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02e9308751
Let's replace the "compat" module in our proposed nsswitch.conf configuration with "files", since it is not 1995 anymore. Fedora and other distros have deprecated and removed NIS support a while back. While others still retain some support I am not sure we should advertise it in our examples. Downstream can of course still use "compat" instead of "files" if they want to, but let's not confuse people who don't care about NIS anymore with this. Also, bring the nsswitch.conf snippet in README in line with what our man pages say. Also see: https://fedoraproject.org/wiki/Changes/retire_NIS_user_space_utils
140 lines
6.2 KiB
XML
140 lines
6.2 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-or-later -->
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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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<listitem><para>The hostname <literal>_outbound</literal> is resolved to the local IPv4 and IPv6
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addresses that are most likely used for communication with other hosts. This is determined by
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requesting a routing decision to the configured default gateways from the kernel and then using the
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local IP addresses selected by this decision. This hostname is only available if there is at least one
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local default gateway configured. This assigns a stable hostname to the local outbound IP addresses,
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useful for referencing them independently of the 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> after <literal>file</literal> and before <literal>dns</literal>.
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This resolves well-known hostnames like <literal>localhost</literal>
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and the machine hostnames locally. It is consistent with the behaviour
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of <command>nss-resolve</command>, and still allows overriding via
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<filename>/etc/hosts</filename>.</para>
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<para>Please keep in mind that <command>nss-myhostname</command> (and <command>nss-resolve</command>) also resolve
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in the other direction — from locally attached IP addresses to
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hostnames. If you rely on that lookup being provided by DNS, you might
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want to order things differently.
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</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: files systemd
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group: files [SUCCESS=merge] systemd
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shadow: files systemd
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gshadow: files systemd
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hosts: mymachines resolve [!UNAVAIL=return] files <command>myhostname</command> dns
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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
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<citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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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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