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2dc6b11d | 1 | <?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*--> |
091a364c | 2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" |
12b42c76 | 3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> |
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4 | |
5 | <!-- | |
6 | This file is part of systemd. | |
7 | ||
8 | Copyright 2014 Tom Gundersen | |
9 | ||
10 | systemd is free software; you can redistribute it and/or modify it | |
11 | under the terms of the GNU Lesser General Public License as published by | |
12 | the Free Software Foundation; either version 2.1 of the License, or | |
13 | (at your option) any later version. | |
14 | ||
15 | systemd is distributed in the hope that it will be useful, but | |
16 | WITHOUT ANY WARRANTY; without even the implied warranty of | |
17 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
18 | Lesser General Public License for more details. | |
19 | ||
20 | You should have received a copy of the GNU Lesser General Public License | |
21 | along with systemd; If not, see <http://www.gnu.org/licenses/>. | |
22 | --> | |
23 | ||
24 | <refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVED'> | |
25 | ||
798d3a52 ZJS |
26 | <refentryinfo> |
27 | <title>systemd-resolved.service</title> | |
28 | <productname>systemd</productname> | |
29 | ||
30 | <authorgroup> | |
31 | <author> | |
32 | <contrib>Developer</contrib> | |
33 | <firstname>Tom</firstname> | |
34 | <surname>Gundersen</surname> | |
35 | <email>teg@jklm.no</email> | |
36 | </author> | |
37 | </authorgroup> | |
38 | </refentryinfo> | |
39 | ||
40 | <refmeta> | |
41 | <refentrytitle>systemd-resolved.service</refentrytitle> | |
42 | <manvolnum>8</manvolnum> | |
43 | </refmeta> | |
44 | ||
45 | <refnamediv> | |
46 | <refname>systemd-resolved.service</refname> | |
47 | <refname>systemd-resolved</refname> | |
48 | <refpurpose>Network Name Resolution manager</refpurpose> | |
49 | </refnamediv> | |
50 | ||
51 | <refsynopsisdiv> | |
52 | <para><filename>systemd-resolved.service</filename></para> | |
12b42c76 | 53 | <para><filename>/usr/lib/systemd/systemd-resolved</filename></para> |
798d3a52 ZJS |
54 | </refsynopsisdiv> |
55 | ||
56 | <refsect1> | |
57 | <title>Description</title> | |
58 | ||
624993ac LP |
59 | <para><command>systemd-resolved</command> is a system service that provides network name resolution to local |
60 | applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR resolver and | |
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61 | responder. Local applications may submit network name resolution requests via three interfaces:</para> |
62 | ||
63 | <itemizedlist> | |
64 | <listitem><para>The native, fully-featured API <command>systemd-resolved</command> exposes on the bus. See the | |
28a0ad81 | 65 | <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved">API Documentation</ulink> for |
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66 | details. Usage of this API is generally recommended to clients as it is asynchronous and fully featured (for |
67 | example, properly returns DNSSEC validation status and interface scope for addresses as necessary for supporting | |
68 | link-local networking).</para></listitem> | |
69 | ||
70 | <listitem><para>The glibc | |
0a07667d | 71 | <citerefentry project='man-pages'><refentrytitle>getaddrinfo</refentrytitle><manvolnum>3</manvolnum></citerefentry> API as defined |
fc549b96 | 72 | by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related resolver functions, |
0a07667d | 73 | including <citerefentry project='man-pages'><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This |
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74 | API is widely supported, including beyond the Linux platform. In its current form it does not expose DNSSEC |
75 | validation status information however, and is synchronous only. This API is backed by the glibc Name Service | |
0a07667d | 76 | Switch (<citerefentry project='man-pages'><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>). Usage of the |
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77 | glibc NSS module <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
78 | is required in order to allow glibc's NSS resolver functions to resolve host names via | |
79 | <command>systemd-resolved</command>.</para></listitem> | |
80 | ||
81 | <listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on IP | |
82 | address 127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly, bypassing any local | |
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83 | API may be directed to this stub, in order to connect them to <command>systemd-resolved</command>. Note however |
