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 the API Documentation[1] for details. Usage of this API is
23 generally recommended to clients as it is asynchronous and fully
24 featured (for example, properly returns DNSSEC validation status
25 and interface scope for addresses as necessary for supporting
26 link-local networking).
27 · The glibc getaddrinfo(3) API as defined by RFC3493[2] 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 host names 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, user request made 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) and the IPv6 address ::1 (which is the local host).
68 · The hostnames "localhost" and "localhost.localdomain" (as well as
70 any hostname ending in ".localhost" or ".localhost.localdomain")
71 are resolved to the IP addresses 127.0.0.1 and ::1.
72 · The hostname "_gateway" is resolved to all current default routing
74 gateway addresses, ordered by their metric. This assigns a stable
75 hostname to the current gateway, useful for referencing it
76 independently of the current network configuration state.
77 · The mappings defined in /etc/hosts are resolved to their configured
79 addresses and back, but they will not affect lookups for
80 non-address types (like MX).
81
83 Lookup requests are routed to the available DNS servers, LLMNR and
84 MulticastDNS interfaces according to the following rules:
85 · Lookups for the special hostname "localhost" are never routed to
87 the network. (A few other, special domains are handled the same
88 way.)
89 · Single-label names are routed to all local interfaces capable of IP
91 multicasting, using the LLMNR protocol. Lookups for IPv4 addresses
92 are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are
93 only sent via LLMNR on IPv6. Lookups for the locally configured
94 host name and the "_gateway" host name are never routed to LLMNR.
95 · Multi-label names with the domain suffix ".local" are routed to all
97 local interfaces capable of IP multicasting, using the MulticastDNS
98 protocol. As with LLMNR IPv4 address lookups are sent via IPv4 and
99 IPv6 address lookups are sent via IPv6.
100 · Other multi-label names are routed to all local interfaces that
102 have a DNS server configured, plus the globally configured DNS
103 server if there is one. Address lookups from the link-local address
104 range are never routed to DNS. Note that by default lookups for
105 domains with the ".local" suffix are not routed to DNS servers,
106 unless the domain is specified explicitly as routing or search
107 domain for the DNS server and interface. This means that on
108 networks where the ".local" domain is defined in a site-specific
109 DNS server, explicit search or routing domains need to be
110 configured to make lookups within this DNS domain work. Note that
111 today it's generally recommended to avoid defining ".local" in a
112 DNS server, as RFC6762[3] reserves this domain for exclusive
113 MulticastDNS use.
114 If lookups are routed to multiple interfaces, the first successful
116 response is returned (thus effectively merging the lookup zones on all
117 matching interfaces). If the lookup failed on all interfaces, the last
118 failing response is returned.
119 Routing of lookups may be influenced by configuring per-interface
121 domain names and other settings. See systemd.network(5) and
122 resolvectl(1) for details. The following query routing logic applies
123 for unicast DNS traffic:
124 · If a name to look up matches (that is: is equal to or has as
126 suffix) any of the configured search or route-only domains of any
127 link (or the globally configured DNS settings), the "best matching"
128 search/route-only domain is determined: the matching one with the
129 most labels. The query is then sent to all DNS servers of any links
130 or the globally configured DNS servers associated with this "best
131 matching" search/route-only domain. (Note that more than one link
132 might have this same "best matching" search/route-only domain
133 configured, in which case the query is sent to all of them in
134 parallel).
135 · If a query does not match any configured search/route-only domain
137 (neither per-link nor global), it is sent to all DNS servers that
138 are configured on links with the "DNS default route" option set, as
139 well as the globally configured DNS server.
140 · If there is no link configured as "DNS default route" and no global
142 DNS server configured, the compiled-in fallback DNS server is used.
143 · Otherwise the query is failed as no suitable DNS servers could be
145 determined.
146 The "DNS default route" option is a boolean setting configurable with
148 resolvectl or in .network files. If not set, it is implicitly
149 determined based on the configured DNS domains for a link: if there's
150 any route-only domain (not matching "~.") it defaults to false,
151 otherwise to true.
152 Effectively this means: in order to preferably route all DNS queries
154 not explicitly matched by search/route-only domain configuration to a
155 specific link, configure a "~." route-only domain on it. This will
156 ensure that other links will not be considered for the queries (unless
157 they too carry such a route-only domain). In order to route all such
158 DNS queries to a specific link only in case no other link is
159 preferable, then set the "DNS default route" option for the link to
160 true, and do not configure a "~." route-only domain on it. Finally, in
161 order to ensure that a specific link never receives any DNS traffic not
162 matching any of its configured search/route-only domains, set the "DNS
163 default route" option for it to false.
