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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<!--
  This file is part of systemd.

  Copyright 2011 Lennart Poettering
  Copyright 2013 Tom Gundersen

  systemd is free software; you can redistribute it and/or modify it
  under the terms of the GNU Lesser General Public License as published by
  the Free Software Foundation; either version 2.1 of the License, or
  (at your option) any later version.

  systemd is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License
  along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->

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

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

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </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 plugin 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 hostname <literal>localhost</literal> is
      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, <literal>myhostname</literal>
    has to be added 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> 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>myhostname</command> correctly:</para>

<programlisting>passwd:         compat mymachines
group:          compat mymachines
shadow:         compat

hosts:          files resolve mymachines <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-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>