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<?xml version='1.0'?>
<!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-resolve" conditional='ENABLE_NSS_RESOLVE'
xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>nss-resolve</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>nss-resolve</refentrytitle>
<manvolnum>8</manvolnum>
</refmeta>
<refnamediv>
<refname>nss-resolve</refname>
<refname>libnss_resolve.so.2</refname>
<refpurpose>Hostname resolution via <filename>systemd-resolved.service</filename></refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>libnss_resolve.so.2</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><command>nss-resolve</command> is a plug-in module for the GNU Name Service Switch (NSS) functionality of the
GNU C Library (<command>glibc</command>) enabling it to resolve hostnames via the
<citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry> local network
name resolution service. It replaces the <command>nss-dns</command> plug-in module that traditionally resolves
hostnames via DNS.</para>
<para>To activate the NSS module, add <literal>resolve [!UNAVAIL=return]</literal> to the line starting
with <literal>hosts:</literal> in <filename>/etc/nsswitch.conf</filename>. Specifically, it is
recommended to place <literal>resolve</literal> early in <filename>/etc/nsswitch.conf</filename>'s
<literal>hosts:</literal> line. It should be before the <literal>files</literal> entry, since
<filename>systemd-resolved</filename> supports <filename>/etc/hosts</filename> internally, but with
caching. To the contrary, it should be after <literal>mymachines</literal>, to give hostnames given to
local VMs and containers precedence over names received over DNS. Finally, we recommend placing
<literal>dns</literal> somewhere after <literal>resolve</literal>, to fall back to
<command>nss-dns</command> if <filename>systemd-resolved.service</filename> is not available.</para>
<para>Note that <command>systemd-resolved</command> will synthesize DNS resource records in a few cases,
for example for <literal>localhost</literal> and the current local hostname, see
<citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
the full list. This duplicates the functionality of
<citerefentry><refentrytitle>nss-myhostname</refentrytitle><manvolnum>8</manvolnum></citerefentry>, but
it is still recommended (see examples below) to keep <command>nss-myhostname</command> configured in
<filename>/etc/nsswitch.conf</filename>, to keep those names resolveable if
<command>systemd-resolved</command> is not running.</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>
<para>Communication between <command>nss-resolve</command> and
<filename>systemd-resolved.service</filename> takes place via the
<filename>/run/systemd/resolve/io.systemd.Resolve</filename> <constant>AF_UNIX</constant> socket.</para>
</refsect1>
<refsect1>
<title>Environment variables</title>
<variablelist class='environment-variables'>
<varlistentry>
<term><varname>$SYSTEMD_NSS_RESOLVE_VALIDATE</varname></term>
<listitem><para>Takes a boolean argument. When false, cryptographic validation of resource records
via DNSSEC will be disabled. This may be useful for testing, or when system time is known to be
unreliable.</para>
<xi:include href="version-info.xml" xpointer="v250"/></listitem>
</varlistentry>
</variablelist>
<variablelist class='environment-variables'>
<varlistentry>
<term><varname>$SYSTEMD_NSS_RESOLVE_SYNTHESIZE</varname></term>
<listitem><para>Takes a boolean argument. When false, synthetic records, e.g. for the local host
name, will not be returned. See section SYNTHETIC RECORDS in
<citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
for more information. This may be useful to query the "public" resource records, independent of the
configuration of the local machine.</para>
<xi:include href="version-info.xml" xpointer="v250"/></listitem>
</varlistentry>
</variablelist>
<variablelist class='environment-variables'>
<varlistentry>
<term><varname>$SYSTEMD_NSS_RESOLVE_CACHE</varname></term>
<listitem><para>Takes a boolean argument. When false, the cache of previously queried records will
not be used by
<citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
</para>
<xi:include href="version-info.xml" xpointer="v250"/></listitem>
</varlistentry>
</variablelist>
<variablelist class='environment-variables'>
<varlistentry>
<term><varname>$SYSTEMD_NSS_RESOLVE_ZONE</varname></term>
<listitem><para>Takes a boolean argument. When false, answers using locally registered public
LLMNR/mDNS resource records will not be returned.</para>
<xi:include href="version-info.xml" xpointer="v250"/></listitem>
</varlistentry>
</variablelist>
<variablelist class='environment-variables'>
<varlistentry>
<term><varname>$SYSTEMD_NSS_RESOLVE_TRUST_ANCHOR</varname></term>
<listitem><para>Takes a boolean argument. When false, answers using locally configured trust anchors
will not be used.</para>
<xi:include href="version-info.xml" xpointer="v250"/></listitem>
</varlistentry>
</variablelist>
<variablelist class='environment-variables'>
<varlistentry>
<term><varname>$SYSTEMD_NSS_RESOLVE_NETWORK</varname></term>
<listitem><para>Takes a boolean argument. When false, answers will be returned without using the
network, i.e. either from local sources or the cache in
<citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
</para>
<xi:include href="version-info.xml" xpointer="v250"/></listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Example</title>
<para>Here is an example <filename>/etc/nsswitch.conf</filename> file that enables
<command>nss-resolve</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 <command>resolve [!UNAVAIL=return]</command> files myhostname dns
networks: files
protocols: db files
services: db files
ethers: db files
rpc: db files
netgroup: nis</programlisting>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>nss-myhostname</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><refentrytitle>systemd.syntax</refentrytitle><manvolnum>5</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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