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 the IP addresses 127.0.0.53 and 127.0.0.54 on the local loopback
39 interface. Programs issuing DNS requests directly, bypassing any
40 local API may be 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 stub resolver on 127.0.0.53 provides the full feature set
48 of the local resolver, which includes offering LLMNR/MulticastDNS
49 resolution. The DNS stub resolver on 127.0.0.54 provides a more
50 limited resolver, that operates in "proxy" mode only, i.e. it will
51 pass most DNS messages relatively unmodified to the current
52 upstream DNS servers and back, but not try to process the messages
53 locally, and hence does not validate DNSSEC, or offer up
54 LLMNR/MulticastDNS. (It will translate to DNS-over-TLS
55 communication if needed however.)
56 The DNS servers contacted are determined from the global settings in
58 /etc/systemd/resolved.conf, the per-link static settings in
59 /etc/systemd/network/*.network files (in case systemd-
60 networkd.service(8) is used), the per-link dynamic settings received
61 over DHCP, information provided via resolvectl(1), and any DNS server
62 information made available by other system services. See
63 resolved.conf(5) and systemd.network(5) for details about systemd's own
64 configuration files for DNS servers. To improve compatibility,
65 /etc/resolv.conf is read in order to discover configured system DNS
66 servers, but only if it is not a symlink to
67 /run/systemd/resolve/stub-resolv.conf, /usr/lib/systemd/resolv.conf or
68 /run/systemd/resolve/resolv.conf (see below).
69
71 systemd-resolved synthesizes DNS resource records (RRs) for the
72 following cases:
73 • The local, configured hostname is resolved to all locally
75 configured IP addresses ordered by their scope, or — if none are
76 configured — the IPv4 address 127.0.0.2 (which is on the local
77 loopback interface) and the IPv6 address ::1 (which is the local
78 host).
79 • The hostnames "localhost" and "localhost.localdomain" as well as
81 any hostname ending in ".localhost" or ".localhost.localdomain" are
82 resolved to the IP addresses 127.0.0.1 and ::1.
83 • The hostname "_gateway" is resolved to all current default routing
85 gateway addresses, ordered by their metric. This assigns a stable
86 hostname to the current gateway, useful for referencing it
87 independently of the current network configuration state.
88 • The hostname "_outbound" is resolved to the local IPv4 and IPv6
90 addresses that are most likely used for communication with other
91 hosts. This is determined by requesting a routing decision to the
92 configured default gateways from the kernel and then using the
93 local IP addresses selected by this decision. This hostname is only
94 available if there is at least one local default gateway
95 configured. This assigns a stable hostname to the local outbound IP
96 addresses, useful for referencing them independently of the current
97 network configuration state.
98 • The mappings defined in /etc/hosts are resolved to their configured
100 addresses and back, but they will not affect lookups for
101 non-address types (like MX). Support for /etc/hosts may be disabled
102 with ReadEtcHosts=no, see resolved.conf(5).
103
105 The lookup requests that systemd-resolved.service receives are routed
106 to the available DNS servers, LLMNR, and MulticastDNS interfaces
107 according to the following rules:
108 • Names for which synthetic records are generated (the local
110 hostname, "localhost" and "localdomain", local gateway, as listed
111 in the previous section) and addresses configured in /etc/hosts are
112 never routed to the network and a reply is sent immediately.
113 • Single-label names are resolved using LLMNR on all local interfaces
115 where LLMNR is enabled. Lookups for IPv4 addresses are only sent
116 via LLMNR on IPv4, and lookups for IPv6 addresses are only sent via
117 LLMNR on IPv6. Note that lookups for single-label synthesized names
118 are not routed to LLMNR, MulticastDNS or unicast DNS.
