man/nss-myhostname.xml

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