1SYSTEMD-RESOLVED.SERVICE(8)systemd-resolved.serviceSYSTEMD-RESOLVED.SERVICE(8)
2
6 systemd-resolved.service, systemd-resolved - Network Name Resolution
7 manager
8
10 systemd-resolved.service
11 /usr/lib/systemd/systemd-resolved
13
15 systemd-resolved is a system service that provides network name
16 resolution to local applications. It implements a caching and
17 validating DNS/DNSSEC stub resolver, as well as an LLMNR and
18 MulticastDNS resolver and responder. Local applications may submit
19 network name resolution requests via three interfaces:
20 • The native, fully-featured API systemd-resolved exposes on the bus,
22 see org.freedesktop.resolve1(5) and org.freedesktop.LogControl1(5)
23 for details. Usage of this API is generally recommended to clients
24 as it is asynchronous and fully featured (for example, properly
25 returns DNSSEC validation status and interface scope for addresses
26 as necessary for supporting link-local networking).
27 • The glibc getaddrinfo(3) API as defined by RFC3493[1] and its
29 related resolver functions, including gethostbyname(3). This API is
30 widely supported, including beyond the Linux platform. In its
31 current form it does not expose DNSSEC validation status
32 information however, and is synchronous only. This API is backed by
33 the glibc Name Service Switch (nss(5)). Usage of the glibc NSS
34 module nss-resolve(8) is required in order to allow glibc's NSS
35 resolver functions to resolve hostnames via systemd-resolved.
36 • Additionally, systemd-resolved provides a local DNS stub listener
38 on IP address 127.0.0.53 on the local loopback interface. Programs
39 issuing DNS requests directly, bypassing any local API may be
40 directed to this stub, in order to connect them to
41 systemd-resolved. Note however that it is strongly recommended that
42 local programs use the glibc NSS or bus APIs instead (as described
43 above), as various network resolution concepts (such as link-local
44 addressing, or LLMNR Unicode domains) cannot be mapped to the
45 unicast DNS protocol.
46 The DNS servers contacted are determined from the global settings in
48 /etc/systemd/resolved.conf, the per-link static settings in
49 /etc/systemd/network/*.network files (in case systemd-
50 networkd.service(8) is used), the per-link dynamic settings received
51 over DHCP, information provided via resolvectl(1), and any DNS server
52 information made available by other system services. See
53 resolved.conf(5) and systemd.network(5) for details about systemd's own
54 configuration files for DNS servers. To improve compatibility,
55 /etc/resolv.conf is read in order to discover configured system DNS
56 servers, but only if it is not a symlink to
57 /run/systemd/resolve/stub-resolv.conf, /usr/lib/systemd/resolv.conf or
58 /run/systemd/resolve/resolv.conf (see below).
59
61 systemd-resolved synthesizes DNS resource records (RRs) for the
62 following cases:
63 • The local, configured hostname is resolved to all locally
65 configured IP addresses ordered by their scope, or — if none are
66 configured — the IPv4 address 127.0.0.2 (which is on the local
67 loopback interface) and the IPv6 address ::1 (which is the local
68 host).
69 • The hostnames "localhost" and "localhost.localdomain" as well as
71 any hostname ending in ".localhost" or ".localhost.localdomain" are
72 resolved to the IP addresses 127.0.0.1 and ::1.
73 • The hostname "_gateway" is resolved to all current default routing
75 gateway addresses, ordered by their metric. This assigns a stable
76 hostname to the current gateway, useful for referencing it
77 independently of the current network configuration state.
78 • The hostname "_outbound" is resolved to the local IPv4 and IPv6
80 addresses that are most likely used for communication with other
81 hosts. This is determined by requesting a routing decision to the
82 configured default gateways from the kernel and then using the
83 local IP addresses selected by this decision. This hostname is only
84 available if there is at least one local default gateway
85 configured. This assigns a stable hostname to the local outbound IP
86 addresses, useful for referencing them independently of the current
87 network configuration state.
88 • The mappings defined in /etc/hosts are resolved to their configured
90 addresses and back, but they will not affect lookups for
91 non-address types (like MX). Support for /etc/hosts may be disabled
92 with ReadEtcHosts=no, see resolved.conf(5).
