1RESOLVED.CONF(5)                 resolved.conf                RESOLVED.CONF(5)
2
3
4

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
13
14       /run/systemd/resolved.conf.d/*.conf
15
16       /usr/lib/systemd/resolved.conf.d/*.conf
17

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/. Files in /etc/
30       are reserved for the local administrator, who may use this logic to
31       override the configuration files installed by vendor packages. The main
32       configuration file is read before any of the configuration directories,
33       and has the lowest precedence; entries in a file in any configuration
34       directory override entries in the single configuration file. Files in
35       the *.conf.d/ configuration subdirectories are sorted by their filename
36       in lexicographic order, regardless of which of the subdirectories they
37       reside in. When multiple files specify the same option, for options
38       which accept just a single value, the entry in the file with the
39       lexicographically latest name takes precedence. For options which
40       accept a list of values, entries are collected as they occur in files
41       sorted lexicographically. It is recommended to prefix all filenames in
42       those subdirectories with a two-digit number and a dash, to simplify
43       the ordering of the files.
44
45       To disable a configuration file supplied by the vendor, the recommended
46       way is to place a symlink to /dev/null in the configuration directory
47       in /etc/, with the same filename as the vendor configuration file.
48

OPTIONS

50       The following options are available in the "[Resolve]" section:
51
52       DNS=
53           A space-separated list of IPv4 and IPv6 addresses to use as system
54           DNS servers. DNS requests are sent to one of the listed DNS servers
55           in parallel to suitable per-link DNS servers acquired from systemd-
56           networkd.service(8) or set at runtime by external applications. For
57           compatibility reasons, if this setting is not specified, the DNS
58           servers listed in /etc/resolv.conf are used instead, if that file
59           exists and any servers are configured in it. This setting defaults
60           to the empty list.
61
62       FallbackDNS=
63           A space-separated list of IPv4 and IPv6 addresses to use as the
64           fallback DNS servers. Any per-link DNS servers obtained from
65           systemd-networkd.service(8) take precedence over this setting, as
66           do any servers set via DNS= above or /etc/resolv.conf. This setting
67           is hence only used if no other DNS server information is known. If
68           this option is not given, a compiled-in list of DNS servers is used
69           instead.
70
71       Domains=
72           A space-separated list of domains. These domains are used as search
73           suffixes when resolving single-label host names (domain names which
74           contain no dot), in order to qualify them into fully-qualified
75           domain names (FQDNs). Search domains are strictly processed in the
76           order they are specified, until the name with the suffix appended
77           is found. For compatibility reasons, if this setting is not
78           specified, the search domains listed in /etc/resolv.conf are used
79           instead, if that file exists and any domains are configured in it.
80           This setting defaults to the empty list.
81
82           Specified domain names may optionally be prefixed with "~". In this
83           case they do not define a search path, but preferably direct DNS
84           queries for the indicated domains to the DNS servers configured
85           with the system DNS= setting (see above), in case additional,
86           suitable per-link DNS servers are known. If no per-link DNS servers
87           are known using the "~" syntax has no effect. Use the construct
88           "~."  (which is composed of "~" to indicate a routing domain and
89           "."  to indicate the DNS root domain that is the implied suffix of
90           all DNS domains) to use the system DNS server defined with DNS=
91           preferably for all domains.
92
93       LLMNR=
94           Takes a boolean argument or "resolve". Controls Link-Local
95           Multicast Name Resolution support (RFC 4795[1]) on the local host.
96           If true, enables full LLMNR responder and resolver support. If
97           false, disables both. If set to "resolve", only resolution support
98           is enabled, but responding is disabled. Note that systemd-
99           networkd.service(8) also maintains per-link LLMNR settings. LLMNR
100           will be enabled on a link only if the per-link and the global
101           setting is on.
102
103       MulticastDNS=
104           Takes a boolean argument or "resolve". Controls Multicast DNS
105           support (RFC 6762[2]) on the local host. If true, enables full
106           Multicast DNS responder and resolver support. If false, disables
107           both. If set to "resolve", only resolution support is enabled, but
108           responding is disabled. Note that systemd-networkd.service(8) also
109           maintains per-link Multicast DNS settings. Multicast DNS will be
110           enabled on a link only if the per-link and the global setting is
111           on.
112
113       DNSSEC=
114           Takes a boolean argument or "allow-downgrade". If true all DNS
115           lookups are DNSSEC-validated locally (excluding LLMNR and Multicast
116           DNS). If the response to a lookup request is detected to be invalid
117           a lookup failure is returned to applications. Note that this mode
118           requires a DNS server that supports DNSSEC. If the DNS server does
119           not properly support DNSSEC all validations will fail. If set to
120           "allow-downgrade" DNSSEC validation is attempted, but if the server
121           does not support DNSSEC properly, DNSSEC mode is automatically
122           disabled. Note that this mode makes DNSSEC validation vulnerable to
123           "downgrade" attacks, where an attacker might be able to trigger a
