1RESOLVED.CONF(5)                 resolved.conf                RESOLVED.CONF(5)
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NAME

6       resolved.conf, resolved.conf.d - Network Name Resolution configuration
7       files
8

SYNOPSIS

10       /etc/systemd/resolved.conf
11
12       /etc/systemd/resolved.conf.d/*.conf
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14       /run/systemd/resolved.conf.d/*.conf
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16       /usr/lib/systemd/resolved.conf.d/*.conf
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DESCRIPTION

19       These configuration files control local DNS and LLMNR name resolution.
20

CONFIGURATION DIRECTORIES AND PRECEDENCE

22       The default configuration is defined during compilation, so a
23       configuration file is only needed when it is necessary to deviate from
24       those defaults. By default, the configuration file in /etc/systemd/
25       contains commented out entries showing the defaults as a guide to the
26       administrator. This file can be edited to create local overrides.
27
28       When packages need to customize the configuration, they can install
29       configuration snippets in /usr/lib/systemd/*.conf.d/ or
30       /usr/local/lib/systemd/*.conf.d/. The main configuration file is read
31       before any of the configuration directories, and has the lowest
32       precedence; entries in a file in any configuration directory override
33       entries in the single configuration file. Files in the *.conf.d/
34       configuration subdirectories are sorted by their filename in
35       lexicographic order, regardless of in which of the subdirectories they
36       reside. When multiple files specify the same option, for options which
37       accept just a single value, the entry in the file with the
38       lexicographically latest name takes precedence. For options which
39       accept a list of values, entries are collected as they occur in files
40       sorted lexicographically.
41
42       Files in /etc/ are reserved for the local administrator, who may use
43       this logic to override the configuration files installed by vendor
44       packages. It is recommended to prefix all filenames in those
45       subdirectories with a two-digit number and a dash, to simplify the
46       ordering of the files.
47
48       To disable a configuration file supplied by the vendor, the recommended
49       way is to place a symlink to /dev/null in the configuration directory
50       in /etc/, with the same filename as the vendor configuration file.
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OPTIONS

