"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
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>
<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. In addition it 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>.</para>
-
- <para>The glibc NSS module
- <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> is required to
- permit glibc's NSS resolver functions to resolve host names via <command>systemd-resolved</command>.</para>
-
- <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,
- and the per-link dynamic settings received over DHCP. See
- <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
- and
- <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
- for details. 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 above).</para>
+ 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, 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 RRs for the following cases:</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
is on the local loopback) and the IPv6 address ::1 (which is the
local host).</para></listitem>
- <listitem><para>The hostname <literal>localhost</literal> is
- resolved to the IP addresses 127.0.0.1 and
- ::1.</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
+ <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
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
+ <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
+ 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.</para></listitem>
per-interface domains are exclusively routed to the matching
interfaces.</para>
- <para>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>.</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>systemd-resolve --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>systemd-resolve --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>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>resolv.conf</refentrytitle><manvolnum>5</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>
projects / thirdparty / systemd.git / blobdiff