-<?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
--->
+<!-- SPDX-License-Identifier: LGPL-2.1+ -->
<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>
<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>
+ <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 native, fully-featured API <command>systemd-resolved</command> exposes on the bus,
+ see
+ <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+ 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
+ <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 hostnames 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>
+ <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>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/stub-resolv.conf</filename> or
+ <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>,
+ <filename>/usr/lib/systemd/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>
+ </refsect1>
+
+ <refsect1>
+ <title>Synthetic Records</title>
+
+ <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>
+ <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>
+ </refsect1>
+
+ <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>
+ <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
+ <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 hostname and the
+ <literal>_gateway</literal> hostname 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>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>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>
- <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>
+ <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>
<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
+ <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>
+ <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
+ 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>
+ <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>
<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>
+ <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>
+ <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>
<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>
+ 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>
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