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<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
This file is part of systemd.
Copyright 2014 Tom Gundersen
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->
<refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVED'>
<refentryinfo>
<title>systemd-resolved.service</title>
<productname>systemd</productname>
<authorgroup>
<author>
<contrib>Developer</contrib>
<firstname>Tom</firstname>
<surname>Gundersen</surname>
<email>teg@jklm.no</email>
</author>
</authorgroup>
</refentryinfo>
<refmeta>
<refentrytitle>systemd-resolved.service</refentrytitle>
<manvolnum>8</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd-resolved.service</refname>
<refname>systemd-resolved</refname>
<refpurpose>Network Name Resolution manager</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>systemd-resolved.service</filename></para>
<para><filename>/usr/lib/systemd/systemd-resolved</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><command>systemd-resolved</command> is a system service that provides network name resolution to local
applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR resolver and
responder. In addition it maintains the <filename>/run/systemd/resolve/resolv.conf</filename> file for
compatibility with traditional Linux programs. This file may be symlinked from
<filename>/etc/resolv.conf</filename>.</para>
<para>The glibc NSS module
<citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> is required to
permit glibc's NSS resolver functions to resolve host names via <command>systemd-resolved</command>.</para>
<para>The DNS servers contacted are determined from the global
settings in <filename>/etc/systemd/resolved.conf</filename>, the
per-link static settings in <filename>/etc/systemd/network/*.network</filename> files,
and the per-link dynamic settings received over DHCP. See
<citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
and
<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details. To improve compatibility,
<filename>/etc/resolv.conf</filename> is read in order to discover
configured system DNS servers, but only if it is not a symlink
to <filename>/run/systemd/resolve/resolv.conf</filename> (see above).</para>
<para><command>systemd-resolved</command> synthesizes DNS RRs for the following cases:</para>
<itemizedlist>
<listitem><para>The local, configured hostname is resolved to
all locally configured IP addresses ordered by their scope, or
— if none are configured — the IPv4 address 127.0.0.2 (which
is on the local loopback) and the IPv6 address ::1 (which is the
local host).</para></listitem>
<listitem><para>The hostnames <literal>localhost</literal> and
<literal>localhost.localdomain</literal> (as well as any hostname
ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
<listitem><para>The hostname <literal>gateway</literal> is
resolved to all current default routing gateway addresses,
ordered by their metric. This assigns a stable hostname to the
current gateway, useful for referencing it independently of the
current network configuration state.</para></listitem>
<listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved to their configured
addresses and back.</para></listitem>
</itemizedlist>
<para>Lookup requests are routed to the available DNS servers
and LLMNR interfaces according to the following rules:</para>
<itemizedlist>
<listitem><para>Lookups for the special hostname
<literal>localhost</literal> are never routed to the
network. (A few other, special domains are handled the same way.)</para></listitem>
<listitem><para>Single-label names are routed to all local
interfaces capable of IP multicasting, using the LLMNR
protocol. Lookups for IPv4 addresses are only sent via LLMNR on
IPv4, and lookups for IPv6 addresses are only sent via LLMNR on
IPv6. Lookups for the locally configured host name and the
<literal>gateway</literal> host name are never routed to
LLMNR.</para></listitem>
<listitem><para>Multi-label names are routed to all local
interfaces that have a DNS sever configured, plus the globally
configured DNS server if there is one. Address lookups from the
link-local address range are never routed to
DNS.</para></listitem>
</itemizedlist>
<para>If lookups are routed to multiple interfaces, the first
successful response is returned (thus effectively merging the
lookup zones on all matching interfaces). If the lookup failed on
all interfaces, the last failing response is returned.</para>
<para>Routing of lookups may be influenced by configuring
per-interface domain names. See
<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details. Lookups for a hostname ending in one of the
per-interface domains are exclusively routed to the matching
interfaces.</para>
<para>Note that <filename>/run/systemd/resolve/resolv.conf</filename> should not be used directly by applications,
but only through a symlink from <filename>/etc/resolv.conf</filename>.</para>
<para>See the <ulink url="http://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API
Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para>
</refsect1>
<refsect1>
<title>Signals</title>
<variablelist>
<varlistentry>
<term><constant>SIGUSR1</constant></term>
<listitem><para>Upon reception of the SIGUSR1 process signal <command>systemd-resolved</command> will dump the
contents of all DNS resource record caches it maintains into the system logs.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>SIGUSR2</constant></term>
<listitem><para>Upon reception of the SIGUSR2 process signal <command>systemd-resolved</command> will flush all
caches it maintains. Note that it should normally not be necessary to request this explicitly – except for
debugging purposes – as <command>systemd-resolved</command> flushes the caches automatically anyway any time
the host's network configuration changes.</para></listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-resolve</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
|