53       The following options are available in the [Resolve] section:
54
55       DNS=
56           A space-separated list of IPv4 and IPv6 addresses to use as system
57           DNS servers. Each address can optionally take a port number
58           separated with ":", a network interface name or index separated
59           with "%", and a Server Name Indication (SNI) separated with "#".
60           When IPv6 address is specified with a port number, then the address
61           must be in the square brackets. That is, the acceptable full
62           formats are "111.222.333.444:9953%ifname#example.com" for IPv4 and
63           "[1111:2222::3333]:9953%ifname#example.com" for IPv6. DNS requests
64           are sent to one of the listed DNS servers in parallel to suitable
65           per-link DNS servers acquired from systemd-networkd.service(8) or
66           set at runtime by external applications. For compatibility reasons,
67           if this setting is not specified, the DNS servers listed in
68           /etc/resolv.conf are used instead, if that file exists and any
69           servers are configured in it. This setting defaults to the empty
70           list.
71
72       FallbackDNS=
73           A space-separated list of IPv4 and IPv6 addresses to use as the
74           fallback DNS servers. Please see DNS= for acceptable format of
75           adddresses. Any per-link DNS servers obtained from systemd-
76           networkd.service(8) take precedence over this setting, as do any
77           servers set via DNS= above or /etc/resolv.conf. This setting is
78           hence only used if no other DNS server information is known. If
79           this option is not given, a compiled-in list of DNS servers is used
80           instead.
81
82       Domains=
83           A space-separated list of domains optionally prefixed with "~",
84           used for two distinct purposes described below. Defaults to the
85           empty list.
86
87           Any domains not prefixed with "~" are used as search suffixes when
88           resolving single-label hostnames (domain names which contain no
89           dot), in order to qualify them into fully-qualified domain names
90           (FQDNs). These "search domains" are strictly processed in the order
91           they are specified in, until the name with the suffix appended is
92           found. For compatibility reasons, if this setting is not specified,
93           the search domains listed in /etc/resolv.conf with the search
94           keyword are used instead, if that file exists and any domains are
95           configured in it.
96
97           The domains prefixed with "~" are called "routing domains". All
98           domains listed here (both search domains and routing domains after
99           removing the "~" prefix) define a search path that preferably
100           directs DNS queries to this interface. This search path has an
101           effect only when suitable per-link DNS servers are known. Such
102           servers may be defined through the DNS= setting (see above) and
103           dynamically at run time, for example from DHCP leases. If no
104           per-link DNS servers are known, routing domains have no effect.
105
106           Use the construct "~."  (which is composed from "~" to indicate a
107           routing domain and "."  to indicate the DNS root domain that is the
108           implied suffix of all DNS domains) to use the DNS servers defined
109           for this link preferably for all domains.
110
111       LLMNR=
112           Takes a boolean argument or "resolve". Controls Link-Local
113           Multicast Name Resolution support (RFC 4795[1]) on the local host.
114           If true, enables full LLMNR responder and resolver support. If
115           false, disables both. If set to "resolve", only resolution support
116           is enabled, but responding is disabled. Note that systemd-
117           networkd.service(8) also maintains per-link LLMNR settings. LLMNR
118           will be enabled on a link only if the per-link and the global
119           setting is on.
120
121       MulticastDNS=
122           Takes a boolean argument or "resolve". Controls Multicast DNS
123           support (RFC 6762[2]) on the local host. If true, enables full
124           Multicast DNS responder and resolver support. If false, disables
125           both. If set to "resolve", only resolution support is enabled, but
126           responding is disabled. Note that systemd-networkd.service(8) also
127           maintains per-link Multicast DNS settings. Multicast DNS will be
128           enabled on a link only if the per-link and the global setting is
129           on.
130
131       DNSSEC=
132           Takes a boolean argument or "allow-downgrade". If true all DNS
133           lookups are DNSSEC-validated locally (excluding LLMNR and Multicast
134           DNS). If the response to a lookup request is detected to be invalid
135           a lookup failure is returned to applications. Note that this mode
136           requires a DNS server that supports DNSSEC. If the DNS server does
137           not properly support DNSSEC all validations will fail. If set to
138           "allow-downgrade" DNSSEC validation is attempted, but if the server
139           does not support DNSSEC properly, DNSSEC mode is automatically
140           disabled. Note that this mode makes DNSSEC validation vulnerable to
141           "downgrade" attacks, where an attacker might be able to trigger a
142           downgrade to non-DNSSEC mode by synthesizing a DNS response that
143           suggests DNSSEC was not supported. If set to false, DNS lookups are
144           not DNSSEC validated.
145
146           Note that DNSSEC validation requires retrieval of additional DNS
147           data, and thus results in a small DNS look-up time penalty.
148
149           DNSSEC requires knowledge of "trust anchors" to prove data
150           integrity. The trust anchor for the Internet root domain is built
151           into the resolver, additional trust anchors may be defined with
152           dnssec-trust-anchors.d(5). Trust anchors may change at regular
153           intervals, and old trust anchors may be revoked. In such a case
154           DNSSEC validation is not possible until new trust anchors are
155           configured locally or the resolver software package is updated with
156           the new root trust anchor. In effect, when the built-in trust
157           anchor is revoked and DNSSEC= is true, all further lookups will
158           fail, as it cannot be proved anymore whether lookups are correctly
159           signed, or validly unsigned. If DNSSEC= is set to "allow-downgrade"
160           the resolver will automatically turn off DNSSEC validation in such
161           a case.
162
163           Client programs looking up DNS data will be informed whether
