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-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:         compat systemd
97group:          compat [SUCCESS=merge] systemd
98shadow:         compat systemd
99gshadow:        files systemd
100
101
102hosts:          mymachines resolve [!UNAVAIL=return] files <command>myhostname</command> dns
103networks:       files
104
105protocols:      db files
106services:       db files
107ethers:         db files
108rpc:            db files
109
110netgroup:       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
118127.0.0.2       STREAM
119127.0.0.2       DGRAM
120127.0.0.2       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>
139