236 lines
13 KiB
XML
236 lines
13 KiB
XML
<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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This file is part of systemd.
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Copyright 2014 Tom Gundersen
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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-->
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<refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE'>
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<refentryinfo>
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<title>systemd-resolved.service</title>
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<productname>systemd</productname>
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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Tom</firstname>
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<surname>Gundersen</surname>
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<email>teg@jklm.no</email>
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</author>
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</authorgroup>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd-resolved.service</refentrytitle>
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<manvolnum>8</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd-resolved.service</refname>
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<refname>systemd-resolved</refname>
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<refpurpose>Network Name Resolution manager</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>systemd-resolved.service</filename></para>
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<para><filename>/usr/lib/systemd/systemd-resolved</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><command>systemd-resolved</command> is a system service that provides network name resolution to local
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applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR resolver and
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responder. Local applications may submit network name resolution requests via three interfaces:</para>
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<itemizedlist>
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<listitem><para>The native, fully-featured API <command>systemd-resolved</command> exposes on the bus. See the
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<ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved">API Documentation</ulink> for
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details. Usage of this API is generally recommended to clients as it is asynchronous and fully featured (for
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example, properly returns DNSSEC validation status and interface scope for addresses as necessary for supporting
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link-local networking).</para></listitem>
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<listitem><para>The glibc
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<citerefentry project='man-pages'><refentrytitle>getaddrinfo</refentrytitle><manvolnum>3</manvolnum></citerefentry> API as defined
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by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related resolver functions,
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including <citerefentry project='man-pages'><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This
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API is widely supported, including beyond the Linux platform. In its current form it does not expose DNSSEC
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validation status information however, and is synchronous only. This API is backed by the glibc Name Service
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Switch (<citerefentry project='man-pages'><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>). Usage of the
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glibc NSS module <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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is required in order to allow glibc's NSS resolver functions to resolve host names via
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<command>systemd-resolved</command>.</para></listitem>
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<listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on IP
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address 127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly, bypassing any local
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API may be directed to this stub, in order to connect them to <command>systemd-resolved</command>. Note however
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that it is strongly recommended that local programs use the glibc NSS or bus APIs instead (as described above),
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as various network resolution concepts (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped
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to the unicast DNS protocol.</para></listitem>
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</itemizedlist>
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<para>The DNS servers contacted are determined from the global settings in
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<filename>/etc/systemd/resolved.conf</filename>, the per-link static settings in
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<filename>/etc/systemd/network/*.network</filename> files, the per-link dynamic settings received over DHCP and any
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DNS server information made available by other system services. See
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<citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
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<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details
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about systemd's own configuration files for DNS servers. To improve compatibility,
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<filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but only if it is
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not a symlink to <filename>/run/systemd/resolve/resolv.conf</filename> (see below).</para>
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<para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following cases:</para>
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<itemizedlist>
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<listitem><para>The local, configured hostname is resolved to
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all locally configured IP addresses ordered by their scope, or
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— if none are configured — the IPv4 address 127.0.0.2 (which
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is on the local loopback) and the IPv6 address ::1 (which is the
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local host).</para></listitem>
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<listitem><para>The hostnames <literal>localhost</literal> and
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<literal>localhost.localdomain</literal> (as well as any hostname
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ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
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are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
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<listitem><para>The hostname <literal>gateway</literal> is
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resolved to all current default routing gateway addresses,
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ordered by their metric. This assigns a stable hostname to the
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current gateway, useful for referencing it independently of the
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current network configuration state.</para></listitem>
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<listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved
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to their configured addresses and back, but they will not affect lookups for
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non-address types (like MX).</para></listitem>
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</itemizedlist>
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<para>Lookup requests are routed to the available DNS servers
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and LLMNR interfaces according to the following rules:</para>
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<itemizedlist>
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<listitem><para>Lookups for the special hostname
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<literal>localhost</literal> are never routed to the
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network. (A few other, special domains are handled the same way.)</para></listitem>
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<listitem><para>Single-label names are routed to all local
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interfaces capable of IP multicasting, using the LLMNR
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protocol. Lookups for IPv4 addresses are only sent via LLMNR on
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IPv4, and lookups for IPv6 addresses are only sent via LLMNR on
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IPv6. Lookups for the locally configured host name and the
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<literal>gateway</literal> host name are never routed to
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LLMNR.</para></listitem>
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<listitem><para>Multi-label names are routed to all local
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interfaces that have a DNS server configured, plus the globally
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configured DNS server if there is one. Address lookups from the
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link-local address range are never routed to
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DNS.</para></listitem>
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</itemizedlist>
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<para>If lookups are routed to multiple interfaces, the first
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successful response is returned (thus effectively merging the
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lookup zones on all matching interfaces). If the lookup failed on
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all interfaces, the last failing response is returned.</para>
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<para>Routing of lookups may be influenced by configuring
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per-interface domain names. See
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<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for details. Lookups for a hostname ending in one of the
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per-interface domains are exclusively routed to the matching
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interfaces.</para>
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<para>See the <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API
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Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para>
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</refsect1>
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<refsect1>
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<title><filename>/etc/resolv.conf</filename></title>
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<para>Three modes of handling <filename>/etc/resolv.conf</filename> (see
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<citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are
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supported:</para>
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<itemizedlist>
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<listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists
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the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from
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<filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to
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<command>systemd-resolved</command>. This mode of operation is recommended.</para></listitem>
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<listitem><para><command>systemd-resolved</command> maintains the
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<filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux
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programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept up-to-date,
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containing information about all known DNS servers. Note the file format's limitations: it does not know a
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concept of per-interface DNS servers and hence only contains system-wide DNS server definitions. Note that
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<filename>/run/systemd/resolve/resolv.conf</filename> should not be used directly by applications, but only
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through a symlink from <filename>/etc/resolv.conf</filename>. If this mode of operation is used local clients
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that bypass any local DNS API will also bypass <command>systemd-resolved</command> and will talk directly to the
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known DNS servers.</para> </listitem>
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<listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages, in which
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case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode of operation
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<command>systemd-resolved</command> is consumer rather than provider of this configuration
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file. </para></listitem>
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</itemizedlist>
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<para>Note that the selected mode of operation for this file is detected fully automatically, depending on whether
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<filename>/etc/resolv.conf</filename> is a symlink to <filename>/run/systemd/resolve/resolv.conf</filename> or
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lists 127.0.0.53 as DNS server.</para>
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</refsect1>
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<refsect1>
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<title>Signals</title>
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<variablelist>
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<varlistentry>
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<term><constant>SIGUSR1</constant></term>
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<listitem><para>Upon reception of the SIGUSR1 process signal <command>systemd-resolved</command> will dump the
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contents of all DNS resource record caches it maintains into the system logs.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>SIGUSR2</constant></term>
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<listitem><para>Upon reception of the SIGUSR2 process signal <command>systemd-resolved</command> will flush all
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caches it maintains. Note that it should normally not be necessary to request this explicitly – except for
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debugging purposes – as <command>systemd-resolved</command> flushes the caches automatically anyway any time
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the host's network configuration changes.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-resolve</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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