119 • Queries for the address records (A and AAAA) of single-label
121 non-synthesized names are resolved via unicast DNS using search
122 domains. For any interface which defines search domains, such
123 look-ups are routed to the servers defined for that interface,
124 suffixed with each of those search domains. When global search
125 domains are defined, such look-ups are routed to the global
126 servers. For each search domain, queries are performed by suffixing
127 the name with each of the search domains in turn. Additionally,
128 lookup of single-label names via unicast DNS may be enabled with
129 the ResolveUnicastSingleLabel=yes setting. The details of which
130 servers are queried and how the final reply is chosen are described
131 below. Note that this means that address queries for single-label
132 names are never sent out to remote DNS servers by default, and
133 resolution is only possible if search domains are defined.
134 • Multi-label names with the domain suffix ".local" are resolved
136 using MulticastDNS on all local interfaces where MulticastDNS is
137 enabled. As with LLMNR, IPv4 address lookups are sent via IPv4 and
138 IPv6 address lookups are sent via IPv6.
139 • Queries for multi-label names are routed via unicast DNS on local
141 interfaces that have a DNS server configured, plus the globally
142 configured DNS servers if there are any. Which interfaces are used
143 is determined by the routing logic based on search and route-only
144 domains, described below. Note that by default, lookups for domains
145 with the ".local" suffix are not routed to DNS servers, unless the
146 domain is specified explicitly as routing or search domain for the
147 DNS server and interface. This means that on networks where the
148 ".local" domain is defined in a site-specific DNS server, explicit
149 search or routing domains need to be configured to make lookups
150 work within this DNS domain. Note that these days, it's generally
151 recommended to avoid defining ".local" in a DNS server, as
152 RFC6762[2] reserves this domain for exclusive MulticastDNS use.
153 • Address lookups (reverse lookups) are routed similarly to
155 multi-label names, with the exception that addresses from the
156 link-local address range are never routed to unicast DNS and are
157 only resolved using LLMNR and MulticastDNS (when enabled).
158 If lookups are routed to multiple interfaces, the first successful
160 response is returned (thus effectively merging the lookup zones on all
161 matching interfaces). If the lookup failed on all interfaces, the last
162 failing response is returned.
163 Routing of lookups is determined by the per-interface routing domains
165 (search and route-only) and global search domains. See
166 systemd.network(5) and resolvectl(1) for a description how those
167 settings are set dynamically and the discussion of Domains= in
168 resolved.conf(5) for a description of globally configured DNS settings.
169 The following query routing logic applies for unicast DNS lookups
171 initiated by systemd-resolved.service:
172 • If a name to look up matches (that is: is equal to or has as
174 suffix) any of the configured routing domains (search or
175 route-only) of any link, or the globally configured DNS settings,
176 "best matching" routing domain is determined: the matching one with
177 the most labels. The query is then sent to all DNS servers of any
178 links or the globally configured DNS servers associated with this
179 "best matching" routing domain. (Note that more than one link might
180 have this same "best matching" routing domain configured, in which
181 case the query is sent to all of them in parallel).
182 In case of single-label names, when search domains are defined, the
184 same logic applies, except that the name is first suffixed by each
185 of the search domains in turn. Note that this search logic doesn't
186 apply to any names with at least one dot. Also see the discussion
187 about compatibility with the traditional glibc resolver below.
188 • If a query does not match any configured routing domain (either
190 per-link or global), it is sent to all DNS servers that are
191 configured on links with the DefaultRoute= option set, as well as
192 the globally configured DNS server.
193 • If there is no link configured as DefaultRoute= and no global DNS
195 server configured, one of the compiled-in fallback DNS servers is
196 used.
197 • Otherwise the unicast DNS query fails, as no suitable DNS servers
199 can be determined.
200 The DefaultRoute= option is a boolean setting configurable with
202 resolvectl or in .network files. If not set, it is implicitly
203 determined based on the configured DNS domains for a link: if there's a
204 route-only domain other than "~.", it defaults to false, otherwise to
205 true.
