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authorLuke Shumaker <lukeshu@sbcglobal.net>2016-12-17 03:04:41 -0500
committerLuke Shumaker <lukeshu@sbcglobal.net>2016-12-17 03:04:41 -0500
commitfd6ea8a3f4999133f8ac036a23584c3e5f9e9b3f (patch)
tree6cdd53846655b04b178b4b8057c915c61a9cb525 /man/systemd-resolved.service.xml
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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 project='man-pages'><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 project='man-pages'><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 project='man-pages'><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 project='man-pages'><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>