man/nss-myhostname.xml

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   8   Copyright 2011 Lennart Poettering
   9   Copyright 2013 Tom Gundersen
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  24
  25 <refentry id="nss-myhostname" conditional='HAVE_MYHOSTNAME'>
  26
  27   <refentryinfo>
  28     <title>nss-myhostname</title>
  29     <productname>systemd</productname>
  30
  31     <authorgroup>
  32       <author>
  33         <contrib>Developer</contrib>
  34         <firstname>Lennart</firstname>
  35         <surname>Poettering</surname>
  36         <email>lennart@poettering.net</email>
  37       </author>
  38     </authorgroup>
  39   </refentryinfo>
  40
  41   <refmeta>
  42     <refentrytitle>nss-myhostname</refentrytitle>
  43     <manvolnum>8</manvolnum>
  44   </refmeta>
  45
  46   <refnamediv>
  47     <refname>nss-myhostname</refname>
  48     <refname>libnss_myhostname.so.2</refname>
  49     <refpurpose>Provide hostname resolution for the locally
  50     configured system hostname.</refpurpose>
  51   </refnamediv>
  52
  53   <refsynopsisdiv>
  54     <para><filename>libnss_myhostname.so.2</filename></para>
  55   </refsynopsisdiv>
  56
  57   <refsect1>
  58     <title>Description</title>
  59
  60     <para><command>nss-myhostname</command> is a plug-in module for the GNU Name Service Switch (NSS) functionality of
  61     the GNU C Library (<command>glibc</command>), primarily providing hostname resolution for the locally configured
  62     system hostname as returned by
  63     <citerefentry><refentrytitle>gethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>.  The precise
  64     hostnames resolved by this module are:</para>
  65
  66     <itemizedlist>
  67       <listitem><para>The local, configured hostname is resolved to
  68       all locally configured IP addresses ordered by their scope, or
  69       — if none are configured — the IPv4 address 127.0.0.2 (which
  70       is on the local loopback) and the IPv6 address ::1 (which is the
  71       local host).</para></listitem>
  72
  73       <listitem><para>The hostnames <literal>localhost</literal> and
  74       <literal>localhost.localdomain</literal> (as well as any hostname
  75       ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
  76       are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
  77
  78       <listitem><para>The hostname <literal>gateway</literal> is
  79       resolved to all current default routing gateway addresses,
  80       ordered by their metric. This assigns a stable hostname to the
  81       current gateway, useful for referencing it independently of the
  82       current network configuration state.</para></listitem>
  83     </itemizedlist>
  84
  85     <para>Various software relies on an always-resolvable local
  86     hostname. When using dynamic hostnames, this is traditionally
  87     achieved by patching <filename>/etc/hosts</filename> at the same
  88     time as changing the hostname. This is problematic since it
  89     requires a writable <filename>/etc</filename> file system and is
  90     fragile because the file might be edited by the administrator at
  91     the same time. With <command>nss-myhostname</command> enabled,
  92     changing <filename>/etc/hosts</filename> is unnecessary, and on
  93     many systems, the file becomes entirely optional.</para>
  94
  95     <para>To activate the NSS modules, add <literal>myhostname</literal> to the line starting with
  96     <literal>hosts:</literal> in <filename>/etc/nsswitch.conf</filename>.</para>
  97
  98     <para>It is recommended to place <literal>myhostname</literal> last in the <filename>nsswitch.conf</filename>'
  99     <literal>hosts:</literal> line to make sure that this mapping is only used as fallback, and that any DNS or
 100     <filename>/etc/hosts</filename> based mapping takes precedence.</para>
 101   </refsect1>
 102
 103   <refsect1>
 104     <title>Example</title>
 105
 106     <para>Here is an example <filename>/etc/nsswitch.conf</filename> file that enables
 107     <command>nss-myhostname</command> correctly:</para>
 108
 109 <programlisting>passwd:         compat mymachines systemd
 110 group:          compat mymachines systemd
 111 shadow:         compat
 112
 113 hosts:          files mymachines resolve [!UNAVAIL=return] dns <command>myhostname</command>
 114 networks:       files
 115
 116 protocols:      db files
 117 services:       db files
 118 ethers:         db files
 119 rpc:            db files
 120
 121 netgroup:       nis</programlisting>
 122
 123     <para>To test, use <command>glibc</command>'s <command>getent</command> tool:</para>
 124
 125     <programlisting>$ getent ahosts `hostname`
 126 ::1       STREAM omega
 127 ::1       DGRAM
 128 ::1       RAW
 129 127.0.0.2       STREAM
 130 127.0.0.2       DGRAM
 131 127.0.0.2       RAW</programlisting>
 132
 133     <para>In this case, the local hostname is <varname>omega</varname>.</para>
 134
 135   </refsect1>
 136
 137   <refsect1>
 138     <title>See Also</title>
 139     <para>
 140       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 141       <citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 142       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 143       <citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 144       <citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 145       <citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 146     </para>
 147   </refsect1>
 148
 149 </refentry>