93
95 The lookup requests that systemd-resolved.service receives are routed
96 to the available DNS servers, LLMNR, and MulticastDNS interfaces
97 according to the following rules:
98 • Names for which synthetic records are generated (the local
100 hostname, "localhost" and "localdomain", local gateway, as listed
101 in the previous section) and addresses configured in /etc/hosts are
102 never routed to the network and a reply is sent immediately.
103 • Single-label names are resolved using LLMNR on all local interfaces
105 where LLMNR is enabled. Lookups for IPv4 addresses are only sent
106 via LLMNR on IPv4, and lookups for IPv6 addresses are only sent via
107 LLMNR on IPv6. Note that lookups for single-label synthesized names
108 are not routed to LLMNR, MulticastDNS or unicast DNS.
109 • Queries for the address records (A and AAAA) of single-label
111 non-synthesized names are resolved via unicast DNS using search
112 domains. For any interface which defines search domains, such
113 look-ups are routed to the servers defined for that interface,
114 suffixed with each of those search domains. When global search
115 domains are defined, such look-ups are routed to the global
116 servers. For each search domain, queries are performed by suffixing
117 the name with each of the search domains in turn. Additionally,
118 lookup of single-label names via unicast DNS may be enabled with
119 the ResolveUnicastSingleLabel=yes setting. The details of which
120 servers are queried and how the final reply is chosen are described
121 below. Note that this means that address queries for single-label
122 names are never sent out to remote DNS servers by default, and
123 resolution is only possible if search domains are defined.
124 • Multi-label names with the domain suffix ".local" are resolved
126 using MulticastDNS on all local interfaces where MulticastDNS is
127 enabled. As with LLMNR, IPv4 address lookups are sent via IPv4 and
128 IPv6 address lookups are sent via IPv6.
129 • Queries for multi-label names are routed via unicast DNS on local
131 interfaces that have a DNS server configured, plus the globally
132 configured DNS servers if there are any. Which interfaces are used
133 is determined by the routing logic based on search and route-only
134 domains, described below. Note that by default, lookups for domains
135 with the ".local" suffix are not routed to DNS servers, unless the
136 domain is specified explicitly as routing or search domain for the
137 DNS server and interface. This means that on networks where the
138 ".local" domain is defined in a site-specific DNS server, explicit
139 search or routing domains need to be configured to make lookups
140 work within this DNS domain. Note that these days, it's generally
141 recommended to avoid defining ".local" in a DNS server, as
142 RFC6762[2] reserves this domain for exclusive MulticastDNS use.
143 • Address lookups (reverse lookups) are routed similarly to
145 multi-label names, with the exception that addresses from the
146 link-local address range are never routed to unicast DNS and are
147 only resolved using LLMNR and MulticastDNS (when enabled).
148 If lookups are routed to multiple interfaces, the first successful
150 response is returned (thus effectively merging the lookup zones on all
151 matching interfaces). If the lookup failed on all interfaces, the last
152 failing response is returned.
153 Routing of lookups is determined by the per-interface routing domains
155 (search and route-only) and global search domains. See
156 systemd.network(5) and resolvectl(1) for a description how those
157 settings are set dynamically and the discussion of Domains= in
158 resolved.conf(5) for a description of globally configured DNS settings.
159 The following query routing logic applies for unicast DNS lookups
161 initiated by systemd-resolved.service:
162 • If a name to look up matches (that is: is equal to or has as
164 suffix) any of the configured routing domains (search or
165 route-only) of any link, or the globally configured DNS settings,
166 "best matching" routing domain is determined: the matching one with
167 the most labels. The query is then sent to all DNS servers of any
168 links or the globally configured DNS servers associated with this
169 "best matching" routing domain. (Note that more than one link might
170 have this same "best matching" routing domain configured, in which
171 case the query is sent to all of them in parallel).
172 In case of single-label names, when search domains are defined, the
174 same logic applies, except that the name is first suffixed by each
175 of the search domains in turn. Note that this search logic doesn't
176 apply to any names with at least one dot. Also see the discussion
177 about compatibility with the traditional glibc resolver below.
178 • If a query does not match any configured routing domain (either
180 per-link or global), it is sent to all DNS servers that are
181 configured on links with the DefaultRoute= option set, as well as
182 the globally configured DNS server.
183 • If there is no link configured as DefaultRoute= and no global DNS
185 server configured, one of the compiled-in fallback DNS servers is
186 used.