206 Effectively this means: in order to support single-label
208 non-synthesized names, define appropriate search domains. In order to
209 preferably route all DNS queries not explicitly matched by routing
210 domain configuration to a specific link, configure a "~." route-only
211 domain on it. This will ensure that other links will not be considered
212 for these queries (unless they too carry such a routing domain). In
213 order to route all such DNS queries to a specific link only if no other
214 link is preferred, set the DefaultRoute= option for the link to true
215 and do not configure a "~." route-only domain on it. Finally, in order
216 to ensure that a specific link never receives any DNS traffic not
217 matching any of its configured routing domains, set the DefaultRoute=
218 option for it to false.
219 See org.freedesktop.resolve1(5) for information about the D-Bus APIs
221 systemd-resolved provides.
222
224 This section provides a short summary of differences in the stub
225 resolver implemented by nss-resolve(8) together with systemd-resolved
226 and the traditional stub resolver implemented in nss-dns.
227 • Some names are always resolved internally (see Synthetic Records
229 above). Traditionally they would be resolved by nss-files if
230 provided in /etc/hosts. But note that the details of how a query is
231 constructed are under the control of the client library. nss-dns
232 will first try to resolve names using search domains and even if
233 those queries are routed to systemd-resolved, it will send them out
234 over the network using the usual rules for multi-label name routing
235 [3].
236 • Single-label names are not resolved for A and AAAA records using
238 unicast DNS (unless overridden with ResolveUnicastSingleLabel=, see
239 resolved.conf(5)). This is similar to the no-tld-query option being
240 set in resolv.conf(5).
241 • Search domains are not used for suffixing of multi-label names.
243 (Search domains are nevertheless used for lookup routing, for names
244 that were originally specified as single-label or multi-label.) Any
245 name with at least one dot is always interpreted as a FQDN.
246 nss-dns would resolve names both as relative (using search domains)
247 and absolute FQDN names. Some names would be resolved as relative
248 first, and after that query has failed, as absolute, while other
249 names would be resolved in opposite order. The ndots option in
250 /etc/resolv.conf was used to control how many dots the name needs
251 to have to be resolved as relative first. This stub resolver does
252 not implement this at all: multi-label names are only resolved as
253 FQDNs.[4]
254 • This resolver has a notion of the special ".local" domain used for
256 MulticastDNS, and will not route queries with that suffix to
257 unicast DNS servers unless explicitly configured, see above. Also,
258 reverse lookups for link-local addresses are not sent to unicast
259 DNS servers.
260 • This resolver reads and caches /etc/hosts internally. (In other
262 words, nss-resolve replaces nss-files in addition to nss-dns).
263 Entries in /etc/hosts have highest priority.
264 • This resolver also implements LLMNR and MulticastDNS in addition to
266 the classic unicast DNS protocol, and will resolve single-label
267 names using LLMNR (when enabled) and names ending in ".local" using
268 MulticastDNS (when enabled).
269 • Environment variables $LOCALDOMAIN and $RES_OPTIONS described in
271 resolv.conf(5) are not supported currently.
272
274 Four modes of handling /etc/resolv.conf (see resolv.conf(5)) are
275 supported:
276 • systemd-resolved maintains the
278 /run/systemd/resolve/stub-resolv.conf file for compatibility with
279 traditional Linux programs. This file may be symlinked from
280 /etc/resolv.conf. This file lists the 127.0.0.53 DNS stub (see
281 above) as the only DNS server. It also contains a list of search
282 domains that are in use by systemd-resolved. The list of search
283 domains is always kept up-to-date. Note that
284 /run/systemd/resolve/stub-resolv.conf should not be used directly
285 by applications, but only through a symlink from /etc/resolv.conf.
286 This file may be symlinked from /etc/resolv.conf in order to
287 connect all local clients that bypass local DNS APIs to
288 systemd-resolved with correct search domains settings. This mode of
289 operation is recommended.
290 • A static file /usr/lib/systemd/resolv.conf is provided that lists
292 the 127.0.0.53 DNS stub (see above) as only DNS server. This file
293 may be symlinked from /etc/resolv.conf in order to connect all
294 local clients that bypass local DNS APIs to systemd-resolved. This
295 file does not contain any search domains.
