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 synthetizes 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 mappings defined in /etc/hosts are resolved to their configured
80 addresses and back, but they will not affect lookups for
81 non-address types (like MX). Support for /etc/hosts may be disabled
82 with ReadEtcHosts=no, see resolved.conf(5).
83
85 Lookup requests are routed to the available DNS servers, LLMNR, and
86 MulticastDNS interfaces according to the following rules:
87 · Names for which synthetic records are generated (the local
89 hostname, "localhost" and "localdomain", local gateway, as listed
90 in the previous section) and addresses configured in /etc/hosts are
91 never routed to the network and a reply is sent immediately.
92 · Single-label names are resolved using LLMNR on all local interfaces
94 where LLMNR is enabled. Lookups for IPv4 addresses are only sent
95 via LLMNR on IPv4, and lookups for IPv6 addresses are only sent via
96 LLMNR on IPv6. Note that lookups for single-label synthetized names
97 are not routed to LLMNR, MulticastDNS or unicast DNS.
98 · Queries for the address records (A and AAAA) of single-label
100 non-synthetized names are resolved via unicast DNS using search
101 domains. For any interface which defines search domains, such
102 look-ups are routed to that interface, suffixed with each of the
103 search domains defined on that interface in turn. When global
104 search domains are defined, such look-ups are routed to all
105 interfaces, suffixed by each of the global search domains in turn.
106 Additionally, lookup of single-label names via unicast DNS may be
107 enabled with the ResolveUnicastSingleLabel=yes setting. The details
108 of which servers are queried and how the final reply is chosen are
109 described below. Note that this means that address queries for
110 single-label names are never sent out to remote DNS servers by
111 default, and resoulution is only possible if search domains are
112 defined.
113 · Multi-label names with the domain suffix ".local" are resolved
115 using MulticastDNS on all local interfaces where MulticastDNS is
116 enabled. As with LLMNR, IPv4 address lookups are sent via IPv4 and
117 IPv6 address lookups are sent via IPv6.
118 · Queries for multi-label names are routed via unicast DNS on local
120 interfaces that have a DNS server configured, plus the globally
121 configured DNS servers if there are any. Which interfaces are used
122 is determined by the routing logic based on search and route-only
123 domains, described below. Note that by default, lookups for domains
124 with the ".local" suffix are not routed to DNS servers, unless the
125 domain is specified explicitly as routing or search domain for the
126 DNS server and interface. This means that on networks where the
127 ".local" domain is defined in a site-specific DNS server, explicit
128 search or routing domains need to be configured to make lookups
129 work within this DNS domain. Note that these days, it's generally
130 recommended to avoid defining ".local" in a DNS server, as
131 RFC6762[2] reserves this domain for exclusive MulticastDNS use.
132 · Address lookups (reverse lookups) are routed similarly to
134 multi-label names, with the exception that addresses from the
135 link-local address range are never routed to unicast DNS and are
136 only resolved using LLMNR and MulticastDNS (when enabled).
137 If lookups are routed to multiple interfaces, the first successful
139 response is returned (thus effectively merging the lookup zones on all
140 matching interfaces). If the lookup failed on all interfaces, the last
141 failing response is returned.
142 Routing of lookups is determined by the per-interface routing domains
144 (search and route-only) and global search domains. See
145 systemd.network(5) and resolvectl(1) for a description how those
146 settings are set dynamically and the discussion of Domains= in
147 resolved.conf(5) for a description of globally configured DNS settings.
148 The following query routing logic applies for unicast DNS traffic:
150 · If a name to look up matches (that is: is equal to or has as
152 suffix) any of the configured routing domains (search or
153 route-only) of any link, or the globally configured DNS settings,
154 "best matching" routing domain is determined: the matching one with
155 the most labels. The query is then sent to all DNS servers of any
156 links or the globally configured DNS servers associated with this
157 "best matching" routing domain. (Note that more than one link might
158 have this same "best matching" routing domain configured, in which
159 case the query is sent to all of them in parallel).
160 In case of single-label names, when search domains are defined, the
162 same logic applies, except that the name is first suffixed by each
163 of the search domains in turn. Note that this search logic doesn't
164 apply to any names with at least one dot. Also see the discussion
165 about compatiblity with the traditional glibc resolver below.
