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1 | <?xml version='1.0'?> | |
2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" | |
3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> | |
4 | ||
5 | <!-- | |
6 | SPDX-License-Identifier: LGPL-2.1+ | |
7 | --> | |
8 | ||
9 | <refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE'> | |
10 | ||
11 | <refentryinfo> | |
12 | <title>systemd-resolved.service</title> | |
13 | <productname>systemd</productname> | |
14 | </refentryinfo> | |
15 | ||
16 | <refmeta> | |
17 | <refentrytitle>systemd-resolved.service</refentrytitle> | |
18 | <manvolnum>8</manvolnum> | |
19 | </refmeta> | |
20 | ||
21 | <refnamediv> | |
22 | <refname>systemd-resolved.service</refname> | |
23 | <refname>systemd-resolved</refname> | |
24 | <refpurpose>Network Name Resolution manager</refpurpose> | |
25 | </refnamediv> | |
26 | ||
27 | <refsynopsisdiv> | |
28 | <para><filename>systemd-resolved.service</filename></para> | |
29 | <para><filename>/usr/lib/systemd/systemd-resolved</filename></para> | |
30 | </refsynopsisdiv> | |
31 | ||
32 | <refsect1> | |
33 | <title>Description</title> | |
34 | ||
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> | |
38 | ||
39 | <itemizedlist> | |
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> | |
45 | ||
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> | |
56 | ||
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> | |
63 | </itemizedlist> | |
64 | ||
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 | |
70 | system services. See | |
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> | |
77 | ||
78 | <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following cases:</para> | |
79 | ||
80 | <itemizedlist> | |
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> | |
86 | ||
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> | |
91 | ||
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> | |
97 | ||
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> | |
101 | </itemizedlist> | |
102 | ||
103 | <para>Lookup requests are routed to the available DNS servers, LLMNR and MulticastDNS interfaces according to the | |
104 | following rules:</para> | |
105 | ||
106 | <itemizedlist> | |
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> | |
109 | ||
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> | |
114 | ||
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> | |
118 | ||
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> | |
128 | </itemizedlist> | |
129 | ||
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> | |
134 | ||
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 | |
140 | interfaces.</para> | |
141 | ||
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> | |
144 | ||
145 | </refsect1> | |
146 | ||
147 | <refsect1> | |
148 | <title><filename>/etc/resolv.conf</filename></title> | |
149 | ||
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 | |
152 | supported:</para> | |
153 | ||
154 | <itemizedlist> | |
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> | |
165 | ||
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> | |
170 | ||
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> | |
180 | ||
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> | |
185 | </itemizedlist> | |
186 | ||
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> | |
190 | </refsect1> | |
191 | ||
192 | <refsect1> | |
193 | <title>Signals</title> | |
194 | ||
195 | <variablelist> | |
196 | <varlistentry> | |
197 | <term><constant>SIGUSR1</constant></term> | |
198 | ||
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> | |
203 | </varlistentry> | |
204 | ||
205 | <varlistentry> | |
206 | <term><constant>SIGUSR2</constant></term> | |
207 | ||
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> | |
214 | </varlistentry> | |
215 | ||
216 | <varlistentry> | |
217 | <term><constant>SIGRTMIN+1</constant></term> | |
218 | ||
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> | |
228 | </varlistentry> | |
229 | </variablelist> | |
230 | ||
231 | </refsect1> | |
232 | ||
233 | <refsect1> | |
234 | <title>See Also</title> | |
235 | <para> | |
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> | |
245 | </para> | |
246 | </refsect1> | |
247 | ||
248 | </refentry> |