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a51c1048 | 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 | <!-- | |
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6 | SPDX-License-Identifier: LGPL-2.1+ |
7 | ||
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8 | This file is part of systemd. |
9 | ||
10 | Copyright 2014 Tom Gundersen | |
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11 | --> |
12 | ||
1ec57f33 | 13 | <refentry id="resolved.conf" conditional='ENABLE_RESOLVE' |
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14 | xmlns:xi="http://www.w3.org/2001/XInclude"> |
15 | <refentryinfo> | |
16 | <title>resolved.conf</title> | |
17 | <productname>systemd</productname> | |
18 | ||
19 | <authorgroup> | |
20 | <author> | |
21 | <contrib>Developer</contrib> | |
22 | <firstname>Tom</firstname> | |
23 | <surname>Gundersen</surname> | |
24 | <email>teg@jklm.no</email> | |
25 | </author> | |
26 | </authorgroup> | |
27 | </refentryinfo> | |
28 | ||
29 | <refmeta> | |
30 | <refentrytitle>resolved.conf</refentrytitle> | |
31 | <manvolnum>5</manvolnum> | |
32 | </refmeta> | |
33 | ||
34 | <refnamediv> | |
35 | <refname>resolved.conf</refname> | |
36 | <refname>resolved.conf.d</refname> | |
37 | <refpurpose>Network Name Resolution configuration files</refpurpose> | |
38 | </refnamediv> | |
39 | ||
40 | <refsynopsisdiv> | |
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41 | <para><filename>/etc/systemd/resolved.conf</filename></para> |
42 | <para><filename>/etc/systemd/resolved.conf.d/*.conf</filename></para> | |
798d3a52 | 43 | <para><filename>/run/systemd/resolved.conf.d/*.conf</filename></para> |
12b42c76 | 44 | <para><filename>/usr/lib/systemd/resolved.conf.d/*.conf</filename></para> |
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45 | </refsynopsisdiv> |
46 | ||
47 | <refsect1> | |
48 | <title>Description</title> | |
49 | ||
50 | <para>These configuration files control local DNS and LLMNR | |
a8eaaee7 | 51 | name resolution.</para> |
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52 | |
53 | </refsect1> | |
54 | ||
e93549ef | 55 | <xi:include href="standard-conf.xml" xpointer="main-conf" /> |
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56 | |
57 | <refsect1> | |
58 | <title>Options</title> | |
59 | ||
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60 | <para>The following options are available in the <literal>[Resolve]</literal> section:</para> |
61 | ||
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62 | <variablelist class='network-directives'> |
63 | ||
64 | <varlistentry> | |
65 | <term><varname>DNS=</varname></term> | |
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66 | <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as system DNS servers. DNS requests |
67 | are sent to one of the listed DNS servers in parallel to suitable per-link DNS servers acquired from | |
68 | <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> or | |
69 | set at runtime by external applications. For compatibility reasons, if this setting is not specified, the DNS | |
70 | servers listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any servers | |
71 | are configured in it. This setting defaults to the empty list.</para></listitem> | |
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72 | </varlistentry> |
73 | ||
74 | <varlistentry> | |
75 | <term><varname>FallbackDNS=</varname></term> | |
adc800a6 LP |
76 | <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as the fallback DNS servers. Any |
77 | per-link DNS servers obtained from | |
798d3a52 | 78 | <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
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79 | take precedence over this setting, as do any servers set via <varname>DNS=</varname> above or |
80 | <filename>/etc/resolv.conf</filename>. This setting is hence only used if no other DNS server information is | |
81 | known. If this option is not given, a compiled-in list of DNS servers is used instead.</para></listitem> | |
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82 | </varlistentry> |
83 | ||
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84 | <varlistentry> |
85 | <term><varname>Domains=</varname></term> | |
adc800a6 LP |
86 | <listitem><para>A space-separated list of domains. These domains are used as search suffixes when resolving |
87 | single-label host names (domain names which contain no dot), in order to qualify them into fully-qualified | |
88 | domain names (FQDNs). Search domains are strictly processed in the order they are specified, until the name | |
89 | with the suffix appended is found. For compatibility reasons, if this setting is not specified, the search | |
90 | domains listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any domains | |
91 | are configured in it. This setting defaults to the empty list.</para> | |
92 | ||
93 | <para>Specified domain names may optionally be prefixed with <literal>~</literal>. In this case they do not | |
94 | define a search path, but preferably direct DNS queries for the indicated domains to the DNS servers configured | |
