1RESOLVED.CONF(5) resolved.conf RESOLVED.CONF(5)
2
6 resolved.conf, resolved.conf.d - Network Name Resolution configuration
7 files
8
10 /etc/systemd/resolved.conf
11 /etc/systemd/resolved.conf.d/*.conf
13 /run/systemd/resolved.conf.d/*.conf
15 /usr/lib/systemd/resolved.conf.d/*.conf
17
19 These configuration files control local DNS and LLMNR name resolution.
20
22 The default configuration is set during compilation, so configuration
23 is only needed when it is necessary to deviate from those defaults.
24 Initially, the main configuration file in /etc/systemd/ contains
25 commented out entries showing the defaults as a guide to the
26 administrator. Local overrides can be created by editing this file or
27 by creating drop-ins, as described below. Using drop-ins for local
28 configuration is recommended over modifications to the main
29 configuration file.
30 In addition to the "main" configuration file, drop-in configuration
32 snippets are read from /usr/lib/systemd/*.conf.d/,
33 /usr/local/lib/systemd/*.conf.d/, and /etc/systemd/*.conf.d/. Those
34 drop-ins have higher precedence and override the main configuration
35 file. Files in the *.conf.d/ configuration subdirectories are sorted by
36 their filename in lexicographic order, regardless of in which of the
37 subdirectories they reside. When multiple files specify the same
38 option, for options which accept just a single value, the entry in the
39 file sorted last takes precedence, and for options which accept a list
40 of values, entries are collected as they occur in the sorted files.
41 When packages need to customize the configuration, they can install
43 drop-ins under /usr/. Files in /etc/ are reserved for the local
44 administrator, who may use this logic to override the configuration
45 files installed by vendor packages. Drop-ins have to be used to
46 override package drop-ins, since the main configuration file has lower
47 precedence. It is recommended to prefix all filenames in those
48 subdirectories with a two-digit number and a dash, to simplify the
49 ordering of the files.
50 To disable a configuration file supplied by the vendor, the recommended
52 way is to place a symlink to /dev/null in the configuration directory
53 in /etc/, with the same filename as the vendor configuration file.
54
56 The following options are available in the [Resolve] section:
57 DNS=
59 A space-separated list of IPv4 and IPv6 addresses to use as system
60 DNS servers. Each address can optionally take a port number
61 separated with ":", a network interface name or index separated
62 with "%", and a Server Name Indication (SNI) separated with "#".
63 When IPv6 address is specified with a port number, then the address
64 must be in the square brackets. That is, the acceptable full
65 formats are "111.222.333.444:9953%ifname#example.com" for IPv4 and
66 "[1111:2222::3333]:9953%ifname#example.com" for IPv6. DNS requests
67 are sent to one of the listed DNS servers in parallel to suitable
68 per-link DNS servers acquired from systemd-networkd.service(8) or
69 set at runtime by external applications. For compatibility reasons,
70 if this setting is not specified, the DNS servers listed in
71 /etc/resolv.conf are used instead, if that file exists and any
72 servers are configured in it. This setting defaults to the empty
73 list.
74 FallbackDNS=
76 A space-separated list of IPv4 and IPv6 addresses to use as the
77 fallback DNS servers. Please see DNS= for acceptable format of
78 addresses. Any per-link DNS servers obtained from systemd-
79 networkd.service(8) take precedence over this setting, as do any
80 servers set via DNS= above or /etc/resolv.conf. This setting is
81 hence only used if no other DNS server information is known. If
82 this option is not given, a compiled-in list of DNS servers is used
83 instead.
84 Domains=
86 A space-separated list of domains optionally prefixed with "~",
87 used for two distinct purposes described below. Defaults to the
88 empty list.
89 Any domains not prefixed with "~" are used as search suffixes when
91 resolving single-label hostnames (domain names which contain no
92 dot), in order to qualify them into fully-qualified domain names
93 (FQDNs). These "search domains" are strictly processed in the order
94 they are specified in, until the name with the suffix appended is
95 found. For compatibility reasons, if this setting is not specified,
96 the search domains listed in /etc/resolv.conf with the search
97 keyword are used instead, if that file exists and any domains are
98 configured in it.
99 The domains prefixed with "~" are called "routing domains". All
101 domains listed here (both search domains and routing domains after
102 removing the "~" prefix) define a search path that preferably
103 directs DNS queries to this interface. This search path has an
104 effect only when suitable per-link DNS servers are known. Such
105 servers may be defined through the DNS= setting (see above) and
106 dynamically at run time, for example from DHCP leases. If no
107 per-link DNS servers are known, routing domains have no effect.
108 Use the construct "~." (which is composed from "~" to indicate a
110 routing domain and "." to indicate the DNS root domain that is the
111 implied suffix of all DNS domains) to use the DNS servers defined
112 for this link preferably for all domains.
