| <?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>$SYSTEMD_NSS_RESOLVE_CACHE</varname></term> |
| |
| <listitem><para>Takes a boolean argument. When false, the cache of previously queried records will |
| not be used by <command>systemd-resolved</command>.</para></listitem> |
| </varlistentry> |
| </variablelist> |
| |
| <variablelist class='environment-variables'> |
| <varlistentry> |
| <term><varname>$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 <command>systemd-resolved</command>. |
| </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> |