95 | with the system <varname>DNS=</varname> setting (see above), in case additional, suitable per-link DNS servers | |
96 | are known. If no per-link DNS servers are known using the <literal>~</literal> syntax has no effect. Use the | |
97 | construct <literal>~.</literal> (which is composed of <literal>~</literal> to indicate a routing domain and | |
98 | <literal>.</literal> to indicate the DNS root domain that is the implied suffix of all DNS domains) to use the | |
99 | system DNS server defined with <varname>DNS=</varname> preferably for all domains.</para></listitem> | |
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100 | </varlistentry> |
101 | ||
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102 | <varlistentry> |
103 | <term><varname>LLMNR=</varname></term> | |
104 | <listitem><para>Takes a boolean argument or | |
105 | <literal>resolve</literal>. Controls Link-Local Multicast Name | |
106 | Resolution support (<ulink | |
107 | url="https://tools.ietf.org/html/rfc4795">RFC 4794</ulink>) on | |
b938cb90 | 108 | the local host. If true, enables full LLMNR responder and |
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109 | resolver support. If false, disables both. If set to |
110 | <literal>resolve</literal>, only resolution support is enabled, | |
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111 | but responding is disabled. Note that |
112 | <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
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113 | also maintains per-link LLMNR settings. LLMNR will be |
114 | enabled on a link only if the per-link and the | |
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115 | global setting is on.</para></listitem> |
116 | </varlistentry> | |
117 | ||
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118 | <varlistentry> |
119 | <term><varname>MulticastDNS=</varname></term> | |
120 | <listitem><para>Takes a boolean argument or | |
121 | <literal>resolve</literal>. Controls Multicast DNS support (<ulink | |
122 | url="https://tools.ietf.org/html/rfc6762">RFC 6762</ulink>) on | |
123 | the local host. If true, enables full Multicast DNS responder and | |
124 | resolver support. If false, disables both. If set to | |
125 | <literal>resolve</literal>, only resolution support is enabled, | |
126 | but responding is disabled. Note that | |
127 | <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
128 | also maintains per-link Multicast DNS settings. Multicast DNS will be | |
129 | enabled on a link only if the per-link and the | |
130 | global setting is on.</para></listitem> | |
131 | </varlistentry> | |
132 | ||
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133 | <varlistentry> |
134 | <term><varname>DNSSEC=</varname></term> | |
135 | <listitem><para>Takes a boolean argument or | |
1ed8c0fb | 136 | <literal>allow-downgrade</literal>. If true all DNS lookups are |
b83d91c0 | 137 | DNSSEC-validated locally (excluding LLMNR and Multicast |
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138 | DNS). If the response to a lookup request is detected to be invalid |
139 | a lookup failure is returned to applications. Note that | |
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140 | this mode requires a DNS server that supports DNSSEC. If the |
141 | DNS server does not properly support DNSSEC all validations | |
1ed8c0fb | 142 | will fail. If set to <literal>allow-downgrade</literal> DNSSEC |
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143 | validation is attempted, but if the server does not support |
144 | DNSSEC properly, DNSSEC mode is automatically disabled. Note | |
145 | that this mode makes DNSSEC validation vulnerable to | |
146 | "downgrade" attacks, where an attacker might be able to | |
147 | trigger a downgrade to non-DNSSEC mode by synthesizing a DNS | |
148 | response that suggests DNSSEC was not supported. If set to | |
149 | false, DNS lookups are not DNSSEC validated.</para> | |
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150 | |
151 | <para>Note that DNSSEC validation requires retrieval of | |
152 | additional DNS data, and thus results in a small DNS look-up | |
153 | time penalty.</para> | |
154 | ||
155 | <para>DNSSEC requires knowledge of "trust anchors" to prove | |
156 | data integrity. The trust anchor for the Internet root domain | |
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157 | is built into the resolver, additional trust anchors may be |
158 | defined with | |
159 | <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>. | |
c542f805 | 160 | Trust anchors may change at regular intervals, and old trust |
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161 | anchors may be revoked. In such a case DNSSEC validation is |
162 | not possible until new trust anchors are configured locally or | |
163 | the resolver software package is updated with the new root | |
164 | trust anchor. In effect, when the built-in trust anchor is | |
165 | revoked and <varname>DNSSEC=</varname> is true, all further | |