113 LLMNR=
115 Takes a boolean argument or "resolve". Controls Link-Local
116 Multicast Name Resolution support (RFC 4795[1]) on the local host.
117 If true, enables full LLMNR responder and resolver support. If
118 false, disables both. If set to "resolve", only resolution support
119 is enabled, but responding is disabled. Note that systemd-
120 networkd.service(8) also maintains per-link LLMNR settings. LLMNR
121 will be enabled on a link only if the per-link and the global
122 setting is on.
123 MulticastDNS=
125 Takes a boolean argument or "resolve". Controls Multicast DNS
126 support (RFC 6762[2]) on the local host. If true, enables full
127 Multicast DNS responder and resolver support. If false, disables
128 both. If set to "resolve", only resolution support is enabled, but
129 responding is disabled. Note that systemd-networkd.service(8) also
130 maintains per-link Multicast DNS settings. Multicast DNS will be
131 enabled on a link only if the per-link and the global setting is
132 on.
133 DNSSEC=
135 Takes a boolean argument or "allow-downgrade". If true all DNS
136 lookups are DNSSEC-validated locally (excluding LLMNR and Multicast
137 DNS). If the response to a lookup request is detected to be invalid
138 a lookup failure is returned to applications. Note that this mode
139 requires a DNS server that supports DNSSEC. If the DNS server does
140 not properly support DNSSEC all validations will fail. If set to
141 "allow-downgrade" DNSSEC validation is attempted, but if the server
142 does not support DNSSEC properly, DNSSEC mode is automatically
143 disabled. Note that this mode makes DNSSEC validation vulnerable to
144 "downgrade" attacks, where an attacker might be able to trigger a
145 downgrade to non-DNSSEC mode by synthesizing a DNS response that
146 suggests DNSSEC was not supported. If set to false, DNS lookups are
147 not DNSSEC validated.
148 Note that DNSSEC validation requires retrieval of additional DNS
150 data, and thus results in a small DNS look-up time penalty.
151 DNSSEC requires knowledge of "trust anchors" to prove data
153 integrity. The trust anchor for the Internet root domain is built
154 into the resolver, additional trust anchors may be defined with
155 dnssec-trust-anchors.d(5). Trust anchors may change at regular
156 intervals, and old trust anchors may be revoked. In such a case
157 DNSSEC validation is not possible until new trust anchors are
158 configured locally or the resolver software package is updated with
159 the new root trust anchor. In effect, when the built-in trust
160 anchor is revoked and DNSSEC= is true, all further lookups will
161 fail, as it cannot be proved anymore whether lookups are correctly
162 signed, or validly unsigned. If DNSSEC= is set to "allow-downgrade"
163 the resolver will automatically turn off DNSSEC validation in such
164 a case.
165 Client programs looking up DNS data will be informed whether
167 lookups could be verified using DNSSEC, or whether the returned
168 data could not be verified (either because the data was found
169 unsigned in the DNS, or the DNS server did not support DNSSEC or no
170 appropriate trust anchors were known). In the latter case it is
171 assumed that client programs employ a secondary scheme to validate
172 the returned DNS data, should this be required.
173 It is recommended to set DNSSEC= to true on systems where it is
175 known that the DNS server supports DNSSEC correctly, and where
176 software or trust anchor updates happen regularly. On other systems
177 it is recommended to set DNSSEC= to "allow-downgrade".
178 In addition to this global DNSSEC setting systemd-
180 networkd.service(8) also maintains per-link DNSSEC settings. For
181 system DNS servers (see above), only the global DNSSEC setting is
182 in effect. For per-link DNS servers the per-link setting is in
183 effect, unless it is unset in which case the global setting is used
184 instead.
185 Site-private DNS zones generally conflict with DNSSEC operation,
187 unless a negative (if the private zone is not signed) or positive
188 (if the private zone is signed) trust anchor is configured for
189 them. If "allow-downgrade" mode is selected, it is attempted to
190 detect site-private DNS zones using top-level domains (TLDs) that
191 are not known by the DNS root server. This logic does not work in
192 all private zone setups.
193 Defaults to "no".
195 DNSOverTLS=
197 Takes a boolean argument or "opportunistic". If true all
198 connections to the server will be encrypted. Note that this mode
199 requires a DNS server that supports DNS-over-TLS and has a valid
200 certificate. If the hostname was specified in DNS= by using the
201 format "address#server_name" it is used to validate its certificate
202 and also to enable Server Name Indication (SNI) when opening a TLS
203 connection. Otherwise the certificate is checked against the
204 server's IP. If the DNS server does not support DNS-over-TLS all
205 DNS requests will fail.
206 When set to "opportunistic" DNS request are attempted to send
208 encrypted with DNS-over-TLS. If the DNS server does not support
209 TLS, DNS-over-TLS is disabled. Note that this mode makes
210 DNS-over-TLS vulnerable to "downgrade" attacks, where an attacker
211 might be able to trigger a downgrade to non-encrypted mode by
212 synthesizing a response that suggests DNS-over-TLS was not
213 supported. If set to false, DNS lookups are send over UDP.
