systemd/man/nss-myhostname.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
        "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

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<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>
                <refname>libnss_myhostname.so.2</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 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 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 mymachines <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><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</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>