2 <!DOCTYPE refentry PUBLIC
"-//OASIS//DTD DocBook XML V4.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
6 SPDX-License-Identifier: LGPL-2.1+
9 <refentry id=
"systemd-resolved.service" conditional='ENABLE_RESOLVE'
>
12 <title>systemd-resolved.service
</title>
13 <productname>systemd
</productname>
17 <refentrytitle>systemd-resolved.service
</refentrytitle>
18 <manvolnum>8</manvolnum>
22 <refname>systemd-resolved.service
</refname>
23 <refname>systemd-resolved
</refname>
24 <refpurpose>Network Name Resolution manager
</refpurpose>
28 <para><filename>systemd-resolved.service
</filename></para>
29 <para><filename>/usr/lib/systemd/systemd-resolved
</filename></para>
33 <title>Description
</title>
35 <para><command>systemd-resolved
</command> is a system service that provides network name resolution to local
36 applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR and MulticastDNS
37 resolver and responder. Local applications may submit network name resolution requests via three interfaces:
</para>
40 <listitem><para>The native, fully-featured API
<command>systemd-resolved
</command> exposes on the bus. See the
41 <ulink url=
"https://www.freedesktop.org/wiki/Software/systemd/resolved">API Documentation
</ulink> for
42 details. Usage of this API is generally recommended to clients as it is asynchronous and fully featured (for
43 example, properly returns DNSSEC validation status and interface scope for addresses as necessary for supporting
44 link-local networking).
</para></listitem>
46 <listitem><para>The glibc
47 <citerefentry project='man-pages'
><refentrytitle>getaddrinfo
</refentrytitle><manvolnum>3</manvolnum></citerefentry> API as defined
48 by
<ulink url=
"https://tools.ietf.org/html/rfc3493">RFC3493
</ulink> and its related resolver functions,
49 including
<citerefentry project='man-pages'
><refentrytitle>gethostbyname
</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This
50 API is widely supported, including beyond the Linux platform. In its current form it does not expose DNSSEC
51 validation status information however, and is synchronous only. This API is backed by the glibc Name Service
52 Switch (
<citerefentry project='man-pages'
><refentrytitle>nss
</refentrytitle><manvolnum>5</manvolnum></citerefentry>). Usage of the
53 glibc NSS module
<citerefentry><refentrytitle>nss-resolve
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
54 is required in order to allow glibc's NSS resolver functions to resolve host names via
55 <command>systemd-resolved
</command>.
</para></listitem>
57 <listitem><para>Additionally,
<command>systemd-resolved
</command> provides a local DNS stub listener on IP
58 address
127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly, bypassing any local
59 API may be directed to this stub, in order to connect them to
<command>systemd-resolved
</command>. Note however
60 that it is strongly recommended that local programs use the glibc NSS or bus APIs instead (as described above),
61 as various network resolution concepts (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped
62 to the unicast DNS protocol.
</para></listitem>
65 <para>The DNS servers contacted are determined from the global settings in
66 <filename>/etc/systemd/resolved.conf
</filename>, the per-link static settings in
67 <filename>/etc/systemd/network/*.network
</filename> files (in case
68 <citerefentry><refentrytitle>systemd-networkd.service
</refentrytitle><manvolnum>8</manvolnum></citerefentry> is
69 used), the per-link dynamic settings received over DHCP, and any DNS server information made available by other
71 <citerefentry><refentrytitle>resolved.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
72 <citerefentry><refentrytitle>systemd.network
</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details
73 about systemd's own configuration files for DNS servers. To improve compatibility,
74 <filename>/etc/resolv.conf
</filename> is read in order to discover configured system DNS servers, but only if it is
75 not a symlink to
<filename>/run/systemd/resolve/stub-resolv.conf
</filename> or
76 <filename>/run/systemd/resolve/resolv.conf
</filename> (see below).
</para>
78 <para><command>systemd-resolved
</command> synthesizes DNS resource records (RRs) for the following cases:
</para>
81 <listitem><para>The local, configured hostname is resolved to
82 all locally configured IP addresses ordered by their scope, or
83 — if none are configured — the IPv4 address
127.0.0.2 (which
84 is on the local loopback) and the IPv6 address ::
1 (which is the
85 local host).