214 Note that DNS-over-TLS requires additional data to be send for
216 setting up an encrypted connection, and thus results in a small DNS
217 look-up time penalty.
218 Note that in "opportunistic" mode the resolver is not capable of
220 authenticating the server, so it is vulnerable to
221 "man-in-the-middle" attacks.
222 In addition to this global DNSOverTLS= setting systemd-
224 networkd.service(8) also maintains per-link DNSOverTLS= settings.
225 For system DNS servers (see above), only the global DNSOverTLS=
226 setting is in effect. For per-link DNS servers the per-link setting
227 is in effect, unless it is unset in which case the global setting
228 is used instead.
229 Defaults to "no".
231 Cache=
233 Takes a boolean or "no-negative" as argument. If "yes" (the
234 default), resolving a domain name which already got queried earlier
235 will return the previous result as long as it is still valid, and
236 thus does not result in a new network request. Be aware that
237 turning off caching comes at a performance penalty, which is
238 particularly high when DNSSEC is used. If "no-negative", only
239 positive answers are cached.
240 Note that caching is turned off by default for host-local DNS
242 servers. See CacheFromLocalhost= for details.
243 CacheFromLocalhost=
245 Takes a boolean as argument. If "no" (the default), and response
246 cames from host-local IP address (such as 127.0.0.1 or ::1), the
247 result wouldn't be cached in order to avoid potential duplicate
248 local caching.
249 DNSStubListener=
251 Takes a boolean argument or one of "udp" and "tcp". If "udp", a DNS
252 stub resolver will listen for UDP requests on addresses 127.0.0.53
253 and 127.0.0.54, port 53. If "tcp", the stub will listen for TCP
254 requests on the same addresses and port. If "yes" (the default),
255 the stub listens for both UDP and TCP requests. If "no", the stub
256 listener is disabled.
257 The DNS stub resolver on 127.0.0.53 provides the full feature set
259 of the local resolver, which includes offering LLMNR/MulticastDNS
260 resolution. The DNS stub resolver on 127.0.0.54 provides a more
261 limited resolver, that operates in "proxy" mode only, i.e. it will
262 pass most DNS messages relatively unmodified to the current
263 upstream DNS servers and back, but not try to process the messages
264 locally, and hence does not validate DNSSEC, or offer up
265 LLMNR/MulticastDNS. (It will translate to DNS-over-TLS
266 communication if needed however.)
267 Note that the DNS stub listener is turned off implicitly when its
269 listening address and port are already in use.
270 DNSStubListenerExtra=
272 Takes an IPv4 or IPv6 address to listen on. The address may be
273 optionally prefixed with a protocol name ("udp" or "tcp") separated
274 with ":". If the protocol is not specified, the service will listen
275 on both UDP and TCP. It may be also optionally suffixed by a
276 numeric port number with separator ":". When an IPv6 address is
277 specified with a port number, then the address must be in the
278 square brackets. If the port is not specified, then the service
279 uses port 53. Note that this is independent of the primary DNS stub
280 configured with DNSStubListener=, and only configures additional
281 sockets to listen on. This option can be specified multiple times.
282 If an empty string is assigned, then the all previous assignments
283 are cleared. Defaults to unset.
284 Examples:
286 DNSStubListenerExtra=192.168.10.10
288 DNSStubListenerExtra=2001:db8:0:f102::10
289 DNSStubListenerExtra=192.168.10.11:9953
290 DNSStubListenerExtra=[2001:db8:0:f102::11]:9953
291 DNSStubListenerExtra=tcp:192.168.10.12
292 DNSStubListenerExtra=udp:2001:db8:0:f102::12
293 DNSStubListenerExtra=tcp:192.168.10.13:9953
294 DNSStubListenerExtra=udp:[2001:db8:0:f102::13]:9953
295 ReadEtcHosts=
298 Takes a boolean argument. If "yes" (the default), systemd-resolved
299 will read /etc/hosts, and try to resolve hosts or address by using
300 the entries in the file before sending query to DNS servers.
301 ResolveUnicastSingleLabel=
303 Takes a boolean argument. When false (the default),
304 systemd-resolved will not resolve A and AAAA queries for
305 single-label names over classic DNS. Note that such names may still
306 be resolved if search domains are specified (see Domains= above),
307 or using other mechanisms, in particular via LLMNR or from
308 /etc/hosts. When true, queries for single-label names will be
309 forwarded to global DNS servers even if no search domains are
310 defined.
311 This option is provided for compatibility with configurations where
313 public DNS servers are not used. Forwarding single-label names to
314 servers not under your control is not standard-conformant, see IAB
315 Statement[3], and may create a privacy and security risk.
316
318 systemd(1), systemd-resolved.service(8), systemd-networkd.service(8),
319 dnssec-trust-anchors.d(5), resolv.conf(5)
320
322 1. RFC 4795
323 https://tools.ietf.org/html/rfc4795
324 2. RFC 6762
326 https://tools.ietf.org/html/rfc6762
327 3. IAB Statement
329 https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/
330systemd 250 RESOLVED.CONF(5)