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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-10 20:49:52 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-10 20:49:52 +0000
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Adding upstream version 255.4.upstream/255.4
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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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/4.2/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>
+ <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</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>