164           lookups could be verified using DNSSEC, or whether the returned
165           data could not be verified (either because the data was found
166           unsigned in the DNS, or the DNS server did not support DNSSEC or no
167           appropriate trust anchors were known). In the latter case it is
168           assumed that client programs employ a secondary scheme to validate
169           the returned DNS data, should this be required.
170
171           It is recommended to set DNSSEC= to true on systems where it is
172           known that the DNS server supports DNSSEC correctly, and where
173           software or trust anchor updates happen regularly. On other systems
174           it is recommended to set DNSSEC= to "allow-downgrade".
175
176           In addition to this global DNSSEC setting systemd-
177           networkd.service(8) also maintains per-link DNSSEC settings. For
178           system DNS servers (see above), only the global DNSSEC setting is
179           in effect. For per-link DNS servers the per-link setting is in
180           effect, unless it is unset in which case the global setting is used
181           instead.
182
183           Site-private DNS zones generally conflict with DNSSEC operation,
184           unless a negative (if the private zone is not signed) or positive
185           (if the private zone is signed) trust anchor is configured for
186           them. If "allow-downgrade" mode is selected, it is attempted to
187           detect site-private DNS zones using top-level domains (TLDs) that
188           are not known by the DNS root server. This logic does not work in
189           all private zone setups.
190
191           Defaults to "allow-downgrade"
192
193       DNSOverTLS=
194           Takes a boolean argument or "opportunistic". If true all
195           connections to the server will be encrypted. Note that this mode
196           requires a DNS server that supports DNS-over-TLS and has a valid
197           certificate. If the hostname was specified in DNS= by using the
198           format format "address#server_name" it is used to validate its
199           certificate and also to enable Server Name Indication (SNI) when
200           opening a TLS connection. Otherwise the certificate is checked
201           against the server's IP. If the DNS server does not support
202           DNS-over-TLS all DNS requests will fail.
203
204           When set to "opportunistic" DNS request are attempted to send
205           encrypted with DNS-over-TLS. If the DNS server does not support
206           TLS, DNS-over-TLS is disabled. Note that this mode makes
207           DNS-over-TLS vulnerable to "downgrade" attacks, where an attacker
208           might be able to trigger a downgrade to non-encrypted mode by
209           synthesizing a response that suggests DNS-over-TLS was not
210           supported. If set to false, DNS lookups are send over UDP.
211
212           Note that DNS-over-TLS requires additional data to be send for
213           setting up an encrypted connection, and thus results in a small DNS
214           look-up time penalty.
215
216           Note that in "opportunistic" mode the resolver is not capable of
217           authenticating the server, so it is vulnerable to
218           "man-in-the-middle" attacks.
219
220           In addition to this global DNSOverTLS setting systemd-
221           networkd.service(8) also maintains per-link DNSOverTLS settings.
222           For system DNS servers (see above), only the global DNSOverTLS
223           setting is in effect. For per-link DNS servers the per-link setting
224           is in effect, unless it is unset in which case the global setting
225           is used instead.
226
227           Defaults to off.
228
229       Cache=
230           Takes a boolean or "no-negative" as argument. If "yes" (the
231           default), resolving a domain name which already got queried earlier
232           will return the previous result as long as it is still valid, and
233           thus does not result in a new network request. Be aware that
234           turning off caching comes at a performance penalty, which is
235           particularly high when DNSSEC is used. If "no-negative", only
236           positive answers are cached.
237
238           Note that caching is turned off implicitly if the configured DNS
239           server is on a host-local IP address (such as 127.0.0.1 or ::1), in
240           order to avoid duplicate local caching.
241
242       DNSStubListener=
243           Takes a boolean argument or one of "udp" and "tcp". If "udp", a DNS
244           stub resolver will listen for UDP requests on address 127.0.0.53
245           port 53. If "tcp", the stub will listen for TCP requests on the
246           same address and port. If "yes" (the default), the stub listens for
247           both UDP and TCP requests. If "no", the stub listener is disabled.
248
249           Note that the DNS stub listener is turned off implicitly when its
250           listening address and port are already in use.
251
252       ReadEtcHosts=
253           Takes a boolean argument. If "yes" (the default), systemd-resolved
254           will read /etc/hosts, and try to resolve hosts or address by using
255           the entries in the file before sending query to DNS servers.
256
257       ResolveUnicastSingleLabel=
258           Takes a boolean argument. When false (the default),
259           systemd-resolved will not resolve A and AAAA queries for
260           single-label names over classic DNS. Note that such names may still
261           be resolved if search domains are specified (see Domains= above),
262           or using other mechanisms, in particular via LLMNR or from
263           /etc/hosts. When true, queries for single-label names will be
264           forwarded to global DNS servers even if no search domains are
265           defined.
266
267           This option is provided for compatibility with configurations where
268           public DNS servers are not used. Forwarding single-label names to
269           servers not under your control is not standard-conformant, see IAB
270           Statement[3], and may create a privacy and security risk.
271

SEE ALSO

273       systemd(1), systemd-resolved.service(8), systemd-networkd.service(8),
274       dnssec-trust-anchors.d(5), resolv.conf(4)
275

NOTES

277        1. RFC 4795
278           https://tools.ietf.org/html/rfc4795
279
280        2. RFC 6762
281           https://tools.ietf.org/html/rfc6762
282
283        3. IAB Statement
284           https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/
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288systemd 246                                                   RESOLVED.CONF(5)
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