man/nss-myhostname.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4
   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7
   8   This file is part of systemd.
   9
  10   Copyright 2011 Lennart Poettering
  11   Copyright 2013 Tom Gundersen
  12 -->
  13
  14 <refentry id="nss-myhostname" conditional='ENABLE_MYHOSTNAME'>
  15
  16   <refentryinfo>
  17     <title>nss-myhostname</title>
  18     <productname>systemd</productname>
  19
  20     <authorgroup>
  21       <author>
  22         <contrib>Developer</contrib>
  23         <firstname>Lennart</firstname>
  24         <surname>Poettering</surname>
  25         <email>lennart@poettering.net</email>
  26       </author>
  27     </authorgroup>
  28   </refentryinfo>
  29
  30   <refmeta>
  31     <refentrytitle>nss-myhostname</refentrytitle>
  32     <manvolnum>8</manvolnum>
  33   </refmeta>
  34
  35   <refnamediv>
  36     <refname>nss-myhostname</refname>
  37     <refname>libnss_myhostname.so.2</refname>
  38     <refpurpose>Provide hostname resolution for the locally
  39     configured system hostname.</refpurpose>
  40   </refnamediv>
  41
  42   <refsynopsisdiv>
  43     <para><filename>libnss_myhostname.so.2</filename></para>
  44   </refsynopsisdiv>
  45
  46   <refsect1>
  47     <title>Description</title>
  48
  49     <para><command>nss-myhostname</command> is a plug-in module for the GNU Name Service Switch (NSS) functionality of
  50     the GNU C Library (<command>glibc</command>), primarily providing hostname resolution for the locally configured
  51     system hostname as returned by
  52     <citerefentry><refentrytitle>gethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>.  The precise
  53     hostnames resolved by this module are:</para>
  54
  55     <itemizedlist>
  56       <listitem><para>The local, configured hostname is resolved to
  57       all locally configured IP addresses ordered by their scope, or
  58       — if none are configured — the IPv4 address 127.0.0.2 (which
  59       is on the local loopback) and the IPv6 address ::1 (which is the
  60       local host).</para></listitem>
  61
  62       <listitem><para>The hostnames <literal>localhost</literal> and
  63       <literal>localhost.localdomain</literal> (as well as any hostname
  64       ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
  65       are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
  66
  67       <listitem><para>The hostname <literal>_gateway</literal> is
  68       resolved to all current default routing gateway addresses,
  69       ordered by their metric. This assigns a stable hostname to the
  70       current gateway, useful for referencing it independently of the
  71       current network configuration state.</para></listitem>
  72     </itemizedlist>
  73
  74     <para>Various software relies on an always-resolvable local
  75     hostname. When using dynamic hostnames, this is traditionally
  76     achieved by patching <filename>/etc/hosts</filename> at the same
  77     time as changing the hostname. This is problematic since it
  78     requires a writable <filename>/etc</filename> file system and is
  79     fragile because the file might be edited by the administrator at
  80     the same time. With <command>nss-myhostname</command> enabled,
  81     changing <filename>/etc/hosts</filename> is unnecessary, and on
  82     many systems, the file becomes entirely optional.</para>
  83
  84     <para>To activate the NSS modules, add <literal>myhostname</literal> to the line starting with
  85     <literal>hosts:</literal> in <filename>/etc/nsswitch.conf</filename>.</para>
  86
  87     <para>It is recommended to place <literal>myhostname</literal> last in the <filename>nsswitch.conf</filename>'
  88     <literal>hosts:</literal> line to make sure that this mapping is only used as fallback, and that any DNS or
  89     <filename>/etc/hosts</filename> based mapping takes precedence.</para>
  90   </refsect1>
  91
  92   <refsect1>
  93     <title>Example</title>
  94
  95     <para>Here is an example <filename>/etc/nsswitch.conf</filename> file that enables
  96     <command>nss-myhostname</command> correctly:</para>
  97
  98 <programlisting>passwd:         compat mymachines systemd
  99 group:          compat mymachines systemd
 100 shadow:         compat
 101
 102 hosts:          files mymachines resolve [!UNAVAIL=return] dns <command>myhostname</command>
 103 networks:       files
 104
 105 protocols:      db files
 106 services:       db files
 107 ethers:         db files
 108 rpc:            db files
 109
 110 netgroup:       nis</programlisting>
 111
 112     <para>To test, use <command>glibc</command>'s <command>getent</command> tool:</para>
 113
 114     <programlisting>$ getent ahosts `hostname`
 115 ::1       STREAM omega
 116 ::1       DGRAM
 117 ::1       RAW
 118 127.0.0.2       STREAM
 119 127.0.0.2       DGRAM
 120 127.0.0.2       RAW</programlisting>
 121
 122     <para>In this case, the local hostname is <varname>omega</varname>.</para>
 123
 124   </refsect1>
 125
 126   <refsect1>
 127     <title>See Also</title>
 128     <para>
 129       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 130       <citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 131       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 132       <citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 133       <citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 134       <citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 135     </para>
 136   </refsect1>
 137
 138 </refentry>