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-or-later -->

<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>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> either between <literal>resolve</literal>
    and "traditional" modules like <literal>dns</literal>, or after them. In the first version, well-known
    names like <literal>localhost</literal> and the machine hostname are given higher priority than the
    external configuration. This is recommended when the external DNS servers and network are not absolutely
    trusted. In the second version, external configuration is given higher priority and
    <command>nss-myhostname</command> only provides a fallback mechanism. This might be suitable in closely
    controlled networks, for example on a company LAN.</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 systemd
group:          compat [SUCCESS=merge] systemd
shadow:         compat

# Either (untrusted network, see above):
hosts:          mymachines resolve [!UNAVAIL=return] files <command>myhostname</command> dns
# Or (only trusted networks):
hosts:          mymachines resolve [!UNAVAIL=return] files 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>