166 · If a query does not match any configured routing domain (either
168 per-link or global), it is sent to all DNS servers that are
169 configured on links with the DefaultRoute= option set, as well as
170 the globally configured DNS server.
171 · If there is no link configured as DefaultRoute= and no global DNS
173 server configured, one of the compiled-in fallback DNS servers is
174 used.
175 · Otherwise the unicast DNS query fails, as no suitable DNS servers
177 can be determined.
178 The DefaultRoute= option is a boolean setting configurable with
180 resolvectl or in .network files. If not set, it is implicitly
181 determined based on the configured DNS domains for a link: if there's a
182 route-only domain other than "~.", it defaults to false, otherwise to
183 true.
184 Effectively this means: in order to support single-label
186 non-synthetized names, define appropriate search domains. In order to
187 preferably route all DNS queries not explicitly matched by routing
188 domain configuration to a specific link, configure a "~." route-only
189 domain on it. This will ensure that other links will not be considered
190 for these queries (unless they too carry such a routing domain). In
191 order to route all such DNS queries to a specific link only if no other
192 link is preferred, set the DefaultRoute= option for the link to true
193 and do not configure a "~." route-only domain on it. Finally, in order
194 to ensure that a specific link never receives any DNS traffic not
195 matching any of its configured routing domains, set the DefaultRoute=
196 option for it to false.
197 See the resolved D-Bus API Documentation[3] for information about the
199 APIs systemd-resolved provides.
200
202 This section provides a short summary of differences in the stub
203 resolver implemented by nss-resolve(8) together with systemd-resolved
204 and the tranditional stub resolver implemented in nss-dns(8).
205 · Some names are always resolved internally (see Synthetic Records
207 above). Traditionally they would be resolved by nss-files, and only
208 if provided in /etc/hosts.
209 · Single-label names are not resolved for A and AAAA records using
211 unicast DNS (unless overriden with ResolveUnicastSingleLabel=, see
212 resolved.conf(5)). This is similar to the no-tld-query option being
213 set in resolv.conf(5).
214 · Search domains are not used for suffixing of multi-label names.
216 (Search domains are nevertheless used for lookup routing, for names
217 that were originally specified as single-label or multi-label.) Any
218 name with at least one dot is always interpreted as a FQDN.
219 nss-dns would resolve names both as relative (using search domains)
220 and absolute FQDN names. Some names would be resolved as relative
221 first, and after that query has failed, as absolute, while other
222 names would be resolved in opposite order. The ndots option in
223 /etc/resolv.conf was used to control how many dots the name needs
224 to have to be resolved as relative first. This stub resolver does
225 not implement this at all: multi-label names are only resolved as
226 FQDNs. (There are currently more than 1500 top-level domain names
227 defined, and new ones are added regularly, often using "attractive"
228 names that are also likely to be used locally. Not looking up
229 multi-label names in this fashion avoids fragility in both
230 directions: a valid global name could be obscured by a local name,
231 and resolution of a relative local name could suddenly break when a
232 new top-level domain is created, or when a new subdomain of a
233 top-level domain in registered. Resolving any given name as either
234 relative or absolute avoids this ambiguity.)
235 · This resolver has a notion of the special ".local" domain used for
237 MulticastDNS, and will not route queries with that suffix to
238 unicast DNS servers unless explicitly configured, see above. Also,
239 reverse lookups for link-local addresses are not sent to unicast
240 DNS servers.
241 · This resolver reads and caches /etc/hosts internally. (In other
243 words, nss-resolve replaces nss-files in addition to nss-dns).
244 Entries in /etc/hosts have highest priority.
245 · This resolver also implements LLMNR and MulticastDNS in addition to
247 the classic unicast DNS protocol, and will resolve single-label
248 names using LLMNR (when enabled) and names ending in ".local" using
249 MulticastDNS (when enabled).
250 · Environment variables $LOCALDOMAIN and $RES_OPTIONS described in
252 resolv.conf(5) are not supported currently.
