129 lines
5.5 KiB
XML
129 lines
5.5 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1+ -->
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<refentry id="nss-myhostname" conditional='ENABLE_NSS_MYHOSTNAME'>
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<refentryinfo>
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<title>nss-myhostname</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>nss-myhostname</refentrytitle>
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<manvolnum>8</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>nss-myhostname</refname>
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<refname>libnss_myhostname.so.2</refname>
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<refpurpose>Hostname resolution for the locally configured system hostname</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>libnss_myhostname.so.2</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><command>nss-myhostname</command> is a plug-in module for the GNU Name Service Switch (NSS) functionality of
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the GNU C Library (<command>glibc</command>), primarily providing hostname resolution for the locally configured
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system hostname as returned by
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<citerefentry><refentrytitle>gethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>. The precise
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hostnames resolved by this module are:</para>
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<itemizedlist>
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<listitem><para>The local, configured hostname is resolved to
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all locally configured IP addresses ordered by their scope, or
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— if none are configured — the IPv4 address 127.0.0.2 (which
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is on the local loopback) and the IPv6 address ::1 (which is the
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local host).</para></listitem>
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<listitem><para>The hostnames <literal>localhost</literal> and
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<literal>localhost.localdomain</literal> (as well as any hostname
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ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
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are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
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<listitem><para>The hostname <literal>_gateway</literal> is
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resolved to all current default routing gateway addresses,
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ordered by their metric. This assigns a stable hostname to the
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current gateway, useful for referencing it independently of the
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current network configuration state.</para></listitem>
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</itemizedlist>
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<para>Various software relies on an always-resolvable local
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hostname. When using dynamic hostnames, this is traditionally
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achieved by patching <filename>/etc/hosts</filename> at the same
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time as changing the hostname. This is problematic since it
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requires a writable <filename>/etc</filename> file system and is
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fragile because the file might be edited by the administrator at
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the same time. With <command>nss-myhostname</command> enabled,
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changing <filename>/etc/hosts</filename> is unnecessary, and on
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many systems, the file becomes entirely optional.</para>
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<para>To activate the NSS modules, add <literal>myhostname</literal> to the line starting with
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<literal>hosts:</literal> in <filename>/etc/nsswitch.conf</filename>.</para>
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<para>It is recommended to place <literal>myhostname</literal> either between <literal>resolve</literal>
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and "traditional" modules like <literal>dns</literal>, or after them. In the first version, well-known
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names like <literal>localhost</literal> and the machine hostname are given higher priority than the
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external configuration. This is recommended when the external DNS servers and network are not absolutely
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trusted. In the second version, external configuration is given higher priority and
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<command>nss-myhostname</command> only provides a fallback mechanism. This might be suitable in closely
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controlled networks, for example on a company LAN.</para>
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</refsect1>
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<refsect1>
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<title>Example</title>
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<para>Here is an example <filename>/etc/nsswitch.conf</filename> file that enables
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<command>nss-myhostname</command> correctly:</para>
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<!-- synchronize with other nss-* man pages and factory/etc/nsswitch.conf -->
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<programlisting>passwd: compat systemd
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group: compat [SUCCESS=merge] systemd
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shadow: compat
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# Either (untrusted network, see above):
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hosts: mymachines resolve [!UNAVAIL=return] files <command>myhostname</command> dns
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# Or (only trusted networks):
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hosts: mymachines resolve [!UNAVAIL=return] files dns <command>myhostname</command>
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networks: files
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protocols: db files
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services: db files
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ethers: db files
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rpc: db files
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netgroup: nis</programlisting>
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<para>To test, use <command>glibc</command>'s <command>getent</command> tool:</para>
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<programlisting>$ getent ahosts `hostname`
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::1 STREAM omega
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::1 DGRAM
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::1 RAW
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127.0.0.2 STREAM
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127.0.0.2 DGRAM
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127.0.0.2 RAW</programlisting>
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<para>In this case, the local hostname is <varname>omega</varname>.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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