</para></listitem>
87 <listitem><para>The hostnames
<literal>localhost
</literal> and
88 <literal>localhost.localdomain
</literal> (as well as any hostname
89 ending in
<literal>.localhost
</literal> or
<literal>.localhost.localdomain
</literal>)
90 are resolved to the IP addresses
127.0.0.1 and ::
1.
</para></listitem>
92 <listitem><para>The hostname
<literal>_gateway
</literal> is
93 resolved to all current default routing gateway addresses,
94 ordered by their metric. This assigns a stable hostname to the
95 current gateway, useful for referencing it independently of the
96 current network configuration state.
</para></listitem>
98 <listitem><para>The mappings defined in
<filename>/etc/hosts
</filename> are resolved
99 to their configured addresses and back, but they will not affect lookups for
100 non-address types (like MX).
</para></listitem>
103 <para>Lookup requests are routed to the available DNS servers, LLMNR and MulticastDNS interfaces according to the
104 following rules:
</para>
107 <listitem><para>Lookups for the special hostname
<literal>localhost
</literal> are never routed to the network. (A
108 few other, special domains are handled the same way.)
</para></listitem>
110 <listitem><para>Single-label names are routed to all local interfaces capable of IP multicasting, using the LLMNR
111 protocol. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are only
112 sent via LLMNR on IPv6. Lookups for the locally configured host name and the
<literal>_gateway
</literal> host
113 name are never routed to LLMNR.
</para></listitem>
115 <listitem><para>Multi-label names with the domain suffix
<literal>.local
</literal> are routed to all local
116 interfaces capable of IP multicasting, using the MulticastDNS protocol. As with LLMNR IPv4 address lookups are
117 sent via IPv4 and IPv6 address lookups are sent via IPv6.
</para></listitem>
119 <listitem><para>Other multi-label names are routed to all local interfaces that have a DNS server configured,
120 plus the globally configured DNS server if there is one. Address lookups from the link-local address range are
121 never routed to DNS. Note that by default lookups for domains with the
<literal>.local
</literal> suffix are not
122 routed to DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server
123 and interface. This means that on networks where the
<literal>.local
</literal> domain is defined in a
124 site-specific DNS server, explicit search or routing domains need to be configured to make lookups within this
125 DNS domain work. Note that today it's generally recommended to avoid defining
<literal>.local
</literal> in a DNS
126 server, as
<ulink url=
"https://tools.ietf.org/html/rfc6762">RFC6762
</ulink> reserves this domain for exclusive
127 MulticastDNS use.
</para></listitem>
130 <para>If lookups are routed to multiple interfaces, the first
131 successful response is returned (thus effectively merging the
132 lookup zones on all matching interfaces). If the lookup failed on
133 all interfaces, the last failing response is returned.
</para>
135 <para>Routing of lookups may be influenced by configuring
136 per-interface domain names. See
137 <citerefentry><refentrytitle>systemd.network
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
138 for details. Lookups for a hostname ending in one of the
139 per-interface domains are exclusively routed to the matching
142 <para>See the
<ulink url=
"https://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API
143 Documentation
</ulink> for information about the APIs
<filename>systemd-resolved
</filename> provides.
</para>
148 <title><filename>/etc/resolv.conf
</filename></title>
150 <para>Four modes of handling
<filename>/etc/resolv.conf
</filename> (see
151 <citerefentry project='man-pages'
><refentrytitle>resolv.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are
155 <listitem><para><command>systemd-resolved
</command> maintains the
156 <filename>/run/systemd/resolve/stub-resolv.conf
</filename> file for compatibility with traditional Linux
157 programs. This file may be symlinked from
<filename>/etc/resolv.conf
</filename>. This file lists the
127.0.0.53
158 DNS stub (see above) as the only DNS server. It also contains a list of search domains that are in use by
159 systemd-resolved. The list of search domains is always kept up-to-date. Note that
160 <filename>/run/systemd/resolve/stub-resolv.conf
</filename> should not be used directly by applications, but only
161 through a symlink from
<filename>/etc/resolv.conf
</filename>. This file may be symlinked from
162 <filename>/etc/resolv.conf
</filename> in order to connect all local clients that bypass local DNS APIs to
163 <command>systemd-resolved
</command> with correct search domains settings. This mode of operation is
164 recommended.
