systemd/man/nss-resolve.xml

<?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'>

  <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></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></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 <filename>systemd-resolved</filename>.</para></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></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></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 <filename>systemd-resolved</filename>.
        </para></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:         compat systemd
group:          compat [SUCCESS=merge] systemd
shadow:         compat 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>