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 plugin for the GNU
  61     Name Service Switch (NSS) functionality of the GNU C Library
  62     (<command>glibc</command>), primarily providing hostname resolution
  63     for the locally configured system hostname as returned by
  64     <citerefentry><refentrytitle>gethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>.
  65     The precise hostnames resolved by this module are:</para>
  66
  67     <itemizedlist>
  68       <listitem><para>The local, configured hostname is resolved to
  69       all locally configured IP addresses ordered by their scope, or
  70       — if none are configured — the IPv4 address 127.0.0.2 (which
  71       is on the local loopback) and the IPv6 address ::1 (which is the
  72       local host).</para></listitem>
  73
  74       <listitem><para>The hostname <literal>localhost</literal> is
  75       resolved to the IP addresses 127.0.0.1 and
  76       ::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
  84     </itemizedlist>
  85
  86     <para>Various software relies on an always-resolvable local
  87     hostname. When using dynamic hostnames, this is traditionally
  88     achieved by patching <filename>/etc/hosts</filename> at the same
  89     time as changing the hostname. This is problematic since it
  90     requires a writable <filename>/etc</filename> file system and is
  91     fragile because the file might be edited by the administrator at
  92     the same time. With <command>nss-myhostname</command> enabled,
  93     changing <filename>/etc/hosts</filename> is unnecessary, and on
  94     many systems, the file becomes entirely optional.</para>
  95
  96     <para>To activate the NSS modules, <literal>myhostname</literal>
  97     has to be added to the line starting with
  98     <literal>hosts:</literal> in
  99     <filename>/etc/nsswitch.conf</filename>.</para>
 100
 101     <para>It is recommended to place <literal>myhostname</literal>
 102     last in the <filename>nsswitch.conf</filename> line to make sure
 103     that this mapping is only used as fallback, and that any DNS or
 104     <filename>/etc/hosts</filename> based mapping takes
 105     precedence.</para>
 106   </refsect1>
 107
 108   <refsect1>
 109     <title>Example</title>
 110
 111     <para>Here is an example <filename>/etc/nsswitch.conf</filename>
 112     file that enables <command>myhostname</command> correctly:</para>
 113
 114 <programlisting>passwd:         compat mymachines
 115 group:          compat mymachines
 116 shadow:         compat
 117
 118 hosts:          files resolve mymachines <command>myhostname</command>
 119 networks:       files
 120
 121 protocols:      db files
 122 services:       db files
 123 ethers:         db files
 124 rpc:            db files
 125
 126 netgroup:       nis</programlisting>
 127
 128     <para>To test, use <command>glibc</command>'s <command>getent</command> tool:</para>
 129
 130     <programlisting>$ getent ahosts `hostname`
 131 ::1       STREAM omega
 132 ::1       DGRAM
 133 ::1       RAW
 134 127.0.0.2       STREAM
 135 127.0.0.2       DGRAM
 136 127.0.0.2       RAW</programlisting>
 137
 138     <para>In this case, the local hostname is <varname>omega</varname>.</para>
 139
 140   </refsect1>
 141
 142   <refsect1>
 143     <title>See Also</title>
 144     <para>
 145       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 146       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 147       <citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 148       <citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 149       <citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 150     </para>
 151   </refsect1>
 152
 153 </refentry>