man/nss-myhostname.xml

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  10   Copyright 2011 Lennart Poettering
  11   Copyright 2013 Tom Gundersen
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  26
  27 <refentry id="nss-myhostname" conditional='ENABLE_MYHOSTNAME'>
  28
  29   <refentryinfo>
  30     <title>nss-myhostname</title>
  31     <productname>systemd</productname>
  32
  33     <authorgroup>
  34       <author>
  35         <contrib>Developer</contrib>
  36         <firstname>Lennart</firstname>
  37         <surname>Poettering</surname>
  38         <email>lennart@poettering.net</email>
  39       </author>
  40     </authorgroup>
  41   </refentryinfo>
  42
  43   <refmeta>
  44     <refentrytitle>nss-myhostname</refentrytitle>
  45     <manvolnum>8</manvolnum>
  46   </refmeta>
  47
  48   <refnamediv>
  49     <refname>nss-myhostname</refname>
  50     <refname>libnss_myhostname.so.2</refname>
  51     <refpurpose>Provide hostname resolution for the locally
  52     configured system hostname.</refpurpose>
  53   </refnamediv>
  54
  55   <refsynopsisdiv>
  56     <para><filename>libnss_myhostname.so.2</filename></para>
  57   </refsynopsisdiv>
  58
  59   <refsect1>
  60     <title>Description</title>
  61
  62     <para><command>nss-myhostname</command> is a plug-in module for the GNU Name Service Switch (NSS) functionality of
  63     the GNU C Library (<command>glibc</command>), primarily providing hostname resolution for the locally configured
  64     system hostname as returned by
  65     <citerefentry><refentrytitle>gethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>.  The precise
  66     hostnames resolved by this module are:</para>
  67
  68     <itemizedlist>
  69       <listitem><para>The local, configured hostname is resolved to
  70       all locally configured IP addresses ordered by their scope, or
  71       — if none are configured — the IPv4 address 127.0.0.2 (which
  72       is on the local loopback) and the IPv6 address ::1 (which is the
  73       local host).</para></listitem>
  74
  75       <listitem><para>The hostnames <literal>localhost</literal> and
  76       <literal>localhost.localdomain</literal> (as well as any hostname
  77       ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
  78       are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
  79
  80       <listitem><para>The hostname <literal>_gateway</literal> is
  81       resolved to all current default routing gateway addresses,
  82       ordered by their metric. This assigns a stable hostname to the
  83       current gateway, useful for referencing it independently of the
  84       current network configuration state.</para></listitem>
  85     </itemizedlist>
  86
  87     <para>Various software relies on an always-resolvable local
  88     hostname. When using dynamic hostnames, this is traditionally
  89     achieved by patching <filename>/etc/hosts</filename> at the same
  90     time as changing the hostname. This is problematic since it
  91     requires a writable <filename>/etc</filename> file system and is
  92     fragile because the file might be edited by the administrator at
  93     the same time. With <command>nss-myhostname</command> enabled,
  94     changing <filename>/etc/hosts</filename> is unnecessary, and on
  95     many systems, the file becomes entirely optional.</para>
  96
  97     <para>To activate the NSS modules, add <literal>myhostname</literal> to the line starting with
  98     <literal>hosts:</literal> in <filename>/etc/nsswitch.conf</filename>.</para>
  99
 100     <para>It is recommended to place <literal>myhostname</literal> last in the <filename>nsswitch.conf</filename>'
 101     <literal>hosts:</literal> line to make sure that this mapping is only used as fallback, and that any DNS or
 102     <filename>/etc/hosts</filename> based mapping takes precedence.</para>
 103   </refsect1>
 104
 105   <refsect1>
 106     <title>Example</title>
 107
 108     <para>Here is an example <filename>/etc/nsswitch.conf</filename> file that enables
 109     <command>nss-myhostname</command> correctly:</para>
 110
 111 <programlisting>passwd:         compat mymachines systemd
 112 group:          compat mymachines systemd
 113 shadow:         compat
 114
 115 hosts:          files mymachines resolve [!UNAVAIL=return] dns <command>myhostname</command>
 116 networks:       files
 117
 118 protocols:      db files
 119 services:       db files
 120 ethers:         db files
 121 rpc:            db files
 122
 123 netgroup:       nis</programlisting>
 124
 125     <para>To test, use <command>glibc</command>'s <command>getent</command> tool:</para>
 126
 127     <programlisting>$ getent ahosts `hostname`
 128 ::1       STREAM omega
 129 ::1       DGRAM
 130 ::1       RAW
 131 127.0.0.2       STREAM
 132 127.0.0.2       DGRAM
 133 127.0.0.2       RAW</programlisting>
 134
 135     <para>In this case, the local hostname is <varname>omega</varname>.</para>
 136
 137   </refsect1>
 138
 139   <refsect1>
 140     <title>See Also</title>
 141     <para>
 142       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 143       <citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 144       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 145       <citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 146       <citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 147       <citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 148     </para>
 149   </refsect1>
 150
 151 </refentry>