166 | lookups will fail, as it cannot be proved anymore whether | |
167 | lookups are correctly signed, or validly unsigned. If | |
519d39de | 168 | <varname>DNSSEC=</varname> is set to |
1ed8c0fb | 169 | <literal>allow-downgrade</literal> the resolver will |
d57d3973 | 170 | automatically turn off DNSSEC validation in such a case.</para> |
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171 | |
172 | <para>Client programs looking up DNS data will be informed | |
173 | whether lookups could be verified using DNSSEC, or whether the | |
174 | returned data could not be verified (either because the data | |
175 | was found unsigned in the DNS, or the DNS server did not | |
176 | support DNSSEC or no appropriate trust anchors were known). In | |
177 | the latter case it is assumed that client programs employ a | |
178 | secondary scheme to validate the returned DNS data, should | |
179 | this be required.</para> | |
180 | ||
181 | <para>It is recommended to set <varname>DNSSEC=</varname> to | |
d57d3973 | 182 | true on systems where it is known that the DNS server supports |
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183 | DNSSEC correctly, and where software or trust anchor updates |
184 | happen regularly. On other systems it is recommended to set | |
185 | <varname>DNSSEC=</varname> to | |
1ed8c0fb | 186 | <literal>allow-downgrade</literal>.</para> |
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187 | |
188 | <para>In addition to this global DNSSEC setting | |
189 | <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
adc800a6 | 190 | also maintains per-link DNSSEC settings. For system DNS |
ad6c0475 | 191 | servers (see above), only the global DNSSEC setting is in |
adc800a6 | 192 | effect. For per-link DNS servers the per-link |
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193 | setting is in effect, unless it is unset in which case the |
194 | global setting is used instead.</para> | |
195 | ||
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196 | <para>Site-private DNS zones generally conflict with DNSSEC |
197 | operation, unless a negative (if the private zone is not | |
198 | signed) or positive (if the private zone is signed) trust | |
199 | anchor is configured for them. If | |
200 | <literal>allow-downgrade</literal> mode is selected, it is | |
201 | attempted to detect site-private DNS zones using top-level | |
202 | domains (TLDs) that are not known by the DNS root server. This | |
203 | logic does not work in all private zone setups.</para> | |
204 | ||
ad6c0475 | 205 | <para>Defaults to off.</para> |
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206 | </listitem> |
207 | </varlistentry> | |
208 | ||
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209 | <varlistentry> |
210 | <term><varname>Cache=</varname></term> | |
5bd73426 LP |
211 | <listitem><para>Takes a boolean argument. If "yes" (the default), resolving a domain name which already got |
212 | queried earlier will return the previous result as long as it is still valid, and thus does not result in a new | |
cfaf4b75 | 213 | network request. Be aware that turning off caching comes at a performance penalty, which is particularly |
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214 | high when DNSSEC is used.</para> |
215 | ||
216 | <para>Note that caching is turned off implicitly if the configured DNS server is on a host-local IP address | |
217 | (such as 127.0.0.1 or ::1), in order to avoid duplicate local caching.</para></listitem> | |
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218 | </varlistentry> |
219 | ||
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220 | <varlistentry> |
221 | <term><varname>DNSStubListener=</varname></term> | |
222 | <listitem><para>Takes a boolean argument or one of <literal>udp</literal> and <literal>tcp</literal>. If | |
223 | <literal>udp</literal> (the default), a DNS stub resolver will listen for UDP requests on address 127.0.0.53 | |
224 | port 53. If <literal>tcp</literal>, the stub will listen for TCP requests on the same address and port. If | |
225 | <literal>yes</literal>, the stub listens for both UDP and TCP requests. If <literal>no</literal>, the stub | |
226 | listener is disabled.</para> | |
227 | ||
228 | <para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already | |
229 | in use.</para></listitem> | |
230 | </varlistentry> | |
231 | ||
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232 | </variablelist> |
233 | </refsect1> | |
234 | ||
235 | <refsect1> | |
236 | <title>See Also</title> | |
237 | <para> | |
238 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
239 | <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
240 | <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
b5a8703f | 241 | <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
524f3e5c | 242 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>4</manvolnum></citerefentry> |
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243 | </para> |
244 | </refsect1> | |
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245 | |
246 | </refentry> |