84 | that it is strongly recommended that local programs use the glibc NSS or bus APIs instead (as described above), | |
85 | as various network resolution concepts (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped | |
86 | to the unicast DNS protocol.</para></listitem> | |
b541146b | 87 | </itemizedlist> |
798d3a52 | 88 | |
b541146b LP |
89 | <para>The DNS servers contacted are determined from the global settings in |
90 | <filename>/etc/systemd/resolved.conf</filename>, the per-link static settings in | |
91 | <filename>/etc/systemd/network/*.network</filename> files, the per-link dynamic settings received over DHCP and any | |
92 | DNS server information made available by other system services. See | |
93 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and | |
94 | <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details | |
95 | about systemd's own configuration files for DNS servers. To improve compatibility, | |
96 | <filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but only if it is | |
97 | not a symlink to <filename>/run/systemd/resolve/resolv.conf</filename> (see below).</para> | |
98 | ||
99 | <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following cases:</para> | |
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100 | |
101 | <itemizedlist> | |
102 | <listitem><para>The local, configured hostname is resolved to | |
103 | all locally configured IP addresses ordered by their scope, or | |
104 | — if none are configured — the IPv4 address 127.0.0.2 (which | |
105 | is on the local loopback) and the IPv6 address ::1 (which is the | |
106 | local host).</para></listitem> | |
107 | ||
63003524 DH |
108 | <listitem><para>The hostnames <literal>localhost</literal> and |
109 | <literal>localhost.localdomain</literal> (as well as any hostname | |
110 | ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>) | |
111 | are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem> | |
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112 | |
113 | <listitem><para>The hostname <literal>gateway</literal> is | |
114 | resolved to all current default routing gateway addresses, | |
115 | ordered by their metric. This assigns a stable hostname to the | |
116 | current gateway, useful for referencing it independently of the | |
117 | current network configuration state.</para></listitem> | |
394bac4f | 118 | |
4050e04b MP |
119 | <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved |
120 | to their configured addresses and back, but they will not affect lookups for | |
121 | non-address types (like MX).</para></listitem> | |
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122 | </itemizedlist> |
123 | ||
124 | <para>Lookup requests are routed to the available DNS servers | |
125 | and LLMNR interfaces according to the following rules:</para> | |
126 | ||
127 | <itemizedlist> | |
128 | <listitem><para>Lookups for the special hostname | |
129 | <literal>localhost</literal> are never routed to the | |
624993ac | 130 | network. (A few other, special domains are handled the same way.)</para></listitem> |
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131 | |
132 | <listitem><para>Single-label names are routed to all local | |
133 | interfaces capable of IP multicasting, using the LLMNR | |
134 | protocol. Lookups for IPv4 addresses are only sent via LLMNR on | |
135 | IPv4, and lookups for IPv6 addresses are only sent via LLMNR on | |
136 | IPv6. Lookups for the locally configured host name and the | |
137 | <literal>gateway</literal> host name are never routed to | |
138 | LLMNR.</para></listitem> | |
139 | ||
140 | <listitem><para>Multi-label names are routed to all local | |
e223f799 | 141 | interfaces that have a DNS server configured, plus the globally |
2dc6b11d | 142 | configured DNS server if there is one. Address lookups from the |
7f3fdb7f | 143 | link-local address range are never routed to |
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144 | DNS.</para></listitem> |
145 | </itemizedlist> | |
146 | ||
147 | <para>If lookups are routed to multiple interfaces, the first | |
148 | successful response is returned (thus effectively merging the | |
149 | lookup zones on all matching interfaces). If the lookup failed on | |
b938cb90 | 150 | all interfaces, the last failing response is returned.</para> |
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151 | |
152 | <para>Routing of lookups may be influenced by configuring | |
b938cb90 | 153 | per-interface domain names. See |
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154 | <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> |