187 • Otherwise the unicast DNS query fails, as no suitable DNS servers
189 can be determined.
190 The DefaultRoute= option is a boolean setting configurable with
192 resolvectl or in .network files. If not set, it is implicitly
193 determined based on the configured DNS domains for a link: if there's a
194 route-only domain other than "~.", it defaults to false, otherwise to
195 true.
196 Effectively this means: in order to support single-label
198 non-synthesized names, define appropriate search domains. In order to
199 preferably route all DNS queries not explicitly matched by routing
200 domain configuration to a specific link, configure a "~." route-only
201 domain on it. This will ensure that other links will not be considered
202 for these queries (unless they too carry such a routing domain). In
203 order to route all such DNS queries to a specific link only if no other
204 link is preferred, set the DefaultRoute= option for the link to true
205 and do not configure a "~." route-only domain on it. Finally, in order
206 to ensure that a specific link never receives any DNS traffic not
207 matching any of its configured routing domains, set the DefaultRoute=
208 option for it to false.
209 See org.freedesktop.resolve1(5) for information about the D-Bus APIs
211 systemd-resolved provides.
212
214 This section provides a short summary of differences in the stub
215 resolver implemented by nss-resolve(8) together with systemd-resolved
216 and the traditional stub resolver implemented in nss-dns.
217 • Some names are always resolved internally (see Synthetic Records
219 above). Traditionally they would be resolved by nss-files if
220 provided in /etc/hosts. But note that the details of how a query is
221 constructed are under the control of the client library. nss-dns
222 will first try to resolve names using search domains and even if
223 those queries are routed to systemd-resolved, it will send them out
224 over the network using the usual rules for multi-label name routing
225 [3].
226 • Single-label names are not resolved for A and AAAA records using
228 unicast DNS (unless overridden with ResolveUnicastSingleLabel=, see
229 resolved.conf(5)). This is similar to the no-tld-query option being
230 set in resolv.conf(5).
231 • Search domains are not used for suffixing of multi-label names.
233 (Search domains are nevertheless used for lookup routing, for names
234 that were originally specified as single-label or multi-label.) Any
235 name with at least one dot is always interpreted as a FQDN.
236 nss-dns would resolve names both as relative (using search domains)
237 and absolute FQDN names. Some names would be resolved as relative
238 first, and after that query has failed, as absolute, while other
239 names would be resolved in opposite order. The ndots option in
240 /etc/resolv.conf was used to control how many dots the name needs
241 to have to be resolved as relative first. This stub resolver does
242 not implement this at all: multi-label names are only resolved as
243 FQDNs.[4]
244 • This resolver has a notion of the special ".local" domain used for
246 MulticastDNS, and will not route queries with that suffix to
247 unicast DNS servers unless explicitly configured, see above. Also,
248 reverse lookups for link-local addresses are not sent to unicast
249 DNS servers.
250 • This resolver reads and caches /etc/hosts internally. (In other
252 words, nss-resolve replaces nss-files in addition to nss-dns).
253 Entries in /etc/hosts have highest priority.
254 • This resolver also implements LLMNR and MulticastDNS in addition to
256 the classic unicast DNS protocol, and will resolve single-label
257 names using LLMNR (when enabled) and names ending in ".local" using
258 MulticastDNS (when enabled).
259 • Environment variables $LOCALDOMAIN and $RES_OPTIONS described in
261 resolv.conf(5) are not supported currently.
262
264 Four modes of handling /etc/resolv.conf (see resolv.conf(5)) are
265 supported:
266 • systemd-resolved maintains the
268 /run/systemd/resolve/stub-resolv.conf file for compatibility with
269 traditional Linux programs. This file may be symlinked from
270 /etc/resolv.conf. This file lists the 127.0.0.53 DNS stub (see
271 above) as the only DNS server. It also contains a list of search
272 domains that are in use by systemd-resolved. The list of search
273 domains is always kept up-to-date. Note that
274 /run/systemd/resolve/stub-resolv.conf should not be used directly
275 by applications, but only through a symlink from /etc/resolv.conf.
276 This file may be symlinked from /etc/resolv.conf in order to
277 connect all local clients that bypass local DNS APIs to
278 systemd-resolved with correct search domains settings. This mode of
279 operation is recommended.
