man/nss-resolve.xml

   1 <?xml version='1.0'?>
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
   5
   6 <refentry id="nss-resolve" conditional='ENABLE_NSS_RESOLVE'>
   7
   8   <refentryinfo>
   9     <title>nss-resolve</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>nss-resolve</refentrytitle>
  15     <manvolnum>8</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>nss-resolve</refname>
  20     <refname>libnss_resolve.so.2</refname>
  21     <refpurpose>Hostname resolution via <filename>systemd-resolved.service</filename></refpurpose>
  22   </refnamediv>
  23
  24   <refsynopsisdiv>
  25     <para><filename>libnss_resolve.so.2</filename></para>
  26   </refsynopsisdiv>
  27
  28   <refsect1>
  29     <title>Description</title>
  30
  31     <para><command>nss-resolve</command> is a plug-in module for the GNU Name Service Switch (NSS) functionality of the
  32     GNU C Library (<command>glibc</command>) enabling it to resolve hostnames via the
  33     <citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry> local network
  34     name resolution service. It replaces the <command>nss-dns</command> plug-in module that traditionally resolves
  35     hostnames via DNS.</para>
  36
  37     <para>To activate the NSS module, add <literal>resolve [!UNAVAIL=return]</literal> to the line starting
  38     with <literal>hosts:</literal> in <filename>/etc/nsswitch.conf</filename>. Specifically, it is
  39     recommended to place <literal>resolve</literal> early in <filename>/etc/nsswitch.conf</filename>'s
  40     <literal>hosts:</literal> line. It should be before the <literal>files</literal> entry, since
  41     <filename>systemd-resolved</filename> supports <filename>/etc/hosts</filename> internally, but with
  42     caching. To the contrary, it should be after <literal>mymachines</literal>, to give hostnames given to
  43     local VMs and containers precedence over names received over DNS. Finally, we recommend placing
  44     <literal>dns</literal> somewhere after <literal>resolve</literal>, to fall back to
  45     <command>nss-dns</command> if <filename>systemd-resolved.service</filename> is not available.</para>
  46
  47     <para>Note that <command>systemd-resolved</command> will synthesize DNS resource records in a few cases,
  48     for example for <literal>localhost</literal> and the current local hostname, see
  49     <citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
  50     the full list. This duplicates the functionality of
  51     <citerefentry><refentrytitle>nss-myhostname</refentrytitle><manvolnum>8</manvolnum></citerefentry>, but
  52     it is still recommended (see examples below) to keep <command>nss-myhostname</command> configured in
  53     <filename>/etc/nsswitch.conf</filename>, to keep those names resolveable if
  54     <command>systemd-resolved</command> is not running.</para>
  55   </refsect1>
  56
  57   <refsect1>
  58     <title>Example</title>
  59
  60     <para>Here is an example <filename>/etc/nsswitch.conf</filename> file that enables <command>nss-resolve</command>
  61     correctly:</para>
  62
  63     <!-- synchronize with other nss-* man pages and factory/etc/nsswitch.conf -->
  64 <programlisting>passwd:         compat systemd
  65 group:          compat [SUCCESS=merge] systemd
  66 shadow:         compat
  67
  68 hosts:          mymachines <command>resolve [!UNAVAIL=return]</command> files myhostname dns
  69 networks:       files
  70
  71 protocols:      db files
  72 services:       db files
  73 ethers:         db files
  74 rpc:            db files
  75
  76 netgroup:       nis</programlisting>
  77   </refsect1>
  78
  79   <refsect1>
  80     <title>See Also</title>
  81     <para>
  82       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
  83       <citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
  84       <citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
  85       <citerefentry><refentrytitle>nss-myhostname</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
  86       <citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
  87       <citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  88     </para>
  89   </refsect1>
  90
  91 </refentry>