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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<refentry id="nss-myhostname" conditional='ENABLE_NSS_MYHOSTNAME'>
<refentryinfo>
<title>nss-myhostname</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>nss-myhostname</refentrytitle>
<manvolnum>8</manvolnum>
</refmeta>
<refnamediv>
<refname>nss-myhostname</refname>
<refname>libnss_myhostname.so.2</refname>
<refpurpose>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 plug-in module 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 IP addresses ordered by their scope, 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 hostnames <literal>localhost</literal> and
<literal>localhost.localdomain</literal> (as well as any hostname
ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
are 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>
<listitem><para>The hostname <literal>_outbound</literal> is resolved to the local IPv4 and IPv6
addresses that are most likely used for communication with other hosts. This is determined by
requesting a routing decision to the configured default gateways from the kernel and then using the
local IP addresses selected by this decision. This hostname is only available if there is at least one
local default gateway configured. This assigns a stable hostname to the local outbound IP addresses,
useful for referencing them 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 unnecessary, and on
many systems, the file becomes entirely optional.</para>
<para>To activate the NSS modules, add <literal>myhostname</literal> to the line starting with
<literal>hosts:</literal> in <filename>/etc/nsswitch.conf</filename>.</para>
<para>It is recommended to place <literal>myhostname</literal> after <literal>file</literal> and before <literal>dns</literal>.
This resolves well-known hostnames like <literal>localhost</literal>
and the machine hostnames locally. It is consistent with the behaviour
of <command>nss-resolve</command>, and still allows overriding via
<filename>/etc/hosts</filename>.</para>
<para>Please keep in mind that <command>nss-myhostname</command> (and <command>nss-resolve</command>) also resolve
in the other direction — from locally attached IP addresses to
hostnames. If you rely on that lookup being provided by DNS, you might
want to order things differently.
</para>
</refsect1>
<refsect1>
<title>Example</title>
<para>Here is an example <filename>/etc/nsswitch.conf</filename> file that enables
<command>nss-myhostname</command> correctly:</para>
<!-- synchronize with other nss-* man pages and factory/etc/nsswitch.conf -->
<programlisting>passwd: files systemd
group: files [SUCCESS=merge] systemd
shadow: files systemd
gshadow: files systemd
hosts: mymachines resolve [!UNAVAIL=return] files <command>myhostname</command> dns
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
<citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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><simplelist type="inline">
<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
<member><citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
<member><citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
</simplelist></para>
</refsect1>
</refentry>
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