literal>, and use <literal>-</literal> as the replacement for all
      punctuation characters and whitespace.</para></listitem>

      <listitem><para>Try to avoid creating repeated <literal>-</literal>, as well as <literal>-</literal> as
      the first or last char.</para></listitem>

      <listitem><para>Limit the hostname to 63 chars, which is the length of a DNS label.</para></listitem>

      <listitem><para>If after stripping special chars the empty string is the result, you can pass this
      as-is to <filename>systemd-hostnamed</filename> in which case it will automatically use
      <literal>&FALLBACK_HOSTNAME;</literal>.</para></listitem>

      <listitem><para>Uppercase charaacters should be replaced with their lowercase equivalents.
      </para></listitem>
    </itemizedlist></para>

    <para>Note that while <filename>systemd-hostnamed</filename> applies some checks to the hostname you pass
    they are much looser than the recommendations above. For example, <filename>systemd-hostnamed</filename>
    will also accept <literal>_</literal> in the hostname, but we recommend not using this to avoid clashes
    with DNS-SD service types. Also <filename>systemd-hostnamed</filename> allows longer hostnames, but
    because of the DNS label limitations, we recommend not making use of this.</para>

    <para>Here are a couple of example conversions:
    <itemizedlist>
      <listitem><para><literal>Lennart's PC</literal> → <literal>lennarts-pc</literal></para></listitem>
      <listitem><para><literal>Müllers Computer</literal> → <literal>muellers-computer</literal></para></listitem>
      <listitem><para><literal>Voran!</literal> → <literal>voran</literal></para></listitem>
      <listitem><para><literal>Es war einmal ein Männlein</literal> → <literal>es-war-einmal-ein-maennlein</literal></para></listitem>
      <listitem><para><literal>Jawoll. Ist doch wahr!</literal> → <literal>jawoll-ist-doch-wahr</literal></para></listitem>
      <listitem><para><literal>レナート</literal> → <literal>localhost</literal></para></listitem>
      <listitem><para><literal>...zack!!! zack!...</literal> → <literal>zack-zack</literal></para></listitem>
    </itemizedlist></para>

    <para>Of course, an already valid internet hostname label you enter and pass through this
    conversion should stay unmodified, so that users have direct control of it, if they want -- by simply
    ignoring the fact that the pretty hostname is pretty and just edit it as if it was the normal internet
    name.</para>
  </refsect1>

  <refsect1>
    <title>Versioning</title>

    <para>These D-Bus interfaces follow <ulink url="http://0pointer.de/blog/projects/versioning-dbus.html">
    the usual interface versioning guidelines</ulink>.</para>
  </refsect1>

  <refsect1>
    <title>See also</title>

    <para>David Zeuthen's original Fedora
    <ulink url="https://fedoraproject.org/wiki/Features/BetterHostname">Feature page about xdg-hostname</ulink></para>
  </refsect1>
</refentry>