124           downgrade to non-DNSSEC mode by synthesizing a DNS response that
125           suggests DNSSEC was not supported. If set to false, DNS lookups are
126           not DNSSEC validated.
127
128           Note that DNSSEC validation requires retrieval of additional DNS
129           data, and thus results in a small DNS look-up time penalty.
130
131           DNSSEC requires knowledge of "trust anchors" to prove data
132           integrity. The trust anchor for the Internet root domain is built
133           into the resolver, additional trust anchors may be defined with
134           dnssec-trust-anchors.d(5). Trust anchors may change at regular
135           intervals, and old trust anchors may be revoked. In such a case
136           DNSSEC validation is not possible until new trust anchors are
137           configured locally or the resolver software package is updated with
138           the new root trust anchor. In effect, when the built-in trust
139           anchor is revoked and DNSSEC= is true, all further lookups will
140           fail, as it cannot be proved anymore whether lookups are correctly
141           signed, or validly unsigned. If DNSSEC= is set to "allow-downgrade"
142           the resolver will automatically turn off DNSSEC validation in such
143           a case.
144
145           Client programs looking up DNS data will be informed whether
146           lookups could be verified using DNSSEC, or whether the returned
147           data could not be verified (either because the data was found
148           unsigned in the DNS, or the DNS server did not support DNSSEC or no
149           appropriate trust anchors were known). In the latter case it is
150           assumed that client programs employ a secondary scheme to validate
151           the returned DNS data, should this be required.
152
153           It is recommended to set DNSSEC= to true on systems where it is
154           known that the DNS server supports DNSSEC correctly, and where
155           software or trust anchor updates happen regularly. On other systems
156           it is recommended to set DNSSEC= to "allow-downgrade".
157
158           In addition to this global DNSSEC setting systemd-
159           networkd.service(8) also maintains per-link DNSSEC settings. For
160           system DNS servers (see above), only the global DNSSEC setting is
161           in effect. For per-link DNS servers the per-link setting is in
162           effect, unless it is unset in which case the global setting is used
163           instead.
164
165           Site-private DNS zones generally conflict with DNSSEC operation,
166           unless a negative (if the private zone is not signed) or positive
167           (if the private zone is signed) trust anchor is configured for
168           them. If "allow-downgrade" mode is selected, it is attempted to
169           detect site-private DNS zones using top-level domains (TLDs) that
170           are not known by the DNS root server. This logic does not work in
171           all private zone setups.
172
173           Defaults to "allow-downgrade"
174
175       DNSOverTLS=
176           Takes false or "opportunistic". When set to "opportunistic" DNS
177           request are attempted to send encrypted with DNS-over-TLS. If the
178           DNS server does not support TLS, DNS-over-TLS is disabled. Note
179           that this mode makes DNS-over-TLS vulnerable to "downgrade"
180           attacks, where an attacker might be able to trigger a downgrade to
181           non-encrypted mode by synthesizing a response that suggests
182           DNS-over-TLS was not supported. If set to false, DNS lookups are
183           send over UDP.
184
185           Note that DNS-over-TLS requires additional data to be send for
186           setting up an encrypted connection, and thus results in a small DNS
187           look-up time penalty.
188
189           Note as the resolver is not capable of authenticating the server,
190           it is vulnerable for "man-in-the-middle" attacks.
191
192           In addition to this global DNSOverTLS setting systemd-
193           networkd.service(8) also maintains per-link DNSOverTLS settings.
194           For system DNS servers (see above), only the global DNSOverTLS
195           setting is in effect. For per-link DNS servers the per-link setting
196           is in effect, unless it is unset in which case the global setting
197           is used instead.
198
199           Defaults to off.
200
201       Cache=
202           Takes a boolean argument. If "yes" (the default), resolving a
203           domain name which already got queried earlier will return the
204           previous result as long as it is still valid, and thus does not
205           result in a new network request. Be aware that turning off caching
206           comes at a performance penalty, which is particularly high when
207           DNSSEC is used.
208
209           Note that caching is turned off implicitly if the configured DNS
210           server is on a host-local IP address (such as 127.0.0.1 or ::1), in
211           order to avoid duplicate local caching.
212
213       DNSStubListener=
214           Takes a boolean argument or one of "udp" and "tcp". If "udp" (the
215           default), a DNS stub resolver will listen for UDP requests on
216           address 127.0.0.53 port 53. If "tcp", the stub will listen for TCP
217           requests on the same address and port. If "yes", the stub listens
218           for both UDP and TCP requests. If "no", the stub listener is
219           disabled.
220
221           Note that the DNS stub listener is turned off implicitly when its
222           listening address and port are already in use.
223

SEE ALSO

225       systemd(1), systemd-resolved.service(8), systemd-networkd.service(8),
226       dnssec-trust-anchors.d(5), resolv.conf(4)
227

NOTES

229        1. RFC 4795
230           https://tools.ietf.org/html/rfc4795
231
232        2. RFC 6762
233           https://tools.ietf.org/html/rfc6762
234
235
236
237systemd 239                                                   RESOLVED.CONF(5)
Impressum