164 See the resolved D-Bus API Documentation[1] for information about the
166 APIs systemd-resolved provides.
167
169 Four modes of handling /etc/resolv.conf (see resolv.conf(5)) are
170 supported:
171 · systemd-resolved maintains the
173 /run/systemd/resolve/stub-resolv.conf file for compatibility with
174 traditional Linux programs. This file may be symlinked from
175 /etc/resolv.conf. This file lists the 127.0.0.53 DNS stub (see
176 above) as the only DNS server. It also contains a list of search
177 domains that are in use by systemd-resolved. The list of search
178 domains is always kept up-to-date. Note that
179 /run/systemd/resolve/stub-resolv.conf should not be used directly
180 by applications, but only through a symlink from /etc/resolv.conf.
181 This file may be symlinked from /etc/resolv.conf in order to
182 connect all local clients that bypass local DNS APIs to
183 systemd-resolved with correct search domains settings. This mode of
184 operation is recommended.
185 · A static file /usr/lib/systemd/resolv.conf is provided that lists
187 the 127.0.0.53 DNS stub (see above) as only DNS server. This file
188 may be symlinked from /etc/resolv.conf in order to connect all
189 local clients that bypass local DNS APIs to systemd-resolved. This
190 file does not contain any search domains.
191 · systemd-resolved maintains the /run/systemd/resolve/resolv.conf
193 file for compatibility with traditional Linux programs. This file
194 may be symlinked from /etc/resolv.conf and is always kept
195 up-to-date, containing information about all known DNS servers.
196 Note the file format's limitations: it does not know a concept of
197 per-interface DNS servers and hence only contains system-wide DNS
198 server definitions. Note that /run/systemd/resolve/resolv.conf
199 should not be used directly by applications, but only through a
200 symlink from /etc/resolv.conf. If this mode of operation is used
201 local clients that bypass any local DNS API will also bypass
202 systemd-resolved and will talk directly to the known DNS servers.
203 · Alternatively, /etc/resolv.conf may be managed by other packages,
205 in which case systemd-resolved will read it for DNS configuration
206 data. In this mode of operation systemd-resolved is consumer rather
207 than provider of this configuration file.
208 Note that the selected mode of operation for this file is detected
210 fully automatically, depending on whether /etc/resolv.conf is a symlink
211 to /run/systemd/resolve/resolv.conf or lists 127.0.0.53 as DNS server.
212
214 SIGUSR1
215 Upon reception of the SIGUSR1 process signal systemd-resolved will
216 dump the contents of all DNS resource record caches it maintains,
217 as well as all feature level information it learnt about configured
218 DNS servers into the system logs.
219 SIGUSR2
221 Upon reception of the SIGUSR2 process signal systemd-resolved will
222 flush all caches it maintains. Note that it should normally not be
223 necessary to request this explicitly – except for debugging
224 purposes – as systemd-resolved flushes the caches automatically
225 anyway any time the host's network configuration changes. Sending
226 this signal to systemd-resolved is equivalent to the resolvectl
227 flush-caches command, however the latter is recommended since it
228 operates in a synchronous way.
229 SIGRTMIN+1
231 Upon reception of the SIGRTMIN+1 process signal systemd-resolved
232 will forget everything it learnt about the configured DNS servers.
233 Specifically any information about server feature support is
234 flushed out, and the server feature probing logic is restarted on
235 the next request, starting with the most fully featured level. Note
236 that it should normally not be necessary to request this explicitly
237 – except for debugging purposes – as systemd-resolved automatically
238 forgets learnt information any time the DNS server configuration
239 changes. Sending this signal to systemd-resolved is equivalent to
240 the resolvectl reset-server-features command, however the latter is
241 recommended since it operates in a synchronous way.
242
244 systemd(1), resolved.conf(5), dnssec-trust-anchors.d(5), nss-
245 resolve(8), resolvectl(1), resolv.conf(5), hosts(5),
246 systemd.network(5), systemd-networkd.service(8)
247
249 1. API Documentation
250 https://www.freedesktop.org/wiki/Software/systemd/resolved
251 2. RFC3493
253 https://tools.ietf.org/html/rfc3493
254 3. RFC6762
256 https://tools.ietf.org/html/rfc6762
257systemd 245 SYSTEMD-RESOLVED.SERVICE(8)