155 | for details. Lookups for a hostname ending in one of the | |
156 | per-interface domains are exclusively routed to the matching | |
157 | interfaces.</para> | |
158 | ||
28a0ad81 | 159 | <para>See the <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API |
a0956ed0 LP |
160 | Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para> |
161 | ||
798d3a52 ZJS |
162 | </refsect1> |
163 | ||
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164 | <refsect1> |
165 | <title><filename>/etc/resolv.conf</filename></title> | |
166 | ||
167 | <para>Three modes of handling <filename>/etc/resolv.conf</filename> (see | |
0a07667d | 168 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are |
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169 | supported:</para> |
170 | ||
171 | <itemizedlist> | |
172 | <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists | |
173 | the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from | |
174 | <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to | |
175 | <command>systemd-resolved</command>. This mode of operation is recommended.</para></listitem> | |
176 | ||
177 | <listitem><para><command>systemd-resolved</command> maintains the | |
178 | <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux | |
179 | programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept up-to-date, | |
180 | containing information about all known DNS servers. Note the file format's limitations: it does not know a | |
181 | concept of per-interface DNS servers and hence only contains system-wide DNS server definitions. Note that | |
182 | <filename>/run/systemd/resolve/resolv.conf</filename> should not be used directly by applications, but only | |
183 | through a symlink from <filename>/etc/resolv.conf</filename>. If this mode of operation is used local clients | |
184 | that bypass any local DNS API will also bypass <command>systemd-resolved</command> and will talk directly to the | |
185 | known DNS servers.</para> </listitem> | |
186 | ||
187 | <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages, in which | |
188 | case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode of operation | |
189 | <command>systemd-resolved</command> is consumer rather than provider of this configuration | |
190 | file. </para></listitem> | |
191 | </itemizedlist> | |
192 | ||
193 | <para>Note that the selected mode of operation for this file is detected fully automatically, depending on whether | |
194 | <filename>/etc/resolv.conf</filename> is a symlink to <filename>/run/systemd/resolve/resolv.conf</filename> or | |
195 | lists 127.0.0.53 as DNS server.</para> | |
196 | </refsect1> | |
197 | ||
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198 | <refsect1> |
199 | <title>Signals</title> | |
200 | ||
201 | <variablelist> | |
202 | <varlistentry> | |
203 | <term><constant>SIGUSR1</constant></term> | |
204 | ||
205 | <listitem><para>Upon reception of the SIGUSR1 process signal <command>systemd-resolved</command> will dump the | |
206 | contents of all DNS resource record caches it maintains into the system logs.</para></listitem> | |
207 | </varlistentry> | |
208 | ||
209 | <varlistentry> | |
210 | <term><constant>SIGUSR2</constant></term> | |
211 | ||
212 | <listitem><para>Upon reception of the SIGUSR2 process signal <command>systemd-resolved</command> will flush all | |
213 | caches it maintains. Note that it should normally not be necessary to request this explicitly – except for | |
214 | debugging purposes – as <command>systemd-resolved</command> flushes the caches automatically anyway any time | |
215 | the host's network configuration changes.</para></listitem> | |
216 | </varlistentry> | |
217 | </variablelist> | |
218 | </refsect1> | |
219 | ||
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220 | <refsect1> |
221 | <title>See Also</title> | |
222 | <para> | |
223 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
224 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
b5a8703f | 225 | <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
437293cf | 226 | <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
624993ac | 227 | <citerefentry><refentrytitle>systemd-resolve</refentrytitle><manvolnum>1</manvolnum></citerefentry>, |
1c18f60a | 228 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
394bac4f | 229 | <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
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230 | <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
231 | <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
232 | </para> | |
233 | </refsect1> | |
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234 | |
235 | </refentry> |