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<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
SPDX-License-Identifier: LGPL-2.1+
-->
<refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE'>
<refentryinfo>
<title>systemd-resolved.service</title>
<productname>systemd</productname>
</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 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
<ulink url="https://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 (in case
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> is
used), 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/stub-resolv.conf</filename> or
<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, but they will not affect lookups for
non-address types (like MX).</para></listitem>
</itemizedlist>
<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 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
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="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>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 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
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 <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 <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>
<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>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>,
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
|
