man/nss-myhostname.xml

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