296 • systemd-resolved maintains the /run/systemd/resolve/resolv.conf
298 file for compatibility with traditional Linux programs. This file
299 may be symlinked from /etc/resolv.conf and is always kept
300 up-to-date, containing information about all known DNS servers.
301 Note the file format's limitations: it does not know a concept of
302 per-interface DNS servers and hence only contains system-wide DNS
303 server definitions. Note that /run/systemd/resolve/resolv.conf
304 should not be used directly by applications, but only through a
305 symlink from /etc/resolv.conf. If this mode of operation is used
306 local clients that bypass any local DNS API will also bypass
307 systemd-resolved and will talk directly to the known DNS servers.
308 • Alternatively, /etc/resolv.conf may be managed by other packages,
310 in which case systemd-resolved will read it for DNS configuration
311 data. In this mode of operation systemd-resolved is consumer rather
312 than provider of this configuration file.
313 Note that the selected mode of operation for this file is detected
315 fully automatically, depending on whether /etc/resolv.conf is a symlink
316 to /run/systemd/resolve/resolv.conf or lists 127.0.0.53 as DNS server.
317
319 SIGUSR1
320 Upon reception of the SIGUSR1 process signal systemd-resolved will
321 dump the contents of all DNS resource record caches it maintains,
322 as well as all feature level information it learnt about configured
323 DNS servers into the system logs.
324 SIGUSR2
326 Upon reception of the SIGUSR2 process signal systemd-resolved will
327 flush all caches it maintains. Note that it should normally not be
328 necessary to request this explicitly – except for debugging
329 purposes – as systemd-resolved flushes the caches automatically
330 anyway any time the host's network configuration changes. Sending
331 this signal to systemd-resolved is equivalent to the resolvectl
332 flush-caches command, however the latter is recommended since it
333 operates in a synchronous way.
334 SIGRTMIN+1
336 Upon reception of the SIGRTMIN+1 process signal systemd-resolved
337 will forget everything it learnt about the configured DNS servers.
338 Specifically any information about server feature support is
339 flushed out, and the server feature probing logic is restarted on
340 the next request, starting with the most fully featured level. Note
341 that it should normally not be necessary to request this explicitly
342 – except for debugging purposes – as systemd-resolved automatically
343 forgets learnt information any time the DNS server configuration
344 changes. Sending this signal to systemd-resolved is equivalent to
345 the resolvectl reset-server-features command, however the latter is
346 recommended since it operates in a synchronous way.
347
349 systemd(1), resolved.conf(5), dnssec-trust-anchors.d(5), nss-
350 resolve(8), resolvectl(1), resolv.conf(5), hosts(5),
351 systemd.network(5), systemd-networkd.service(8)
352
354 1. RFC3493
355 https://tools.ietf.org/html/rfc3493
356 2. RFC6762
358 https://tools.ietf.org/html/rfc6762
359 3. For example, if /etc/resolv.conf has
361 nameserver 127.0.0.53
363 search foobar.com barbar.com
364 and we look up "localhost", nss-dns will send the following queries
367 to systemd-resolved listening on 127.0.0.53:53: first
368 "localhost.foobar.com", then "localhost.barbar.com", and finally
369 "localhost". If (hopefully) the first two queries fail, systemd-
370 resolved will synthesize an answer for the third query.
371 When using nss-dns with any search domains, it is thus crucial to
373 always configure nss-files with higher priority and provide
374 mappings for names that should not be resolved using search
375 domains.
376 4. There are currently more than 1500 top-level domain names defined,
378 and new ones are added regularly, often using "attractive" names
379 that are also likely to be used locally. Not looking up multi-label
380 names in this fashion avoids fragility in both directions: a valid
381 global name could be obscured by a local name, and resolution of a
382 relative local name could suddenly break when a new top-level
383 domain is created, or when a new subdomain of a top-level domain in
384 registered. Resolving any given name as either relative or absolute
385 avoids this ambiguity.
386systemd 251 SYSTEMD-RESOLVED.SERVICE(8)