280 • A static file /usr/lib/systemd/resolv.conf is provided that lists
282 the 127.0.0.53 DNS stub (see above) as only DNS server. This file
283 may be symlinked from /etc/resolv.conf in order to connect all
284 local clients that bypass local DNS APIs to systemd-resolved. This
285 file does not contain any search domains.
286 • systemd-resolved maintains the /run/systemd/resolve/resolv.conf
288 file for compatibility with traditional Linux programs. This file
289 may be symlinked from /etc/resolv.conf and is always kept
290 up-to-date, containing information about all known DNS servers.
291 Note the file format's limitations: it does not know a concept of
292 per-interface DNS servers and hence only contains system-wide DNS
293 server definitions. Note that /run/systemd/resolve/resolv.conf
294 should not be used directly by applications, but only through a
295 symlink from /etc/resolv.conf. If this mode of operation is used
296 local clients that bypass any local DNS API will also bypass
297 systemd-resolved and will talk directly to the known DNS servers.
298 • Alternatively, /etc/resolv.conf may be managed by other packages,
300 in which case systemd-resolved will read it for DNS configuration
301 data. In this mode of operation systemd-resolved is consumer rather
302 than provider of this configuration file.
303 Note that the selected mode of operation for this file is detected
305 fully automatically, depending on whether /etc/resolv.conf is a symlink
306 to /run/systemd/resolve/resolv.conf or lists 127.0.0.53 as DNS server.
307
309 SIGUSR1
310 Upon reception of the SIGUSR1 process signal systemd-resolved will
311 dump the contents of all DNS resource record caches it maintains,
312 as well as all feature level information it learnt about configured
313 DNS servers into the system logs.
314 SIGUSR2
316 Upon reception of the SIGUSR2 process signal systemd-resolved will
317 flush all caches it maintains. Note that it should normally not be
318 necessary to request this explicitly – except for debugging
319 purposes – as systemd-resolved flushes the caches automatically
320 anyway any time the host's network configuration changes. Sending
321 this signal to systemd-resolved is equivalent to the resolvectl
322 flush-caches command, however the latter is recommended since it
323 operates in a synchronous way.
324 SIGRTMIN+1
326 Upon reception of the SIGRTMIN+1 process signal systemd-resolved
327 will forget everything it learnt about the configured DNS servers.
328 Specifically any information about server feature support is
329 flushed out, and the server feature probing logic is restarted on
330 the next request, starting with the most fully featured level. Note
331 that it should normally not be necessary to request this explicitly
332 – except for debugging purposes – as systemd-resolved automatically
333 forgets learnt information any time the DNS server configuration
334 changes. Sending this signal to systemd-resolved is equivalent to
335 the resolvectl reset-server-features command, however the latter is
336 recommended since it operates in a synchronous way.
337
339 systemd(1), resolved.conf(5), dnssec-trust-anchors.d(5), nss-
340 resolve(8), resolvectl(1), resolv.conf(5), hosts(5),
341 systemd.network(5), systemd-networkd.service(8)
342
344 1. RFC3493
345 https://tools.ietf.org/html/rfc3493
346 2. RFC6762
348 https://tools.ietf.org/html/rfc6762
349 3. For example, if /etc/resolv.conf has
351 nameserver 127.0.0.53
353 search foobar.com barbar.com
354 and we look up "localhost", nss-dns will send the following queries
357 to systemd-resolved listening on 127.0.0.53:53: first
358 "localhost.foobar.com", then "localhost.barbar.com", and finally
359 "localhost". If (hopefully) the first two queries fail, systemd-
360 resolved will synthesize an answer for the third query.
361 When using nss-dns with any search domains, it is thus crucial to
363 always configure nss-files with higher priority and provide
364 mappings for names that should not be resolved using search
365 domains.
366 4. There are currently more than 1500 top-level domain names defined,
368 and new ones are added regularly, often using "attractive" names
369 that are also likely to be used locally. Not looking up multi-label
370 names in this fashion avoids fragility in both directions: a valid
371 global name could be obscured by a local name, and resolution of a
372 relative local name could suddenly break when a new top-level
373 domain is created, or when a new subdomain of a top-level domain in
374 registered. Resolving any given name as either relative or absolute
375 avoids this ambiguity.
376systemd 249 SYSTEMD-RESOLVED.SERVICE(8)