systemd/man/nss-myhostname.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1+ -->

<refentry id="nss-myhostname" conditional='ENABLE_NSS_MYHOSTNAME'>

  <refentryinfo>
    <title>nss-myhostname</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>nss-myhostname</refentrytitle>
    <manvolnum>8</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>nss-myhostname</refname>
    <refname>libnss_myhostname.so.2</refname>
    <refpurpose>Provide hostname resolution for the locally
    configured system hostname.</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename>libnss_myhostname.so.2</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><command>nss-myhostname</command> is a plug-in module for the GNU Name Service Switch (NSS) functionality of
    the GNU C Library (<command>glibc</command>), primarily providing hostname resolution for the locally configured
    system hostname as returned by
    <citerefentry><refentrytitle>gethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>.  The precise
    hostnames resolved by this module are:</para>

    <itemizedlist>
      <listitem><para>The local, configured hostname is resolved to
      all locally configured IP addresses ordered by their scope, or
      — if none are configured — the IPv4 address 127.0.0.2 (which
      is on the local loopback) and the IPv6 address ::1 (which is the
      local host).</para></listitem>

      <listitem><para>The hostnames <literal>localhost</literal> and
      <literal>localhost.localdomain</literal> (as well as any hostname
      ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
      are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>

      <listitem><para>The hostname <literal>_gateway</literal> is
      resolved to all current default routing gateway addresses,
      ordered by their metric. This assigns a stable hostname to the
      current gateway, useful for referencing it independently of the
      current network configuration state.</para></listitem>
    </itemizedlist>

    <para>Various software relies on an always-resolvable local
    hostname. When using dynamic hostnames, this is traditionally
    achieved by patching <filename>/etc/hosts</filename> at the same
    time as changing the hostname. This is problematic since it
    requires a writable <filename>/etc</filename> file system and is
    fragile because the file might be edited by the administrator at
    the same time. With <command>nss-myhostname</command> enabled,
    changing <filename>/etc/hosts</filename> is unnecessary, and on
    many systems, the file becomes entirely optional.</para>

    <para>To activate the NSS modules, add <literal>myhostname</literal> to the line starting with
    <literal>hosts:</literal> in <filename>/etc/nsswitch.conf</filename>.</para>

    <para>It is recommended to place <literal>myhostname</literal> last in the <filename>nsswitch.conf</filename>'
    <literal>hosts:</literal> line to make sure that this mapping is only used as fallback, and that any DNS or
    <filename>/etc/hosts</filename> based mapping takes precedence.</para>
  </refsect1>

  <refsect1>
    <title>Example</title>

    <para>Here is an example <filename>/etc/nsswitch.conf</filename> file that enables
    <command>nss-myhostname</command> correctly:</para>

    <!-- synchronize with other nss-* man pages and factory/etc/nsswitch.conf -->
<programlisting>passwd:         compat mymachines systemd
group:          compat mymachines systemd
shadow:         compat

hosts:          files mymachines resolve [!UNAVAIL=return] dns <command>myhostname</command>
networks:       files

protocols:      db files
services:       db files
ethers:         db files
rpc:            db files

netgroup:       nis</programlisting>

    <para>To test, use <command>glibc</command>'s <command>getent</command> tool:</para>

    <programlisting>$ getent ahosts `hostname`
::1       STREAM omega
::1       DGRAM
::1       RAW
127.0.0.2       STREAM
127.0.0.2       DGRAM
127.0.0.2       RAW</programlisting>

    <para>In this case, the local hostname is <varname>omega</varname>.</para>

  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>