<?xml version='1.0'?> <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/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><simplelist type="inline"> <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member> <member><citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry></member> <member><citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry></member> <member><citerefentry><refentrytitle>nss-myhostname</refentrytitle><manvolnum>8</manvolnum></citerefentry></member> <member><citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry></member> <member><citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry></member> <member><citerefentry><refentrytitle>systemd.syntax</refentrytitle><manvolnum>5</manvolnum></citerefentry></member> </simplelist></para> </refsect1> </refentry>