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authorLuke Shumaker <lukeshu@sbcglobal.net>2016-10-22 17:33:34 -0400
committerLuke Shumaker <lukeshu@sbcglobal.net>2016-10-22 17:33:34 -0400
commit4bd618f4666fb6629f5426b915ce856114667b88 (patch)
tree8721fc6a02f15f4b42c6281dec7586ede1b25522 /src/grp-resolve/systemd-resolved/systemd-resolved.service.xml
parentacbe2e3a644cb0fea2e9ead61cae2128f97413bb (diff)
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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
+ Lesser General Public License for more details.
+
+ 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. Local applications may submit network name resolution requests via three interfaces:</para>
+
+ <itemizedlist>
+ <listitem><para>The native, fully-featured API <command>systemd-resolved</command> exposes on the bus. See the
+ <ulink url="http://www.freedesktop.org/wiki/Software/systemd/resolved">API Documentation</ulink> for
+ details. Usage of this API is generally recommended to clients as it is asynchronous and fully featured (for
+ example, properly returns DNSSEC validation status and interface scope for addresses as necessary for supporting
+ link-local networking).</para></listitem>
+
+ <listitem><para>The glibc
+ <citerefentry><refentrytitle>getaddrinfo</refentrytitle><manvolnum>3</manvolnum></citerefentry> API as defined
+ by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related resolver functions,
+ including <citerefentry><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This
+ API is widely supported, including beyond the Linux platform. In its current form it does not expose DNSSEC
+ validation status information however, and is synchronous only. This API is backed by the glibc Name Service
+ Switch (<citerefentry><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>). Usage of the
+ glibc NSS module <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ is required in order to allow glibc's NSS resolver functions to resolve host names via
+ <command>systemd-resolved</command>.</para></listitem>
+
+ <listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on IP
+ address 127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly, bypassing any local
+ API may be directed to this stub, in order to connect them to <command>systemd-resolved</command>. Note however
+ that it is strongly recommended that local programs use the glibc NSS or bus APIs instead (as described above),
+ as various network resolution concepts (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped
+ to the unicast DNS protocol.</para></listitem>
+ </itemizedlist>
+
+ <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, the per-link dynamic settings received over DHCP and any
+ DNS server information made available by other system services. See
+ <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
+ <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details
+ about systemd's own configuration files for DNS servers. 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 below).</para>
+
+ <para><command>systemd-resolved</command> synthesizes DNS resource records (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>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><filename>/etc/resolv.conf</filename></title>
+
+ <para>Three modes of handling <filename>/etc/resolv.conf</filename> (see
+ <citerefentry><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are
+ supported:</para>
+
+ <itemizedlist>
+ <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists
+ the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from
+ <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to
+ <command>systemd-resolved</command>. This mode of operation is recommended.</para></listitem>
+
+ <listitem><para><command>systemd-resolved</command> 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> and is always kept up-to-date,
+ containing information about all known DNS servers. Note the file format's limitations: it does not know a
+ concept of per-interface DNS servers and hence only contains system-wide DNS server definitions. 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>. If this mode of operation is used local clients
+ that bypass any local DNS API will also bypass <command>systemd-resolved</command> and will talk directly to the
+ known DNS servers.</para> </listitem>
+
+ <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages, in which
+ case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode of operation
+ <command>systemd-resolved</command> is consumer rather than provider of this configuration
+ file. </para></listitem>
+ </itemizedlist>
+
+ <para>Note that the selected mode of operation for this file is detected fully automatically, depending on whether
+ <filename>/etc/resolv.conf</filename> is a symlink to <filename>/run/systemd/resolve/resolv.conf</filename> or
+ lists 127.0.0.53 as DNS server.</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>