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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
This file is part of systemd.
Copyright 2011 Lennart Poettering
Copyright 2013 Tom Gundersen
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->
<refentry id="nss-myhostname" conditional='HAVE_MYHOSTNAME'>
<refentryinfo>
<title>nss-myhostname</title>
<productname>systemd</productname>
<authorgroup>
<author>
<contrib>Developer</contrib>
<firstname>Lennart</firstname>
<surname>Poettering</surname>
<email>lennart@poettering.net</email>
</author>
</authorgroup>
</refentryinfo>
<refmeta>
<refentrytitle>nss-myhostname</refentrytitle>
<manvolnum>8</manvolnum>
</refmeta>
<refnamediv>
<refname>nss-myhostname</refname>
<refpurpose>Provide hostname resolution for the locally
configured system hostname.</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>libnss_myhostname.so.2</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><command>nss-myhostname</command> is a plugin
for the GNU Name Service Switch (NSS) functionality of
the GNU C Library (<command>glibc</command>) primarily
providing hostname resolution for the locally
configured system hostname as returned by
<citerefentry><refentrytitle>gethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>. The
precise hostnames resolved by this module are:</para>
<itemizedlist>
<listitem><para>The local, configured hostname
is resolved to all locally configured public
IP addresses, or -- if none are configured --
the IPv4 address 127.0.0.2 (which is on the
local loopback) and the IPv6 address ::1
(which is the local host).</para></listitem>
<listitem><para>The hostname
<literal>localhost</literal> is resolved to
the IP addresses 127.0.0.1 and
::1.</para></listitem>
<listitem><para>The hostname
<literal>gateway</literal> is resolved to all
current default routing gateway addresses,
ordered by their metric. This assigns a stable
hostname to the current gateway, useful for
referencing it independently of the current
network configuration state.</para></listitem>
</itemizedlist>
<para>Various software relies on an always-resolvable
local hostname. When using dynamic hostnames, this is
traditionally achieved by patching
<filename>/etc/hosts</filename> at the same time as
changing the hostname. This is problematic since it
requires a writable <filename>/etc</filename> file
system and is fragile because the file might be edited
by the administrator at the same time. With
<command>nss-myhostname</command> enabled changing
<filename>/etc/hosts</filename> is unncessary, and on
many systems the file becomes entirely optional.</para>
<para>To activate the NSS modules,
<literal>myhostname</literal> has to be added to the
line starting with <literal>hosts:</literal> in
<filename>/etc/nsswitch.conf</filename>.</para>
<para>It is recommended to place
<literal>myhostname</literal> last in the
<filename>nsswitch.conf</filename> line to make sure
that this mapping is only used as fallback, and any
DNS or <filename>/etc/hosts</filename> based mapping
takes precedence.</para>
</refsect1>
<refsect1>
<title>Example</title>
<para>Here's an example
<filename>/etc/nsswitch.conf</filename> file, that
enables <command>myhostname</command>
correctly:</para>
<programlisting>passwd: compat
group: compat
shadow: compat
hosts: files dns <command>myhostname</command>
networks: files
protocols: db files
services: db files
ethers: db files
rpc: db files
netgroup: nis</programlisting>
<para>To test, use <command>glibc</command>'s <command>getent</command> tool:</para>
<programlisting>$ getent ahosts `hostname`
::1 STREAM omega
::1 DGRAM
::1 RAW
127.0.0.2 STREAM
127.0.0.2 DGRAM
127.0.0.2 RAW</programlisting>
<para>In this case the local hostname is <varname>omega</varname>.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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