-<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
-<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+<?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+ -->
-<!--
- 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.
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- systemd is distributed in the hope that it will be useful, but
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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
- along with systemd; If not, see <http://www.gnu.org/licenses/>.
--->
-
-<refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVED'>
+<refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE'>
<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>
<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>
+ applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR and MulticastDNS
+ 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
<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
+ <filename>/etc/systemd/network/*.network</filename> files (in case
+ <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> is
+ used), the per-link dynamic settings received over DHCP, user request made via
+ <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, 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>
+ not a symlink to <filename>/run/systemd/resolve/stub-resolv.conf</filename>,
+ <filename>/usr/lib/systemd/resolv.conf</filename> or <filename>/run/systemd/resolve/resolv.conf</filename> (see
+ below).</para>
+
+ </refsect1>
+
+ <refsect1>
+ <title>Synthetic Records</title>
<para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following cases:</para>
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
+ <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
to their configured addresses and back, but they will not affect lookups for
non-address types (like MX).</para></listitem>
</itemizedlist>
+ </refsect1>
- <para>Lookup requests are routed to the available DNS servers
- and LLMNR interfaces according to the following rules:</para>
+ <refsect1>
+ <title>Protocols and Routing</title>
+
+ <para>Lookup requests are routed to the available DNS servers, LLMNR and MulticastDNS 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>
+ <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 with the domain suffix <literal>.local</literal> are routed to all local
+ interfaces capable of IP multicasting, using the MulticastDNS protocol. As with LLMNR IPv4 address lookups are
+ sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem>
+
+ <listitem><para>Other multi-label names are routed to all local interfaces that have a DNS server configured,
+ plus the globally configured DNS server if there is one. Address lookups from the link-local address range are
+ never routed to DNS. Note that by default lookups for domains with the <literal>.local</literal> suffix are not
+ routed to DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server
+ and interface. This means that on networks where the <literal>.local</literal> domain is defined in a
+ site-specific DNS server, explicit search or routing domains need to be configured to make lookups within this
+ DNS domain work. Note that today it's generally recommended to avoid defining <literal>.local</literal> in a DNS
+ server, as <ulink url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive
+ MulticastDNS use.</para></listitem>
</itemizedlist>
<para>If lookups are routed to multiple interfaces, the first
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>Routing of lookups may be influenced by configuring per-interface domain names and other settings. See
+ <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
+ <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for details. The
+ following query routing logic applies for unicast DNS traffic:</para>
+
+ <itemizedlist>
+ <listitem><para>If a name to look up matches (that is: is equal to or has as suffix) any of the configured search
+ or route-only domains of any link (or the globally configured DNS settings), the "best matching"
+ search/route-only domain is determined: the matching one with the most labels. The query is then sent to all DNS
+ servers of any links or the globally configured DNS servers associated with this "best matching"
+ search/route-only domain. (Note that more than one link might have this same "best matching" search/route-only
+ domain configured, in which case the query is sent to all of them in parallel).</para></listitem>
+
+ <listitem><para>If a query does not match any configured search/route-only domain (neither per-link nor global),
+ it is sent to all DNS servers that are configured on links with the "DNS default route" option set, as well as
+ the globally configured DNS server.</para></listitem>
+
+ <listitem><para>If there is no link configured as "DNS default route" and no global DNS server configured, the
+ compiled-in fallback DNS server is used.</para></listitem>
+
+ <listitem><para>Otherwise the query is failed as no suitable DNS servers could be determined.</para></listitem>
+ </itemizedlist>
+
+ <para>The "DNS default route" option is a boolean setting configurable with <command>resolvectl</command> or in
+ <filename>.network</filename> files. If not set, it is implicitly determined based on the configured DNS domains
+ for a link: if there's any route-only domain (not matching <literal>~.</literal>) it defaults to false, otherwise
+ to true.</para>
+
+ <para>Effectively this means: in order to preferably route all DNS queries not explicitly matched by
+ search/route-only domain configuration to a specific link, configure a <literal>~.</literal> route-only domain on
+ it. This will ensure that other links will not be considered for the queries (unless they too carry such a
+ route-only domain). In order to route all such DNS queries to a specific link only in case no other link is
+ preferable, then set the "DNS default route" option for the link to true, and do not configure a
+ <literal>~.</literal> route-only domain on it. Finally, in order to ensure that a specific link never receives any
+ DNS traffic not matching any of its configured search/route-only domains, set the "DNS default route" option for it
+ to false.</para>
<para>See the <ulink url="https://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
+ <para>Four 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><command>systemd-resolved</command> maintains the
+ <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional Linux
+ programs. This file may be symlinked from <filename>/etc/resolv.conf</filename>. This file lists the 127.0.0.53
+ DNS stub (see above) as the only DNS server. It also contains a list of search domains that are in use by
+ systemd-resolved. The list of search domains is always kept up-to-date. Note that
+ <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not be used directly by applications, but only
+ through a symlink from <filename>/etc/resolv.conf</filename>. 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> with correct search domains settings. This mode of operation is
+ recommended.</para></listitem>
+
<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>
+ <command>systemd-resolved</command>. This file does not contain any search domains.</para></listitem>
<listitem><para><command>systemd-resolved</command> maintains the
<filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux
<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>
+ <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal
+ <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it maintains, as
+ well as all feature level information it learnt about configured DNS servers 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>
+ <listitem><para>Upon reception of the <constant>SIGUSR2</constant> 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. Sending this signal
+ to <command>systemd-resolved</command> is equivalent to the <command>resolvectl flush-caches</command>
+ command, however the latter is recommended since it operates in a synchronous way.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><constant>SIGRTMIN+1</constant></term>
+
+ <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal
+ <command>systemd-resolved</command> will forget everything it learnt about the configured DNS
+ servers. Specifically any information about server feature support is flushed out, and the server feature
+ probing logic is restarted on the next request, starting with the most fully featured level. Note that it
+ should normally not be necessary to request this explicitly – except for debugging purposes – as
+ <command>systemd-resolved</command> automatically forgets learnt information any time the DNS server
+ configuration changes. Sending this signal to <command>systemd-resolved</command> is equivalent to the
+ <command>resolvectl reset-server-features</command> command, however the latter is recommended since it
+ operates in a synchronous way.</para></listitem>
</varlistentry>
</variablelist>
+
</refsect1>
<refsect1>
<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><refentrytitle>resolvectl</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>,
projects / thirdparty / systemd.git / blobdiff