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[thirdparty/systemd.git] / man / nss-resolve.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<!--
  SPDX-License-Identifier: LGPL-2.1+

  This file is part of systemd.

  Copyright 2011 Lennart Poettering
  Copyright 2013 Tom Gundersen
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<refentry id="nss-resolve" conditional='ENABLE_RESOLVE'>

  <refentryinfo>
    <title>nss-resolve</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>nss-resolve</refentrytitle>
    <manvolnum>8</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>nss-resolve</refname>
    <refname>libnss_resolve.so.2</refname>
    <refpurpose>Provide 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 host names 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</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 (but
    after the <literal>files</literal> or <literal>mymachines</literal> entries), right before the
    <literal>dns</literal> entry if it exists, followed by <literal>[!UNAVAIL=return]</literal>, to ensure DNS queries
    are always routed via
    <citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry> if it is
    running, but are routed to <command>nss-dns</command> if this service 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 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>
  </refsect1>

  <refsect1>
    <title>Example</title>

    <para>Here is an example <filename>/etc/nsswitch.conf</filename> file that enables <command>nss-resolve</command>
    correctly:</para>

<programlisting>passwd:         compat mymachines systemd
group:          compat mymachines systemd
shadow:         compat

hosts:          files mymachines <command>resolve [!UNAVAIL=return]</command> dns myhostname
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>
    </para>
  </refsect1>

</refentry>