253
255 Four modes of handling /etc/resolv.conf (see resolv.conf(5)) are
256 supported:
257 · systemd-resolved maintains the
259 /run/systemd/resolve/stub-resolv.conf file for compatibility with
260 traditional Linux programs. This file may be symlinked from
261 /etc/resolv.conf. This file lists the 127.0.0.53 DNS stub (see
262 above) as the only DNS server. It also contains a list of search
263 domains that are in use by systemd-resolved. The list of search
264 domains is always kept up-to-date. Note that
265 /run/systemd/resolve/stub-resolv.conf should not be used directly
266 by applications, but only through a symlink from /etc/resolv.conf.
267 This file may be symlinked from /etc/resolv.conf in order to
268 connect all local clients that bypass local DNS APIs to
269 systemd-resolved with correct search domains settings. This mode of
270 operation is recommended.
271 · A static file /usr/lib/systemd/resolv.conf is provided that lists
273 the 127.0.0.53 DNS stub (see above) as only DNS server. This file
274 may be symlinked from /etc/resolv.conf in order to connect all
275 local clients that bypass local DNS APIs to systemd-resolved. This
276 file does not contain any search domains.
277 · systemd-resolved maintains the /run/systemd/resolve/resolv.conf
279 file for compatibility with traditional Linux programs. This file
280 may be symlinked from /etc/resolv.conf and is always kept
281 up-to-date, containing information about all known DNS servers.
282 Note the file format's limitations: it does not know a concept of
283 per-interface DNS servers and hence only contains system-wide DNS
284 server definitions. Note that /run/systemd/resolve/resolv.conf
285 should not be used directly by applications, but only through a
286 symlink from /etc/resolv.conf. If this mode of operation is used
287 local clients that bypass any local DNS API will also bypass
288 systemd-resolved and will talk directly to the known DNS servers.
289 · Alternatively, /etc/resolv.conf may be managed by other packages,
291 in which case systemd-resolved will read it for DNS configuration
292 data. In this mode of operation systemd-resolved is consumer rather
293 than provider of this configuration file.
294 Note that the selected mode of operation for this file is detected
296 fully automatically, depending on whether /etc/resolv.conf is a symlink
297 to /run/systemd/resolve/resolv.conf or lists 127.0.0.53 as DNS server.
298
300 SIGUSR1
301 Upon reception of the SIGUSR1 process signal systemd-resolved will
302 dump the contents of all DNS resource record caches it maintains,
303 as well as all feature level information it learnt about configured
304 DNS servers into the system logs.
305 SIGUSR2
307 Upon reception of the SIGUSR2 process signal systemd-resolved will
308 flush all caches it maintains. Note that it should normally not be
309 necessary to request this explicitly – except for debugging
310 purposes – as systemd-resolved flushes the caches automatically
311 anyway any time the host's network configuration changes. Sending
312 this signal to systemd-resolved is equivalent to the resolvectl
313 flush-caches command, however the latter is recommended since it
314 operates in a synchronous way.
315 SIGRTMIN+1
317 Upon reception of the SIGRTMIN+1 process signal systemd-resolved
318 will forget everything it learnt about the configured DNS servers.
319 Specifically any information about server feature support is
320 flushed out, and the server feature probing logic is restarted on
321 the next request, starting with the most fully featured level. Note
322 that it should normally not be necessary to request this explicitly
323 – except for debugging purposes – as systemd-resolved automatically
324 forgets learnt information any time the DNS server configuration
325 changes. Sending this signal to systemd-resolved is equivalent to
326 the resolvectl reset-server-features command, however the latter is
327 recommended since it operates in a synchronous way.
328
330 systemd(1), resolved.conf(5), dnssec-trust-anchors.d(5), nss-
331 resolve(8), resolvectl(1), resolv.conf(5), hosts(5),
332 systemd.network(5), systemd-networkd.service(8)
333
335 1. RFC3493
336 https://tools.ietf.org/html/rfc3493
337 2. RFC6762
339 https://tools.ietf.org/html/rfc6762
340 3. resolved D-Bus API Documentation
342 https://www.freedesktop.org/wiki/Software/systemd/resolved
343systemd 246 SYSTEMD-RESOLVED.SERVICE(8)