</para></listitem>
166 <listitem><para>A static file
<filename>/usr/lib/systemd/resolv.conf
</filename> is provided that lists
167 the
127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from
168 <filename>/etc/resolv.conf
</filename> in order to connect all local clients that bypass local DNS APIs to
169 <command>systemd-resolved
</command>. This file does not contain any search domains.
</para></listitem>
171 <listitem><para><command>systemd-resolved
</command> maintains the
172 <filename>/run/systemd/resolve/resolv.conf
</filename> file for compatibility with traditional Linux
173 programs. This file may be symlinked from
<filename>/etc/resolv.conf
</filename> and is always kept up-to-date,
174 containing information about all known DNS servers. Note the file format's limitations: it does not know a
175 concept of per-interface DNS servers and hence only contains system-wide DNS server definitions. Note that
176 <filename>/run/systemd/resolve/resolv.conf
</filename> should not be used directly by applications, but only
177 through a symlink from
<filename>/etc/resolv.conf
</filename>. If this mode of operation is used local clients
178 that bypass any local DNS API will also bypass
<command>systemd-resolved
</command> and will talk directly to the
179 known DNS servers.
</para> </listitem>
181 <listitem><para>Alternatively,
<filename>/etc/resolv.conf
</filename> may be managed by other packages, in which
182 case
<command>systemd-resolved
</command> will read it for DNS configuration data. In this mode of operation
183 <command>systemd-resolved
</command> is consumer rather than provider of this configuration
184 file.
</para></listitem>
187 <para>Note that the selected mode of operation for this file is detected fully automatically, depending on whether
188 <filename>/etc/resolv.conf
</filename> is a symlink to
<filename>/run/systemd/resolve/resolv.conf
</filename> or
189 lists
127.0.0.53 as DNS server.
</para>
193 <title>Signals
</title>
197 <term><constant>SIGUSR1
</constant></term>
199 <listitem><para>Upon reception of the
<constant>SIGUSR1
</constant> process signal
200 <command>systemd-resolved
</command> will dump the contents of all DNS resource record caches it maintains, as
201 well as all feature level information it learnt about configured DNS servers into the system
202 logs.
</para></listitem>
206 <term><constant>SIGUSR2
</constant></term>
208 <listitem><para>Upon reception of the
<constant>SIGUSR2
</constant> process signal
209 <command>systemd-resolved
</command> will flush all caches it maintains. Note that it should normally not be
210 necessary to request this explicitly – except for debugging purposes – as
<command>systemd-resolved
</command>
211 flushes the caches automatically anyway any time the host's network configuration changes. Sending this signal
212 to
<command>systemd-resolved
</command> is equivalent to the
<command>resolvectl --flush-caches
</command>
213 command, however the latter is recommended since it operates in a synchronous way.
</para></listitem>
217 <term><constant>SIGRTMIN+
1</constant></term>
219 <listitem><para>Upon reception of the
<constant>SIGRTMIN+
1</constant> process signal
220 <command>systemd-resolved
</command> will forget everything it learnt about the configured DNS
221 servers. Specifically any information about server feature support is flushed out, and the server feature
222 probing logic is restarted on the next request, starting with the most fully featured level. Note that it
223 should normally not be necessary to request this explicitly – except for debugging purposes – as
224 <command>systemd-resolved
</command> automatically forgets learnt information any time the DNS server
225 configuration changes. Sending this signal to
<command>systemd-resolved
</command> is equivalent to the
226 <command>resolvectl --reset-server-features
</command> command, however the latter is recommended since it
227 operates in a synchronous way.
</para></listitem>
234 <title>See Also
</title>
236 <citerefentry><refentrytitle>systemd
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
237 <citerefentry><refentrytitle>resolved.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
238 <citerefentry><refentrytitle>dnssec-trust-anchors.d
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
239 <citerefentry><refentrytitle>nss-resolve
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
240 <citerefentry><refentrytitle>resolvectl
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
241 <citerefentry project='man-pages'
><refentrytitle>resolv.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
242 <citerefentry project='man-pages'
><refentrytitle>hosts
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
243 <citerefentry><refentrytitle>systemd.network
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
244 <citerefentry><refentrytitle>systemd-networkd.service
</refentrytitle><manvolnum>8</manvolnum></citerefentry>