man/nss-myhostname.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
   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>Hostname resolution for the locally configured system hostname</refpurpose>
  22   </refnamediv>
  23
  24   <refsynopsisdiv>
  25     <para><filename>libnss_myhostname.so.2</filename></para>
  26   </refsynopsisdiv>
  27
  28   <refsect1>
  29     <title>Description</title>
  30
  31     <para><command>nss-myhostname</command> is a plug-in module for the GNU Name Service Switch (NSS) functionality of
  32     the GNU C Library (<command>glibc</command>), primarily providing hostname resolution for the locally configured
  33     system hostname as returned by
  34     <citerefentry><refentrytitle>gethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>.  The precise
  35     hostnames resolved by this module are:</para>
  36
  37     <itemizedlist>
  38       <listitem><para>The local, configured hostname is resolved to
  39       all locally configured IP addresses ordered by their scope, or
  40       — if none are configured — the IPv4 address 127.0.0.2 (which
  41       is on the local loopback) and the IPv6 address ::1 (which is the
  42       local host).</para></listitem>
  43
  44       <listitem><para>The hostnames <literal>localhost</literal> and
  45       <literal>localhost.localdomain</literal> (as well as any hostname
  46       ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
  47       are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
  48
  49       <listitem><para>The hostname <literal>_gateway</literal> is
  50       resolved to all current default routing gateway addresses,
  51       ordered by their metric. This assigns a stable hostname to the
  52       current gateway, useful for referencing it independently of the
  53       current network configuration state.</para></listitem>
  54
  55       <listitem><para>The hostname <literal>_outbound</literal> is resolved to the local IPv4 and IPv6
  56       addresses that are most likely used for communication with other hosts. This is determined by
  57       requesting a routing decision to the configured default gateways from the kernel and then using the
  58       local IP addresses selected by this decision. This hostname is only available if there is at least one
  59       local default gateway configured. This assigns a stable hostname to the local outbound IP addresses,
  60       useful for referencing them independently of the current network configuration state.</para></listitem>
  61     </itemizedlist>
  62
  63     <para>Various software relies on an always-resolvable local
  64     hostname. When using dynamic hostnames, this is traditionally
  65     achieved by patching <filename>/etc/hosts</filename> at the same
  66     time as changing the hostname. This is problematic since it
  67     requires a writable <filename>/etc/</filename> file system and is
  68     fragile because the file might be edited by the administrator at
  69     the same time. With <command>nss-myhostname</command> enabled,
  70     changing <filename>/etc/hosts</filename> is unnecessary, and on
  71     many systems, the file becomes entirely optional.</para>
  72
  73     <para>To activate the NSS modules, add <literal>myhostname</literal> to the line starting with
  74     <literal>hosts:</literal> in <filename>/etc/nsswitch.conf</filename>.</para>
  75
  76     <para>It is recommended to place <literal>myhostname</literal> after <literal>file</literal> and before <literal>dns</literal>.
  77     This resolves well-known hostnames like <literal>localhost</literal>
  78     and the machine hostnames locally. It is consistent with the behaviour
  79     of <command>nss-resolve</command>, and still allows overriding via
  80     <filename>/etc/hosts</filename>.</para>
  81
  82     <para>Please keep in mind that <command>nss-myhostname</command> (and <command>nss-resolve</command>) also resolve
  83     in the other direction — from locally attached IP addresses to
  84     hostnames. If you rely on that lookup being provided by DNS, you might
  85     want to order things differently.
  86     </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     <!-- synchronize with other nss-* man pages and factory/etc/nsswitch.conf -->
  96 <programlisting>passwd:         files systemd
  97 group:          files [SUCCESS=merge] systemd
  98 shadow:         files systemd
  99 gshadow:        files systemd
 100
 101 hosts:          mymachines resolve [!UNAVAIL=return] files <command>myhostname</command> dns
 102 networks:       files
 103
 104 protocols:      db files
 105 services:       db files
 106 ethers:         db files
 107 rpc:            db files
 108
 109 netgroup:       nis</programlisting>
 110
 111     <para>To test, use <command>glibc</command>'s
 112     <citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 113     tool:</para>
 114
 115     <programlisting>$ getent ahosts `hostname`
 116 ::1       STREAM omega
 117 ::1       DGRAM
 118 ::1       RAW
 119 127.0.0.2       STREAM
 120 127.0.0.2       DGRAM
 121 127.0.0.2       RAW</programlisting>
 122
 123     <para>In this case, the local hostname is <varname>omega</varname>.</para>
 124
 125   </refsect1>
 126
 127   <refsect1>
 128     <title>See Also</title>
 129     <para><simplelist type="inline">
 130       <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 131       <member><citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 132       <member><citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 133       <member><citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 134       <member><citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
 135       <member><citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 136     </simplelist></para>
 137   </refsect1>
 138
 139 </refentry>