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https://github.com/systemd/systemd
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166 lines
7.5 KiB
XML
166 lines
7.5 KiB
XML
<?xml version='1.0'?>
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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-resolve" conditional='ENABLE_NSS_RESOLVE'>
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<refentryinfo>
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<title>nss-resolve</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>nss-resolve</refentrytitle>
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<manvolnum>8</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>nss-resolve</refname>
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<refname>libnss_resolve.so.2</refname>
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<refpurpose>Hostname resolution via <filename>systemd-resolved.service</filename></refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>libnss_resolve.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-resolve</command> is a plug-in module for the GNU Name Service Switch (NSS) functionality of the
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GNU C Library (<command>glibc</command>) enabling it to resolve hostnames via the
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<citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry> local network
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name resolution service. It replaces the <command>nss-dns</command> plug-in module that traditionally resolves
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hostnames via DNS.</para>
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<para>To activate the NSS module, add <literal>resolve [!UNAVAIL=return]</literal> to the line starting
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with <literal>hosts:</literal> in <filename>/etc/nsswitch.conf</filename>. Specifically, it is
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recommended to place <literal>resolve</literal> early in <filename>/etc/nsswitch.conf</filename>'s
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<literal>hosts:</literal> line. It should be before the <literal>files</literal> entry, since
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<filename>systemd-resolved</filename> supports <filename>/etc/hosts</filename> internally, but with
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caching. To the contrary, it should be after <literal>mymachines</literal>, to give hostnames given to
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local VMs and containers precedence over names received over DNS. Finally, we recommend placing
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<literal>dns</literal> somewhere after <literal>resolve</literal>, to fall back to
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<command>nss-dns</command> if <filename>systemd-resolved.service</filename> is not available.</para>
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<para>Note that <command>systemd-resolved</command> will synthesize DNS resource records in a few cases,
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for example for <literal>localhost</literal> and the current local hostname, see
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<citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
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the full list. This duplicates the functionality of
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<citerefentry><refentrytitle>nss-myhostname</refentrytitle><manvolnum>8</manvolnum></citerefentry>, but
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it is still recommended (see examples below) to keep <command>nss-myhostname</command> configured in
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<filename>/etc/nsswitch.conf</filename>, to keep those names resolveable if
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<command>systemd-resolved</command> is not running.</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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<para>Communication between <command>nss-resolve</command> and
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<filename>systemd-resolved.service</filename> takes place via the
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<filename>/run/systemd/resolve/io.systemd.Resolve</filename> <constant>AF_UNIX</constant> socket.</para>
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</refsect1>
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<refsect1>
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<title>Environment variables</title>
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<variablelist class='environment-variables'>
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<varlistentry>
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<term><varname>$SYSTEMD_NSS_RESOLVE_VALIDATE</varname></term>
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<listitem><para>Takes a boolean argument. When false, cryptographic validation of resource records
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via DNSSEC will be disabled. This may be useful for testing, or when system time is known to be
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unreliable.</para></listitem>
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</varlistentry>
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</variablelist>
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<variablelist class='environment-variables'>
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<varlistentry>
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<term><varname>$SYSTEMD_NSS_RESOLVE_SYNTHESIZE</varname></term>
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<listitem><para>Takes a boolean argument. When false, synthetic records, e.g. for the local host
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name, will not be returned. See section SYNTHETIC RECORDS in
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<citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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for more information. This may be useful to query the "public" resource records, independent of the
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configuration of the local machine.</para></listitem>
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</varlistentry>
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</variablelist>
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<variablelist class='environment-variables'>
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<varlistentry>
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<term><varname>$SYSTEMD_NSS_RESOLVE_CACHE</varname></term>
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<listitem><para>Takes a boolean argument. When false, the cache of previously queried records will
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not be used by <command>systemd-resolved</command>.</para></listitem>
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</varlistentry>
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</variablelist>
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<variablelist class='environment-variables'>
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<varlistentry>
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<term><varname>$SYSTEMD_NSS_RESOLVE_ZONE</varname></term>
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<listitem><para>Takes a boolean argument. When false, answers using locally registered public
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LLMNR/mDNS resource records will not be returned.</para></listitem>
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</varlistentry>
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</variablelist>
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<variablelist class='environment-variables'>
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<varlistentry>
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<term><varname>$SYSTEMD_NSS_RESOLVE_TRUST_ANCHOR</varname></term>
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<listitem><para>Takes a boolean argument. When false, answers using locally configured trust anchors
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will not be used.</para></listitem>
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</varlistentry>
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</variablelist>
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<variablelist class='environment-variables'>
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<varlistentry>
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<term><varname>$SYSTEMD_NSS_RESOLVE_NETWORK</varname></term>
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<listitem><para>Takes a boolean argument. When false, answers will be returned without using the
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network, i.e. either from local sources or the cache in <command>systemd-resolved</command>.
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</para></listitem>
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</varlistentry>
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</variablelist>
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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-resolve</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 systemd
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gshadow: files systemd
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hosts: mymachines <command>resolve [!UNAVAIL=return]</command> files myhostname 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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</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>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>nss-myhostname</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><refentrytitle>systemd.syntax</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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